BMES Final 821 (Dr. Marek Swoboda) By : Jayet Moon (13052081)

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BMES 821 Final ANSWERS Jayet Moon(13052081)

Basic Parameters

The actual differential ECG signal that appears between the electrodes in any lead configuration is

limited to ±5 mV in magnitude and 0.05 Hz to 150 Hz(100Hz) in frequency. The magnitude of this

actual ECG signal, together with the resolution required from the ECG signal, determines the dynamic

range requirement of the front-end. The frequency content of this signal determines the bandwidth

requirements of the analog front-end.The skin-electrode interface gives an additional dc offset of

approximately 300 mV. This offset must be manipulated such that the signal chain is not saturated. In

addition to these two signals, the human body can pick up large interference signals from power lines,

fluorescent lights. This interference can manifest itself as either a normal-mode signal or a common-

mode signal. Normal-mode interference can be mitigated by a software-implemented, 50-Hz/60-Hz

notch filter. Common-mode interference, on the other hand, is generally countered in one of three

ways,Increasing the isolation of the ground of the front-end electronics from the earth ground as much

as possible,Increasing the common-mode rejection of the signal processing circuitry (on the order of

100 dB),Driving the patient body with an out-of-phase common-mode signal (also called as the right

leg drive)

Figure 1 ECG Signal Charateristics[1]

BMES Final 821 (Dr. Marek Swoboda) By : Jayet Moon (13052081)

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Figure 2 Power Spectum of ECG[2]

The power spectral densities are calculated using the fast Fourier transform on a quarter of a second

portion of an FM recorded ECG, centered on the peak of the R wave. In a typical QRS complex of

normal duration, virtually all of the power is contained in frequencies below 30 Hz with peak power

occurring in the range of 2 to 15 Hz. It should be noted that the frequency corresponding to the location

of the peak is the heart rate of the patient,with virtually all the power contained in frequencies below 12

Hz with peak power located at about 4 Hz. Finally, notches in some QRS complexes was associated

with a broadened distribution of power in the power spectral density even though the tail from 30-100

Hz contained relatively less power than that contained in the frequency band below 30 Hz.[3]

Figure 3 ECG wave

BMES Final 821 (Dr. Marek Swoboda) By : Jayet Moon (13052081)

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The different waves that comprise the ECG represent the sequence of depolarization and repolarization

of the atria and ventricles. The ECG is recorded at a speed of 25 mm/sec, and the voltages are

calibrated so that 1 mV = 10 mm in the vertical direction. Therefore, each small 1-mm square

represents 0.04 sec (40 msec) in time and 0.1 mV in voltage. The P wave represents the wave of

depolarization that spreads from the SA node throughout the atria, and is usually 0.08 to 0.1 seconds

(80-100 ms) in duration . The QRS complex represents ventricular depolarization. Ventricular rate can

be calculated by determining the time interval between QRS complexes. The isoelectric period (ST

segment) following the QRS is the time at which the entire ventricle is depolarized and roughly

corresponds to the plateau phase of the ventricular action potential The duration of the QRS complex is

normally 0.06 to 0.1 seconds. The T wave represents ventricular repolarization and is longer in duration

than depolarization (i.e., conduction of the repolarization wave is slower than the wave of

depolarization). The Q-T interval represents the time for both ventricular depolarization and

repolarization to occur, and therefore roughly estimates the duration of an average ventricular action

potential. This interval can range from 0.2 to 0.4 seconds depending upon heart rate [4]

PATHOPHYSIOLOGY ECG Signals[5]

BMES Final 821 (Dr. Marek Swoboda) By : Jayet Moon (13052081)

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Target Signal Characteristics

We have to measure Aortic Pressure Rise in initial 50ms, so,we assume minimal time delay in pressure

transmission to Aorta by Left Ventricle and that the first derivative of ventricular pressure is

effectively and instantaneously transmitted to Aorta.

Figure 4 Ventricular Pressure derivative Spectrum of Primate Fast Fourier Transform[6]

The upper figure shows that even for primates the power spectrum dies down at frequencies greater

than 30Hz (40Hz Maximum), the human dP/dT is expected to be lower.

Figure 5 dP/dT for Humans[7] Figure 6 LV dp/dT for healthy Humans[7]

BMES Final 821 (Dr. Marek Swoboda) By : Jayet Moon (13052081)

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Figure 7 Lv dp/dt by incorporating 90 phase shift (obtained by differentiator circuit used

later)[8]

Also,the Aortic pressure itself and its derivative can be measured and calculated.

