Determinants of systolic pressure variation in patients ventilated after vascular surgery

Objective: To discover the predominant determinant of systolic pressure variation during positive-pressure ventilation in mechanically ventilated patients after a vascular surgical procedure. Design: Case control study. Setting: Postanesthesia care unit at a university hospital. Participants: Eleven patients who were sedated during mechanical ventilation after abdominal aortic surgery. Interventions: Radial arterial pressure and airway pressure were simultaneously recorded. The systolic pressure variation was measured as the mean difference between the maximal and minimal systolic pressure values during five consecutive mechanical breaths. The delta down was measured as the difference between the systolic blood pressure during apnea and the minimal values of the systolic pressure after one mechanical breath. The velocity time integral, which is closely related to stroke volume, was measured throughout the systolic pressure measurements. After the first set of measurement, 8 of the 11 patients were volume loaded with colloïd solution (15 mL/kg). The same measurements were collected after this loading. Measurements and Main Results: Positive correlation was found between changes in velocity time integral and the magnitude of both systolic pressure variation (r = 0.73) and delta down (r = 0.80). Volume loading did not significantly modify systolic blood pressure. However, it did significantly decrease systolic pressure variation and delta down. The corresponding changes in velocity time integral provoked by mechanical ventilation decreased significantly as well. Conclusions: The decrease in systolic pressure provoked by positive-pressure inspiration reflects simultaneous decreases in stroke volume. This suggests that a decrease in left ventricular filling, associated with positive-pressure inspiration, is responsible for systolic pressure variation. This finding confirms the interest in considering systolic pressure variation to provide reliable information about the responsiveness of the heart to preload variations.