Dynamic Aspects ofAcuteMitral Regurgitation: Effects ofVentricular Volume, Pressure andContractility ontheEffective Regurgitant Orifice Area

SUMMARY Thedynamics ofacute mitral regurgitation werestudied insixopen-chest dogsinwhomaportion oftheanterior leaflet wasexcised. Phasic mitral andaortic flows weremeasured electromagnetically and left ventricular filling volume, regurgitant volume (RV)andforward stroke volume (SV) werecalculated. The systolic pressuregradient (SPG)between theleft ventricle (LV)andleft atrium (LA)wasobtained fromhighfidelity pressuretransducers. Theeffective mitral regurgitant orifice area(MRA)was calculated fromthe hydraulic equation ofGorlin. Volumeinfusion resulted insignificant increases inbothleft atrial andleft ventricular pressures;thus, the SPGwasunchanged andtheincrease inRV wasdueprimarily totheincrease inMRA.Angiotensin infused to raise arterial pressureresulted ingreater increments inleft ventricular thanleft atrial pressure, sothat SPG rosesignificantly. Theincrease inRVwasduetoincreases inbothMRA andSPG.Norepinephrine infusion increased systolic left ventricular pressureandSPG,while left ventricular end-diastolic pressureandleft atrial pressurediminished. Despite asignificant increase inSPG,RVdidnotincrease, duetoasubstantial decrease inMRA.Thus, angiotensin andvolume infusion induced asubstantial increase inregurgitation duetotheincreaseinMRA,while augmentation ofcontractility after norepinephrine infusion resulted ina decrease in regurgitation through reduction ofMRA.Thesefindings support theclinical viewthatmaintaining asmall LV withsustained myocardial contractility will reduce mitral regurgitation. Alternatively, left ventricular dilatation can enhance mitral regurgitation byincreasing theeffective regurgitant orifice independent ofSPG. NORMAL MITRALVALVEFUNCTIONdepends onthemechanical integrity ofthemitral annulus, valve leaflets, chordae tendineae, papillary muscles andthecontraction ofthefree left ventricular (LV) wall. Thefactors that determine theregurgitant flow inmitral insufficiency arethesystolic pressure gradient (SPG)between theleft ventricle andleft atrium, thesize oftheregurgitant orifice ortheareaof thedeficit inmitral closure, andtheduration ofthe regurgitation orthelength ofventricular systole.'s 2It hasbeentraditionally accepted that theregurgitant orifice isfixed under different circulatory states,' exceptintheclinical syndrome ofpapillary muscle dysfunction inwhich theproperties ofthesupporting structures maychange withtime.3 However, recent studies ofexperimental acute mitral insufficiency in thedoghavesuggested that thesize oftheregurgitant orifice isnotfixed.' Thepresent study wasdesigned to define theeffects ofalterations inventricular volume, pressure loading andmyocardial contractility onthe regurgitation ofexperimental acute mitral insufficiency,andtodetermine ifthesize oftheregurgitant orifice isaltered bythese interventions.