Figure 7 First Curve shows Aortic Pressre dP/dT and second curve shows Aortic Pressure [19]

Figure 7.1 The Arterial Pressure Power Spectrum[20]

BMES Final 821 (Dr. Marek Swoboda) By : Jayet Moon (13052081)

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Range: appx +1500mmhg/s to -1500 mmhg/s of ventricular dP/dT in humans[7] while that of

Aortic dP/dT is +1000mmHg/s to -400mmHg/s[19]

Their spectral analysis suggests that both the derivatives peak at lower frequencies of less than 12

Hz,with harmonics extending upto 40 Hz or more.[7][8][19][20]

Spectrum analysis of the ECG signal reveals that most of the frequencies present in the QRS complex

lie near 7-20 Hz and hence a filter with a band−pass of, say, 5 to 40 Hz would maximise the QRS

energy without losing peak power and compromising signal.

Configuration Transducers and Electrodes

LV Catheterization to insert tubing in Aorta, using fluid filled invasive pressure catheter is the method

I am using.It gives us constant monitoring,minimal delay,accuracy within a fixed bandwidth and is

useful to measure additional parameters like flow etc.We use a external strain gauge piezoelectric

transducer that converts pressure signal into electrical signal by generation of voltage by standardized

ceramic crystal when force is applied.External transducer can be re-zeroed anytime after the

measurement,facilitating calibration.

A fluid-filled, balloon-tipped catheter is required for insertion into the aorta. The balloon provides a

closed system from which pressure measurements may be made, and intraaortic pressure near the

ventricular opening is determined. The balloon is attached to a fluid-filled catheter and connected to a

pressure transducer and bridge amplifier.

The ideal balloon should be:

• infinitely thin

• infinitely flexible (i.e. conforms to the ventricular geometry)

• non-elastic (i.e. elastic energy is not expended in compressing the structure of the balloon itself)

• highly responsive (i.e. it should have a linear frequency response to > 40 Hz

Figure 8

BMES Final 821 (Dr. Marek Swoboda) By : Jayet Moon (13052081)

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We use a Gould Statham P23GB strain gauge pressure transducer coupled to the catheter. The tubing is

427516 - BD Intramedic™ Polyethylene Tubing (Sterile) (PE 50) 12 ft (12/sp) with inside

diameter 0.53mm.Alternatively a ADInstruments make #170423 Teflon catheter is, for use with

small balloons (#170403 and #170404) can be used.

Electrodes used are radiolucent foam electrodes. Radiotransparency of the electrodes enables them to

be situated on the chest wall throughout the diagnostic procedures without interfering with the

radiographic image(X Ray).Since the procedure involves catheterization, it may involve the need for

XRay.

This electrode utilizes a low chloride, conductive wet gel which provides instant electrical contact for

high quality tracings and is radiolucent. The foam substrate repels fluids and conforms easily to skin.

Its small footprint provides easy lead placement and the teardrop shape is easy to remove from the liner

as well as the patient. These are manufactured by Covidien LLC( Kendall™ Foam Wet Gel Electrodes,

1601500 Radiolucent).

We use the 12-lead ECG system, consisting of the following 12 leads, which are:

I, II, III

aVR, aVL, aVF

V1, V2, V3, V4, V5, V6

Of these 12 leads, the first six are derived from the same three measurement points..

Two of the limb leads (I, II, III) could reflect the frontal plane components, whereas one precordial lead

could be chosen for the anterior-posterior component. The combination should be sufficient to describe

completely the electric heart vector. (The lead V2 is directed closest to the x axis. It is roughly

orthogonal to the standard limb plane, which is close to the frontal plane.) To the extent that the cardiac

source can be described as a dipole, the 12-lead ECG system could be thought to have three

independent leads and nine redundant leads.

The main reason for recording all 12 leads is that it enhances pattern recognition. This combination of

leads gives an opportunity to compare the projections of the resultant vectors in two orthogonal planes

and at different angles. This is further facilitated when the polarity of the lead aVR can be changed.

P.T.O

BMES Final 821 (Dr. Marek Swoboda) By : Jayet Moon (13052081)

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Signal Conditioning

Figure 9 ECG Signal Conditioning Circuit[9]

Inferring from the ECG spectrum as shown earler, it can be seen that almost all power is concentrated

between 0- 150 Hz and this spectrum is enough to give a faithful ECG waveform.. A typical value of

the cut-off frequency is 0.05Hz. According to the IEC specification, the bandwidth of the ECG required

is from 0.5Hz to 150Hz(we may use 100Hz).

