The increase in pulmonary arterial pressure (Ppa) during acute hypoxia is mainly related to alveolar hypoxaemia inducing vasoconstriction of precapillary pulmonary arterioles [1,2] whereas during chronic hypoxia, mechanical and hormonal factors make the vessels adapt to hypoxia and modify their response to chemical stimulus [3]. Furthermore, the temporal profile of the hypoxaemic stimulus may have a substantial effect on the pulmonary vascular response [4] as suggested by experimental studies allowing that repeated exposure to intermittent hypoxaemia [5] enhances the pulmonary hypoxic pressor response.Previous studies [6] in obstructive sleep apnoea (OSAS) patients have shown that repeated desaturations from obstructive apnoeas induce a transient increase in Ppa that, together with daytime hypoxaemia, is considered to be the key factor responsible for daytime pulmonary hypertension [7,8]. Recent evidence in animals [9][10][11] has demonstrated that repetitive hypoxic stimuli, simulating the chemical changes produced by apnoeas lead to systemic hypertension either during the exposure or during the recovery period. A more direct approach to the pulmonary vascular effects of repetitive hypoxaemia has been used by IWASE et al. [12]. Using a canine model they have demonstrated a progressive increase in systemic and pulmonary arterial pressure during repetitive hypoxaemic exposure. On the basis of these latter findings a trend in the pulmonary arterial pressure response to repetitive hypoxaemia similar to the one produced experimentally may occur in patients with OSAS.The two aims of the present study were: 1) to ascertain whether recurrent apnoea-related oxygen desaturations are associated with a progressive rise in Ppa during the night in OSAS patients; 2) if there was a rise in the expected direction, to determine whether diurnal and nocturnal variables would predict the degree of progressive nocturnal increase in Ppa.
Patients and methods
PatientsThirty five consecutive patients referred to the sleep laboratory for habitual snoring and possible sleep apnoea We have, therefore, examined the nocturnal trend of Ppa in seven obstructive sleep apnoea syndrome (OSAS) patients and in five snorers. Mean Ppa was measured before, at the start, at the end and after the selected apnoeas. The analysis was performed for each 1 h period for at least 7 h throughout the night on at least 10 randomly selected apnoeas per hour. In snorers, 100 randomly chosen values were measured during every hour of the night. In the morning after the nocturnal study, the Ppa responses to acute hypoxia and hypercapnia were measured.No Ppa changes throughout the 7 h were found during sleep in snorers (Ppa slope:-0.002±0.10 mmHg·h -1 ). In OSAS patients a small but significant increase in Ppa throughout the night was noted, affecting the values before (Ppa slope: 0.7±0.16 mm Hg·h -1 ), at the start of apnoea (Ppa slope: 0.53±0.1 mmHg·h -1 ) as well as at the end (Ppa slope: 0.44±0.08 mmHg·h -1 ) and in the postapnoeic period (Ppa slope: 0.55±0.1 mmHg...