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The relationship between instantaneous distending pressure and diameter of the pulmonary artery was studied in 18 living thoracotomized dogs. An electrical caliper with adequate recording characteristics was developed for instantaneous diameter measurement. Control observations were made over a range of pressures induced by rapid, right heart dextran-infusion and were compared with those obtained during norepinephrine administration. Results indicate: a) the pulse contours of the pulmonary artery pressure and diameter are essentially identical, indicating negligible inertance and viscous resistance of the vessel wall, b) the mean change in average radius during a cardiac cycle was ±7.8% ± 2.86 S.D. ± .32 S.E.M. (0.48%/cm H2O pulse pressure) under control conditions, c) the ratio of change in radius to pulse pressure, R/P, showed a significant decrease during norepinephrine administration when compared to control values within the same pressure range (P < .01) and d) the cross-sectional area of the main pulmonary artery exceeded that of the right and left combined. Note: (With the Technical Assistance of Alfred G. T. Casper) Submitted on August 25, 1959

Page 94: Dali J. Patel, Donald B. Schilder and Alexander J. Mallos, “Mechanical properties and dimensions of the major pulmonary arteries.” Column 1, paragraph 5, line 1 should read: The mean value for ∆R/∆P × 10+3 in control animals. Column 2, table 1, line 9 should read: ∆R/∆P × 10+3.