ECG Filtering

Signal processing is a huge challenge since the actual signal value will be 0.5mV in an offset

environment of 300mV. Other factors like AC power-supply interference, RF interference from surgery

equipment, and implanted devices like pace makers and physiological monitoring systems can also

impact accuracy.

Removal of baseline wander

Baseline wander is a low-frequency component present in the ECG system. This is due to offset

voltages in the electrodes, respiration, and body movement. This can cause problems in the analysis of

the ECG waveform. The offset also limits the maximum value of gain which can be obtained from the

instrumentation amplifier. At higher gains, the signal can saturate. This noise can be removed by:

The cut-off frequency should be such that the ECG is undistorted while the baseline wander must be

removed. A typical value of the cut-off frequency is 0.05Hz. Since this cut-off frequency is very low,

this method requires bulky capacitors.Two stages of gain are implemented since the offset can saturate

at the output of the instrumentation amplifier. The two-stage filter also makes the system more

complex. This system requires a low resolution ADC, typically 8 to 16 bits of resolution.

BMES Final 821 (Dr. Marek Swoboda) By : Jayet Moon (13052081)

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Removal of common mode noise

Interference usually manifests as common mode noise across both terminals of the differential

amplifier. This noise can be removed by the following methods:

Isolate the front-end ground electronics from the digital system as much as possible. Effective

system level design is extremely important in terms of the overall noise rejection.

Use instrumentation amplifiers with very high common mode rejection ratios on the order of

100dB

Drive the patient body with an inverted common mode signal. The right leg of the patient is

driven with a signal which is the inverted average of Lead_I, Lead_II, and Lead_III. Scaling the

suitably prevents common mode noise from being coupled into the system.

Shield the device using metallic shields to prevent high frequency RF from being coupled into

the system.

Use shielded cables to acquire the ECG which are driven with a common voltage to reduce

noise from being coupled.

Apart from the above methods, a number of software algorithms are present for the removal of

noise after the signal has been acquired.

Removal of power-line noise

Figure 10 ECG with 50Hz noise

The amplitude of power-line noise is very large and generally gets coupled into the system, despite care

to prevent common-mode noise in the digital domain. Power-line noise is removed by implementing a

notch filter at 50/60Hz in the digital domain [10]

BMES Final 821 (Dr. Marek Swoboda) By : Jayet Moon (13052081)

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SignaConditioning for Pressure Transducer:

CATHETER PRESURE TRANSDUCER SYSTEM:

Figure 11 Pressure Monitor Circuit on 5V Supply.[9]

Differential Circuit for dP/dT

Figure 12 and 13: Differential Band Pass Circuit and its Bode Plot [8]

Z= R1 + 1/jwC1

G=-jwC1R1/(1+jwc1R1)(1+jwC2R2)

System is a differentiator for low frequencies where wC2R2 <<1 and wC1R1<<1

BMES Final 821 (Dr. Marek Swoboda) By : Jayet Moon (13052081)

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The high frequency signal is attenuated in proportionto frequency. The system also introduces a phase

angle which is zero at low frequency and -90 at higher frequencies.

R2=3M Ohms

C1= 0.1 uF

R1= 15K ohms

C2=540 pF [8]

f1= 0.5 Hz corresponding to 30 bpm

f2 = 100 Hz

A/D Conversion

In order to faithfully record and recreate the signal, all of the frequencies with significant power must

be captured as long as the frequency response of the recording system is higher than the highest

frequency component of the signal shown in the Fourier Transform, the signal can be accurately

recreated and analyzed. The highest significant frequency content of pressure signal is slightly over 40

Hz. This frequency is critical to accurately measuring dP/dtmax, occurring at the point when the slope

of the waveform is the steepest. Thus, if frequency response were to be reduced to a value much below

40 Hz, the information in the higher frequency spikes on the Fourier Analysis would be lost and the

dP/dtmax value would be incorrect.

12-bit A to D conversion (212=4096).Pressure signal can be digitized with 12-bit resolution at a

sample rate of 500 Hz The A/D converter repeatedly measures the voltage and approximates it to the

nearest binary. In this case the nearest value is within 1/ 4096th of the input range of 10 volts (approx.

