neonatal head and torso vibration c the author(s) 2016 xx...

Neonatal head and torso vibrationexposure during inter-hospital transfer

Journal TitleXX(X):1–15c©The Author(s) 2016

Reprints and permission:sagepub.co.uk/journalsPermissions.navDOI: 10.1177/ToBeAssignedwww.sagepub.com/

Laurence Blaxter 1, Mildrid Yeo 2, Donal McNally 1, John Crowe 1, Caroline Henry 2, Sarah Hill 2,Neil Mansfield 3, Andrew Leslie 4, Don Sharkey 3,4

AbstractInter-hospital transport of premature infants is increasingly common given the centralisation of neonatal intensivecare. However, it is known to be associated with anomalously increased morbidity, most notably brain injury, and withincreased mortality from multifactorial causes. Surprisingly, there have been relatively few previous studies investigatingthe levels of mechanical shock and vibration hazard present during this vehicular transport pathway.Using a custom inertial datalogger, and analysis software, we quantify vibration and linear head acceleration. Mountingmultiple inertial sensing units on the forehead and torso of neonatal patients and a preterm manikin, and on the chassisof transport incubators over the duration of inter-site transfers, we find that the resonant frequency of the mattress andharness system currently used to secure neonates inside incubators is ∼9Hz. This couples to vehicle chassis vibration,increasing vibration exposure to the neonate. The vibration exposure per journey (A(8) using the ISO2631 standard)was at least 20% of the action point value of current EU regulations over all 12 neonatal transports studied, reaching70% in two cases. Direct injury risk from linear head acceleration (HIC15) was negligible.Although the overall hazard was similar, vibration isolation differed substantially between sponge and air mattresses,with a manikin. Using a GPS datalogger alongside inertial sensors, vibration increased with vehicle speed only above60km/h. These preliminary findings suggest there is scope to engineer better systems for transferring sick infants, thuspotentially improving their outcomes.

Keywordsvibration hazard, shock hazard, linear head acceleration, neonatal, brain injury, monitoring, intra-ventricularhaemorrhage

Introduction

Newborn inter-hospital transfer is important in order toprovide the centralised neonatal intensive care deliveredinmany countries. In the UK, over 16,000 neonatal transfersoccur every year and these are on the increase. Babies aretransferred within a trolley mounted incubator, which isheavily laden with equipment. Secure mechanical fixationof this unit gives gives improved safety in the event of acrash or rapid deceleration, and is provided by mountingpoints in the ambulance floor into which the trolley isthen clamped. However, although these precautions mitigateagainst injury in the event of a road traffic accident, evenin the absence of such events, a number of studies havedemonstrated an increased rate of mortality and morbidityfollowing neonatal transfer between hospital sites (1; 2;3). Of particular concern is the association with braininjury, for example Levene et al found an anomalouslyhigh incidence of Intra-ventricular Haemorrhage (IVH) inoutborn and transferred, compared with inborn neonates (4).These findings have led to questions regarding other lessobvious hazards encountered during transport (5). As theincubators, equipment and staff used during transports arevery similar to those in the Neonatal Intensive Care Unit(NICU), and therefore the care provided remains unchanged,it is suspected that vibration and linear acceleration (i.e.mechanical shock) conditions encountered during transport

may be one of the key injury mechanisms. Indeed, anumber of animal studies have highlighted the adverse healthimplications of vibration upon the respiratory system (6) andcardiovascular status (7). In a study of mechanical resonancein the adult brain, Laksai et al stated “our findings suggesta dangerous frequency, around which relative brain motionis maximized” (8). Grosek et al (9) found “an associationbetween daytime road transport and higher heart rate andperipheral blood leukocyte counts, which may be related tosome particular (but unidentified) stress of road ambulancetransfer” in a study of 48 transports. Hypothetically, thismayin turn result in fluctuating blood flow to brain. Fluctuations

1Bioengineering Research Group, Faculty of Engineering, University ofNottingham, University Park, Nottingham, UK.2Academic Child Health, School of Medicine, E Floor East Block,Queen’s Medical Centre, Nottingham, UK.3Dyson School of Design Engineering, Imperial College London,Exhibition Road, London, UK.4CenTre Neonatal Transport Service, Leicester and NottinghamUniversity Hospitals NHS Trusts, UK.

Corresponding author:John Crowe, University of Nottingham, Tower Building, University Park,Nottingham, Nottinghamshire, NG7 2RD, UK.

Email: [email protected]

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in cerebral blood flow are an important mechanism for IVHand subsequent poor neurological outcome (10)

In order to be able to mitigate against the deleteriouseffects of transport, improved knowledge of the damagemechanism would be beneficial. The relative hazard posedby ‘shocks’ (that cause rapid relative motion betweenskull and brain and hence damage to connective tissue),versus relatively long-term continuous vibration (and itseffect upon the body’s control systems, which may producephysiological responses) are unclear.

A number of previous studies have quantified vibrationexposure during inter-hospital transport. Shenai et al (22)studied vibration during neonatal transports using anabdominal accelerometer. RMS vibration power spectraldensity was computed, concluding that vibration exposurereaches high levels compared to adult workplace limits.Campbell et al (21) measured similar vibration magnitudes,their analysis method differing from Shenai by use of amulti axis accelerometer, whilst Gajendragadkar et al (18)used manikins together with 3 axis accelerometers (on themanikin’s head and transport incubator chassis) to measureboth vibration exposure during different transport routes, andthe isolation properties of mattress configurations. However,these studies did not use standardised methodologiesto quantify the hazards associated with whole bodyvibration: Shenai was limited by single axis measurements;Gajendragadkar failed to validate manikin models againstpatients; and all three failed to use standardised axisdependent frequency weightings.

This study aims to provide a baseline assessment of theexposure of neonates to head and torso vibrations, and linearaccelerations, that is comparable to both current assessmentsof vibration in the workplace (ISO 2631-1:1997), and totraumatic head injury in vehicle accidents (HIC15). Althoughextrapolating these adult standards to unwell neonates maybe challenging to interpret, it does offer a means of analysisthat is well grounded.

