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

Although it is known that the vasculatures of the brain and the forearm are sensitive to changes in arterial P-CO2, 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; (V) over bar P) and brachial blood flow ( BBF) to hypercapnia. We compared the sensitivities of (V) over barP 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 (V) over barP would be more sensitive than BBF to hypercapnia. Nine healthy males ( 30.1 +/- 5.2 yr, mean +/- SD) participated. Euoxic hypercapnia ( endtidal PO2 = 88 Torr, end-tidal PCO2 = 9 Torr above resting) was achieved by using the technique of dynamic end- tidal forcing. (V) over bar P was measured by transcranial Doppler ultrasound as an index of CBF, whereas BBF was measured in the brachial artery by echo Doppler. (V) over bar 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 (V) over bar P and BBF. During hypercapnia, (V) over bar P and BBF increased by 34 +/- 8 and 14 +/- 8%, respectively. (V) over bar P remained elevated throughout the hypercapnic period, but BBF returned to baseline levels by 60 min. The (V) over barP CO2 sensitivity was greater than BBF ( 4 +/- 1 vs. 2 +/- 1%/ Torr; P < 0.05). Our findings confirm that (V) over bar P has a greater sensitivity than BBF in response to hypercapnia and show an adaptive response of BBF that is not evident in (V) over bar P.