Evaluation of aortic wall distensibility by aortic pressure-dimension relation: effects of nifedipine on aortic wall

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Abstract
The relation between aortic pressure and dimension was studied before and after nifedipine infusion in eight anaesthetised dogs. An electromagnetic flowmeter was positioned in the proximal ascending aorta and one pair of ultrasonic dimension gauges attached to the thoracic aorta. A Millar micromanometer was positioned just below the dimension gauges. A constrictor was placed around the thoracic aorta distal to the dimension gauges to produce an abrupt rise in aortic pressure for 15 s. After complete recovery nifedipine (0.75 μg·kg−1·min−1) was infused for 10 min and the same procedure repeated. The ratio (△D:△P) of the difference (ΔD) between maximum and minimum dimensions (D) to the pulse pressure (ΔP) and the percentage distensibility (ΔD/ ΔP/minimum D) were decreased significantly after aortic constriction (0.037(0.013) to 0.019(0.004) mm·mmHg−1 and 0.247(0.075) to 0.121(0.033)%·mmHg−1, p−1, p<0.05), suggesting an increase in aortic distensibility by nifedipine. When mean aortic pressure or stroke volume before and after nifedipine was compared at the same mean dimension, which was reduced by 5% of the control mean dimension, stroke volume increased to 128%(p<0.01). Total systemic resistance was significantly reduced by 14.5%(p<0.05) after nifedipine, but mean aortic pressure tended to be maintained at a higher value man before nifedipine, probably owing to an increase in cardiac output. Thus nifedipine increased stroke volume and seemed to attenuate the decrease in systemic perfusion pressure when compared with that at the same aortic dimension within the physiological range of the decrease in aortic pressure. These events were accompanied by an increase in aortic pressure distensibility.