A method for the assessment of phasic vagal influence on tidal volume

Vagal influence related to lung volume changes results in reduction in tidal volume during spontaneous breathing due primarily to premature termination of inspiration. The strength of this vagal influence was traditionally assessed by the duration of apnea following lung inflation, a method recently shown to be inadequate and potentially misleading. An alternate method is described utilizing analysis of the volume tracing of spontaneous breaths and the tracheal pressure tracing during the first breath following airway occlusion at FRC. A formula was devised which, on the basis of previous observations, should predict the tidal volume to be obtained in the absence of phasic vagal influence. The formula was tested in four pentobarbital-anesthetized rabbits using a technique of vagal cooling which rapidly eliminated the vagal influence under study. It was found that the tidal volume obtained following vagal block could be accurately predicted provided allowances were made for the vagally mediated terminal inhibition during spontaneous breathing and the relative stiffness of the respiratory system at high lung volumes.