Head-up tilt test is useful for exploring neurally mediated
syncope.
Adenosine is an
ATP derivative implicated in cardiovascular disturbances that occur during head-up tilt test. The aim of the present study was to investigate the impact of
hyperoxia on
adenosine plasma level and on hemodynamic changes induced by head-up tilt testing.Seventeen healthy male volunteers (mean age 35 ± 11 years) were included in the study. The experiment consisted of 2 head-up tilt tests, 1 session with subjects breathing, through a mask, medical air (FiO2 = 21%) and 1 session with administration of pure
oxygen (FiO2 = 100%) in double-blind manner. Investigations included continuous monitoring of hemodynamic data and measurement of plasma
adenosine levels.No
presyncope or
syncope was found in 15 of the 17 volunteers. In these subjects, a slight decrease in systolic blood pressure was recorded during orthostatic stress performed under medical air exposure. In contrast,
hyperoxia led to increased systolic blood pressure during orthostatic stress when compared with medical air. Furthermore, mean
adenosine plasma levels decreased during hyperoxic exposure before (0.31 ± 0.08 μM) and during head-up tilt test (0.33 ± 0.09 μM) when compared with baseline (0.6 ± 0.1 μM).
Adenosine plasma level was unchanged during medical air exposure at rest (0.6 ± 0.1 μM), and slightly decreased during orthostatic stress. In 2 volunteers, the head-up tilt test induced a
loss of consciousness when breathing air. In these subjects,
adenosine plasma level increased during orthostatic stress. In contrast, during hyperoxic exposure, the head-up tilt test did not induce
presyncope or
syncope. In these 2 volunteers, biological study demonstrated a decrease in
adenosine plasma level at both baseline and during orthostatic stress for hyperoxic exposure compared with medical air.These results suggest that
hyperoxia was able to increase blood pressure during head-up tilt test via a decrease in plasma
adenosine concentration. Our results also suggest that
adenosine receptor antagonists are worth trying in
neurocardiogenic syncope.