Use of acetylcholine to measure total vascular pressure-volume relationship in dogs

To define total vascular capacitance, we used acetylcholine to arrest the heart and measured mean circulatory filling pressure (MCFP) during controlled hemorrhage and volume loading in 12 splenectomized dogs after ganglion blockade with hexamethonium. We also examined the gross pathological and histoslogical changes in the lungs. Controlled hemorrhage (n = 12) of 5 and 10 ml/kg decreased MCFP from 6.8 ± 0.1 to 4.9 ± 0.3 and 3.6 ± 0.2 mmHg, respectively. Volume loading of 5 (n = 8) and 10 ml/kg (n = 4) increased MCFP to 9.3 ± 0.2 and 12.1 ± 0.1 mmHg, respectively. At MCFPs below 5 mmHg, the pressure (P)-volume (V) relationship was not linear [(P = P₀e(kV), where k is slope of ln (MCFP) vs. V, k = 0.061, R² = 0.998]. At MCFPs between 5 and 12 mmHg, the pressure-volume relationship was linear (slope = 0.479 mmHg·ml^-1·kg^-1, R² = 0.992) and total vascular compliance was 2.09 ml·mmHg^-1·kg^-1. There were no changes in heart rate, cardiac output, right atrial, pulmonary artery, and pulmonary artery wedge pressures when values at base line were compared with those measured 15 min after each arrest. There were no changes in arterial gas measurements or acid-base balance, and there was no evidence of atelectasis or interstitial or intra-alveolar edema. We conclude that the total body pressure-volume relationship in the presence of ganglion blockade had a nonlinear configuration. The use of acetylcholine to arrest the heart, four times with hexamethonium in reflex-blocked animals, did not result in changes in hemodynamics or pulmonary function.