Effects of hypovolaemia or isoprenaline infusion on the sympathetic reflex response to PEEP ventilation in rats

The aim of this investigation was to study vagally mediated sympathetic reflex responses to mechanical ventilation with positive end-expiratory pressure (PEEP), during hypovolaemia or during high inotropic stimulation of the heart by isoprenaline infusion. Renal sympathetic nerve activity (RSNA), heart rate and mean arterial pressure were studied during mechanical ventilation with zero end-expiratory pressure (ZEEP) and 5 and 10 cmH2O PEEP in chloralose anaesthetized Wistar rats. Experiments were performed on two groups of rats: eight animals were subjected to 10% blood volume depletion, and seven animals to infusion of isoprenaline. PEEP ventilation was applied during control conditions and during hypovolaemia or isoprenaline infusion before and after vagotomy. In the intact (control) situation, there was a significant increase in RSNA from ZEEP to 10 PEEP in both groups (+75%, +51%). During hypovolaemia or isoprenaline infusion, PEEP ventilation did not induce any significant increase in RSNA in both groups after vagotomy, 10 PEEP induced a significant increase in both groups (+56%, +54%). These results indicated that under conditions of hypovolaemia or increased cardiac inotropism, PEEP ventilation may elicit a vagally mediated reflex with an inhibitory action on sympathetic activity, in turn probably caused by an activation of left ventricular receptors.