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Published byPerla Icke Modified over 10 years ago
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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 () = (0.05 to 0.1) • 10 = 0.5 – 1 sec
Time required for rise to 63% Time required for fall to 37% In Pul. System = Compliance • Resistance = (0.05 to 0.1) • 10 = 0.5 – 1 sec
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Airway Pressure Equation of Motion Paw = V(t) / C + R . V(t) + PEEP + PEEPi •
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Airway Pressure Sites of Measurement
Directly at proximal airway At the inspiratory valve At the expiratory valve
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Airway Pressure Sites of Measurement
Directly at proximal airway The best approximation Technical difficulty Hostile environment
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Airway Pressure Sites of Measurement
Directly at proximal airway At the inspiratory valve To approximate airway pressure during expiration
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Airway Pressure Sites 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 > 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-PEEP Effects Overinflation Failure to trigger Barotrauma
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Measurement technique
Auto-PEEP Measurement technique
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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
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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 Pressure Auto-PEEP Measurement
Airway flow & esophageal pressure trace Auto-PEEP = Change in esophageal pressure to reverse flow direction Passive exhalation
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Auto-PEEP Measurement
Esophageal Pressure Auto-PEEP Measurement Flow Peso
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Flow Inspiratory Volume ventilation Value by Peak Flow Rate button
Waveform by Waveform select button
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Flow Inspiratory Pressure ventilation Value : V = (P / R) · (e-t / )
Waveform:
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Flow Expiratory Palv , RA , V = –(Palv / R) · (e-t / )
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Flow waveform application
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 compliance Decrease
Abdominal distension Chest wall edema Chest wall burn Thoracic deformities Muscle tone
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Chest wall compliance Increase
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 compliance Decrease
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 > RI
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Airway resistance Increased
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 average of pressures in a cycle Pressure ventilation (PIP – PEEP) · (TI / Ttot) + PEEP Volume ventilation 0.5 · (PIP – PEEP) · (TI / Ttot) + PEEP
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Mean Airway Pressure 14 cmH2O
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Mean Airway Pressure Typical 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 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
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Normal PV Loop
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PV Loop in Acute Lung Injury
UIP LIP
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PEEP > LIP , Pplat < UIP
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 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
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Volume Ventilation Parameters Interaction
Run VVPI Program
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