Hemodynamic Effects of Preload and Sodium Nitroprusside in Patients Subjected to Coronary Bypass Surgery

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SUMMARY To differentiate between the hemodynamic effects of afterload reduction produced by sodium nitroprusside (NP) and the effects of preload, left ventricular function curves were constructed during and without NP infusion at the same, normal left ventricular filling pressure (LVFP) levels for 20 patients 8 hours after coronary bypass grafting and for 10 of these patients preoperatively.

Preoperatively, when the patients had normal myocardial performance, NP, in reducing both preload and afterload, decreased the stroke index (SI) (P < 0.05). However, SI and the cardiac index (CI) were 14% and 25% higher, respectively, with than without NP at identical LVFPs (P < 0.02). NP did not displace the left ventricular stroke work index (LVSWI) from a single function curve, nor did it affect myocardial oxygen consumption (MVO2, described as rate-pressure-product) if the cardiac work index (CWI) was unchanged.

Postoperatively, when the patients' myocardial performance was generally moderately or severely reduced, NP as such did not change SI or CI. However, at constant LVFP level, SI was 29% and CI was 31% greater during NP infusion (P < 0.001); the more horizontal the control ventricular function curve, the more LVSWI was augmented by NP. Postoperatively, NP increased CWI significantly without affecting MVO2 at constant LVFP.

Thus, NP is beneficial even at low preload levels, but the greatest enhancement in blood flow and the smallest diminution in systemic or pulmonary arterial pressures are achieved if LVFP is unchanged or increased while the NP infusion rate is accelerated stepwise to its optimum. At a constant filling pressure, NP produces an enhancement of myocardial pumping performance equal to that caused by a 7 mm Hg increase in the wedge pressure, but during NP infusion, changes in arterial pressures and MVO2 are minimal.

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