A secondary objective is to begin to address the questionas to whether transport could be made safer, for example bya redesign of the incubator or its components, or by exertingrouting and speed control over the ambulance’s journey.

Therefore we will also assess the contribution of themattress type, the road type, and the vehicle speed upon thevibration exposure to the neonate.

Method

Design and assembly of a datalogger for MEMSinertial sensors

As no off-the-shelf device suitable for use in these studiescould be found, a datalogger was custom built to recordacceleration and angular rotation rate over the course ofneonatal transports and routine vehicle transfers betweenUK hospital sites. This was a two part device, consistingof 1) a base unit containing the power supply (sufficientfor 24 hours operation) and data storage elements, and 2) aset of swappable plug in cable assemblies (2 meters long)incorporating Micro Electro-Mechanical System (MEMS)sensors.

These sensor cables incorporated two small (1cm x 3cmx 2cm) enclosures spaced along their length, with a thirdsmaller sensor board placed at one end, and encapsulatedin biocompatible (ISO10993) silicone resin. Each enclosurecontained inertial sensors: a three axis accelerometer(ADXL345, Analog devices, Northwood, MA, USA.), anda three axis rate gyroscope with build in temperaturesensor (ITG-3200, Invensense Inc., San Jose, CA, USA.).To reduce the size of the resin encapsulated sensor forforehead attachment (where a large sensor could potentiallyinduce resonant modes), a combined accelerometer and rategyroscope IC was used (LSM330, ST microelectronics,Geneva, Switzerland), this sensor had a 14mm outer diameterand 3 gram encapsulated mass. Fig1 is a labelled image ofthe complete system. By incorporating multiple sensor unitsinto a single assembly, aligned datasets could be recorded bya single datalogger, dramatically improving ease of use andreducing the potential for mislabelling in the field.

Fig 2 is a labelled image of the system installed onthe 800 gram, 25 week neonatal manikin used in allmanikin transports (a LifeForm micro-preemie, Nasco Inc,Fort Atkinson, WI, USA.). The LifeForm micro-preemieis designed to mechanically simulate a neonate, and wasemployed in the study for ethical reasons where non-standardprocedures may have posed a risk. However, mechanicalaccuracy in terms of vibration transfer is not claimed by themanufacturer.

A “S.K.I.P” harness (SKIP Harness Ltd., 447A RiddellRoad, Glendowie, Auckland, New Zealand), which isroutinely used as a patient safety restraint is seen in thisimage.

Manikin and neonatal transports used identical incubator(Draeger Air Shields TI500), and sensor configurations, withone sensor enclosure being attached to the incubator chassis(referred to as the chassis site), one to the torso in contactwith the ribcage, and finally the silicone resin encapsulatedsensor being secured to the centre of the forehead.

The accelerometers and rate gyroscopes were configuredto the largest possible measurement ranges of±16g and±2000◦s−1 respectively. These were found to be more thanadequate for the conditions encountered during transports,with the magnitude of the largest acceleration being 6.5g

and largest angular rate being 800◦s−1. It was not possibleto run all the sensors at their maximum sampling ratesdue to hardware limitations, so the head mounted sensorwas prioritised, with a high sensor bandwidth of 800Hzused for the LSM330 accelerometer (Table1). It should benoted that this is still lower than the 1kHz specified in theHIC15 standard, meaning that HIC15 impacts may have beenslightly underestimated by the sensor.

Processing of MEMS sensor dataAnalyses of vibration and linear acceleration hazardpotential were conducted using the ISO 2631-1:1997 (11)and HIC (Head Impact Criterion) (12; 13) standardsrespectively. MEMS accelerometers have significant (∼0.1g)zero acceleration offsets due to manufacturing tolerances,and their sensitivity typically differs from the publishedspecification by several percent. Offsets typically changewith both sensor temperature and device ageing, and so highaccuracy calibration can be challenging. For this reason an

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Figure 1. Labelled image of the inertial datalogging system.

Figure 2. Labelled image of the sensors installed on a neonatal manikin.

Location Sensor type Manufacturer Part No.Sample

rate (Hz)Anti-aliasing

filter(Hz)Rawbits

Forehead AccelerometerST Micro. LSM330

1600 800 12Rate Gyro &Temperature

380 100 16

Torso &Chassis

AccelerometerAnaloguedevices

ADXL345 100 50 13

Rate Gyro &Temperature

Invensense ITG-3200 100 42 16

Table 1. Summary of sensor configuration.

accelerometer gain and offset compensation algorithm wasused to estimate gain and offset correction terms for eachaccelerometer sensor axis on a per journey basis. The simple6 point accelerometer calibration process described by

Titterton and Weston (14) was trialled for sensor calibration,but required each sensor to be held stationary for severalseconds in six different orientations, a complex procedurethat was found to be unreliable when attempted in the field.



For this reason a “calibration free” approach was taken,requiring no explicit calibration manoeuvres at any point.The built in fminsearch function from the GNU-Octaveenvironment was used to optimise a six component vector(consisting of 3 offset terms and 3 gain terms) so as tominimize a cost function defined as the median differencebetween the lengths of the corrected acceleration vectors and1g. The procedure converges on the correct gain and offsetvalues under the condition that the median true accelerationis of magnitude 1g, and performed reliably in real worldapplication to the recorded datasets.

ISO 2631-1 vibration analysis For the application of ISO2631 vibration weighting, the horizontal plane frequencyweighting function, Wd, and vertical plane function, Wk,were used with torso and forehead data. The motion sicknessfunction, Wf , was not considered as its appropriatenesswas unclear, and head weighting used Wk as opposed tothe Wj head vibration function, as there was a soft pillowbeneath the head in all transports. Weightings were appliedusing scripts running in the GNU-Octave environment, withfilter coefficients generated using the procedures outlinedbyRimell and Mansfield (15).

