Factors Affecting Shunting in Experimental Atrial Septal Defects in Dogs
Pulmonary and aortic blood flows were measured with electromagnetic flowmeters in ten unanesthetized dogs with chronic atrial septal defects. The calculated ratio of pulmonary to systemic flow reflects net bidirectional shunting. Data were obtained during alterations in rate produced by both atrial and ventricular pacing and during changes in vascular resistance produced by drug administration. Results indicate that control ratio of pulmonary to systemic flow depends on the size of the defect. Pacing either atrium at rates between 210 and 240 beats/min invariably produced both a sustained augmentation of pulmonary flow and a diminished aortic flow. Ventricular pacing had a similar qualitative response. Angiotensin, methoxamine, and phenylephrine depressed aortic and augmented pulmonary flow. Nitroglycerin, histamine, and isoproterenol reduced the ratio of pulmonary to systemic flow.
These data indicate that the degree of shunting is dynamic in nature. Increasing heart rate and consequent shortening of diastole appears to impair filling of the left ventricle more than that of the right and results in an increased left-to-right shunt. Drugs which elevate peripheral vascular resistance increase the ratio of pulmonary to systemic flow. Conversely, lowering the systemice resistance decreases the ratio.
- pulmonary blood flow
- electromagnetic flowmeter
- systemic blood flow
- cardiac pacing
- Received July 15, 1968.
- © 1969 American Heart Association, Inc.