How do you measure lung compliance on a ventilator?

From the Guidelines

Lung compliance on a ventilator is typically measured using the overdistension and lung collapse (OD-CL) method, which involves assessing regional compliance changes during a decremental PEEP trial.

Key Considerations

• The OD-CL method requires driving pressure measurement to estimate regional compliance and should be performed in volume-controlled ventilation or pressure-controlled mode with constant support level and sufficient time for equilibration between alveolar and airway pressures at end-inspiration and end-expiration 1.
• The chosen PEEP range affects results, and a standardized PEEP window (e.g., from 24 to 6 cm H2O) is preferable to improve reliability and inter-patient comparisons 1.
• Lung compliance can also be estimated using the driving pressure (ΔP = Pplat – PEEP), which is the ratio of tidal volume to compliance, and values exceeding 15 cmH2O are of particular concern 1.

Measurement Details

• The OD-CL method involves assessing regional compliance changes during a decremental PEEP trial, and the optimal PEEP is usually considered the one found at the intersection of the collapse and overdistension curves 1.
• Driving pressure measurement can be automatically collected by the EIT machine or added by the operator in post-processing, and it is essential to use a consistent PEEP range and steps for intra-patient comparisons 1.
• Tidal volume should be titrated according to the patient's lung compliance, and values exceeding 15 cmH2O of driving pressure are of particular concern 1.

Clinical Implications

• Measuring lung compliance is crucial in patients with acute respiratory distress syndrome (ARDS) to guide ventilatory strategies and minimize lung injury 1.
• The use of lower tidal volumes and higher PEEP levels can reduce hospital mortality in patients with ARDS, and driving pressure predicts outcomes better than any other ventilatory parameters 1.

From the Research

Measuring Lung Compliance on a Ventilator

To measure lung compliance on a ventilator, several methods can be employed, including:

• Using esophageal pressure measurements with an air-filled balloon catheter or a fluid-filled catheter, such as a Salem double-lumen stomach tube, to separate lung and chest wall compliance 2
• Measuring airway pressures at the Y-piece and in the trachea with fluid-filled pressure lines to account for endotracheal tube resistance 2
• Utilizing a pulse method to measure respiratory system compliance during inflation, which involves measuring flow with a pneumotachometer and transthoracic pressure with a strip chart recorder 3
• Compensating for circuit compressible volume in lung protective ventilation, which can be achieved by measuring the compliance of the breathing circuit with the same parameters and ventilation technique used in lung protective ventilation 4

Factors Affecting Lung Compliance Measurements

Several factors can affect the accuracy of lung compliance measurements, including:

• Compliance and resistance of the test lung, which can cause variations in tidal volume delivery beyond the declared range 5
• Placement site of the flow sensor in the ventilator circuit, which can influence the accuracy of tidal volume delivery 5
• Ventilator settings, such as peak inspiratory pressure, positive end-expiratory pressure, and rate, which can have a modest effect on mechanics readout 6
• Simulated lung severity, which can affect the bias in tidal volume, compliance, and resistance measurements 6

Importance of Accurate Lung Compliance Measurements

Accurate lung compliance measurements are crucial for:

• Making informed decisions about ventilator settings 2
• Ensuring the delivery of accurate tidal volumes 5
• Monitoring changes in lung mechanics, which can be critical in patients with severe lung pathologies 4