We studied coupling between the heart and arterial system in patients undergoing elective noncardiac surgery. Left ventricular end-systolic (ESV) and end-diastolic volumes (EDV) were measured by transesophageal echocardiography, and left ventricular end-systolic pressure (Pes) was estimated from the radial artery pressure tracing. Studies include two clinical trials. First trial: To investigate whether E'max is a clinically useful measurement of ventricular function or not, the correlation of E'max to Emax was accomplished in 10 patients. Emax was calculated during afterload reduction (nicardipine 30μg/kg), and E'max was approximated as the ratio of mean arterial pressure (MAP) and ESV based on the simplifying approximations, i.e., Pes is close to MAP and x-axis intercept (Vo) is zero. The correlation coefficient obtained between E'max and Emax was 0.97, suggesting that E'max, like Emax, is a useful tool to estimate left ventricular performance. Second trial: Ea was approximated as MAP/(EDV-ESV). In 222 (74%) of the 299 measurements obtained in 68 subjects without heart disease (normal group), Ea/E'max was nearly 0.5, which is a condition for a maximal mechanical efficiency, while in 61 measurements (20%) Ea was almost equal to E'max (Ea/E'max=1), which is a condition for maximal stroke work from a given end-diastolic volume. In contrast, in 32 subjects with heart disease (cardiac group), Ea/E'max was nearly 0.5 in 43 (30%) of the 143 measurements, while in 49 measurements (34%) Ea/E'max was nearly 1. In addition, although the value of Ea/E'max over 2, which represents severe heart failure, was not observed in normal group, Ea/E'max was over 2 in 18 measurements (13%) in cardiac group. Thus, the present results suggest that, as reported previously in awake patients, ventriculoarterial coupling is set toward higher left ventricular work efficiency in surgical patients without heart disease, whereas in patients with heart disease, ventricular and arterial properties are so matched as to maximize stroke work at the expense of the work efficiency.