Dynamic blood pressure control and middle cerebral artery mean blood velocity variability at rest and during exercise in humans

Cardiac failure and ischaemic heart disease patients receive standard of care cardiac β₁-adrenergic blockade medication. Such medication reduces cardiac output and cerebral blood flow. It is unknown whether the β₁-adrenergic blockade-induced reduction of cardiac output in the presence of an exercise-induced reduction in cardiac-arterial baroreflex gain affects cerebral blood flow variability. This study evaluated the influence of cardiac output variability on beat-to-beat middle cerebral artery mean blood velocity (MCA Vmean) during exercise with and without cardiac β₁-adrenergic blockade. Eight men (22 ± 1 years; mean ± SE) performed 15 min bouts of moderate (105 ± 11 W) and heavy (162 ± 8 W) intensity cycling before and after cardio-selective β₁-adrenergic blockade (0.15 mg kg⁻¹ metoprolol). The relationship between changes in cardiac output or mean arterial pressure (MAP) and MCA Vmean as well as cardiac-arterial baroreflex gain were evaluated using transfer function analysis. Both exercise intensities decreased the low frequency (LF) transfer function gain between cardiac output and MCA Vmean (P < 0.05) with no significant influence of β₁-blockade. In contrast, the LF transfer function gain between MAP and MCA Vmean remained stable also with no significant influence of metoprolol (P > 0.05). The LF transfer function gain between MAP and HR, an index of cardiac-arterial baroreflex gain, decreased from rest to heavy exercise with and without β₁-blockade (P < 0.05). These findings suggest that the exercise intensity related reduction in cardiac-arterial baroreflex function at its operating point does not influence the dynamic control of MCA Vmean, even when the ability of exercise-induced increase in cardiac output is reduced by cardiac β₁-adrenergic blockade.