Analysis of the effects of changes in peripheral resistance on cardiac output and Pv is complex because both the cardiac and vascular function curves shift. When peripheral resistance increases (
Fig. 19.11 ), the vascular function curve is rotated counterclockwise, but it converges on the same Pv axis intercept as the control curve does. Note that vasoconstriction causes a counterclockwise rotation of the vascular function curve in
Fig. 19.11
Fig. 19.5 . The direction of rotation differs because the axes for the vascular function curves were switched in these two figures, as explained earlier. The cardiac function curve in
Fig. 19.11 is also shifted downward because at any given Pv, the heart is able to pump less blood against the greater cardiac afterload imposed by the increased peripheral resistance. Because both curves in
Fig. 19.11 are displaced downward, the new equilibrium point (point B) is below the control point (point A); that is, an increase in peripheral resistance diminishes cardiac output.
Whether point B falls directly below point A or lies slightly to the right or left of it depends on the magnitude of the shift in each curve. For example, if a given increase •Fig. 19.10 Moderateorsevereheartfailureshiftsthecardiacfunctioncurvesdownwardandtotheright.Beforechangesinbloodvolume,cardiacoutputdecreasesandcentralvenouspressurerises(fromcontrolequilibriumpointAtopointBorpointC).Aftertheincreaseinbloodvolumethatusuallyoccursinheartfailure,thevascularfunctioncurveisshiftedtotheright.Hence,centralvenouspressuremaybeelevatedwithnoreductionincardiacoutput(pointD)or,insevereheartfailure,withsomereductionincardiacoutput(pointE).
Heart failure isageneraltermthatappliestoconditionsinwhichthepumpingcapabilityoftheheartisimpairedtotheextentthatthetissuesofthebodyarenotadequatelyperfused.Inheartfailure,myocardialcontractilityisimpaired.Heartfailuremaybeacuteorchronic.Consequently,inagraphofcardiacandvascularfunctioncurves,thecardiacfunctioncurveisshifteddownwardandtotheright,asdepictedin
Fig.19.10
Acuteheartfailuremaybecausedbytoxicconcentrationsofdrugsorbycertainpathologicalconditionssuchascoronaryarteryocclusion.Inacuteheartfailure,bloodvolumedoesnotchangeimmediately.In
Fig.19.10 ,therefore,theequilibriumpointshiftsfromtheintersection(pointA)ofthenormalcurvestotheintersection(pointBorpointC)ofthenormalvascularfunctioncurve.
Chronicheartfailuremayoccurinconditionssuchasessentialhypertensionorischemicheartdisease.Inchronicheartfailure,boththecardiacfunctionandvascularfunctioncurvesshift.Thevascularfunctioncurveshiftsbecauseofanincreaseinbloodvolumecausedinpartbyfluidretentionbythekidneys.ThefluidretentionisrelatedtotheconcomitantreductioninglomerularfiltrationrateandthedecreasedrenalexcretionofNaClandwater(seealso ).Theresultanthypervolemiaisreflectedbyarightwardshiftofthevascularfunctioncurve,asshownin
Fig.19.10.
Hence,withmoderatedegreesofheartfailure,Pviselevated,butcardiacoutputmaybenormal(pointD).Withmoreseveredegreesofheartfailure,Pvisstillelevated,butcardiacoutputissubnormal(pointE).