As ISO 2631 defines sensor axes relative to a standardbody position, movement of the patient and practicalconsiderations when mounting the transducers on the patientcreated a significant challenge. This problem was overcomeusing a signal processing algorithm (similar to those usedin auto-pilot systems) to track orientation and transformthe acceleration data into vertical and horizontal planecomponents that correspond with ISO 2631 weightingschemes. The three axis sensor data from the accelerometerand rate gyroscope on each sensor were transformed from“sensor body” (x,y,z) or local sensor axis co-ordinates to afixed “world” co-ordinate system (N,E,D) of fixed horizontal(North, East) and vertical (Down) axes.

The orientation tracking algorithm is illustrated schemat-ically in Fig 3. An Extended Kalman filter (EKF) wasused, with a 7 component state vector consisting of threegyroscope bias terms (bx,y,z, the estimated rate gyroscopeoutput in a stationary state), and estimated attitude quater-nion (sensor body relative to world co-ordinate system). Inmost previous strap-down IMU systems, a magnetometerhas been used to allow orientation around the vertical axis(yaw) to be determined. No magnetometer was used here, asabsolute heading was not required for sensor re-orientation.

The orientation tracker is based on the algorithm describedby Foxlin ((16) Fig 2, the “Direct Kalman” approach),with modifications: no magnetometer; and the accelerometermeasurement error covariance matrix scaled using theabsolute difference between the magnitude of the measuredacceleration and a 1g acceleration (i.e. 9.8ms−2). Thiscovariance scaling reduced the influence of prolongedvehicle acceleration on gyroscope bias and orientation; aprolonged period of acceleration leads to a non verticalacceleration vector (biassing the filter), but also to anacceleration with magnitude greater than 1g, allowing suchacceleration events to be identified and assigned a high errorwhen input into the EKF.

Once the acceleration vectors had been rotated into theNED frame (Aworld in the figure), it was assumed that the

patient was in a reclining position, and that the ambulancechassis was horizontal. Although neither of these conditionswill be completely satisfied in practice, it was judged that anydeviations of the vehicle from horizontal were likely to beof short duration and limited tilt angle (likely<5◦), makingthem a less significant cause of mis-modelling than imperfectpatient orientation relative to a true reclined position, anerror source that cannot easily be controlled. It should alsobe noted that the ISO 2631 specification states that “thesensitive axes of transducers may deviate from the preferredaxes by up to 15◦ where necessary”. In the world or NEDframe, North was treated as patient z axis, East as patienty, and Down as the patient x axis. The ISO 2631 weightingfilters could then be applied (using the algorithm describedby Rimell and Mansfield (15)), giving weighted outputs atthe raw accelerometer sample rates.

Further windowing of weighted data into 10 secondintervals, followed by calculation of the RMS vibration on aper window basis was then undertaken, allowing analysis ofvibration as a function of road class and vehicle speed, withthe ten second period being chosen based upon considerationof typical vehicle acceleration. Calculation of crest factor andeight hour equivalent vibration exposure, or A(8) proceededas per ISO 2631 (7.2.3).

HIC15 analysis of linear head acceleration The HeadImpact Criterion (HIC) is defined by the NHTSA (NationalHighway Traffic Safety Administration) (13), Chapter 2.HIC is the acceleration ing units integrated with respectto time over a rolling window, and raised to the power of2.5 following normalisation with respect to the integrationperiod (equation1). HIC can be defined over arbitrarywindow lengths, these usually being denoted in ms using asubscript. HIC15 was implemented as 15ms is a commonwindow length, used in for example the EEVC workinggroup report (17).

HIC = (t2 − t1)

[∫ t2

t1a(t)dt

t2 − t1

]2.5

(1)

HIC15 was assessed using the head sensor accelerometer.A rolling window method was used to process the Aworld

accelerometer data (which was gain and offset compensatedand rotated into world space, but not changed in magnitudeby the quaternion rotation), with local maxima of HIC abovea threshold of 0.3 being logged.

Although extrapolation of HIC15 thresholds down toneonatal patients is difficult, extrapolation to newborn and12 month old subjects has been attempted, for exampleEppinger et al suggested a limit value of 390 for 12 monthold subjects (13), and noted that values as low as 200have been suggested elsewhere. Using the formulaλHIC =λ2.5σf − λ−1.5

L , whereλHIC is the scaling factor for HIC,λσf is for tissue mechanical maturity, andλL is for length,Johannsen et al (19) discuss scaling of HIC with age (fromthe validated Q3 dummy, and with data from the CASPERproject). Using tensile strength data for a 22 week foetusand the dimensions of the manikin used in this study gives ascaling factor ofλHIC = 0.13. Applying this scaling factorto those validated for the Q3 dummy gives injury thresholdsof HIC15=99 and 130 for 20% and 50% risks of an AIS 3+injury respectively. The European Enhanced Vehicle-Safety


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Figure 3. Flow diagram showing data flow through the orientation tracker.

Committee (EEVC) working group 12 and 18 report (17)reaches very similar figures using the CHILD project dataset.

Analysis of vehicle speed and road class An off-the-shelfGPS datalogger (M-1200E, Holux Technology Ltd, Taiwan),installed in the vehicle cab recorded vehicle position andvelocity over the course of all journeys. As data collectiontook place over several months, a correlation technique(between vibration and GPS vehicle speed) was employedto ensure correct data alignment (to within 5 seconds).

Road classification used the open access“nominatim” server run by openstreetmap(nominatim.openstreetmap.org/reverse) to convert GPSpositions to location descriptors, which were then passedback to the server to find the nearest object described asa “way object” (implying a road). As this takes severalseconds, optimised querying was employed: the raw GPSdata (a 1Hz time series record) was interpolated to aphysically uniform sequence of points, one per 100m oftravel, and binary search applied to identify transition pointsbetween road classes with minimal queries. Retrieved “wayobjects”, passed back from the server as xml files, werefinally processed with string matching, and classified as oneof nine road types; motorway, expressway, trunk, primary,secondary, tertiary, tertiary link, service and unclassified.

