Eq. 19.1 ), also affects pulse pressure. This relationship is illustrated in
Fig. 17.16 . When cardiac output and TPR are constant, a decrease in arterial compliance results in an increase in pulse pressure. Diminished arterial compliance also imposes a greater workload on the left ventricle (i.e., increased afterload), even if stroke volume, TPR, and Pa are equal in the two individuals.
As previously discussed, if the heart rate and stroke volume remain constant, an increase in TPR causes Pa to increase.
•Fig. 17.15 Effect of a Change in Stroke Volume on Pulse Pressure in a System in Which Arterial Compliance Remains Constant Over the Prevailing Range of Pressures and Volumes. Alargerincrementinbloodvolume,whereby(V4− V3)> (V2− V1),resultsingreatermeanbloodpressure(PB > PA)andagreaterpulsepressure,sothat(P4− P3)> (P2− P1).
When arterial compliance is constant, an increase in TPR leads to proportional increases in systolic and diastolic pressure so that the pulse pressure is unchanged (Fig. 17.17A ). However, arterial compliance is not linear. As Pa increases and the artery is stressed, compliance decreases (see
Fig.
17.17B ). Because of the decrease in arterial compliance with increased Pa, pulse pressure increases when Pa is elevated.
Effect of Arterial Compliance on Myocardial Energy Consumption
The increased cardiac energy requirement imposed by a rigid arterial system is illustrated in
Fig. 17.18 . In the data depicted in
Fig. 17.18 , the cardiac output from the left ventricle either was allowed to flow through the natural route (the aorta) or was directed through a stiff plastic tube to the peripheral arteries. In this experiment, the TPR values were virtually identical, regardless of which pathway •Fig. 17.16 Foragivenvolumeincrement(V2− V1),reducedarterialcompliance(complianceB[LowCa]< complianceA[HighCa])resultsinincreasedpulsepressure,whereby(P4− P1)> (P3− P2).Pa,meanarterialpressure.
•Fig. 17.17 Comparisonoftheeffectsofagivenchangeinperipheralresistanceonpulsepressure(P)whenthepressure-volumecurveforthearterialsystemiseitherrectilinear(A) orcurvilinear(B). Theincrementinarterialvolumeisthesameforbothconditions;thatis,(V4−V3)=(V2−V1).PressureABVolumeP1V1V2V3V4P3P6P4P2–P5–PressureP1V1V2V3V4P3P6P4P2–P5– •Fig. 17.18 Therelationshipbetweenmyocardialoxygencon-sumption(1mL/100g/beat)andstrokevolume(inmilliliters)inananesthetizeddogwhosecardiacoutputcouldbepumpedbytheleftventricleeitherthroughtheaortaorthroughastiffplastictubetotheperipheralarteries.(ModifiedfromKellyRP,TuninR,KassDA.Circ Res. 1992;71:490.)Plastic tubing Native aorta Stroke volume (mL) 5 0 0.05 O 2 consumption (mL) O2/100 g/beat)0.1 10 15 was selected. The results showed that for any given stroke volume, myocardial oxygen consumption was substantially greater when the blood was diverted through the plastic tubing than when it flowed through the aorta. The increased oxygen consumption indicates that the left ventricle has to expend significantly more energy to pump blood through a less compliant conduit than through a more compliant conduit.