Effects of lung time constant, gas analyser delay and rise time on measurements of respiratory dead-space

This study evaluated effects of mechanical time constants (τm) of the respiratory system, delays between flow and CO2 partial pressure signals and rise time of the CO2 analyser on dead-space measurements. A computer model simulated low alveolar dead-space, high alveolar dead-space, 0.2 ≤ τm ≤ 3.6 s and varying ventilation–perfusion ratios . CO2 expirograms were recorded from the model under each condition and from 22 anaesthetized intubated patients. signals were shifted with respect to flow to produce varying time delays and anatomic and physiological dead-spaces were calculated. The CO2 analyser was simulated as a critically damped second-order system with 10–90% rise times of 25–400 ms. The error in measured dead-space increases approximately 2.5% per 10 ms signal delay for normal lungs (τm = 1 s), but has low sensitivity (0.58% per 10 ms) to the rise time of the CO2 analyser. Sensitivity of physiological dead-space, but not anatomic dead-space to delay is decreased in high alveolar dead-space and abnormal distribution. Shorter τm increase the error sensitivity of both physiological and anatomic dead-spaces to both delay and rise time. and flow should be well synchronized, particularly when τm are short, to avoid dead-space errors.