Evaluation of Aortic Wall Distensibility by Aortic Pressure-Dimension Relationship : Effects of Nifedipine on Aortic Wall

Six anesthetized dogs were studied to analyze the relation between aortic pressure (P) and dimension (D) with and without Nifedipine (NF). An electromagnetic flowmeter was instrumented around the proximal ascending aorta (Ao) and one pair of ultrasonic dimension gauge around the proximal third of the discending Ao. A Millar's micro-transducer was placed just below the dimension gauge. A constrictor was placed around the thoracic Ao distal to the dimension gauge for acute constriction of Ao (Ao. OC). After control recording, Ao was constricted and AoP increased to 137.7% of the control level for 5 sec, and then Ao. OC was released abruptly. After complete recovery, NF (0.75γ/kg/min) was infused for 10min, and the same procedure was repeated. %Distensibility (percent change (%△D) in AoD/pulse pressure) was obtained on each stage. After Ao. OC, %Distensibility was decreased significantly from 0.267±0.077 to 0.132±0.029 (p＜0.02), suggesting that %Distensibility was dependent on the afterload. Beat-to-beat mean P (Y-axis)-mean D (X-axis) relation was obtained during the release from the Ao. OC. This mean P-D relation discribed a convex-upward curve and was fitted by an exponential function. The slope (S) of the mean P-lnD relation was 372±102 before NF, while after NF the mean P-lnD line shifted to the left and S was significantly lowered (304±129 :P＜0.05). When mean AoP or stroke volume (SV) before and after NF was compared at the same level of mean D which was reduced to 5% of the control mean D, SV was incresed to 130%(P＜0.02) and the mean AoP also tended to be increased (105% : NS). Total systemic resistance was significantly reduced by 14.5% (P＜0.05) after NF, but systemic perfusion pressure was maintained at the higher level, probabhy due to increase in cardiac output.