To explore the influence of road class and vehicle speedupon vibration, the 10 second RMS weighted vibrationintervals were matched to the corresponding road class andvehicle speed, and then binned to calculate the total numberof intervals (and thus travel duration) falling within a rangeof RMS vibrations (0.1ms−2 increments), and vehicle speeds(10km/h increments) for the entire dataset.

Selection of patient and manikin transports

A convenience sample was recruited, consisting of infantsrequiring transfer by the CenTre transport service betweenTrent Perinatal Network neonatal units. Patient inclusioncriteria were pragmatic, including all infants who were caredfor in a neonatal unit and required transfer, but excludingthose undergoing palliative care. Manikin studies took placeon ambulance journeys when no patient being transferred(i.e. before or after transferring a patient), allowing thesejourneys to serve as direct validators: using the sametransport system, vehicle, and routes. The CenTre serviceroutinely uses a sponge mattress for transfer and a “S.K.I.P.”harness as the patient safety restraint.

To investigate the influence of mattress type uponvibration, the manikin studies were divided into threegroups, which used: the stock Draeger incubator mattresses,a polyurethane foam material (sponge) also used in allneonatal patient transports; gel mattresses (Squishon, PhilipsHealthcare); and air mattresses (Repose Babynest, FrontierTherapeutics, Blackwood, UK).

The journeys studied were within a group of 16 hospitalsites, and all but four of the routes were unique. The UKtransport fleet consists of numerous vehicle types, but tominimised confounding effects, transports were constrainedto a single vehicle make and model: Volkswagen LT-35 (3Ldiesel). The LT-35 has dual wishbone front suspension witha leaf sprung rear live axle, a layout used in most of the UKvehicle fleet, and identical to the Chevrolet P-30 employedin a previous study by Shenai et al (22).

Informed consent was obtained in writing from thepatient’s parents or legal guardian before all neonatal patienttransports commenced.



Results

Data was recorded from a total of 35 ambulance journeys, 12of which were neonatal transports, with the others using themanikin. Total duration was 34 hours, with journey distancesbeing between 45km and 200km. The 12 neonatal transportsinvolved a total of 10 patients, birth gestation range 24+2 to41+2 weeks, and weight range 0.9-4.6 kg, median 1.9 kg.

The forehead sensor data from three manikin journeyswas found to be corrupted due to intermittent faults with thesensor cabling, leaving 32 journeys with data from all threesensor sites. Out of the manikin transports, 10 (including 8with forehead sensor data) used a sponge mattress, 7 (6 withforehead sensor data) a gel mattress, and 6 an air mattress.

Frequency space vibration analysis

Fig 4 shows a spectrogram (using ten second bins) ofvibration at the forehead site over the course of a typicalmanikin transport with sponge mattress. The x, y, and zsensor axes are mapped to red, green, and blue respectivelyin the image, meaning that the colour separation indicatesvibration along different axes.

Several features correspond to events noted during theambulance journey: 1) periods of very low vibration whenthe vehicle was stationary (vertical bands, e.g. at time A),with the engine idling during some of these (e.g. time B),this leading to horizontal line C at approximately 7Hz with14Hz harmonic D; and 2) short periods of broad spectrumnoise. There are two short periods of broad noise, at timesE and F, which correspond to times when the manikin anddatalogging equipment was repositioned, leading to shortmechanical shocks. The vibration occurs along two distinctbands; G at 1.5Hz, and a band spanning 6.5Hz (H) to 13Hz(I). This second band has clear colour separation betweenH and I, indicating that the vibration axis is frequencydependent and thus this band may be the result of multipleresonances. Most vibration energy was concentrated between5Hz and 20Hz. The fact that colour of the engine vibrationharmonics during of idling matches that of the bands atapproximately 7Hz and 14Hz (when the vehicle is in motion)suggests that these bands may originate from the engine, asthe vibration direction is similar. The lower frequency bandG appears to arise from resonance of the vehicle suspension.

In Figs5 and6, vibration spectral density has been plottedfollowing subdivision of the data according to sensor site,axis (horizontal plane or x/y, and vertical or y axis), andjourney type. Journeys were divided into four types; threemanikin types according to mattress configuration (sponge,gel, or air), and neonatal transports as a fourth group (theseused the sponge mattress). For each sensor site and each axis,RMS vibration spectral density was calculated by averagingthe appropriate data from all of the journeys of each ofthe four journey types. These types are indicated accordingto the figure key. Two very distinct vibration peaks canbe seen in the figures. The lower frequency peak alignsclosely with the band identified earlier in the spectrogramas G, and likely originating from resonance of the vehiclesuspension. The higher frequency peak spans the 7 to 14Hzregion which was earlier identified as likely originating fromthe engine and transmission. The small spike at 26Hz inthe neonatal transports was found to have originated from

an incubator ventilation fan which was turned off duringmanikin transports. The vertical axis spectrum at the torsosite recorded during neonatal transport (Fig5b) shows verygood agreement to the results published by Shenail et al ((22)Fig 1) who used a piezoelectric accelerometer with a 2Hz to30Hz bandwidth at the same sensor site.

Compared to the incubator chassis, spectral density in the3 to 20Hz range is up to four times higher at the torso andforehead sensor sites, indicating that vibration is amplifiedover this spectral range. This finding is in agreement with aprevious study by Gajendragadkar et al (18), in which themattress system amplified vibration magnitude by factorsof between 2.2 to 3.4 (sponge) and 1.1 to 2.1 (gel type).However, this previous study is not directly comparable, asit compared RMS values computed from 50Hz low passfiltered acceleration data.

To analyse exposure hazard, a model was used consistingof an input “chassis level” function (the RMS vibrationspectral density at the chassis site), passed through afrequency dependant weighting function (Wk or Wd as perISO 2631), and finally through a site and configurationdependent gain function (computed from the study data)modelling resonant effects between the incubator chassisand the neonate or manikin. Overall RMS vibration at thehead and torso sites could then be calculated for the variousmattress configurations. Compared to a direct comparisonof weighted RMS vibration between study groups, thismethod reduces confounding effects from journey to journeyvariance in chassis vibration, allows for insight into theorigin of the vibration hazard, and enables evaluation ofconfigurations that were not part of the original dataset. Itmay thus be possible to extend the analysis to configurationssuch as neonate with air mattress, although this was notattempted as differences between the manikin and neonateswere minimal.

