Haemodynamic changes during human pregnancy
The aim of this work was to investigate the physiological adaptations that occur in the maternal cardiovascular system during singleton and twin pregnancy. The cardiovascular system has been shown to undergo major adaptations during human pregnancy. Most investigators agreed that cardiac output increased during pregnancy however there was no unanimity regarding the extent and timing of this increase nor about the physiological mechanisms underlying it. Even less was known about the haemodynamic readjustments following delivery and about the alterations in multiple pregnancy. Further study has been limited by the lack of an accurate noninvasive technique which is applicable and reproducible during pregnancy. Cross-sectional echocardiography combined with Doppler ultrasound measurement of blood velocity provides a noninvasive method for measuring cardiac output at a number of locations within the heart and great vessels. Preliminary investigations revealed that cardiac outputs determined by this method correlated closely with those measured by the direct Fick technique in non-pregnant subjects. In addition the method was highly reproducible in both pregnant and nonpregnant subjects. M-mode echocardiography allows accurate and noninvasive measurements of cardiac chamber size and ventricular function. These measurements were also found to be highly reproducible in pregnant and non-pregnant subjects. Using these techniques the aims of this thesis were to investigate the extent, timing and mechanisms of the changes in cardiac output during singleton and twin pregnancy. Echocardiographic investigations were performed prior to and during singleton pregnancy, during the first 6 months after singleton pregnancy, and during and 6 months after twin pregnancy. All subjects were studied in the left semi-lateral position. The results suggested that; (1) During singleton pregnancy cardiac output was increased early in the first trimester and continued to rise until 24 weeks gestation when values were 43-49% above pre-pregnant control values. Thereafter there was no further change. Heart rate and left ventricular function increased during the first trimester. Left atrial and left ventricular end-diastolic dimensions increased during the second trimester suggesting an increase in venous return. Cardiac valve cross-sectional areas and left ventricular wall thickness also increased during pregnancy. After delivery cardiac output had fallen to non-pregnant values by 2 weeks. This was associated with reductions in left ventricular performance and left atrial and left ventricular end-diastolic dimensions. The decrease in valve cross-sectional areas and left ventricular wall thickness was not evident until later in the puerperium. (2) During twin pregnancy cardiac output was increased by 20 weeks gestation and thereafter showed no further change. Maximum cardiac output was 59-62% above postnatal control values. This increase was greater than that recorded during singleton pregnancy due to a relatively greater increase in heart rate. Twin pregnancy was also associated with a greater increase in left atrial dimension. The results of these studies shed light on some of the the unanswered questions in the field of maternal haemodynamics. The noninvasive Doppler techniques used allowed accurate and reproducible measurements of cardiac output in pregnant subjects. This work has important implications for the future investigation of cardiac and hypertensive disorders during pregnancy.