0.2 mV, or 0.002-volt increments). If the transducer was calibrated with a full range of zero to 200 mm

Hg (user defined). Thus, 0 mm Hg= −5 V=0 bits and 200 mm Hg=+5 V=12 bits=4096. The resolution

in physiological units is 1/4096th of 200 mm Hg, or approx. 0.05 mm Hg.

When the signal is reconstructed, the pressure values will always be within 0.05 mm Hg of the actual

value at the time of measurement. Given that physiologic pressure sensing devices do not have that

level of accuracy or precision, and such precision[11]

P.T.O

BMES Final 821 (Dr. Marek Swoboda) By : Jayet Moon (13052081)

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Block Diagram

Figure 15 Simplified System Diagram

Figure 16 ECG,LV dP/dT, Brachial artery pressure and LV Pressure[12]

BMES Final 821 (Dr. Marek Swoboda) By : Jayet Moon (13052081)

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As can be seen from the schematic above the QRS Pulse of the ECG leading the ventricular contraction

triggers the rise of the dP/dt waveform.Thus ECG detected first .To capture this R peak, a R peak

detector is connected to the circuit which goes high whenever the R peak appears.Assuming the delay

between catheter pressure circuit and ECG circuit is minimal, then the timer is triggered for a 50

milliseconds and it shows the voltage rise across the counter then resets till next cycle.

The detailed ECG and Pressure Measurement diagrams are shown earlier, here they are simplified for a

holistic view.

Calibration

Figure 17 Calibration of Pressure Transducer Sytem [13]

We may choose to have a double point or limited calibration over single point calibration because our

measurand is not fixed and varies over a range,although limited.

The catheter is inserted into the pressure generator. Two Statham P23Db transducers are used; the

reference transducer is attached to the dome of the pressure generator and the other transducer to the

hub of the catheter to be tested. Their signals are amplified and recorded.The upper panel is a log-log

plot of amplitude ratio versus frequency for catheter. Phase response is shown in the lower panel.

Figure 18 [13]

BMES Final 821 (Dr. Marek Swoboda) By : Jayet Moon (13052081)

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The pressure transducer must be zeroed, calibrated, and leveled to the appropriate position on the

patient. The initial step in this process is to expose the transducer to atmospheric pressure by

opening the adjacent stopcock to air, pressing the zero pressure button on the monitor, and thus

establishing the zero pressure reference value. The transducer now has a reference—ambient

atmospheric pressure—against which all intravascular pressures are measured. This process

underscores the fact that all pressures displayed on the monitor are referenced to atmospheric

pressure, outside the body. it is this air–fluid interface at the level of the stopcock that is the zero

pressure locus. This point must be aligned with a specific position on the patient to ensure the

correct transducer level.

When a significant change in pressure occurs, the zero reference value should be rechecked. This

can be accomplished quickly, by opening the stopcock and exposing the transducer to atmospheric

pressure. The monitor should be inspected to ensure that the pressure trace overlies the zero

pressure line on the display screen and the digital pressure value equals zero. Note that checking the

zero value is different from establishing the zero reference, which is done at the beginning of the

monitoring procedure. If the stopcock is exposed to atmospheric pressure and pressure is not equal

to zero, baseline drift of the transducer‘s electrical circuit may have occurred. This transducer drift

is caused by problems with membrane dome coupling to the electronic pressure transducing

elements, as well as other technical problems with the transducer, the attached electrical cable, or

the monitor itself.

Transducer calibration was the next step following the zeroing procedure. Calibration is an

adjustment of system gain to ensure the proper response to a known reference pressure value.

Traditionally, this has been performed using a mercury manometer as the standard.

The final step in transducer setup is leveling the pressure monitoring zero point to the appropriate

position on the patient. In general, zeroing and leveling the transducer are accomplished at the same

time, prior to initiating patient monitoring. However, these are two distinct procedures. Zeroing

exposes the transducer to ambient, atmospheric pressure via an open stopcock. Leveling assigns this

zero reference point to a specific position on the patient‘s body.

P.T.O

BMES Final 821 (Dr. Marek Swoboda) By : Jayet Moon (13052081)

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Figure 19

Practical evaluation of dynamic response characteristics of a catheter–transducer system.

