Pulmonary Mechanics and Graphics during Mechanical Ventilation

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Pulmonary Mechanics and Graphics during
Mechanical Ventilation
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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

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Objectives

• Evaluate lung function
• Assess response to therapy
• Optimize mechanical support

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Exponential Decay
y
37
13.5
5
TC
y y0 . e (-t / TC)
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Exponential Rise
y
95
86.5
63
TC
y yf . (1 - e (-t / TC))
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Time Constant (?)

• Time required for rise to 63
• Time required for fall to 37
• In Pul. System ? Compliance Resistance

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Airway Pressure

• Equation of MotionPaw V(t) / C R . V(t)
  PEEP PEEPi

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Airway PressureSites of Measurement

• Directly at proximal airway
• At the inspiratory valve
• At the expiratory valve

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Airway PressureSites of Measurement

• Directly at proximal airway
• The best approximation
• Technical difficulty
• Hostile environment

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Airway PressureSites of Measurement

• Directly at proximal airway
• At the inspiratory valve
• To approximate airway pressure during expiration

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Airway PressureSites of Measurement

• Directly at proximal airway
• At the inspiratory valve
• At the expiratory valve
• To approximate airway pressure during
  inspiration

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A typical airway pressure waveform Volume
ventilation
PIP
PPlat
Linear increase
End-exp. Pause (Auto-PEEP)
Initial rise
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Peak Alveolar Pressure (Pplat)

• Palv can not be measured directly
• If flow is present, during inspiration Paw gt
  Pplat
• Measurement by end-inspiratory hold

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Peak Inspiratory Pressure (PIP)
PPlat
PZ
Pressure at Zero Flow
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Peak Alveolar Pressure (Pplat)Uses

• Prevention of overinflation Pplat ? 34 cmH2O
• Compliance calculation CStat VT / (PPlat
  PEEP)
• Resistance calculation RI (PIP PPlat) / VI

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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

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Auto-PEEPEffects

• Overinflation
• Failure to trigger
• Barotrauma

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Auto-PEEP Measurement technique
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Auto-PEEPInfluencing factors

• Ventilator settings RR VT TPlat IE TE
• Lung function Resistance Compliance
• auto-PEEP VT / (C (eTe/? 1))Te Exp. Time
  , ? Exp. Time constant , C Compliance

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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

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Esophageal PressureAuto-PEEP Measurement

• Airway flow esophageal pressure trace
• Auto-PEEP Change in esophageal pressure to
  reverse flow direction
• Passive exhalation

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Esophageal Pressure Auto-PEEP Measurement
Flow
Peso
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FlowInspiratory

• Volume ventilation
• Value by Peak Flow Rate button
• Waveform by Waveform select button

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FlowInspiratory

• Pressure ventilation
• Value V (?P / R) (e-t / ?)
• Waveform

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FlowExpiratory

• Palv , RA , ?
• V (Palv / R) (e-t / ?)

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Flow waveformapplication

• Detection of Auto-PEEP
• 1) Expiratory waveform not return to baseline
  (no quantification)
• 2) May be falsely negative

Flow at end-expiration
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Flow waveform application

• Dips in exp. flow during assisted ventilation or
  PSV Insufficient trigger effort

Auto-PEEP
Inspiratory effort
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Volume

• Measurement Integration of expiratory flow
  waveform

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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

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Chest wall compliance(Ccw)

• Changes in Peso during passive inflation
• Normal range 100 200 ml/cmH2O

400 ml
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Chest wall complianceDecrease

• Abdominal distension
• Chest wall edema
• Chest wall burn
• Thoracic deformities
• ?Muscle tone

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Chest wall complianceIncrease

• Flail Chest
• Muscle paralysis

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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
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Lung complianceDecrease

• Pulmonary edema
• ARDS
• Pneumothorax
• Consolidation
• Atelectasis
• Pulmonary fibrosis

• Pneumonectomy
• Bronchial intubation
• Hyperinflation
• Pleural effusion
• Abdominal distension
• Chest wall deformity

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Airway resistance

• Volume ventilation RI (PIP PPlat) / VI RE
  (Pplat PEEP) / VEXP
• Intubated mechanically ventilated RI ? 10
  cmH2O/L/sec RE gt RI

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Airway resistanceIncreased

• Bronchospasm
• Secretions
• Small ID tracheal tube
• Mucosal edema

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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)

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Mean Airway Pressure ? 14 cmH2O
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Mean Airway PressureTypical values

• Normal lung 5 10 cmH2O
• ARDS 15 30 cmH2O
• COPD 10 20 cmH2O

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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

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PV Loop

• Normal shape Sigmoidal
• Hysteresis Inflation vs. deflation
• In acute lung injuryInitial flat segment LIP
  Linear portion UIP
• LIP Closing volume in normal subjects
• UIP Overdistension
• Best use of PV loop To guide ventilator
  management PEEP gt LIP , Pplat lt UIP

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Normal PV Loop
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PV Loop in Acute Lung Injury
UIP
LIP
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PEEP gt 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

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PV LoopRole 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

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Volume Ventilation Parameters Interaction
Run VVPI Program