Pressure-flow loops and instantaneous input impedance in the thoracic aorta: Another way to assess the effect of aortic bypass graft implantation on myocardial, brain, and subdiaphragmatic perfusion

Background: The serious disturbances in ventriculoarterial coupling after thoracic aorta bypass grafting are addressed through aortic entry impedance in the frequency domain from flow-pressure waves. We designed a method for synthesizing pressure and flow waves to evaluate opposal to aortic flow along the cardiac cycle, addressing myocardial, brain, and visceral tissue perfusions from pressure-flow hysteresis loops and forward-backward aortic entry impedance in the ascending aorta, transverse aortic arch, and distal descending aorta, respectively, before and after extra-anatomic grafting of the descending aorta in the swine. Methods: Twelve pigs underwent extra-anatomic grafting (woven double-velour prosthesis, 18-mm diameter), bypassing the descending aorta. Periarterial flow and endovascular pressure signals were mathematically synthesized (error minimization) to yield continuous functions of flow, pressure along the cardiac cycle before treatment for mean hemodynamics, pressure-flow hysteresis loops, and aortic entry impedance. Results: Grafting of the descending aorta overshadowed pressure-flow hysteresis loops in the ascending aorta by shortening maximum pressure delay on maximum flow and diastolic flow reversal. Clamping of the descending aorta substantially restored hemodynamics in the ascending aorta, although the diastolic flow decrease was accelerated. Identical processes developed in the transverse aorta. Subdiaphragmatic descending aortic flow was flattened after grafting and restored, although thickened, after clamping of the descending aorta. Flow wave peak was framed by a diastolic aortic entry impedance peak, which was damped along the transverse aortic arch (aortic entry impedance peak in the ascending aorta, 1700 +/- 102 kN . s . m(-5); aortic entry impedance peak in the descending aorta, 292 +/- 45 kN . s . m(-5); P < .05). After grafting, the aortic entry impedance peak was transferred to early systole (aortic entry impedance peak in the transverse aortic arch, 2104 +/- 94 kN S s . m(-5); aortic entry impedance peak in the descending aorta, 450 +/- 75 kN . s . m(-5); P <.05). Clamping of the descending aorta attenuated the early systolic aortic entry impedance peak (aortic entry impedance peak in the transverse aortic arch, 1269 +/- 104 kN . s . m(-5); aortic entry impedance peak in the descending aorta, 491 +/- 75 kN . s . m(-5) P < .05), although aortic entry impedance in the descending aorta remained higher than before grafting (P < .05). Specifically, the backward flow ascending aorta to coronary trunks generated a backward aortic entry impedance peak (2234 +/- 350 kN . s . m(-5)) superimposed onto the forward aortic entry impedance peak with asymptotic boundaries that diminished after grafting and further enlarged after clamping of the descending aorta. Conclusions: Hemodynamic opposition of grafting of the descending aorta are specific to the aortic site and cardiac cycle and are dependent on clamping of the descending aorta. Our approach to thoracic aorta hemodynamics could enable optimization of bypass grafting.