The systolic pressure variation (SPV), which is the difference between the maximal and minimal systolic blood pressure (SP) during one ventilatory cycle, was studied in ten patients during posterior spine fusion. To minimize the blood loss, deliberate hypotension to a mean blood pressure of 50 mmHg was introduced by a continuous infusion of sodium nitroprusside. SPV was further divided into two components, delta up and delta down, using SP during a short apnea as a reference point. All hemodynamic parameters were measured at the beginning of anesthesia, 15 minutes after induction of hypotension, before cessation of nitroprusside infusion, and 15 minutes after the end of the hypotensive period. During the hypotensive period (166 +/- 53 minutes), cardiac output (CO) decreased significantly from 4.83 +/- 1.36 L/min to 3.86 +/- 1.07 L/min (p less than 0.05). Heart rate (HR), central venous pressure (CVP), and pulmonary capillary wedge pressure (PCWP) did not change during this period and bore no correlation to the changes in CO. The only variables that changed during the hypotensive period, in addition to CO, were SPV (from 13.1 +/- 4.9 mmHg to 16.9 +/- 5.1 mmHg, p less than 0.02), and delta down (from 6.0 +/- 3.8 mmHg to 9.9 +/- 6.3 mmHg, p less than 0.05). The delta down segment was the only hemodynamic variable whose changes during the hypotensive period showed a significant (p less than 0.018) correlation with the changes in CO. delta down reflects the degree of decrease in left ventricular stroke output in response to a positive pressure breath, and thus is a sensitive indicator of preload.(ABSTRACT TRUNCATED AT 250 WORDS)