Fig 5c shows a 45% reduction in the amplitude of the1.5Hz peak during neonatal transport. It would appear thatdrivers may have been more attentive to road conditionsand vehicle vibration during patient transport, a hypothesissupported by the reduction in both frequency and amplitudeof the higher frequency (engine vibration related) peak. Forthis reason two “chassis level” spectral density functionswere generated, one from manikin transports, the other fromneonatal transports.

Application of the frequency weighting functions Wkand Wd to the chassis vibration is illustrated in Fig7 as the “Weighted level”. The peak in vertical axisweighted vibration spectral density at around 12Hz canbe seen to dominate this damage potential function.Mattress configurations, driving styles, or vibration dampingarrangements which minimize the amplitude of this peak,would appear to be a promising route forward if a reductionin overall vibration exposure level is desired.

The vibration gain functions were calculated betweenthe incubator chassis, and the forehead and torso sensorsby dividing torso and head spectral density functions bythe chassis spectral density. This allowed for assessmentboth of the effects of different mattress configurations uponvibration attenuation, and of the validity of the manikin asa mechanical model through comparison with the neonatalpatients.


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Figure 8. Vertical (z) acceleration gain between incubator chassis and neonatal and manikin head sensor.

Fig 8 plots vibration gain between the incubator chassis,and both the manikin (with sponge mattress) and the neonatalpatients. Although the two gain functions deviate by up to3dB (∼40%) in places, there is a less than 1dB (∼10%)deviation over the 10Hz to 18Hz frequency band, wheregain is highest and the majority of the damaging vibration isconcentrated (Fig5c). The spike around 26Hz in the neonataltrace is caused by the raw data peak identified earlier (andoriginates from a fan). Given the close match between gains,it appears that the manikin is an appropriate model for theassessment of neonatal vibration exposure. The close matchis also notable given the large range of patient masses acrossthe study group (0.9-4.6 kg), which did not overlap with the0.8 kg manikin mass. This indicates that patient mass onlyweakly affects vibration damping, and that the heterogeneity

of the sample group is unlikely to have a significant impacton the results..

Vibration gain (in decibels) was also calculated betweenthe incubator chassis and the forehead and torso sensor forthe four different journey types, and is plotted in Figs9 and10. The plot lines (left hand side axes), are vibration gainsin dB, and the dB scaled chassis sensor vibration spectraldensity, or “Chassis level”, is plotted as a shaded background(right hand axes). A gain peak (indicating resonance) isseen at 7Hz for the air mattress and around 10Hz for theother mattress configurations, with a peak gain of between3 and 5dB in the vertical axis for both the sponge and gelmattress, and the neonatal transports, with a considerablyhigher peak for the air mattress in the vertical axis (10dB forthe head at 7Hz). The response rolls off at higher frequenciesat approximately 15dB per decade in the vertical axis (i.e.


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Figure 10. Horizontal plane (x, y) acceleration gain between incubator chassis and head (a), torso (b). Plotted versus frequency forall four journey types.

slightly lower than a first order roll off), and around 20dBper decade in the horizontal from 5 to 35Hz (first order).

It is notable that, although there is a 7dB peak at 14Hz inthe horizontal axis for the gel mattress which is not presentfor the sponge mattress, the vibration gain of the gel mattressis similar to the sponge mattress, with the gel gain beingwithin 2dB of the sponge over most of the plotted frequencyrange, thus suggesting that the sponge and gel mattressesare mechanically similar. It is suspected that the large 7dBdeviation in horizontal plane attenuation at the head sitearound 7Hz (where the large resonance peak is stronglydamped in the neonates) may be related to the differentprocedures used to secure the neonates as compared to the

manikins; for the neonatal transports, air lines etc. need to beattached, changing the mechanical coupling to the head.

Another significant feature seen in the two vertical axisgain plots is that peak gain occurs at a lower frequency thanpeak “Chassis level” (i.e. chassis vibration spectral densityduring manikin transports) for all four transport types. Forneonatal transports the peak vertical vibration density, or“Chassis level (neonatal)”, occurs at approximately 12Hz,lower than the 14Hz peak of the “Chassis level”, androughly coinciding with the resonance peak of both thesponge and gel mattress with manikin, and the neonate (withsponge mattress) configurations, meaning incubator chassisvibration will drive the mattress and manikin/neonate system



on resonance. However, as the peak in air mattress gain isat a considerably lower frequency of 7Hz, it avoids such aneffect.

Finally, the two frequency weighted chassis vibrationspectral density functions (Fig7) were scaled using thevibration gain functions (Figs9 and10), and the overall RMSvibration at the head and torso sites calculated for the fourmattress configurations (manikin with sponge, gel, and air,neonate with sponge) and two chassis level functions (formanikin and neonate driving styles).

The resultant RMS vibration is plotted in Fig11. Severalfeatures are seen: the vertical axis vibration dominatesoverall exposure; the small changes in driving style duringthe neonatal transports reduced vibration magnitude byapproximately 30%; the air mattress results in an increase invibration exposure with the neonatal driving style; and thereis virtually no difference between the manikin with spongemattress, manikin with gel mattress and neonate with spongemattress. Use of an air mattress results in only a modest (∼

15%) reduction in vibration with the manikin driving style.RMS vertical axis vibration is well above the 0.315ms−2

comfort limit specified in ISO 2631, with horizontal planevibration falling well below this limit.

Thus, as well as illustrating the severity of the vibrationexperienced during neonatal transport, these preliminaryresults also suggest that replacement of sponge mattresseswith air or gel mattresses may not be beneficial; the gelmattress does not result in any significant reduction invibration exposure, and the air mattress results in slightlyincreased vibration exposure when the vehicle is drivenwithin a low engine rpm range. Furthermore, the gel mattresshas a considerably higher density (approx 1gram/ml) thanthe sponge (approximately 0.2grams/ml), presenting anincreased risk of injury in the event of a road traffic accident.

