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Effect of antihypertensive agents on the arterial partial pressure of oxygen and venous admixture after cardiac surgery

Wood, Gordon MD, FRCPC

Clinical Investigations

Objective  To determine whether stopping nitroglycerin and sodium nitroprusside (both vasodilators) infusions in hypertensive, postcardiac surgical patients requiring a high FIO2 improves PaO2 and venous admixture.

Design  Prospective, clinical trial.

Setting  Intensive care unit in a university-affiliated hospital.

Patients  Thirty postcardiac surgical patients who, because of high FIO2 requirements, did not meet the criteria for weaning from mechanical ventilation and who were receiving infusions of nitroglycerin and/or sodium nitroprusside to control blood pressure.

Interventions  PaO2, venous admixture, and oxygen transport data were determined at baseline using arterial and mixed venous blood gas samples and hemodynamic values from a pulmonary artery catheter. The nitroglycerin and sodium nitroprusside infusions were stopped, and intravenous boluses of labetalol were administered to maintain a target blood pressure. After the vasodilator infusions were stopped, the baseline measurements were repeated to redetermine PaO2, venous admixture, and oxygen transport values.

Measurements and Main Results  Results included a mean increase in PaO2 from 79.3 +/- 15 torr (10.5 +/- 2.0 kPa) to 118.3 +/- 38 torr (15.7 +/- 5.1 kPa) and a mean decrease in venous admixture from 26.4 +/- 5.8% to 17.6 +/- 5.6% when the vasodilators were stopped. All 30 patients had an increase in PaO2 and a decrease in venous admixture. Because of the improvement in oxygenation, 28 of the 30 patients met the criteria for weaning from mechanical ventilation once nitroglycerin and sodium nitroprusside were stopped or decreased. Labetalol was well tolerated in this group of patients who had preserved ventricular function.

Conclusions  Substituting labetalol for nitroglycerin and sodium nitroprusside improves arterial oxygenation and venous admixture in hypertensive postcardiac surgical patients who require a high FIO2. This change in therapy may allow patients to be weaned from mechanical ventilation sooner. (Crit Care Med 1997; 25:1807-1812)

From the Department of Anaesthesia and Critical Care Medicine, Queen's University, Kingston, ON, Canada.

Supported, in part, by a grant from The Physician Services Incorporated Foundation.

Address requests for reprints to: Gordon Wood, MD, Department of Anaesthesia and Critical Care Medicine, Douglas 2, Kingston General Hospital, 76 Stuart Street, Kingston, ON K7L 3N6, Canada.

© Williams & Wilkins 1997. All Rights Reserved.