Aortic input impedance and left ventricular energetics in acute isovolumic anaemia

Abstract

Ascending aortic impedance spectra and left ventricular energetics were studied before and for one hour following the rapid induction of anaemia in seven dogs anaesthetised with halothane N₂O/O₂. A control group of five dogs were similarly studied, using the same protocol, but mimicking the induction of anaemia by bleeding and immediate replacement of the shed blood. Anaemia (Hb 6.6 g·dl⁻¹), induced by an isovolumic exchange-transfusion with Dextran 70, resulted in a significant decrease in the impedance modulus at zero frequency, and a slight reduction in the characteristic impedance (Z₀). No consistent changes were observed in the phase angle of impedance. The reduction in systemic vascular resistance after the induction of anaemia was associated with a significant rise in the heart rate and stroke volume, and significant increases were observed in both the steady and pulsatile components of left ventricular work. The administration of propranolol (0.2 mg·kg⁻¹) 60 min after dextran-exchange resulted in a reversion of these indices to the pre-exchange control values. No significant changes were observed in the pulsatile to total work ratio. The absence of any significant increase in this ratio demonstrates that the efficiency with which the heart is ‘decoupled’ from the peripheral vascular bed by the impedance characteristics of the aorta is not compromised by the hyperdynamic circulatory state that is the consequence of an acutely induced anaemia. The reversion of the indices of left ventricular energetics to the pre-anaemia control values is interpreted as corroborative evidence for the existence of increased cardiac sympathetic drive during acute isovolumic anaemia.