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 (τ_m) of the respiratory system, delays between flow and CO ₂ 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 ≤ τ_m ≤ 3.6 s and varying ventilation-perfusion ratios (V̇/Q̇). 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 (τ_m = 1 s), but has low sensitivity (0.58% per 10 ms) to the rise time of the CO ₂ analyser. Sensitivity of physiological dead-space, but not anatomic dead-space to delay is decreased in high alveolar dead-space and abnormal V̇/Q̇ distribution. Shorter τ_m 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 τ_m are short, to avoid dead-space errors.