Anychangeincontractilitythataffectsthetwoventriclesdifferentlyaltersthedistributionofbloodvolumeinthetwovascularsystems.Ifacoronaryarterytotheleftventriclebecomesoccluded,leftventricularcontractilityisimpaired,andacute left ventricular failure ensues.Intheinstantafterocclusion,leftatrialpressuredoesnotchange,andtheleftventriclebeginstopumpatadiminishedrateofflow.Iftherightventricleisnotaffectedbytheacutecoronaryarteryocclusion,therightventricleinitiallycontinuestopumpthenormalflow.Thedisparaterightandleftventricularoutputsresultinaprogressiveincreaseinleftatrialpressureandaprogressivedecreaseinrightatrialpressure.Therefore,leftventricularoutputincreasestowardthenormalvalue,andrightventricularoutputfallsbelowthenormalvalue.Thisprocesscontinuesuntiltheoutputsofthetwoventriclesagainbecomeequal.Atthisnewequilibrium,theoutputofthetwoventriclesissubnormal.Theelevationinleftatrialpressureisaccompaniedbyanequalelevationinpulmonaryvenouspressure,whichcanhaveseriousclinicalconsequences.Thehighpulmonaryvenouspressurecanincreaselungstiffnessandleadtorespiratorydistressbyincreasingthemechanicalworkofpulmonaryventilation(see ).Furthermore,thehighpulmonaryvenouspressurecausesanelevatethehydrostaticpressureinthepulmonarycapillariesandmayleadtothetransudationoffluidfromthepulmonarycapillariestothepulmonaryinterstitiumorintothealveoli(pulmonary edema), whichmaybelethal.
Two basic principles to remember about ventricular function are that (1) the left ventricle pumps blood through the systemic vasculature, and (2) the right ventricle pumps blood through the pulmonary vasculature. However, these principles do not necessarily imply that both ventricles are essential to perfuse the systemic and pulmonary vascular beds adequately. To better understand the relationships between the two ventricles and the two vascular beds, the right ventricular function is examined in more detail as follows.
In the circulatory system model shown in
Fig. 19.12, consider the hemodynamic consequences that would occur if the right ventricle suddenly ceased its pump function but instead served merely as a passive, low-resistance conduit between the systemic veins and the pulmonary arteries. Under these conditions, the only functional pump would be the left ventricle, which would then be required to pump blood through both the systemic and pulmonary resistances (for the purposes of this discussion, consider the resistance to the flow of blood through the inactive right ventricle to be negligible).
Normally, pulmonary vascular resistance is approximately 10% as great as systemic vascular resistance. Because the two resistances are in series with one another, total resistance would be 10% greater than systemic resistance alone (see ). In a normal cardiovascular system, a 10% (and hence left ventricular afterload) by approximately 10%. This increase would not drastically affect left ventricular function. Under certain conditions, however, this increase in Pa could significantly alter the function of the cardiovascular system. If the 10% increase in total resistance is achieved by adding a small degree of resistance (i.e., pulmonary vascular resistance) to that of the much larger systemic resistance, and if the pulmonary vascular resistance is separated from the systemic resistance by a large degree of compliance (the combined systemic venous and pulmonary arterial compliance), the 10% increase in total resistance could drastically impair operation of the cardiovascular system.
The simulated effects of inactivating the pumping action of the right ventricle in a hydraulic analogue of the circula tory system are shown in