The renal resistance index is increased in mild-to-moderate obstructive sleep apnoea and is reduced under continuous positive airway pressure

Background. Impaired renal function has recently been reported in obstructive sleep apnoea (OSA). The underlying mechanisms, however, are not entirely understood. This study investigated the influence of mild-to-moderate OSA and its treatment on renal haemodynamics as assessed by the renal resistance index (RRI). Methods. RRI has been measured by colour duplex ultrasound in 64 patients with newly diagnosed mild-to-moderate OSA and 61 controls without OSA at baseline and follow-up after 9.9 months. Treatment with continuous positive airway pressure was offered to all patients with OSA (apnoea/hypopnoea index >= 5/h). Results. Increased values of RRI (>= 1 SD [8.9%] above the age-adjusted normal value) were found in 41 out of 64 (64.0%) OSA patients when compared with 20 out of 61 (32.8%) controls (P < 0.001). The corresponding mean RRI was 70.50 +/- 9.01 vs 66.51 +/- 8.33 (P = 0.012). In multivariate analyses, the influence of OSA on RRI was independent from hypertension, diabetes mellitus, age and baseline renal function. At follow-up, RRI decreased only in patients with effective OSA treatment but remained unchanged in ineffectively treated OSA patients and controls. Conclusions. For the first time, this prospective controlled observational study demonstrates an impairment of renal haemodynamics in OSA as measured by an increased RRI. These changes of renal blood flow may identify OSA patients at high risk of declining renal function. Both parenchymal and vascular renal diseases are proposed as pathomechanisms for this association. An effective treatment of OSA resulted in a decreased RRI, suggesting an improvement in renal perfusion. Further studies are needed to elucidate the role of impaired renal haemodynamics in OSA.