Cardiovascular physiology in normal pregnancy: Studies with the dye dilution technique

UTHSC Affiliation

College of Medicine

Document Type

Article

Publication Date

1-1-1954

Publication Title

American Journal of Obstetrics and Gynecology

Volume

67

Issue

4

Abstract

1. 1. A new method for the determination of cardiac output and plasma volume in man based upon the Hamilton dilution principle has been presented. The method is simple, accurate, requires very little equipment, and can be applied clinically. Repeated determinations can be made on the same patient with no discomfort to the patient. It is technically easier and produces less apprehension than any other method available for the determination of cardiac output. With the same procedure, requiring only six minutes, one can measure the plasma volume, blood volume, red-cell volume, cardiac output, mean pulmonary circulation time, pulmonary or "central" blood volume, and the total peripheral resistance. 2. 2. We have made 94 determinations in 31 normal pregnant women. Serial determinations were made on each patient at intervals of four to five weeks during the pregnancy. Determinations were also made immediately following delivery and during the puerperium. On several patients as many as six determinations were made. Eight normal nonpregnant women were selected as controls and determinations were made in the same manner as those in the pregnancy series. The patients were as near basal conditions as possible at the time of each determination. 3. 3. The cardiac output during pregnancy rises rapidly from the end of the first trimester to a maximum at 28 weeks' gestation. This increase amounts to 32 per cent over the nonpregnant levels. Following this peak there is a definite decrease as the pregnancy progresses. The original nonpregnant level is reached at approximately 38 to 40 weeks. Immediately following delivery there is another rapid increase in cardiac output amounting to 29 per cent. The nonpregnant levels are again reached by two weeks post partum. The alterations in cardiac output are produced by stroke volume changes. The stroke volume tends to parallel the minute volume with the heart rate remaining relatively constant. 4. 4. The plasma volume increases during normal pregnancy; however, the change is much slower than that found in the cardiac output. The maximum plasma volume is attained at approximately 32 to 34 weeks' gestation. This increase amounts to 22 per cent at this point. There is no significant decrease in plasma volume during the last trimester. The red-cell volume also increases during pregnancy but to a much less degree. This produces a hemodilution or physiological anemia that is greatest at the point of maximum plasma volume. Therefore, the hematocrit during normal pregnancy is lowest at 32 to 34 weeks' gestation. 5. 5. The circulation time was found to decrease from an average of 13.3 seconds to 11.3 seconds at 34 weeks' gestation. From this point it gradually rose, reaching 15.8 seconds at 40 weeks. 6. 6. The central blood volume was found to be greater during pregnancy than in the nonpregnant state. It tends to follow the blood volume and cardiac output changes, being greatest at the point of maximum cardiac output. 7. 7. The changes in the mean blood pressure and total peripheral resistance are definitely related and tend to change in the same direction. During normal pregnancy the total peripheral resistance and mean blood pressure decrease, reaching the lowest levels at the point of maximum cardiac output. This decrease is thought to be due to the enlarged maternal sinuses, the arteriovenous shunt effect of the placenta, the increased cardiac output, and the decreased viscosity of the blood. 8. 8. It was necessary to establish the normal levels in pregnancy before studying pathological conditions. The comparison between the normal and the abnormal may effect changes in therapeutic measures. It is thought that this new method may prove to have practical value. © 1954.

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