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

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 micrograms.kg-1.min-1) was infused for 10 min and the same procedure repeated. The ratio (delta D:delta P) of the difference (delta D) between maximum and minimum dimensions (D) to the pulse pressure (delta P) and the percentage distensibility (delta D/delta P/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 less than 0.01, respectively), suggesting that these indices depend on afterload. Beat to beat mean pressure-dimension relations during the release of aortic constriction showed a convex upward curve and was fitted by an exponential function with a high correlation coefficient (r = 0.99). The slope of this relation was significantly reduced after nifedipine compared with before nifedipine (379(83) to 330(119) mmHg.cm-1, p less than 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 less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)