advanced pulmonary mechanics during mechanical ventilation
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Advanced Pulmonary Mechanics during Mechanical Ventilation
Points of Discussion
Basics Abnormalities Equation of motion Dynamic Compliance Pressure-volume loop Flow-volume loop Work of breathing Lower and upper
inflection points Hysterexis Intrathoracic
Pressures
Air-leak Air trapping Increased airway
resistance Inadequate flow support Inadequate sensitivity Atelectasis Inadequate PEEP Airway obstruction Over-distension
Tube + Spring Model
Elastic Forces
Resistive Forces
Static and Dynamic Pressures
time
Pressure
PEEP
PIP
Pplat
Alveolar Distending (recoil) Pressure difference (Pdis)
Flow-Resistive Pressure difference (Pres)
Resistive and Elastive Forces
DYNAMIC CHARACTERISTICS:dP = dV / Cdyn
DYNAMIC CHARACTERISTICS:dP = dV / Cdyn
RESISTANCE:dPresistive = R x Flow
RESISTANCE:dPresistive = R x Flow
STATIC COMPLIANCE:dPdistensive = dV / Cst
STATIC COMPLIANCE:dPdistensive = dV / Cst
dP = dPresist. + dP dist.
dP = R x Flow + dV / C st
Assessment of static P-V curveSuper-syringe method:
Stepwise inflation from a big syringe with multiply occlusions at each volumes to record recoil pressureTime consumingCumbersome to
performDifficult to standardizePatient must be
paralysed, connected to a special equipment
Great risk of oxygen desaturation
Volume
Pressure
Assessment of static P-V curveSlow Flow Single Inflation Method
Slow (5-10 lpm) inspiratory flow with large Vt and ZEEP
The inspiratory curve of the dynamic P-V loop closely approximates the static curve
The flow-resistive pressure component could be subtracted
Easy to perform, fast and relatively comfortable
Servillo: AJRCCM 1997
Lu: AJRCCM 1999 Pressure
Volume
LPIflex
UPIflex
inspir
ationStat
ic c
urve
FRC and PV Loop
FRC
VO
LU
ME
TLC
Negative Positive0
DISTENDING PRESSURE
Normal Compliance
FRC
Components of Pressure-Volume Loop
Components of Pressure-Volume Loop
Volume Volume (mL)(mL)
Insp
irat
ion
Expirat
ion
PIP
VT
PPawaw (cm H (cm H22O)O)
Pressure-Volume Loop(Type of Breath)
Pressure-Volume Loop(Type of Breath)
Controlled Assisted Spontaneous
Vol (m
l)
Paw
(cm H2O)
I: InspirationE: Expiration
I
E
E
E
II
PEEP and P-V LoopPEEP and P-V Loop
Volume (mL)
VT
PIPPIPPaw (cm H2O)
PEEPEEP P
Inflection PointsInflection Points
Pressure (cm H2O)
Volume (mL)
Upper Inflection Point
Lower Inflection Point
Upper Inflection Point: Represents pressure resulting in regional overdistension
Lower Inflection Point: Represents minimal pressure for adequate alveolar recruitment
Decreased ComplianceDecreased Compliance
Volu
me(m
l)
Pressure (cm HPressure (cm H22O)O)
NormalNormalPatientPatient
Lung Compliance Changes and the P-V Loop
Lung Compliance Changes and the P-V Loop
Volume (mL)Volume (mL)
PIP levels
Preset VT
PPawaw (cm H (cm H22O)O)
NormalNormal
Volume Targeted Ventilation
DecreasedDecreased
IncreasedIncreased
Lung Compliance Changes and the P-V Loop
Lung Compliance Changes and the P-V Loop
Volume (mL)Volume (mL)
Preset PIP
VT
levels
PPawaw (cm H (cm H22O)O)
NormalNormal
Pre
ssu
re T
arg
ete
d
Ven
tilatio
n
IncreasedIncreased
DecreasedDecreased
HysteresisHysteresis
Volume (ml)
Pressure (cm Pressure (cm HH22O)O)
Abnormal Hysteresis
Normal Hysteresis
Flow-Volume LoopFlow-Volume Loop
Volume (ml)Volume (ml)
Inspiration
Expiration
Flo
w (
L/m
in)
Flo
w (
L/m
in)
PEFR
FRC
PIFR
VT
Positive Pressure Ventilation: The Equation of Motion
In a passive subject, airway pressure represents the entire pressure (P) applied across the respiratory system.
