The effects of superimposed tilt and lower body negative pressure on anterior and posterior cerebral circulations

Abstract

Steady‐state tilt has no effect on cerebrovascular reactivity to increases in the partial pressure of end‐tidal carbon dioxide (PETCO 2). However, the anterior and posterior cerebral circulations may respond differently to a variety of stimuli that alter central blood volume, including lower body negative pressure (LBNP). Little is known about the superimposed effects of head‐up tilt (HUT; decreased central blood volume and intracranial pressure) and head‐down tilt (HDT; increased central blood volume and intracranial pressure), and LBNP on cerebral blood flow (CBF) responses. We hypothesized that (a) cerebral blood velocity (CBV; an index of CBF) responses during LBNP would not change with HUT and HDT, and (b) CBV in the anterior cerebral circulation would decrease to a greater extent compared to posterior CBV during LBNP when controlling PETCO 2. In 13 male participants, we measured CBV in the anterior (middle cerebral artery, MCAv) and posterior (posterior cerebral artery, PCAv) cerebral circulations using transcranial Doppler ultrasound during LBNP stress (−50 mmHg) in three body positions (45°HUT, supine, 45°HDT). PETCO 2 was measured continuously and maintained at constant levels during LBNP through coached breathing. Our main findings were that (a) steady‐state tilt had no effect on CBV responses during LBNP in both the MCA (P = 0.077) and PCA (P = 0.583), and (b) despite controlling for PETCO 2, both the MCAv and PCAv decreased by the same magnitude during LBNP in HUT (P = 0.348), supine (P = 0.694), and HDT (P = 0.407). Here, we demonstrate that there are no differences in anterior and posterior circulations in response to LBNP in different body positions.

Associations between changes in precerebral blood flow and cerebral oximetry in the lower body negative pressure model of hypovolemia in healthy volunteers

“…We did not find a change in in the VA flow with hypovolemia. Although not a consistent finding [21, 22], these results agree with studies indicating regional differences between the areas supplied by the ICA and VA in response to hypovolemia with or without hypotension [13, 23–25], indicating that blood supply to the vital functions of the brainstem may be more strongly regulated. Blood flow in the VA was not associated with changes in ScO 2 (Fig 5, right panel), consistent with the attachment of the ScO 2 -probes over the frontal region of the head reflecting circulation from the anterior and middle cerebral arteries [7].…”

Cerebral blood flow changes during tilt table testing in healthy volunteers, as assessed by Doppler imaging of the carotid and vertebral arteries

“…Almost all of the studies examining cerebral flow during head-up tilt testing in healthy volunteers (in isolation or being the reference of patient studies) were performed with transcranial Doppler ( Yoshimoto et al, 1994 , Schondorf et al, 2001 , Oblak et al, 2002 , Wilson et al, 2002 , Carey et al, 2003 , Razumovsky et al, 2003 , Asahina et al, 2006 , Gonul et al, 2008 , Ocon et al, 2009 , Wang et al, 2010 , Murrell et al, 2011 , Mezei et al, 2013 , Riberholt et al, 2013 , Yang et al, 2015 , Tymko et al, 2016 , Viski et al, 2016 , Castro et al, 2017 ). Recently, a small study of 6 HV ( Sato et al, 2012 ) undergoing HUT was published using extracranial Doppler imaging.…”

Assessment of the brain ischemia during orthostatic stress and lower body negative pressure in air force pilots by near-infrared spectroscopy

“…The cerebral oxygen saturation index StO 2 was obtained for 15°HDT with -25 mmHg of LBNP [49]. Cerebral blood velocity measured with the transcranial Doppler ultrasound techinque while the LBNP (-50 mmHg) in three body positions (45°HUT, supine, 45°HDT) was shown in [50]. The cerebral blood velocity decreased during LBNP in all three tilt positions.…”