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 con sists of two uniform and lossless elastic tubes, one representing arte ries going toward the head and upper limbs and the other (body tube) r epresenting 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 d escending aorta were measured and used to estimate model parameters. U sing the estimated model parameters, we calculated the pressure wavesh ape at the termination of the body tube. Comparison of this model-pred icted pressure with pressure measured in the abdominal aorta near the origin of renal arteries suggested that the end of the body tube (effe ctive reflecting site of the body circulation) corresponds to this maj or branching site of the abdominal aorta. To calculate the length of t he body tube, we used aortic pulse wave velocity estimated from the me asurements of pressure in ascending and abdominal aorta. Calculated bo dy 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 t he 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 thora cic aortic compliance. The ratio of this compliance to the body tube l ength gave an estimate of the effective distributed compliance, i.e., the compliance per unit length that would be observed in the absence o f 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 es timated along the descending thoracic aorta from measurements of press ure and diameter. Thus tube compliance resulted in a physically identi fiable property. This property was contrasted with the ill-defined eff ective compliances of the terminal loads.