BackgroundHypoxia is a trigger for sympathetic activation and autonomic cardiovascular dysfunction. Pulmonary vascular disease (PVD) is associated with hypoxaemia, which increases with altitude. The aim was to investigate how exposure of patients with PVD to hypobaric hypoxia at altitude affects autonomic cardiovascular regulation.MethodsIn a randomised crossover study, patients with PVD were studied for 1 day and one night at an altitude of 2500 m (hypobaric hypoxia) and low altitude at 470 m in a random order. Outcomes were heart rate variability (HRV) in the time domain and in the frequency domain (low frequency (LF)/high frequency (HF) and LF/HF) and heart rate (HR) during day and night and baroreflex sensitivity (BRS).ResultsIn 25 patients with PVD (72% pulmonary arterial hypertension and 28% distal chronic thromboembolic pulmonary hypertension; mean±sdage 60.7±13.6 years), exposure to altitude resulted in significant increases in awake HR by 9.4 bpm (95% confidence interval (CI) 6.3–12.4, p<0.001) and nocturnal HR by 9.0 bpm (95% CI 6.6–11.4, p<0.001) and significant changes in awake and particularly nocturnal HRV indicating decreasing parasympathetic and increasing sympathetic activity (change in daytime LF/HF 1.7 (95% CI 0.6–2.8), p=0.004; nocturnal LF/HF 1.9 (95% CI 0.3–3.4), p=0.022) and a significant decrease in BRS (−2.4·mmHg−1(95% CI −4.3– −0.4, p=0.024)).ConclusionExposure of PVD patients to altitude resulted in a significant change in HRV indicating an increase in sympathetic activity and a decrease in BRS. The relative change in HRV at altitude was more pronounced during sleep.