The effect of continuous positive airway pressure (CPAP) on renal vascular resistance: the influence of renal denervation

Ten healthy volunteers and six renal transplant patients were studied. Using Doppler ultrasonography, the pulsatility index (PI), an index of renovascular resistance, was measured at incremental levels of CPAP (0, 2.5, 5.0 and 7.5 cmH2O).In both groups, the PI increased significantly between 0 and5.0 cmH2O CPAP, with a further increase at 7.5 cmH2O CPAP.We found that CPAP at 5.0 and 7.5 cmH2O caused a significant increase in renovascular resistance in both normal and renal transplant patients. There was no difference in the degree of rise in renovascular resistance between both groups, indicating that the renal nerves do not play a role in altering renal vascular resistance with the application of CPAP.Continuous positive airway pressure (CPAP) is used in the treatment of obstructive sleep apnoea, adult respiratory distress syndrome (ARDS), chronic obstructive pulmonary disease (COPD) and acute cardiogenic pulmonary oedema [1]. Many patients on positive pressure ventilation, especially with the addition of positive end-expiratory pressure (PEEP), develop fluid retention and impaired renal function [2,3] and PEEP is known to reduce urinary output and sodium excretion [2,4]. Reduced renal blood flow has been implicated as a possible mechanism for the development of fluid retention [5]. This fall in renal blood flow may be secondary to a reduction in cardiac output, or inceased renal venous pressure and redistribution of renal blood flow from cortical to medullary regions [6]. Sympathetic activation acting directly via renal nerve stimulation or indirectly via nor-adrenaline release may also play a role [7,8]. The fall in renal blood flow secondary to the application of PEEP in dogs is abolished by renal denervation, suggesting a major modulatory role for renal innervation. Fluid retention also occurs with CPAP, but the extent of changes in renal haemodynamics with CPAP are unknown.We studied a group of normal volunteers to determine possible effects of CPAP on distal