Abstract
It is well established that the improvement in gas exchange that occurs with positive pressure ventilation may come at the expense of a decrease in cardiac output and oxygen delivery. Clinical observation suggests that children and infants may be more resistant than adults to the falls in cardiac output induced by positive airway pressure. The aims of this study were to quantify the effect of a sustained increase in intrathoracic pressure on cardiac output and stroke volume, and to determine whether this change is age-related. Twenty-eight children undergoing general anaesthesia were studied. Cardiac output was derived using pulsed wave Doppler techniques at four different levels of sustained positive airway pressure, and stroke volume was calculated. The relationship between airway pressure and both cardiac output and stroke volume was examined using a general linear model which included age as a continuous variable. Cardiac output decreased with increasing levels of sustained positive airway pressure (P=0.001). The fall in SV for a given airway pressure increased with increasing age (P=0.02). The mechanisms responsible for the increase of the magnitude of the fall in stroke volume with age remain to be elucidated.