Atmospheric Pressure and Blood Circulation

Hypoxia rather than reduction in atmospheric pressure (AtP) is thought to be the driving factor of circulatory alterations at high altitude. Ambient pressure-related changes in hemodynamics have not been studied at low altitude. This research of both cross-sectional and repeated measures observational designs was undertaken in Novosibirsk, located at 200 m above sea level, to test a hypothesis whether weather-related AtP itself influences human blood circulation. The usual daily AtP at this place varies over the year within the range 728–770 mmHg. The range demonstrates a positive secular trend over the last two decades probably due to climate changes in Siberia.

The study was carried out by three ways. The first hospital based investigation was performed to verify whether AtP influences peripheral circulation and arterial wall distensibility that were assessed by venous occlusive plethysmography and oscillometry in 276 untreated patients with primary moderate hypertension. After adjustment for the degree of hypertension and season, correlation and multiple regression analyses yielded an age-dependent direct correlation between the stiffness of limb large arteries and AtP levels on examination day in men, but not women. The association weakened with the degree of hypertension, disappeared with age and was more evident in brachial than in shank arteries.

Second, 57 normotensive and moderately hypertensive individuals of both sexes aged 24–70 yrs were twice examined within three weeks on days with different values of AtP. Peripheral and aortic pulse wave characteristics and carotid-radial pulse wave velocity were measured by applanation tonometry. Subjects examined on day with the higher AtP compared with the day of lower ambient pressure had lower brachial and aortic systolic, diastolic, mean blood pressures, augmentation pressure, augmentation index, and cardiac output. Carotid-radial pulse wave velocity tended to be greater on hyperbaric day than on day of smaller AtP. The latter result supports the data described in the previous paragraph on the decreasing arterial compliance with increasing AtP.

Three, in the hospital based study, the ultrasonic location technique was used to measure intima-media thickness, internal systolic diameter of the brachiocephalic arteries and linear blood velocity in these vessels in 555 patients with initial moderate hypertension at the beginning of treatment course. Correlation analysis revealed a temporal association between these vascular parameters and daily average values of AtP. Thus, common carotid artery intima-media thickness tended to increase on high-pressure days. Besides, the lumina of the common carotid and vertebral arteries were narrowed and, consequently, the blood velocity increased. The relationship is more evident in men than in women and in elderly subjects compared with young. It was observed for AtP on both the day of examination and a day prior. The association is not common as it is nonlinear for the diameter of internal carotid artery and inverse for the external one. This implies different sensitivity of arteries to the factor under study and possible blood redistribution in the arterial tree depending on ambient pressure.

The results are suggestive of vasoconstrictive and stiffening action of high AtP on large arteries. The barosusceptibility of circulation seems to be underestimated even taking into account numerous evidences of an association between barometric conditions and cardiovascular deaths and nonfatal events.

Among the physiological mechanisms underlying the ambient pressure whole-body effects, one can consider a reflexive action of barometric stimuli via receptors in internal ear, cochlear-hypothalamic nervous tract, sympathetic efferent pathways, and smooth muscle fibers in the wall of arteries of muscle and muscle-elastic types. The second likely mechanism is physical pressure-dependent properties of ambient air, i.e. O2 partial pressure, H2O partial pressure (humidity) acting as hypoxic factors, and oxygen solubility in blood serum and extracellular fluid.