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SUMMARY The dynamics of acute mitral regurgitation were studied in six open-chest dogs in whom a portion of the anterior leaflet was excised. Phasic mitral and aortic flows were measured electromagnetically and left ventricular filling volume, regurgitant volume (RV) and forward stroke volume (SV) were calculated. The systolic pressure gradient (SPG) between the left ventricle (LV) and left atrium (LA) was obtained from highfidelity pressure transducers. The effective mitral regurgitant orifice area (MRA) was calculated from the hydraulic equation of Gorlin.Volume infusion resulted in significant increases in both left atrial and left ventricular pressures; thus, the SPG was unchanged and the increase in RV was due primarily to the increase in MRA. Angiotensin infused to raise arterial pressure resulted in greater increments in left ventricular than left atrial pressure, so that SPG rose significantly. The increase in RV was due to increases in both MRA and SPG. Norepinephrine infusion increased systolic left ventricular pressure and SPG, while left ventricular end-diastolic pressure and left atrial pressure diminished. Despite a significant increase in SPG, RV did not increase, due to a substantial decrease in MRA. Thus, angiotensin and volume infusion induced a substantial increase in regurgitation due to the increase in MRA, while augmentation of contractility after norepinephrine infusion resulted in a decrease in regurgitation through reduction of MRA. These findings support the clinical view that maintaining a small LV with sustained myocardial contractility will reduce mitral regurgitation. Alternatively, left ventricular dilatation can enhance mitral regurgitation by increasing the effective regurgitant orifice independent of SPG.NORMAL MITRAL VALVE FUNCTION depends on the mechanical integrity of the mitral annulus, valve leaflets, chordae tendineae, papillary muscles and the contraction of the free left ventricular (LV) wall. The factors that determine the regurgitant flow in mitral insufficiency are the systolic pressure gradient (SPG) between the left ventricle and left atrium, the size of the regurgitant orifice or the area of the deficit in mitral closure, and the duration of the regurgitation or the length of ventricular systole.'s 2 It has been traditionally accepted that the regurgitant orifice is fixed under different circulatory states,' except in the clinical syndrome of papillary muscle dysfunction in which the properties of the supporting structures may change with time.3 However, recent studies of experimental acute mitral insufficiency in the dog have suggested that the size of the regurgitant orifice is not fixed.' The present study was designed to define the effects of alterations in ventricular volume, pressure loading and myocardial contractility on the regurgitation of experimental acute mitral insufficiency, and to determine if the size of the regurgitant orifice is altered by these interventions. Materials and MethodsSix mongrel dogs weighing 20-30 kg were anesthetized with sodium ...

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