HIC15 dataHIC15 values were extremely low across the entire dataset.For this reason a more detailed analysis of the HIC15 datawas not conducted. Out of the 12 transports with neonatalpatients, HIC15 exceeded a threshold of one on only fourjourneys, with a maximum value of 9.3 being seen at theforehead sensor on one journey. The majority of patienttransports saw no HIC15 impacts above a threshold of 0.3,and the maximum HIC15 at the forehead sensor site duringmanikin transports was 13.1. These values are well belowboth the accepted limit value of 700 for adults (13), and thescaled value derived in section of 99 for 20% risk of AIS 3+.

Since the maximum HIC15 values for neonatal (9.3) andmanikin (13.1) transports were an order of magnitude smallerthan these thresholds, it would appear to be highly unlikelythat linear acceleration shocks will directly lead to significanttraumatic head injury.

A(8) exposure statisticsThe A(8) metric is defined in ISO 2631, and measurescumulative Whole Body Vibration (WBV) exposure,normalized to an 8 hour equivalent period using a nonlinearscaling. WBV is defined as the quadratic mean of theweighted horizontal plane and vertical axis vibration. TheEU vibration directive defines a daily exposure action value

of 0.5ms−2 for A(8) occupational exposures to whole-bodyvibration (Griffin et al (20)).

ISO 2631 warns that A(8) may prove inaccurate in thecase of a high crest factor (the ratio of peak filtered vibrationmagnitude to RMS vibration over the exposure interval).A fourth power integration method is suggested as moreappropriate in cases where crest factor is greater than 9, butfourth power exposure values are more difficult to interpret.Crest factor was calculated on a per journey basis, and wasgreater than 9 for the majority of transports (Fig12).

As it was hypothesised that crest factors may be biassed bya small number of acceleration events during vehicle loadingand unloading, the raw acceleration data was processed tolocate instances of high acceleration. This analysis is plottedin Fig 13, and supports the hypothesis, with shock eventsclustering around the start and end of journeys. Note thatsensors were secured to the manikin or neonate prior to thedatalogger being turned on, and before the transport trolleywas then loaded into the transport vehicle. This processwas repeated in reverse order once the vehicle reached itsdestination, with the datalogger turned off prior to sensorremoval. Thus the captured events are believed to representreal accelerations, rather than simply attachment and removalof the sensors. Exclusion of 5% of the data at the beginningand end of each journey to avoid inclusion of these clusterslowered median head crest factor to 7.8 and torso to 8.9. Forthese reasons it would appear that A(8) is an appropriatemeans of assessing cumulative exposure, as median crestfactors over the middle 90% of the transport period (whenthe vehicle was in motion and the patients were exposed tothe majority of the hazardous vibration) are below 9.

A(8) exposures were calculated from the weightedacceleration data on a per journey basis. From Fig14aand14b, 2 journeys saw an A(8) vibration exposure magnitudegreater than 0.5ms−2 at the forehead. These did not includeany neonatal transports. Vibration exposure to the torsowas slightly worse, with 4 transports exceeding 0.5ms−2,although again this included no neonatal transports.Nevertheless, the fact that neonatal exposures approach theworkplace exposure limit for healthy adults would appear tobe a significant cause for concern.

Analysis of vibration versus vehicle speed androad class

Vehicle vibration is plotted versus vehicle speed and roadclass in Fig15. The binning procedure used in this analysiswas outlined in section . As a relatively large period of timewas spent with the vehicle at low speeds on minor roads, alogarithmic colour bar has been used in one of the figures forclarity.

Maximum recorded vehicle speed was 131km/h, with 99%of travel time spent below 112km/h (70mph), and a medianspeed of 75km/h (47mph). Vehicle speed was stronglyrelated to road class; speeds exceeded 90km/h over only avery small proportion of A road travel time, whereas almostall motorway and expressway class travel was between 80and 120km/h, and most minor class road travel was below50km/h. This complicates any analysis of the effect of roadclass upon vibration. At a given speed, vibration might beexpected to be inversely related to road grade, but any such


Blaxter et al 11

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Figure 11. RMS vibration in vertical axis and horizontal plane, after application of ISO 2631 weighting functions and vibration gainto the two chassis vibration density functions. The dashed horizontal black line de-marks the ISO 2631 “comfort limit” of 0.315ms−2.

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effects appear to be small. Comparing Figs15c and 15a,lower class roads have a slightly higher median vibration atvehicle speeds between 5 and 50km/h, suggesting a poorerride on B and lower class roads. Any comparison in ridequality between A and expressway/motorway classes (Fig15b) is impossible as there is no significant overlap inspeed ranges. From Fig15a, the vibration distribution duringperiods when the speed was 0 to 10km/h is very similarto that at speeds below 70km/h. As the engine was usually

idling when the vehicle was stationary (e.g. Fig4), thissuggests that engine vibration may be the most significantvibration source at these speeds.

Although it complicates analysis of the influence of roadtype upon vibration, the very strong dependence of speedupon road type leads to important conclusions regardingchoice of transport route; for example if a speed above85km/h is desirable, then a route involving express-way ormotorway class roads should be chosen, as lower speeds are



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Figure 14. Histogram of A(8) exposure for all journeys.

found on ‘A’ classes. To further investigate the relationshipbetween vehicle speed and exposure, the median andquartiles of the weighted forehead vibration were calculatedas a function of vehicle speed (using all data recorded overall road classes), and are plotted in Fig16 (left subplot).

As speed increases, a continual upward trend in vibrationis evident, with an increasing gradient. To model thepotential influence of vehicle speed upon health risk, thevehicle range (in km) before the cumulative A(8) exposure tothe patient exceeded a threshold of 0.5ms−2 was calculatedfor each vehicle speed. Range at the median exposure levelis plotted as the blue dashed trace (on right subplot), and thequartile as the green dashed trace.

