EVALUATION OF THE SLOPE-VOLUME METHOD AS AN INDEX OF PULMONARY BLOOD VOLUME*

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Abstract
Studies were carried out to test the validity of the Newman slope-volume method as an index or measurement of the volume of blood in the lungs. Indocyanine (cardio-green) dye was injected into the pulmonary artery of the dog, and indicator dilution curves were inscribed at the aortic root using a densitometer with a high frequency response. Sampling was from the aorta rather than the left atrium to ensure adequate mixing of the dye with blood. The Stewart-Hamilton method was used to derive the volume of blood in the lungs and chambers of the left side of the heart; the Newman method was used to calculate the slope volume. In 36 determinations in 16 dogs, the volume of blood in the lungs and left side of the heart was 156 ml (SE 3.9) per 10 kg of body weight, whereas the slope volume was 48 ml (Se 1.7) per 10 kg of body weight. Thus, the slope volume is about 30% of the volume in the lungs and left side of the heart and it is probably about 45% of the pulmonary blood volume. With repeated temporary occlusion of the right main pulmonary artery, the volume of blood in the lungs and left side of the heart decreased from an average of 332 to 229 ml, while the average slope volume decreased from 91 to 80 ml. With occlusion of the arteries to the lower half of the right lung, the former volume decreased from 427 to 335 ml, while the latter decreased from 149 to 139 ml. With the assumption that the Stewart-Hamilton method is valid, it appears that the slope volume neither gives a measurement of the volume of blood in the lungs nor reflects the magnitude of changes induced therein.[long dash]Auth. summ.