Expiratory threshold load under extracorporeal circulation: effects of vagal afferents.

Nine anesthetized dogs breathed against an expiratory threshold load (ETL) applied by switching the expiratory circuit into a column of H2O to a depth of 20-30 cm. Arterial blood gas tensions were maintained in the normal range by placing the dogs under arteriovenous bypass to avoid any uncontrolled chemostimulation. There was an increase in integrated electromyogram activity of the diaphragm with the ETL. This was rarely observed after cold block of the vagus nerves which also reduced the evoked expiratory activity. The ventilatory response to hypercapnia was greatly depressed under loaded breathing whether vagal afferents were intact or blocked by cold. Both inspiratory drive and ventilatory timing were affected, suggesting that the central integration of chemosensitive afferents was altered. Proof of supraspinal projections of proprioceptive inputs from abdominal muscles was provided by the demonstration of changes in ventilatory timing during selective activation of muscle spindles in abdominal muscles by high-frequency mechanical vibration applied to the linea alba. Thus these observations suggest that during ETL breathing, a possible interaction exists between chemoreflex drive and proprioceptive afferents.