Blood flow in internal carotid and vertebral arteries during graded lower body negative pressure in humans

Abstract

New Findings r What is the central question of this study?Recently, the heterogeneity of the cerebral arterial circulation has been argued. Orthostatic tolerance may be associated with an orthostatic stress-induced change in blood flow in vertebral arteries rather than in internal carotid arteries, because vertebral arteries supply blood to the medulla oblongata, which is the location of important cardiac, vasomotor and respiratory control centres. r What is the main finding and its importance?The effect of graded orthostatic stress on vertebral artery blood flow is different from that on internal carotid artery blood flow. This response allows for the possibility that orthostatic tolerance may be associated with haemodynamic changes in posterior rather than anterior cerebral blood flow.Recently, the heterogeneity of the cerebral arterial circulation has been argued, but the characteristics of vertebral artery (VA) and internal carotid artery (ICA) blood flow during graded orthostatic stress remain unknown. We hypothesized that the change in blood flow in VA is not similar to that in ICA blood flow during graded orthostatic stress. We measured blood flows in both ICA and VA during graded lower body negative pressure (LBNP; −20, −35 and −50 mmHg) by using two colour-coded ultrasound systems. The effect of graded orthostatic stress on the VA blood flow was different from that on the ICA blood flow (LBNP × artery, P = 0.006). The change in ICA blood flow was associated with the level of LBNP (r = 0.287, P = 0.029), and a reduction in ICA blood flow from pre-LBNP was observed during −50 mmHg LBNP (from 411 ± 35 to 311 ± 40 ml min −1 , P = 0.044) without symptoms of presyncope. In contrast, VA blood flow was unchanged during graded LBNP compared with the baseline (P = 0.597) relative to the reduction in ICA blood flow and thus there was no relationship between VA blood flow and the level of LBNP (r = 0.167, P = 0.219). These findings suggest that the change in ICA blood flow is due to the level of LBNP during graded orthostatic stress, but the change in VA blood flow is different from that in ICA blood flow across the different levels of LBNP. These findings provide the possibility that posterior cerebral blood flow decreases only during severe orthostatic stress and is therefore more likely to be linked with orthostatic tolerance.

Decreased carotid and vertebral arterial blood-flow velocity in response to orthostatic unload in patients with severe aortic stenosis

“…Sato et al [17] showed that blood flow in the internal carotid artery and medial cerebral artery was reduced during head up tilt test, but vertebral artery blood flow was maintained by dilatation of territories of the vertebrobasilar system. Furthermore, Ogoh et al [18] have recently provided data that the effect of graded orthostatic stress on vertebral artery blood flow is different from that on internal carotid artery blood flow. This response allows for the possibility that orthostatic tolerance may be associated with hemodynamic changes in posterior rather than anterior cerebral blood flow.…”

The role of cerebral oxygenation and regional cerebral blood flow on tolerance to central hypovolemia

“…similar up to the last common level of LBNP, despite LT subjects reaching presyncope at this time point. The present study is one of very few to report CBF responses within the posterior cerebral circulation during central hypovolemia elicited by LBNP, including the PCA or vertebral arteries (VA) (12,33). Autonomic and respiratory control centers are located within the medulla oblongata in the brain stem, which receives blood and oxygen supply through these posterior cerebral arteries (45).…”

“…Deegan et al (12) reported similar responses between mean MCAv and blood flow in the VA during head-up tilt plus LBNP to presyncope, although they combined all 18 subjects into a single group and did not assess potential differences between individuals with varying tolerance to this stress. Most recently, Ogoh et al (33) examined blood flow responses in the VA feeding the posterior cerebral circulation and internal carotid arteries (ICA) feeding the anterior cerebral circulation up to submaximal LBNP of Ϫ50 mmHg. On the basis of the significant, although weak, association between the fall in ICA flow and the magnitude of LBNP (r ϭ 0.29; P ϭ 0.029) vs. no change in VA flow with LBNP (r ϭ 0.167; P ϭ 0.22), these investigators postulated that cerebral perfusion of the posterior regions of the brain would only decrease with severe orthostatic stress, and may be associated with tolerance to central hypovolemia.…”

“…Although traditionally protection of absolute CBF was thought to be essential in determining tolerance to central hypovolemia (24), recent studies have indicated a disconnect between tolerance to LBNP and protection of absolute flow [predominantly assessed by middle cerebral artery velocity (MCAv), an index of global cerebral blood flow] (20,27,28,37). Most recently, Ogoh et al (33) suggested that flow in the posterior cerebral circulation (indexed by flow in the vertebral arteries) feeding the brain stem (specifically, the medulla oblongata) is more likely associated with tolerance to central hypovolemia than flow in the anterior cerebral circulation (indexed by flow in the internal carotid arteries); responses between the two regions with central hypovolemia to presyncope, however, were not evaluated.…”

The effect of an acute increase in central blood volume on the response of cerebral blood flow to acute hypotension

“…In addition to carbon dioxide (CO 2 ), other physiological factors affect cerebral circulation. Also, recent investigations have indicated that CBF control was modified by a change in systemic blood distribution during heat stress, exercise, and orthostatic stress (29,31,32,39,40). For example, orthostatic stress or heat stress decreases intracranial CBF (29,31,39).…”