Cerebral Blood Flow Velocity Reactivity Research Articles

Influence of Acute Fasting on Cerebrovascular Reactivity During Mental Stress

Acute fasting has been adopted in fitness communities, and has been practiced for religious purposes. Short‐term fasting does not affect cognitive function, and therefore may not affect cerebrovascular responsiveness. Cerebral blood flow velocity increases with mental stress, however the combined influences of an acute fast and mental stress on cerebrovascular responses are unknown. The purpose of this study was to describe the effects of a 24‐hour, water‐only fast on cerebral blood flow velocity responses to mental stress in a small cohort of research volunteers. Eight young healthy participants (7 males, average age of 25 yrs) completed the protocol in a fed state (4 hours post‐prandial) and in a fasted state (24–25 hours post‐prandial). Whole blood was collected and analyzed before experimentation for β‐Ketones, total cholesterol, HDL, LDL, triglycerides (TRG), and glucose. With participants in a supine position, we recorded the electrocardiogram, beat‐to‐beat arterial pressure (finger plethysmography), and cerebral artery blood flow velocity (transcranial Doppler). After a 5‐min baseline, participants performed 5‐min of mental stress via mental arithmetic (rapid continuous subtraction). Mental stress reactivity was calculated as the mean stress response for each minute of mental stress minus the corresponding 5‐min baseline. The minute‐by‐minute reactivity was analyzed with repeated measures ANOVA. Blood variables were analyzed with paired t‐tests. Data were expressed as mean ± SE. Baseline blood variables were different in the fasted compared with the fed state: TRG (112±14 fed vs. 68±13 mg/dL fasted; p<.01) Glucose (101±5 fed vs. 79±6 mg/dL fasted; p<.01) and β‐Ketone (.1±.01 fed vs. .5±.2 mmol/L fasted; p<.05). Mean arterial pressures and heart rate reactivities to mental stress were not affected by fasting. Mean cerebral blood flow velocity reactivity to mental stress was reduced in the fasted compared with the fed condition (condition × time, p<.01), with a particular divergent responsiveness during the third minute of mental stress. The results we present are new, but preliminary. We did not attempt to ascribe mechanisms to fasting and cerebral blood flow velocity reactivity during mental stress. The changes we observed with our preliminary data set suggest that fasting, and associated changes of plasma glucose, β‐Ketone and TRG may be associated with reductions of cerebral blood flow velocity during mental stress.Support or Funding InformationSupport was provided by the Michigan Space Grant Consortium and The Portage Health Foundation of Michigan.

Cerebral vasoreactivity decreases overnight in severe obstructive sleep apnea syndrome: A study of cerebral hemodynamics

Background OSAS has been associated with surrogate markers of atherosclerosis and is a known risk factor for stroke. However, there is limited data on the effects of recurring apneas in severe OSAS on cerebral circulation and their consequences on cerebrovascular reactivity and compliance. Objective To evaluate cerebral blood flow velocity (CBFV) changes and vascular compliance in patients with severe obstructive sleep apnea syndrome (OSAS) using transcranial Doppler sonography (TCD) and cerebral pulse transit time (PTT). Methods Seven patients (1 woman, 6 men, mean age 57.4 years) with severe OSAS underwent polysomnography at the sleep laboratory of the Neurology Department of Innsbruck Medical University. TCD was performed continuously during the whole night using a pulsed wave probe and was co-registered with routine polysomnography. Cerebrovascular reactivity was assessed by calculation of apnea and hypopnea-related CBFV changes. Arterial compliance was characterized by PTT derived from phase difference analysis between ECG and TCD signals. Sleep time was dichotomized into periods with high density of consecutive respiratory events (CRE) vs. periods with low density of consecutive respiratory events (non-CRE). TCD measurements of CBFV showed a regular, undulating pattern with flow minima immediately before apneas or hypopneas and maxima closely after their termination, reciprocally to peripheral O 2 saturation. CBFV reactivity was significantly diminished in CRE compared to non-CRE periods. PTT phase differences were reduced in non-CRE, and even more so in CRE periods, compared to initial wake phases. Conclusion We found severe disturbances of cerebrovascular reactivity in OSAS patients. Our data demonstrate loss of vasoreactivity and increase of arterial stiffness, indicated by CBF hyporeactivity and PTT reduction, especially during CRE periods. These changes are likely to impair cerebral circulation and may be detrimental to the endothelium.