The work required to deliver a tidal breath (Wb) = tidal volume (VT) x airway pressure
The pressure (P) associated with the delivery of a tidal breath is defined by the simplified equation of motion of the respiratory system (lungs & chest wall):
P = VT/CR+ VT/Ti x RR + PEEP total
Where CR = compliance of the respiratory system, Ti = inspiratory time and VT/Ti = Flow, RR = resistance of the respiratory system and PEEP total = the alveolar pressure at the end of expiration = external PEEP + auto (or intrinsic) PEEP, if any. Auto PEEP = PEEP total – P extrinsic (PEEP dialed in the ventilator) adds to the inspiratory pressure one needs to
generate a tidal breath.
P elastic P resistive P elastic
Work of BreathingWork of Breathing
A: Resistive Work B: Elastic Work A: Resistive Work B: Elastic Work
Pressure (cm H2O)
Volume (ml)
BBBB
AAAA
Work of BreathingWOB is a major source of caloric expenditure and
oxygen consumptionAppr. 70% to overcome elastic forces, 30% flow-
resistive workPatient work is a one of the most sensitive indicator of
ventilator dependencyComparison of Ventilator and Patient work is a useful
indicator during weaning processWOB may be altered by changes in compliance,
resistance, patient effort, level of support, PEEP, improper Ti, demand system sensitivity, mode setting
Elevated WOB may contraindicate the weaning process
WOB Measurements
PA
B C
D
E
VWOB = ∫0
ti P x Vdt
Elasic work: ABCAResistive work
Inspiratory: ADCAExpiratory: ACEA
Work of Breathing Measurements
WOB = ∫0
ti P x Vdt
Paw: Ventilator Work: The physical force required to move gas into the lung, represents the total work of the resp. system (patient + ventilator)
Peso: Patient Work: done by respiratory muscles, represents the pulmonary work of breathing
Paw-Ptr: Imposed Work by the Endotracheal tube
P-V Loop and WOB
P
V
P
V
P
VNormal ComplianceNormal Resistance
Normal ComplianceIncreased Resistance
Decreased ComplianceNormal Resistance
Work per breath is depicted as a pressure-volume area
Work per breath (Wbreath) = P x tidal volume (VT)
Wmin = wbreath x respiratory rate
Pressure Pressure Pressure
Vol
um
e
Vol
um
e
Vol
um
e
VT
WR = resistive work
WEL = elastic work
The total work of breathing can be partitioned between an elastic and resistive work. By analogy, the pressure needed to inflate a balloon through a straw varies; one needs to overcome the resistance of the straw and the elasticity of the balloon.
Work of Breathing
Intrinsic PEEP and Work of Breathing
Vol
um
e
VT
VT
FRCPressure
PEEPi
Dynamic Hyperinflation
PEEPi = intrinsic or auto PEEP; green triangle = tidal elastic work; red loop = flow resistive work; blue rectangle = work expended in offsetting intrinsic PEEP (an expiratory driver) during inflation
When present, intrinsic PEEP contributes to the work of breaking and can be offset by applying external PEEP.
++
++
Ventilator
₊ ₊
The Pressure and Work of Breathing can be Entirely Provided by the Ventilator (Passive Patient)
The Work of Breathing can be Shared Between the Ventilator and the Patient
PAW
PES
patient machine
time
AC mode
The ventilator generates positive pressure within the airway and the patient’s inspiratory muscles generate negative pressure in the pleural space.