The catheter hub is connected by means of a three-way stopcock to one arm of a

pressure transducer. The tip of the catheter is snugly projected through a hole in rubber stopper,

which is tightly inserted into the cutoff barrel of a 60-mL plastic syringe. The manometer and

catheter are filled with saline solution, care being taken to avoid even small air bubbles, and the

catheter is flushed until the catheter tip and holes are submerged in approximately 30 mL of

saline solution. Next, the plunger is slowly inserted into the syringe, producing an upward

deflection of the pressure trace on the oscilloscope of the recording apparatus. When

the pressure trace comes to rest at the top of the oscilloscope screen, the recorder is turned on

and the plunger is suddenly withdrawn from the syringe barrel. Dynamic response

characteristics are then calculated.

Recordings are obtained by continuously increasing the input frequency of a sine

wave pressure waveform from 2 to 200 Hz. The fluid-filled system resonates at natural frequency at 35-

55 Hz . The point upto which the system shows a flat response is the useful range.Natural Frequency

and Damping Frequency are

ECG Calibration

There must be a full 12 leads recorded and labelled plus a rhythm strip, usually from lead II.

A 1mV signal is introduced for each channel that is recorded

The baseline must be stable and not wandering. Leads must be well attached, even if this means

shaving a hairy chest.

BMES Final 821 (Dr. Marek Swoboda) By : Jayet Moon (13052081)

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There should be little interference from skeletal muscle.

There should be a square wave calibration to show that 1 mV is equivalent to 1 cm in height.

Speed should be 25 mm/sec. Hence 1 large square is 200 msec and 1 small square is 40 msec

In case of offset leads are grounded and system reset by pressing zero.

BONUS QUESTION

BP(t)= BP(t)=0.02*(t-100)^2 +20

Pinitial= 0.02(-100)2 + 20 = 220 mmHg

Pend= 0.02(-50)2 + 20

dP/dT= 220-70/ .05= 3000 mmHg/sec

With a delay of 10ms

PinitiAL = 0.02(-90)2 + 20 = 182mmHg

Pend = 0.02(-100)2 + 20= 70 mmHg

dP/dT= 182-70/.04 = 2800mmHg

error= 200 mmHg/sec

Percentage Error = 20000/3000 = 6.67%

With 20 ms delay

PinitiAL = 0.02(-80)2 + 20 = 182mmHg

Pend = 0.02(-50)2 + 20= 70 mmHg

dP/dT= 148-70/.03 = 2600mmHg

error= 400mmHg

Percentage Error = 40000/3000 = 13.33 %

ALTERNATIVE SOLUTIONS:

1 We can use Micro-Cath by Millar Instruments which has a pressure sensor at the tip and is not fluid

filled. [14][18]

2. Tonometric and Oscillometric techniques can be used to measure Brachial or Radial Pressure . The

mathematical operations and transfer functions that can fairly well predict contractility on basis of these

pressures.Its advantage is that it is non invasive[15][16][17]

BMES Final 821 (Dr. Marek Swoboda) By : Jayet Moon (13052081)

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References:

[1] Karthik Soundarapandian, Mark Berarducci ;Application ReportSBAA160A–March 2009–Revised

April 2010 Analog Front-End Design for ECG Systems Using Delta-Sigma ADCs ,TEXAS

INSTRUMENTS]

[2] J. Webster, Medical Instrumentation: Application and Design 4th

Ed.,Wiley

[3] Vrudhula K. Murthy, Thomas M. Grove, George A. Harvey, and L. Julian Haywood :Clinical

usefulness of ECG Frequency Spectrum Analysis , Proc Annu Symp Comput Appl Med Care. 1978

November 9: : 610–612.]

[4] [http://www.cvphysiology.com/Arrhythmias/A009.htm

[5] Paulev-Zubieta New Human Physiology 2nd Edition Chapter 11: Cardiac Action Potencials and

Arrhythmia

[6] Sarazan RD, Kroehle JP, Main :BW FFT OF left ventricular pressure derivative in rats [J Pharmacol

Toxicol Methods. 2012 Sep;66(2):71-8. doi: 10.1016/j.vascn.2012.05.009. Epub 2012 May 31

dsarazan@datasci.com

[7] W. A. SEED, B.M., PH.D., M. I. M NOBLE, M.D., D.SC., J. M. WALKER, M.D., G. A. H.

MILLER, D.M., J. PIDGEON, M.B., D. REDWOOD, M.A., M.B., R. WANLESS, B.Sc., M.B., M. R.

