Fig. 1.

Experimental protocol. Cardiac output (CO) and arterial blood pressure (ABP) were measured at 2–3 and 11–12 min after the onset of dobutamine infusion, with the obtained responses averaged. Cerebral blood flow (CBF), i.e., internal carotid artery, external carotid artery, and common carotid artery, were measured by Doppler ultrasound at 4–9 min after the onset of dobutamine infusion. Echocardiographic indexes of systolic function were performed 5–10 min after the onset of dobutamine infusion. Heart rate (HR) and end-tidal carbon dioxide pressure ($\mathrm{P}\mathrm{E}{\mathrm{T}}_{\mathrm{C}{\mathrm{O}}_{\mathrm{2}}}$) were recorded continuously throughout the experiment. All measures were obtained at the indicated times for both the low-dose and high-dose dobutamine trials.