Pulmonary
Mechanics and
Graphics during
Mechanical
Ventilation
Definition
• Mechanics:
• Expression of lung
function through
measures of pressure
and flow:
• Derived parameters:
volume, compliance,
resistance, work
• Graphics:
• Plotting one parameter
as a function of time
or as a function of
another parameter
• P-T,F-T,V–T F
-V,P-V
Objectives
• Evaluate lung function
• Assess response to
therapy
• Optimize mechanical support
Exponential Decay
y
37
13.5
5
y = y0 . e (-t / TC)
TC
Exponential Rise
y
95
86.5
63
y = yf
. (1
-e
(-t / TC))
TC
Time Constant ()
• Time required for rise to 63%
• Time required for fall to 37%
• In Pul. System
 = Compliance • Resistance
Airway Pressure
• Equation of Motion
•
.
Paw = V(t) / C + R V(t) + PEEP + PEEPi
Airway Pressure
Sites of Measurement
• Directly at proximal airway
• At the inspiratory valve
• At the expiratory valve
Airway Pressure
Sites of Measurement
•
1)
2)
3)
Directly at proximal airway
The best approximation
Technical difficulty
Hostile environment
Airway Pressure
Sites of Measurement
• Directly at proximal airway
• At the inspiratory valve
To approximate airway pressure
during expiration
Airway Pressure
Sites of Measurement
• Directly at proximal airway
• At the inspiratory valve
• At the expiratory valve
To approximate airway pressure
during inspiration
A typical airway pressure waveform
Volume ventilation
PIP
PPlat
Linear
increase
Initial rise
End-exp. Pause
(Auto-PEEP)
Peak Alveolar Pressure (Pplat)
• Palv can not be measured directly
• If flow is present, during inspiration:
Paw > Pplat
Measurement by end-inspiratory hold
Peak Inspiratory Pressure (PIP)
PPlat
PZ
Pressure at Zero Flow
Peak Alveolar Pressure (Pplat)
Uses
• Prevention of overinflation
Pplat  34 cmH2O
• Compliance calculation
CStat = VT / (PPlat – PEEP)
• Resistance calculation
RI = (PIP – PPlat) / VI
Auto-PEEP
• Short TE  air entrapment
• Auto-PEEP = The averaged pressure by
trapped gas in different lung units
• TE shorter than 3 expiratory time constant
• So it is a potential cause of hyperinflation
Auto-PEEP
Effects
• Overinflation
• Failure to trigger
• Barotrauma
Auto-PEEP
Measurement technique
Auto-PEEP
Influencing factors
• Ventilator settings:
RR – VT – TPlat – I:E – TE
• Lung function:
Resistance – Compliance
• auto-PEEP = VT / (C · (eTe/ – 1))
Te = Exp. Time ,  = Exp. Time constant , C = Compliance
Esophageal Pressure
•
•
•
•
•
•
•
In the lower third(35– 40cm, nares)
Fill then remove all but 0.5 – 1 ml
Baydur maneuver, cardiac oscillation
Pleural pressure changes
Work of breathing
Chest wall compliance
Auto-PEEP
Esophageal Pressure
Auto-PEEP Measurement
• Airway flow & esophageal pressure trace
• Auto-PEEP = Change in esophageal
pressure to reverse flow direction
• Passive exhalation
Esophageal Pressure
Auto-PEEP Measurement
Flow
Peso
Flow
Inspiratory
Volume ventilation
• Value by Peak Flow Rate button
• Waveform by Waveform select button
Flow
Inspiratory
Pressure ventilation
·
• Value : V = (P / R) · (e-t / )
• Waveform:
Flow
Expiratory
• Palv , RA , 
·
• V = –(Palv / R) · (e-t / )
Flow waveform
application
• Detection of Auto-PEEP
1) Expiratory waveform not return to
baseline (no quantification)
2) May be falsely negative
Flow at
endexpiration
Flow waveform
application
• Dips in exp. flow during assisted ventilation
or PSV: Insufficient trigger effort
AutoPEEP
Inspiratory
effort
Volume
• Measurement: Integration of expiratory
flow waveform
Compliance
• VT divided by the pressure required to
produce that volume:
C = V / P = VT / (Pplat – PEEP)
• Range in mechanically ventilated patients:
50 – 100 ml/cmH2O
• 1 / CT = 1 / Ccw + 1 / CL
Chest wall compliance
(Ccw)
• Changes in Peso during passive inflation
• Normal range: 100 – 200 ml/cmH2O
400 ml
Chest wall compliance
Decrease
•
•
•
•
•
Abdominal distension
Chest wall edema
Chest wall burn
Thoracic deformities
Muscle tone
Chest wall compliance
Increase
• Flail Chest
• Muscle paralysis
Lung compliance
• VT divided by transpulmonary pressure (PTP)
• PTP = Pplat – Peso
• Normal range : 100 – 200 ml/cmH2O
30 cmH2O
PTP = Pplat – Peso= 30 – 17 = 13
17 cmH2O
Lung compliance
Decrease
Pulmonary edema
ARDS
Pneumothorax
Consolidation
Atelectasis
Pulmonary fibrosis
Pneumonectomy
Bronchial intubation
Hyperinflation
Pleural effusion
Abdominal distension
Chest wall deformity
Airway resistance
• Volume ventilation
·
RI = (PIP – PPlat) / VI
·
RE = (Pplat – PEEP) / VEXP
• Intubated mechanically ventilated
RI  10 cmH2O/L/sec
RE > RI
Airway resistance
Increased
• Bronchospasm
• Secretions
• Small ID tracheal tube
• Mucosal edema
Mean Airway Pressure
•
•
•
•
Beneficial and detrimental effects of IPPV
Direct relationship to oxygenation
Time averaged of pressures in a cycle
Volume ventilation
0.5 · (PIP – PEEP) · (TI / Ttot) + PEEP
• Pressure ventilation
(PIP – PEEP) · (TI / Ttot) + PEEP
• Mean Alveolar Pressure
·
Mean Airway Pressure + (VE / 60) · (RE –RI)
Mean Airway Pressure  14 cmH2O
Mean Airway Pressure
Typical values
• Normal lung : 5 – 10 cmH2O
• ARDS : 15 – 30 cmH2O
• COPD : 10 – 20 cmH2O
Pressure-Volume Loop
• Static elastic forces of the respiratory
system independent of the dynamic and
viscoelastic properties
• Super-syringe technique
• Constant flow inflation
• Lung and chest wall component
• Chest wall PV: Volume vs. Peso
• Lung PV: Volume vs. PTP
PV Loop
• Normal shape: Sigmoidal
• Hysteresis: Inflation vs. deflation
• In acute lung injury:
Initial flat segment – LIP – Linear portion – UIP
• LIP = Closing volume in normal subjects
• UIP = Overdistension
• Best use of PV loop: To guide ventilator
management
PEEP > LIP , Pplat < UIP
Normal PV Loop
PV Loop in Acute
Lung Injury
UIP
LIP
PEEP > UIP , PPlat
•
•
•
•
•
•
Reduce ventilator associated lung injury
Prevention of overinflation
Increased recruitment of collapsed units
Lower incidence of barotrauma
Higher weaning rate
Higher survival rate
PV Loop
Role of chest wall component
•
•
•
•
Effect on LIP and UIP
PV loop for lung alone: Use of Peso
LIP underestimates the necessary PEEP
Better results with PEEP set above LIP on
deflation PV loop rather inflation
Volume Ventilation Parameters
Interaction
Run VVPI Program