THE EFFECT OF GRADED HEMORRHAGE AND INTRAVASCULAR VOLUME REPLACEMENT ON SYSTOLIC PRESSURE VARIATION IN HUMANS DURING MECHANICAL AND SPONTANEOUS VENTILATION

In dogs and humans, the magnitude of the variation in systolic pressure (SPV) over the respiratory cycle during mechanical ventilation appears to be inversely related to intravascular volume. Also reported to correlate with changes in Volume status is Delta down, the difference between systolic pressure at end-expiration and the lowest value during the respiratory cycle. These variables were examined during graded hemorrhage in eight anesthetized, mechanically ventilated subjects, and seven awake, spontaneously breathing subjects. SPV and Delta down were measured in sequence at baseline, after 500 mL blood removal, after 1000 mL (total) blood removal, after 500 mL hetastarch replacement, after 1000 mL (total) hetastarch replacement, and after 500 mL normal saline (NS). Repeated-measures analysis of variance was used to test the significance of the change in SPV and Delta down among the interventions. During mechanical ventilation, each 500-mL hemorrhage significantly increased SPV and Delta down, and each 500-mL hetastarch infusion significantly decreased SPV and Delta down. After hetastarch, both SPV and Delta down were smaller than at baseline and may explain why the infusion of NS caused nonsignificant reductions in SPV and Delta down. A SPV of 5 mm Hg or less, or a Delta down of 2 mm Hg or less appeared to indicate minimal intravascular volume depletion. During spontaneous ventilation, Delta down could not be determined accurately in several subjects, and SPV did not change in the appropriate direction in all cases of hemorrhage and volume infusion. It was concluded that SPV and Delta down appear to follow shifts in intravascular volume in relatively healthy, mechanically ventilated humans under isoflurane anesthesia; however, the use of SPV during spontaneous ventilation appears problematic, perhaps due to uncontrolled tidal volumes.