To the Editor:—
Rivenes et al. 1have presented evidence that sevoflurane and isoflurane preserve forward output in patients with congenital heart disease. Their conclusion that cardiac output is maintained with little change in contractility with sevoflurane and isoflurane is not accurate. The data in their table 4 indicate that systemic vascular resistance decreases with both agents, whereas cardiac output and preload (left ventricular end-diastolic volume) are unchanged. This result is obtained only if contractility decreases. I used the data in table 4 of Rivenes et al. 1to estimate end-systolic elastance (Ees) 2for the isoflurane cases using a computer model 3modified to simulate the characteristics of pediatric hearts. Input variables were heart rate, systemic vascular resistance, and left ventricular end-diastolic volume. The Ees required to generate the ejection fraction (mean value) from table 4 of Rivenes et al. 1for each isoflurane case and the resulting mean arterial pressure and cardiac index are shown in table 1below.