Effective distributed compliance of the canine descending aorta estimated by modified T-tube model

To estimate descending thoracic aortic compliance in anesthetized open-chest dogs, a modified T-tube arterial model was used. This model consists of two uniform and lossless elastic tubes, one representing arteries going toward the head and upper limbs and the other (body tube) representing descending aortic circulation to the trunk and lower limbs. Each tube terminates with a generalized first-order low-pass filter load. Pressure and flow in the ascending aorta and flow in the upper descending aorta were measured and used to estimate model parameters. Using the estimated model parameters, we calculated the pressure waveshape at the termination of the body tube. Comparison of this model-predicted pressure with pressure measured in the abdominal aorta near the origin of renal arteries suggested that the end of the body tube (effective reflecting site of the body circulation) corresponds to this major branching site of the abdominal aorta. To calculate the length of the body tube, we used aortic pulse wave velocity estimated from the measurements of pressure in ascending and abdominal aorta. Calculated body tube length averaged 30.3 +/- 2.8 cm and approximated the measured length (30.6 +/- 3.0 cm) of the aorta from the arch to the region of the origin of renal arteries. Compliance of the body tube averaged 123 +/- 20 x 10(-6) g-1.cm4.s2 and was interpreted as the descending thoracic aortic compliance. The ratio of this compliance to the body tube length gave an estimate of the effective distributed compliance, i.e., the compliance per unit length that would be observed in the absence of tapering. This ratio averaged 4.10 +/- 0.86 x 10(-6) g-1.cm3.s2 and fell in between the values of local aortic compliance independently estimated along the descending thoracic aorta from measurements of pressure and diameter. Thus tube compliance resulted in a physically identifiable property. This property was contrasted with the ill-defined effective compliances of the terminal loads.