From the figure it can be seen that a vehicle speed below60km/h would increase A(8) exposure, due to the increasedjourney time. Optimal vehicle speed is unclear; the medianrange peaks at a speed of 110km/h, so it appears that vehiclespeed above this results in higher A(8) for a given journey

distance, but there is a considerable amount of oscillationinthe median line. However, vehicle speed in the 80 to 110km/hrange appears to be near optimal.

In summary; simply restricting maximum vehicle speed tothe UK motorway limit of 113km/h, and otherwise drivingat the highest speed road conditions allow is likely to resultin minimization of vibration exposure to patients duringtransport. Transport distances should be limited to below200km (125miles) or shorter if there is likely to be significanttravel at<60km/h (37mph). Routes should be chosen tominimize duration rather than distance.

Conclusions

We have demonstrated that a custom made dataloggingsystem for use with MEMS sensors can be successfullydeployed in the field to measure shock and vibration duringroutine neonatal patient transports between UK hospitalsites. Data has been recorded at an appropriate sample rate


Blaxter et al 13

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Figure 16. Median vibration and quartiles versus vehicle speed (a), and vehicle range before A(8) exposure limit exceeded (b).

for the duration of a total of 35 journeys, of which 12 were neonatal transports, and the remainder inter-site vehicle



transfers using a manikin together with three differentmattress configurations.

Throughout this study, vibration assessment standardsdesigned for adults in the workplace environment have beenemployed. This was done out of necessity, since commonlyagreed methods for assessment of vibration in neonatalpatients do not exist. Unfortunately it is not possible toaccurately extrapolate to significantly lower body masses,and so the precise levels of hazard identified here are unclear,as are the likely mechanisms of damage. Standardisedassessments for neonatal vibration hazard would aid riskquantification enormously.

Data analysis using HIC15 to quantify linear headacceleration hazard, and ISO2631-1:1997 for vibrationhazard, indicates that shock exposure is negligible but thatvibration exposure approaches and sometimes exceeds the0.5ms−2 A(8) “action value” threshold for adults in theworkplace, as defined in EU regulations. These findingsbroadly agree with previous studies of vibration duringneonatal transport, for example Campbell et al found RMSaccelerations between 0.4ms−2 and 5.6ms−2 (21), with thevehicle transports (air transport was also studied) beingtowards the lower end of this range. Shenai et al found valuesbetween 2ms−2 and 6ms−2 (22), higher than in our work.The age (1974 model year) of the vehicle used by Shenaisuggests improvements in vehicle construction may explainthis.

We have not explored influences of vehicle suspensionupon vibration conditions, however the spectral analysisindicates that engine as opposed to wheel vibration is thedominant hazard source, implying that engine mounts are adetermining factor as opposed to vehicle suspension.

Comparison of the incubator chassis to forehead gainfunctions between manikin and neonatal study groups showsgood agreement, with<2dB deviation in the 10Hz to 20Hzband where the majority of vibration hazard is concentrated.This implies that the manikin is an acceptable mechanicalmodel. Manikin data indicates that vibration exposureduring incubator transport is not reduced significantly byreplacement of the current sponge mattresses with air orgel mattresses. Replacement with air mattresses results inincreased vibration exposure under some driving conditions,and the high density of a gel mattress may increase injuryrisk in a road traffic accident.

Using a GPS datalogger in the vehicle cab, the influenceof vehicle speed and road type upon vibration exposure hasbeen investigated. Optimal speed range for minimization ofvibration exposure appears to be between 80 and 110km/h(50 to 68mph), with vehicle speed below 60km/h (37mph)increasing overall exposure due to the longer journey time.

Road type appears to weakly influence vibration, as lowerclass roads (UK ‘B’ and below) give higher WBV for agiven vehicle speed. A reduction in total vibration exposuremight be achieved through optimisation of transport routes,for example longer routes involving ‘A’ and motorway classroads could be adopted in place of shorter routes involvingminor roads, although this concept would require furtheranalysis to assess its feasibility.

As the most hazardous vibration is in the 5 to 20Hzfrequency band, which can be damped with simplemechanical shock absorbers, improved transport trolley

and incubator design may be the most promising routefor reducing patient vibration exposure. The vertical axisresonant frequency of the incubator/mattress system wasapproximately 10Hz with a sponge or gel mattresses, witharound 5dB peak gain between incubator chassis and thetorso and forehead sites. This frequency lies close to the14Hz chassis vibration peak, believed to originate from theengine and transmission, and so is easily excited. Mattressinduced resonance was highlighted in a previous study byGajendragadkar et al (18), who found vibration gain factorswhich are comparable to those measured in this study.However, this is the first study to demonstrate, in realpatients, that the neonatal head is exposed to mechanicalvibration hazard in excess of that simply observed at theincubator chassis.

Changes to the mattress, harness, and incubator system toreduce its resonant frequency and increase damping couldpotentially decrease RMS WBV by reducing: the hazardweighting at resonance (i.e. Wd and Wk); the degree ofresonant driving by vehicle chassis vibration; and the gainatresonance respectively. The air mattress appears promisingin this regard; although the air mattress did not significantlydecrease WBV, the resonant frequency was reduced to 7Hz,with lower gain at the 14Hz chassis vibration peak. A morecompliant mattress system with a peak below 7Hz may resultin an overall decrease in vibration exposure, a hypothesisthat could be tested experimentally with an underinflated airmattress.

Current transport trolleys use a stiff locking mechanismto clamp directly to the ambulance floor. Although thisresults in a significant safety improvement in the event of avehicle accident, vehicle chassis vibration can be transferreddirectly to the incubator. Hypothetically, an improvedcoupling incorporating vibration dampening componentscould dramatically reduce vibration exposure. As the highexposures found in this and several prior studies arepotentially an important mechanism of stress and subsequentbrain injury associated with inter-hospital transport in babies,it is recommended that development of such designs ispursued as a priority.

