Reflex changes in sympathetic nerve activity during mechanical ventilation with PEEP in sino-aortic denervated rats

The aim of this study was to evaluate the role of cardiopulmonary receptors with vagal afferents in the reflex control of sympathetic nerve activity during mechanical ventilation with positive end-expiratory pressure (PEEP). Experiments were performed on 17 chloralose-anaesthetized rats. Changes in renal sympathetic nerve activity (RSNA), heart rate and mean arterial pressure were studied at zero end-expiratory pressure (ZEEP) and at 5 and 10 cm H2O PEEP in intact animals (n = 8), after sino-aortic denervation (n = 17) and after sino-aortic denervation plus vagotomy (n = 10). In the intact animals, 10 cm H2O PEEP induced a significant increase in RSNA (+66%) and heart rate (+10%) while the mean arterial pressure did not change significantly. Sino-aortic denervation increased baseline levels of RSNA, heart rate and mean arterial pressure. After sino-aortic denervation, 10 cm H2O PEEP still induced a significant increase in RSNA (+22%) and heart rate in 13 animals. In four animals, 10 cm H2O PEEP elicited a depressor reflex with a significant decrease in both RSNA (-32%) and heart rate. Bilateral vagotomy after sino-aortic denervation at ZEEP significantly increased RNSA (+15%) and heart rate, indicating an ongoing tonic inhibitory influence on RSNA from vagal afferents in sino-aortic denervated rats. The PEEP did not induce any significant change in RSNA or heart rate after sino-aortic denervation plus vagotomy. The results indicate that cardiopulmonary receptors with vagal afferents contribute to the reflex excitation of the sympathetic nervous system during mechanical ventilation with PEEP. Under certain circumstances, PEEP may also trigger powerful depressor reflexes, mediated by vagal afferents.