Increased Aortic Impedance Precedes Peripheral Vasoconstriction attheEarlyStage of Ventricular Failure inthePacedCanine Model

Background. Aortic input impedance isaltered inpatients withcongestive heart failure. However, little isknownabout whether this vascular responseisan earlychange oralate manifestation ofleft ventricular dysfunction. Methods andResults. Thisinvestigation useda pacedcanine modelofcongestive heartfailure to demonstrate thatabnormal aortic input impedance doesevolve inthesetting ofventricular systolic dysfunction andtoprospectively define thetime courseofchange inaortic input impedance andconduit vessel compliance. Studies wereperformed inclosed-chest conditioned beagles aged1to2yearsthat underwent hemodynamic evaluation atbaseline andafter induction ofleft ventricular dysfunction by rapid ventricular pacing. Within 48hours oftheonsetofrapid ventricular pacing, we observed mildleft ventricular systolic dysfunction with anechocardiographically derived left ventricular ejection fraction of 37%(P<.001 compared withbaseline) measured during interruption ofrapidventricular pacing. Concomitant withthis reduction inleft ventricular systolic function, theaortic input impedance spectrum was shifted abovebaseline inalldogsstudied. Characteristic impedance oftheaortasignificantly increased from121±65dynes- s/cm5 to186±114 dyness/cm5(P<.02), andasignificant increase inthe first modulus ofimpedance from137±43dyness/cm5 to228±139 dynes . s/cm5was observed (P<.05). Although characteristic aortic impedance increased by50%7, there was atthispoint no changein peripheral vascular resistance. Therefore, these abnormalities inaortic input impedance arerepresen- tative ofan earlyvascular change thatevolves inresponsetoventricular systolic dysfunction. Conclusions. Considering theearlyappearanceofthese findings, theresultant impaired powertransfer andreduced conduit vessel compliance likely contribute totheprogression ofabnormal myocardial energetics andsystolic dysfunction characteristic ofventricular failure. (Circulation. 1993;88:2714-2721.)