Ventilator Waveform Interpretation

Part of: Mechanical Ventilation

Pressure Waveform

Peak Pressure

• High peak pressure suggests possibility of airway narrowing (e.g. ET tube being kinked, ventilator tubing being full of fluid or bronchospasm)

Plateau Pressure

• Pressure in the lung after all flow has stopped; directly related to the compliance of the lung parenchyma

$$\begin{array}{rlrl}\text{Plateau Pressure}& =\frac{\text{Tidal Volume}}{\text{Static Compliance}}& \text{Static Pressure}& =\frac{\text{Tidal Volume}}{\text{Driving Pressure}}\\ \text{Driving Pressure}& =\text{Plateau Pressure}-\text{PEEP}& & \end{array}$$

Plateau Decay

• In an inspiratory pause, the pressure will gradually decrease from the initial plateau pressure; this occurs because of:
  • Elastic energy stored in the lung tissue and chest wall is dissipating
  • Lung units with different time constants are exchanging gas
  • Some of the gas is being absorbed into the pulmonary circulation
  • Some of the gas is leaking out of the circuit tubing

Flow Waveform

Maximum Flow

• In pressure control modes, the flow waveform is decelerating
  • A rapid deceleration of this flow indicates either that the lung compliance is poor or that airway resistance is high

End-Inspiratory Flow

• If the end inspiratory flow is positive, then increasing inspiratory time will produce larger tidal volumes

Peak Expiratory Flow

• The rate of expiratory flow is determined by the resistance of the airways and the elastic recoil of the lungs and chest wall
  • A low expiratory peak flow suggests airway obstruction (e.g. bronchospasm) or an over-compliant chest wall
• If expiratory flow is prolonged, the airway resistance must be increased
• If flow does not reach zero by the beginning of the next breath, there must be gas trapping

Pressure-Volume Loop

• Above mode is mandatory with volume as the control variable and flow as constant
• Lower inflection point ($P_{\text{flex}}$): the airway pressure which designates the critical opening pressure (takes more pressure to re-inflate a collapsed alveolus than it takes to distend a deflated one)
• Upper inflection point: corresponds to the deflation of the most hyperinflated lung units

Idealised Pressure-Volume Loop of Volume-Controlled Ventilation

Idealised Pressure-Volume Loop of Pressure-Controlled Ventilation

Realistic Pressure-Volume Loop of Pressure-Controlled Ventilation

Changes with Respiratory Compliance

• As lungs decrease in compliance, in volume-controlled mode higher peak airway pressures will develop and a beaked region will develop indicating alveolar overdistention

• In pressure-controlled mode, the volume generated will decrease resulting in hypoventilation and hypercapnoea

Air Leak

• With an air leak in the circuit, the volume does not return to zero

Patient-Ventilator Dyssynchrony

Work of Breathing

$$\begin{array}{rlr}W& =\int Fdx& \\ W& =\int (P\cdot A)dx& \left(P=\frac{F}{A}\right)\\ W& =\int PdV& (dV=Adx)\end{array}$$

• Hence the work of breathing is the area to the left of the pressure-volume loop
• Anything that increases the convexity of the inspiratory curve or pushes the whole loop to the right increases the work of breathing

Figure 17. Work of Breathing (normal)

Figure 18. Work of Breathing in asthma