Determinants of the cuff-leak test: a physiological study

The cuff-leak test has been proposed as a simple method to predict the occurrence of post-extubation stridor. The test is performed by cuff deflation and measuring the expired tidal volume a few breaths later (V_T). The leak is calculated as the difference between V_T with and without a deflated cuff. However, because the cuff remains deflated throughout the respiratory cycle a volume of gas may also leak during inspiration and therefore this method (conventional) measures the total leak consisting of an inspiratory and expiratory component. The aims of this physiological study were, first, to examine the effects of various variables on total leak and, second, to compare the total leak with that obtained when the inspiratory component was eliminated, leaving only the expiratory leak.

In 15 critically ill patients mechanically ventilated on volume control mode, the cuff-leak volume was measured randomly either by the conventional method (Leak_conv) or by deflating the cuff at the end of inspiration and measuring the V_T of the following expiration (Leak_pause). To investigate the effects of respiratory system mechanics and inspiratory flow, cuff-leak volume was studied by using a lung model, varying the cross-sectional area around the endotracheal tube and model mechanics.

In patients Leak_conv was significantly higher than Leak_pause, averaging 188 ± 159 ml (mean ± SD) and 61 ± 75 ml, respectively. In the model study Leak_conv increased significantly with decreasing inspiratory flow and model compliance. Leak_pause and Leak_conv increased slightly with increasing model resistance, the difference being significant only for Leak_pause. The difference between Leak_conv and Leak_pause increased significantly with decreasing inspiratory flow (V'_I) and model compliance and increasing cross-sectional area around the tube.

We conclude that the cross-sectional area around the endotracheal tube is not the only determinant of the cuff-leak test. System compliance and inspiratory flow significantly affect the test, mainly through an effect on the inspiratory component of the total leak. The expiratory component is slightly influenced by respiratory system resistance.