Ethics

NHS ethical approval was granted for human volunteerdata (Research ethics committee: NRES Committee EastMidlands - Nottingham 2, Ref: 12/EM/0257). Studyregistered with ClinicalTrials.gov (Ref:NCT01851668).Informed consent was obtained in writing from the patient’sparents or legal guardian before all neonatal patienttransports commenced.

Data accessibility

Data available athttps://datadryad.org/resource/doi:10.506

Competing interests

We declare we have no competing interests.


https://datadryad.org/resource/doi:10.5061/dryad.5g2nf

Blaxter et al 15

Funding

Funding for this work was given by the NottinghamHospitals Charity (ref PP-D-SHARKEY MAR13), with partfunding from a University of Nottingham Confidence inConcept award (MCPC 14102).

Acknowledgements

Thanks to Prof. Barrie Hayes-Gill (University of Nottingham),and the Queen’s Medical Centre, Nottingham for guidance andassistance with medical device risk assessment and regulatorymatters.

References

[1] Marlow N, Bennett C, Draper E, Hennessy E, Morgan A,Costeloe K. Perinatal outcomes for extremely preterm babiesin relation to place of birth in England: the EPICure 2 study.Archives of Disease in Childhood-Fetal and Neonatal Edition.2014;p. fetalneonatal–2013.

[2] Mohamed MA, Aly H. Transport of premature infants is asso-ciated with increased risk for intraventricular haemorrhage.Archives of Disease in Childhood-Fetal and Neonatal Edition.2010;95(6):F403–F407.

[3] Bowman E, Doyle LW, Murton LJ, Roy R, Kitchen WH.Increased mortality of preterm infants transferred betweentertiary perinatal centres. BMJ. 1988;297(6656):1098–1100.

[4] Levene M, Fawer C, Lamont R. Risk factors in thedevelopment of intraventricular haemorrhage in the pretermneonate. Archives of disease in childhood. 1982;57(6):410–417.

[5] Shlossman PA, Manley JS, Sciscione AC, Colmorgen G. Ananalysis of neonatal morbidity and mortality in maternal (inutero) and neonatal transports at 24-34 weeks’ gestation.American journal of perinatology. 1997;14(8):449–456.

[6] Shah S, Hudak J, Gad A, Cohen JC, Chander A. Simulatedtransport alters surfactant homeostasis and causes dose-dependent changes in respiratory function in neonatalSprague-Dawley rats. Journal of perinatal medicine.2010;38(5):535–543.

[7] Cochrane DJ, Sartor F, Winwood K, Stannard SR, NariciMV, Rittweger J. A comparison of the physiologic effectsof acute whole-body vibration exercise in young and olderpeople. Archives of physical medicine and rehabilitation.2008;89(5):815–821.

[8] Laksari K, Wu LC, Kurt M, Kuo C, Camarillo DC. Resonanceof human brain under head acceleration. Journal of The RoyalSociety Interface. 2015;12(108):20150331.

[9] Grosek S, Mlakar G, Vidmar I, Ihan A, Primozic J. Heartrate and leukocytes after air and ground transportation inartificially ventilated neonates: a prospective observationalstudy. Intensive care medicine. 2009;35(1):161–165.

[10] Perlman JM, McMenamin JB, Volpe JJ. Fluctuating cerebralblood-flow velocity in respiratory-distress syndrome: relationto the development of intraventricular hemorrhage. NewEngland Journal of Medicine. 1983;309(4):204–209.

[11] ISO 2631-1:1997(E). Mechanical Vibration and Shock-Evaluation of Human Exposure to Whole-Body Vibration.Part I: General Requirements. International StandardisationOrganisation, Geneva.; 1997.

[12] Hutchinson, John and Kaiser, Mark J and Lankarani, HamidM. The head injury criterion (HIC) functional. Appliedmathematics and computation. 1998;96(1):1–16.

[13] Eppinger R, Sun E, Bandak F, Haffner M, Khaewpong N,Maltese M, et al. Development of improved injury criteriafor the assessment of advanced automotive restraint systems–II. National Highway Traffic Safety Administration. 1999;p.1–70.

[14] Titterton, David and Weston, John. Strapdown InertialNavigation Technology (IEE Radar, Sonar, Navigation andAvionics Series). Stevenage: Institution of ElectricalEngineers; 1997.

[15] Rimell, Andrew N and Mansfield, Neil J. Design ofdigital filters for frequency weightings required for riskassessments of workers exposed to vibration. IndustrialHealth. 2007;45(4):512–519.

[16] Foxlin, Eric. Inertial head-tracker sensor fusion by acomplementary separate-bias Kalman filter. In: VirtualReality Annual International Symposium, 1996., Proceedingsof the IEEE 1996. IEEE; 1996. p. 185–194.

[17] WG12, EEVC. 18. Q-dummies Report. AdvancedChild Dummies and Injury Criteria for FrontalImpact. EEVC web site; 2008. Available from:http://www.eevc.org/?site=52.

[18] Gajendragadkar G, Boyd J, Potter D, Mellen BG, Hahn G,Shenai JP. Mechanical vibration in neonatal transport: arandomized study of different mattresses. Pediatric Research.1999;45:198A–198A.

[19] Johannsen H, Trosseille X, Lesire P, Beillas P. Estimating Q-Dummy injury criteria using the CASPER project results andscaling adult reference values. In: Proceedings of IRCOBIConference, Dublin, Ireland; 2012. .

[20] Griffin M. Minimum health and safety requirements forworkers exposed to hand-transmitted vibration and whole-body vibration in the European Union; a review. Occupationaland Environmental Medicine. 2004;61(5):387–397.

[21] Campbell AN, Lightstone AD, Smith JM, Kirpalani H,Perlman M. Mechanical vibration and sound levelsexperienced in neonatal transport. American Journal ofDiseases of Children. 1984;138(10):967–970.

[22] Shenai JP, Johnson GE, Varney RV. Mechanical vibrationinneonatal transport. Pediatrics. 1981;68(1):55–57.


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