The relationship between carotid arterial flow and the left ventricular area is valid to indicate contractility in states of cerebral autoregulation and decreased arterial pressure in sheep

Background: Myocardial contractility can be estimated by noninvasive ultrasound-derived time-varying elastance (E′es). The E es is composed of flow in the internal carotid artery located close to the middle cerebral arteries where autoregulated cerebral flow can be accurately detected. Furthermore most contractility indices are highly dependent on ventricular load. We therefore investigated whether the index E′es is influenced by cerebral autoregulation actuated by decreased arterial pressure which equals also a significant decrease in ventricular load. Methods: Time-varying elastance was measured in nine merino sheep using a conductance/micromanometer technique to reveal the standard index Ees, and by the arterial blood-flow velocity–LV area relationship, resulting in the tested index E′es. The precision to indicate changes in contractility was estimated by the derived indices ςEes and ςE′es. Cerebral microcirculation, systolic myocardial dyssynchrony, tissue oxygen of the brain cortex (p(ti)O2) and cerebral cell damage (L/P ratio) were documented. Following a period of stability, mean arterial pressure (MAP) was decreased to 50 mmHg either by the vasodilator sodium nitroprusside (SNP) or by impaired contractility performed with the cardio selective beta-blocker esmolol. Results: E es indicated precisely the unchanged myocardial contractility following SNP administration. The index E′es was valid to detect the decrease in contractility induced by esmolol whereas the precision decreased due to an increase in systolic dyssynchrony. Conclusions: Our results suggest that autoregulation of cerebral microperfusion and variations in arterial load will not alter E′es measurements. E′es is a suitable measurement to use when diagnosing causes of severe hypotension and selecting the appropriate therapy.