Paw = Airway pressure, Pes= esophageal pressure
Work of breath
Resistive Work
Elastic Work of Lung
Elastic Work of Chest
Paw
Pes
Volume
Work to inflate the chest wall
Inflation Deflation
Relationship Between the Set Pressure Support Level and the Patient’s Breathing Effort
Carrey et al. Chest. 1990;97:150.
The changes in Pes (esophageal pressure) and in the diaphragmatic activity (EMG) associated with the increase in the level of mask pressure (Pmask = pressure support) indicate transfer of the work of breathing from the patient to the ventilator.
Partitioning of the Workload Between the Ventilator and the Patient
How the work of breathing partitions between the patient and the ventilatordepends on:
• Mode of ventilation (e.g., in assist control most of the work is usually done by the ventilator)• Patient effort and synchrony with the mode of ventilation• Specific settings of a given mode (e.g., level of pressure in PS and set rate in SIMV)
Respiratory Mechanics in ARF*
Reduced range of volume
excursion: Low
compliance
Flattering at low and high
volumes: Lower and
upper inflection points
*Bigatello: Br J Anaest 1996
Volume
Pressure
NORMAL
ARDS
Lung Protective Strategy
1. Set PEEP above the lower Pflex to keep the lung open and avoid alveolar collapse
2. Apply small Vt to minimize stretching forces
3. Set Pplat below the upper Pflex to avoid regional overdistension
Volume
Pressure
Abnormalities Air-leak Air trapping Increased airway resistance Inadequate flow support Inadequate sensitivity Atelectasis Inadequate PEEP Airway obstruction Over-distension
Air LeakAir Leak
Volume (ml)
Pressure (cm H2O)
Air Leak
Air LeakAir LeakInspiration
Expiration
Volume (ml)
Flow Flow (L/min)(L/min)
Air Leak in mL
NormalAbnormal
Air LeakVolume (mL)
Time (sec)
Air TrappingAir Trapping
Inspiration
Expiration
NormalNormalPatientPatient
Time (sec)
Flo
w (
L/m
in)
Air TrappingAuto-PEEP
}
Air TrappingAir TrappingInspiration
Expiration
Volume (ml)Volume (ml)
Flow (L/min)
Does not returnDoes not returnto baselineto baseline
NormalAbnormal
Response to Bronchodilator
Response to Bronchodilator
Before
Time (sec)
Flo
w (
L/m
in)
PEFR
After
Long TE
Higher PEFR
Shorter TE
Increased Airway Resistance
Increased Airway Resistance
Inspiration
Expiration
VolumeVolume (ml) (ml)
Flow (L/min)
Decreased PEFR
NormalNormalAbnormal
“Scooped out” pattern
Increased RawIncreased Raw
Pressure (cm Pressure (cm HH22O)O)
Higher PTA
Norm
al S
lope
Norm
al S
lope
Vol (mL)Vol (mL)
Lower S
lope
Lower S
lope
Airway Secretions/Water in the Circuit
Airway Secretions/Water in the Circuit
InspirationInspiration
ExpirationExpiration
Volume (ml)Volume (ml)
Flow Flow (L/min)(L/min)
NormalNormalAbnormalAbnormal
F
VV
F
After SuctionBefore Suction
Airway Obstruction
Optimising PEEP
V
P
PEEP: 3 cmH2O
V
P
PEEP: 8 cmH2O
Inadequate SensitivityInadequate Sensitivity
Volume Volume (mL)(mL)
PPawaw (cm H (cm H22O)O)Increased WOB
Replaced FRC
P
V
Lost FRC
V
P
Atelectasis
OverdistensionOverdistension
Volu
me (
ml)
Pressure (cm HPressure (cm H22O)O)
With little or no change in VTWith little or no change in VT
Paw risesPaw
rises
NormalAbnormal
Overdistension
Overdistension occurs when the volume limit of some components of the lung has been exceeded
Abrupt decrease in compliance at the termination of inspiration
Results in a terminal “Beaking” of the P/V Loop
Volume
Pressure
0.8 Pmax Pmax
Overdistension Index
Volume
Pressure
Cdyn
C20
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