HEMODILUTION DECREASES CRITICAL CLOSING PRESSURE BUT DOES NOT INCREASE THE AORTIC TO DISTAL PRESSURE GRADIENT IN DOGS

Le résumé fourni par la source

INTRODUCTION Hemodilution is used routinely during cardiac surgery and CPB. Its clinical and hemodynamic effects are well documented [1]; however, its effect on the arterial zero-flow ("critical closing") pressure has not been established. Further, it has been proposed [2] that hemodilution may be responsible for the aortic-to-distal arterial pressure gradient often seen after cardiopulmonary bypass. METHODS Animal handling and upkeep complied with institutional norms. Eleven mongrel dogs weighting 16.6 +/- 4.4 Kg were anesthetized with 5 mg/Kg of pentobarbital and 2 ug/Kg of sufentanyl and mechanically ventilated with oxygen and air. Millar catheter-tipped pressure transducers were located in the ascending aorta and in a distal femoral artery. An electromagnetic flowmeter was placed around the ascending aorta. Zero-flow pressure was measured by extrapolation of the arterial pressure decay obtained during a 4-second proximal aortic occlusion. Hemodilution was achieved removing 30 ml/Kg of blood over 5 to 10 minutes and replacing it with warm 0.9% NaCl as required to maintain constant systolic aortic pressure. Hematocrit decreased from 39 +/- 11.2 % to 25.6 +/- 4.95 %. Each animal served as its own control. Paired t test was used. Results are mean +/- standard deviation. RESULTS. Hemodilution decreased zero-flow pressure, from 44 +/- 9 to 36 +/- 7mmHg (p<0.05) in the aorta, and from 51 +/- 2 to 37 +/- 3. 1mmHg (p<0.05) in the distal femoral artery. As expected, hemodilution increased cardiac output from 78 +/- 25 to 142 +/- 57 mL/min/Kg (p<0.05) and decreased vascular resistance from 1.3 +/- 0.5 to 0.8 +/- 0.4 {mmHg/mL/min}Kg, (p<0.05). Diastolic aortic pressure decreased from 86 +/- 17 to 79 +/- 15mmHg (p<0.05), but systolic and mean pressures were unchanged (from 108 +/- 20 to 104 +/- 21mmHg and from 97 +/- 20 to 93 +/- 17mmHg, respectively). Peak systolic femoral pressure was 13.5 +/- 7.2mmHg higher than aortic before, and 17.5 +/- 8.7mmHg after, hemodilution (p>0.1). Mean arterial pressure in the aorta was not significantly different from the distal femoral artery, both before (97 +/- 20 vs 95 +/- 19mmHg, respectively) and after hemodilution (93 +/- 17 vs 91 +/- 17mmHg, respectively). DISCUSSION. Hemodilution decreased significantly the zero-flow pressure. Causes underlying zero-flow pressure are not completely known. It is generally assumed that it results from microvascular collapse, due to vasoconstriction or higher surrounding pressure. However, the fact that hemodilution decreased zero-flow pressure suggests that physical characteristics of the blood, such as viscosity, are at least in part responsible. Contrary to our hypothesis, acute normovolemic hemodilution had no significant effect on the aortic-to-distal arterial pressure difference. This work was financed by grant 1940584 from the National Research Fund (FONDECYT)

Ce résumé expose les affirmations des auteurs. BNTIC ne l’interprète pas comme une validation indépendante des résultats.