Systolic blood pressure at end-expiration measured by the automated systolic pressure variation monitor is equivalent to systolic blood pressure during apnea.

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It is necessary to define a reference systolic arterial blood pressure (RP) to calculate delta-Up (dUp) and delta-Down (dDown) for systolic pressure variation. Most studies define the reference pressure as the average systolic blood pressure during a short period of apnea. We describe an automated systolic pressure variation monitor that measures airway pressure and defines the reference pressure as the systolic blood pressure at end-expiration. The present study compares the reference systolic blood pressure measured at end-expiration by the automated systolic pressure variation monitor and the reference systolic blood pressure measured during apnea to test whether the end-expiration value is an adequate substitute for the value during apnea. After obtaining informed consent, 108 sets of measurements of systolic pressure variation (SPV) were made in 20 intubated, mechanically-ventilated, anesthetized patients by the automated SPV monitor and during apnea. Measurements were taken during periods of hemodynamic stability defined as three consecutive end-expiratory systolic blood pressures within four mmHg of each other. The three systolic pressures at end-expiration were averaged (RPmonitor). Immediately following these measurements, the ventilator was turned off and the systolic blood pressure was measured at 6, 8, 10 and 12 seconds of apnea. The reference pressure during apnea (RPapnea) was defined as the average of the systolic blood pressure at 8, 10 and 12 seconds of apnea. For each measurement set, RPmonitor and the systolic blood pressure at 6 seconds of apnea (SBP6) were compared to RPapnea using Bland-Altman analysis. Bland-Altman analysis for the difference between SBP6 and RPapnea yielded a small bias of -0.3 mmHg with standard deviation of 1.3, indicating that the systolic pressure tends to continue to increase slightly after 6 seconds of apnea. Results were similar for the difference between RPmonitor and RPapnea (-0.2 +/- 3.1 mmHg). dUp and dDown are calculated using the reference pressure. RPmonitor is an average of 0.2 mm Hg less than RPapnea, thus dUp calculated by the automated SPV monitor is an avenge of 0.2 mm Hg greater than dUp measured by the reference pressure during apnea and dDown is 0.2 mm Hg less. Since the bias of -0.2 mmHg is clinically insignificant, there is acceptable agreement between the reference pressure obtained during apnea and that obtained by the automated SPV monitor at end-expiration. The mean difference between RPmonitor and RPapnea is explained by the continued rise in systolic pressure during the period of apnea as demonstrated by the difference between SBP6 and RPapnea.