Effects of positive and negative pressure breathing on plasma renin concentration in the dog

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.

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.

Central transmural venous pressure and plasma arginine vasopressin during negative pressure breathing in man

After overnight food and fluid restriction, 8 normal healthy males were examined in the upright sitting position before (prestudy), during and after (recovery) negative pressure breathing (NPB) with a pressure (P = difference between airway pressure and barometric pressure) of -9.6 +/- 0.5 to -10.4 +/- 0.4 mm Hg for 30 min. Plasma arginine vasopressin (pAVP) did not change significantly comparing prestudy with 10 and 30 min of NPB or comparing recovery with NPB at 10, 20 or 30 min. However, at 20 min of NBP, pAVP was slightly lower than at prestudy (p less than 0.05). Central venous pressure (CVP) decreased significantly during NPB, and central transmural venous pressure (CVP-P) increased significantly from -0.9 +/- 0.8 mm Hg to 3.8 +/- 0.7, 4.3 +/- 0.7 and 4.5 +/- 0.6 mm Hg (p less than 0.001) after 10, 20 and 30 min, respectively. Systolic, diastolic and mean arterial pressure and heart rate did not change significantly during NPB. Diuresis, natriuresis, kaliuresis, osmotic excretion and clearance were slightly increased during the recovery hour after NPB compared to prestudy, while urine osmolality decreased during NPB (n = 6). However, none of these changes were significant. There was no significant correlation between CVP-P and pAVP. In conclusion, -10 mm Hg NPB for 30 min in upright sitting subjects did not change pAVP consistently, while CVP-P was significantly increased and HR and arterial pressures were unchanged. This lends support to the concept that arterial baroreceptors and not cardiopulmonary mechanoreceptors are of importance in regulating AVP secretion in man.

Sympathetic nerve activity and central haemodynamics during mechanical ventilation with positive end-expiratory pressure in rats

The aim of this study was to examine the effects of mechanical ventilation with increasing levels of positive end-expiratory pressure (PEEP) on sympathetic nerve activity (SNA), cardiac output (CO), stroke volume (SV), heart rate (HR), central blood volume (CBV), total peripheral resistance (TPR), mean arterial pressure (MAP), pulse pressure (PP) and right and left atrial transmural pressure in chloralose anaesthetized rats before and after vagotomy. Changing ventilatory pattern from spontaneous breathing (SB) to artificial ventilation with 10 cm H2O PEEP in intact animals caused a significant fall in CO, SV and CBV (42, 48 and 17%, respectively) and an increase in SNA, HR and TPR (90, 13 and 83%, respectively). The MAP increased slightly but significantly from 103 +/- 4 to 107 +/- 4 mmHg while PP decreased from 48 +/- 2 to 37 +/- 3, from spontaneous breathing (SB) to 10 cm H2O PEEP. Transmural left atrial pressure decreased significantly from 4.5 +/- 0.3 to 3.0 +/- 0.4 mmHg. After vagotomy, MAP and CO were significantly lower at 10 cm H2O PEEP and PP and SV were significantly lower at all levels of positive end-expiratory pressure than the corresponding prevagotomy values. In spite of a greater fall in MAP and PP during PEEP after vagotomy, the absolute and relative increase of SNA was significantly lower compared to corresponding prevagotomy values. We conclude that reflex cardiovascular adjustments elicited by ventilation with PEEP are not solely due to unloading of arterial baroreceptors as has been claimed by others. Unloading of cardiac receptors with tonically active inhibitory afferents in the vagi is probably also of great importance for the excitation of the sympathetic nervous system during mechanical ventilation with PEEP.

Renin release after furosemide and ethacrynic acid in man. Evidence for neural reflex control mechanisms

The mechanisms of renin release after furosemide (F) and ethacrynic acid (EA) in man were examined. We evaluated whether acute volume shifts within the low pressure system after F induce renin release via neural pathways. Immersion in a water-bath or beta-blockade reduced the increase of plasma renin concentration after F but not after EA. It is concluded that acute renin release after F but not after EA in man is partially due to neurally mediated reflexes originating from volume receptors in the cardiopulmonary area.

Effect of destruction of the posterior pituitary on the diuresis from left atrial receptors

1. In anaesthetized dogs, stimulation of atrial receptors after destruction of the pituitary gland results in a diuresis. This response was not abolished by the administration of bretylium tosylate and was also observed in a surgically denervated kidney. 2. The diuresis is qualitatively similar to that observed in anaesthetized dogs with intact pituitary glands. 3. It is concluded that the diuresis which results from stimulation of the left atrial receptors is mediated by a blood-borne agent which is not the antidiuretic hormone.