Do autonomic cardiovascular reflexes predict the nocturnal rise in blood pressure in obstructive sleep apnea syndrome?

To investigate the relationship between nocturnal changes in blood pressure (BP) and diurnal cardiovascular reflexes we examined a group of 19 male normotensive obstructive sleep apnea syndrome (OSAS) patients. All patients underwent a full polysomnographic examination including BP monitoring by a finger arterial pressure device (Finapres) and a battery of cardiovascular reflex tests; plasma catecholamine levels at rest were also measured. During sleep, BP increased with an average difference of 15.4 ± 7.5 mmHg for systolic and 8.3 ± 4.6 mmHg for diastolic pressure. Compared with control subjects, OSAS patients had lower values of Valsalva ratio (VR) (1.75 ± 0.4 vs 1.34 ± 0.2, p = 0.0004), E/I ratio (1.35 ± 0.2 vs 1.13 ± 0.9, p = 0.0004) and baroreflex sensitivity index (BRSI) (5.4 ± 2.1 vs 2.7 ± 1.9 mms/mmg, p = 0.0006) and a higher systolic (p = 0.02) and diastolic (p = 0.002) pressure response to tilting-up test. Noradrenaline plasma levels were also significantly higher (345 ± 125 vs 224 ± 92 pg/ml, p = 0.001). No significant correlations were found between the nocturnal rise in BP and the pressure responses during sympathetic manoeuvres or rest levels of noradrenaline. The nocturnal changes in systolic blood pressure during the night were negatively dependent on the diurnal BRSI (r = -0.91, p = 0.0007) and VR (r = -0.70, p = 0.006). We conclude that the high levels of noradrenaline at rest and the altered sympathetic cardiovascular reflexes alone do not account for the nocturnal variation in blood pressure in OSAS. Reduced baroreflex sensitivity and apnea-related vagal impairment appear to be implicated in the nocturnal pressure response to obstructive apneas.