The effect of changes in cardiac output on middle cerebral artery mean blood velocity at rest and during exercise

We examined the relationship between changes in cardiac output (Q) and middle cerebral artery mean blood velocity ( MCA V-mean) in seven healthy volunteer men at rest and during 50% maximal oxygen uptake steady-state submaximal cycling exercise. Reductions in Q were accomplished using lower body negative pressure ( LBNP), while increases in. Q were accomplished using infusions of 25% human serum albumin. Heart rate ( HR), arterial blood pressure and MCA V-mean were continuously recorded. At each stage of LBNP and albumin infusion. Q was measured using an acetylene rebreathing technique. Arterial blood samples were analysed for partial pressure of carbon dioxide tension ( P-a,P-CO2). During exercise HR and. Q were increased above rest ( P < 0.001), while neither MCA V-mean nor P-a,P-CO2 was altered ( P > 0.05). The MCA V-mean and. Q were linearly related at rest ( P < 0.001) and during exercise ( P = 0.035). The slope of the regression relationship between MCA V-mean and. Q at rest was greater ( P = 0.035) than during exercise. In addition, the phase and gain between MCA V-mean and mean arterial pressure in the low frequency range were not altered from rest to exercise indicating that the cerebral autoregulation was maintained. These data suggest that the. Q associated with the changes in central blood volume influence the MCA V-mean at rest and during exercise and its regulation is independent of cerebral autoregulation. It appears that the exercise induced sympathoexcitation and the change in the distribution of. Q between the cerebral and the systemic circulation modifies the relationship between MCA V-mean and.