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Differential sensitivities of cerebral and brachial blood flow to hypercapnia in humans

¹Department of Physiology and Biophysics, ²Cardiac Sciences and Libin Cardiovascular Institute, and ³Department of Clinical Neurosciences, Faculties of ⁴Medicine and ⁵Kinesiology, University of Calgary, Calgary, Alberta, Canada

Submitted 13 July 2006 ; accepted in final form 27 September 2006

Although it is known that the vasculatures of the brain and the forearm are sensitive to changes in arterial PCO₂, previous investigations have not made direct comparisons of the sensitivities of cerebral blood flow (CBF) (middle cerebral artery blood velocity associated with maximum frequency of Doppler shift; P) and brachial blood flow (BBF) to hypercapnia. We compared the sensitivities of P and BBF to hypercapnia in humans. On the basis of the critical importance of the brain for the survival of the organism, we hypothesized that P would be more sensitive than BBF to hypercapnia. Nine healthy males (30.1 ± 5.2 yr, mean ± SD) participated. Euoxic hypercapnia (end-tidal PO₂ = 88 Torr, end-tidal PCO₂ = 9 Torr above resting) was achieved by using the technique of dynamic end-tidal forcing. P was measured by transcranial Doppler ultrasound as an index of CBF, whereas BBF was measured in the brachial artery by echo Doppler. P and BBF were measured during two 60-min trials of hypercapnia, each trial separated by 60 min. Since no differences in the responses were found between trials, data from both trials were averaged to make comparisons between P and BBF. During hypercapnia, P and BBF increased by 34 ± 8 and 14 ± 8%, respectively. P remained elevated throughout the hypercapnic period, but BBF returned to baseline levels by 60 min. The P CO₂ sensitivity was greater than BBF (4 ± 1 vs. 2 ± 1%/Torr; P < 0.05). Our findings confirm that P has a greater sensitivity than BBF in response to hypercapnia and show an adaptive response of BBF that is not evident in P.

cerebral blood flow; sensitivity to carbon dioxide