Journal of Applied Physiology AJP: Renal Physiology
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J Appl Physiol (October 5, 2006). doi:10.1152/japplphysiol.00772.2006
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Submitted on July 13, 2006
Accepted on September 27, 2006

DIFFERENTIAL SENSITIVITIES OF CEREBRAL AND BRACHIAL BLOOD FLOW TO HYPERCAPNIA IN HUMANS

Jimmy S Vantanajal1, Jon C Ashmead1, Todd J Anderson2, Russell T Hepple3, and Marc J. Poulin4*

1 Physiology & Biophysics, University of Calgary, Calgary, Canada
2 Cardiac Sciences, University of Calgary, Calgary, Canada; Libin Cardiovascular Institute, University of Calgary, Calgary, Canada; Faculty of Medicine, University of Calgary, Calgary, Canada
3 Physiology & Biophysics, University of Calgary, Calgary, Canada; Faculty of Medicine, University of Calgary, Calgary, Canada; Faculty of Kinesiology, University of Calgary, Calgary, Canada
4 Physiology & Biophysics, University of Calgary, Calgary, Canada; Clinical Neurosciences, University of Calgary, Calgary, Canada; Faculty of Medicine, University of Calgary, Calgary, Canada; Faculty of Kinesiology, University of Calgary, Calgary, Canada

* To whom correspondence should be addressed. E-mail: poulin{at}ucalgary.ca.

Although 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 (middle cerebral artery blood velocity; {bigtriangledown}P) and brachial blood flow (BBF) to hypercapnia. We compared the sensitivities of {bigtriangledown}P and BBF to hypercapnia in humans. Based on the critical importance of the brain for the survival of the organism, we hypothesized that {bigtriangledown}P would be more sensitive than BBF to hypercapnia. Nine healthy males (30.1±5.2 years, mean±SD) participated. Euoxic hypercapnia (end-tidal PO2 (PETO2) = 88 Torr, end tidal PCO2 (PETCO2) = 9 Torr above resting) was achieved using the technique of dynamic end-tidal forcing. {bigtriangledown}P was measured by transcranial Doppler ultrasound as an index of CBF, whereas BBF was measured in the brachial artery by Echo Doppler. {bigtriangledown}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 {bigtriangledown}P and BBF. During hypercapnia, {bigtriangledown}P and BBF increased by 34±8 and 14±8 %, respectively. {bigtriangledown}P remained elevated throughout the hypercapnic period but BBF returned to baseline levels by 60 min. The {bigtriangledown}P CO2 sensitivity was greater than BBF (4±1 vs 2±1 % Torr-1; P<0.05). Our findings confirm that {bigtriangledown}P has a greater sensitivity than BBF in response to hypercapnia, and show an adaptive response of BBF which is not evident in {bigtriangledown}P.




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