FRANZ, M.D., M. SCHOETTLER, M.D., AND J. SCHAEFER, M.DRelationships between beat-to-

beat interval and the strength of contraction in the healthy and diseased human heart. Circulation 70,

No. 5, 799-805, 1984. Print ISSN: 0009-7322. Online ISSN: 1524-4539 doi: 10.1161/01.CIR.70.5.799

Circulation. 1984;70:799-805]

[8] Physical criteria for measurement of left ventricular pressure and its first derivativeCardiovasc

Res (1971) 5 (1): 32-40; Cardiovasc Res (1971) 5 (1): 32-40.doi: 10.1093/cvr/5.1.32]

[9] AD620 datasheet,ANALOG DEVICES, Low Cost Low Power Instrumentation Amplifier

[10] Ajay Bharadwaj and Umanath Kamath Techniques for accurate ECG signal processing , Cypress

Semiconductor Corp. 2/14/2011 10:12 PM EST

[11] R. Dustan Sarazan , John P. Kroehle, Bradley W. Main: Left ventricular pressure, contractility and

dP/dtmax in nonclinical drug safety assessment studies Data Sciences International (DSI). 119 14th St

NW, Suite 100. St. Paul, MN 55112, USA

[12] A Reale :Evaluation of the contractile state of the human heart from the first derivative of the

apexcardiogram. Circulation (Impact Factor: 15.2). 01/1968; 36(6):933-41. Source: PubMed]

[13] [ADAM C. BELL HERMAN L. FALSETTI, ROBERT E. MATES, ROBYN J. CARROLL,

RAMJI L. GUPTA Analysis and Correction of Pressure Wave Distortion in Fluid-Filled Catheter

Systems Print ISSN: 0009-7322. Online ISSN: 1524-4539 Copyright © 1974 American Heart

BMES Final 821 (Dr. Marek Swoboda) By : Jayet Moon (13052081)

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Association, Inc. All rights reserved. Circulation is published by the American Heart Association, 7272

Greenville Avenue, Dallas, TX 75231 doi: 10.1161/01.CIR.49.1.165 Circulation. 1974;49:165-172]

[14]Zimmer HG, Millar HD.Technology and application of ultraminiature catheter pressure

transducers. Carl-Ludwig-Institute of Physiology, University of Leipzig, Germany.

zimmer@medizin.uni-leipzig.de Can J Cardiol. 1998 Oct;14(10):1259-66.

[15] Kawasaki H, Seki M, Saiki H, Masutani S, Senzaki H Noninvasive assessment of left ventricular

contractility in pediatric patients using the maximum rate of pressure rise in peripheral arteries.. Heart

Vessels. 2012 Jul;27(4):384-90. doi: 10.1007/s00380-011-0162-0. Epub 2011 Jun 17.

[16] Chen-Huan Chen, MD; Erez Nevo, MD, DSc; Barry Fetics, BE; Peter H. Pak, MD; Frank C.P.

Yin, MD, PhD; W. Lowell Maughan, MD; David A. Kass, MD Estimation of Central Aortic Pressure

Waveform by Mathematical Transformation of Radial Tonometry Pressure Validation of Generalized

Transfer Function

[17]Kyeong Min Kim; Dong Soo Lee; June-Key Chung; Myung Chul Lee; Yong Jin Kim,

"Noninvasive measurement of left ventricular contractility using gated myocardial SPECT and arterial

pressure tonometer," Engineering in Medicine and Biology Society, 1998. Proceedings of the 20th

Annual International Conference of the IEEE , vol., no., pp.517,519 vol.1, 29 Oct-1 Nov 1998

doi: 10.1109/IEMBS.1998.745960

[18] http://millar.com/products/clinical/pressure/mikro-cath/mikro-cath-disposable-pressure-catheter

[19] [ Diagnostic Value of the First and Second Derivatives of the Arterial Pressure Pulse in Aortic

Valve Disease and in Hypertrophic Subaortic Stenosis DEAN T. MASON, M.D.; EUGENE

BRAUNWALD, M.D.; JOHN ROSS JR., M.D.; ANDREW G. MORROW, M.D.

Circulation.1964; 30: 90-100doi: 10.1161/01.CIR.30.1.90 ]

[20] [Human autonomic rhythms: vagal cardiac mechanisms in tetraplegic subjects Junken Koh, Troy

E. Brown, Larry A. Beightol, Chang Y. Ha* and Dwain L. Eckberg Departments of Medicine,

Physiology and Spinal Cord Injury, *, pp. 48495 Journal of Physiology (1994), 474.3 ]