Chronic intermittent hypoxia in humans during 28 nights results in blood pressure elevation and increased muscle sympathetic nerve activity

Abstract

Chronic intermittent hypoxia (CIH) is thought to be responsible for the cardiovascular disease associated with obstructive sleep apnea (OSA). Increased sympathetic activation, altered vascular function, and inflammation are all putative mechanisms. We recently reported (Tamisier R, Gilmartin GS, Launois SH, Pepin JL, Nespoulet H, Thomas RJ, Levy P, Weiss JW. J Appl Physiol 107: 17-24, 2009) a new model of CIH in healthy humans that is associated with both increases in blood pressure and augmented peripheral chemosensitivity. We tested the hypothesis that exposure to CIH would also result in augmented muscle sympathetic nerve activity (MSNA) and altered vascular reactivity contributing to blood pressure elevation. We therefore exposed healthy subjects between the ages of 20 and 34 yr (n = 7) to 9 h of nocturnal intermittent hypoxia for 28 consecutive nights. Cardiovascular and hemodynamic variables were recorded at three time points; MSNA was collected before and after exposure. Diastolic blood pressure (71 +/- 1.3 vs. 74 +/- 1.7 mmHg, P < 0.01), MSNA [9.94 +/- 2.0 to 14.63 +/- 1.5 bursts/min (P < 0.05); 16.89 +/- 3.2 to 26.97 +/- 3.3 bursts/100 heartbeats (hb) (P = 0.01)], and forearm vascular resistance (FVR) (35.3 +/- 5.8 vs. 55.3 +/- 6.5 mmHg x ml(-1) x min x 100 g tissue, P = 0.01) all increased significantly after 4 wk of exposure. Forearm blood flow response following ischemia of 15 min (reactive hyperemia) fell below baseline values after 4 wk, following an initial increase after 2 wk of exposure. From these results we conclude that the increased blood pressure following prolonged exposure to CIH in healthy humans is associated with sympathetic activation and augmented FVR.

Figures

Fig. 1.

Four-week exposure protocol. Measurements were made at 3 time points: before hypoxic exposure, after 2 wk of hypoxic exposure, and after 4 wk of hypoxic exposure. MSNA, muscle sympathetic nerve activity; MAP, mean arterial pressure; HR, heart rate.

Fig. 2.

Representative plot of oxygen saturation from 1 subject over 1 h of an 8-h nocturnal exposure.

Fig. 3.

Systolic blood pressure and HR before (○) and after (●) 4 wk of exposure.

Fig. 4.

Forearm blood flow (A) and forearm vascular resistance (B) at baseline and after 2 and 4 wk of exposure. **P < 0.01.

Fig. 5.

Representative recording of 90 s of leg MSNA before exposure (baseline) and after 4 wk of intermittent nocturnal hypoxia.

Fig. 6.

Plot of individual data points for subjects' MSNA [expressed in bursts/min (A) and in bursts/100 heartbeats (hb) (B)] before exposure and after 4 wk of intermittent nocturnal hypoxia.

Fig. 7.

Evolution of the set point for diastolic blood pressure and MSNA (bursts/min) before (○) and after (●) 4 wk of exposure. *P < 0.05.

Fig. 8.

Reactive hyperemia (15-min ischemia) before exposure (baseline) and after 2 wk and 4 wk of exposure.