The immature fetal heart has limited diastolic and systolic function in the first trimester, limiting its ability to accom-modate changes in cardiac preload and afterload. Cardiac development during the following weeks results in improved cardiac function throughout the cardiac cycle.
Enhanced diastolic function is manifested by changes in intracardiac and extracardiac flow velocity waveforms – there is change from a monophasic to a biphasic flow velocity profile during ventricular inflow by 8 – 10 weeks, which is followed by elevation of the E/A ratio. The latter changes occur in association with a decrease of pulsatility in precordial venous Doppler flow velocity waveform and
Figure 11.7
Development of cardiac and extracardiac functionality with gestation. Illustrated is the approximate temporal relationship of arterial and venous flow velocity changes and cardiac function with advancing gestational age. UV, umbilical vein; EDF, end-diastolic flow; UA, umbilical artery;
MCA, middle cerebral artery.
8 13 15 17 20 24 28 32 36 40 Gestational age (weeks)
EDF MCA EDF UA
Decreasing cerebral impedance Decreasing placental impedance
Steady increase in cardiac compliance and contractility Heart rate
Steady decrease in baseline heart rate
Steadily rising E/A ratio Exponentially rising E/A ratio
Exponentially rising cardiac output Steadily rising cardiac output Increasing impedance lower body Decreasing precordial venous indexes UV pulsations
Decreasing pulmonary venous indexes
Constant UV flow
6. Nagueh SF , Mikati I , Kopelen HA et al. Doppler estimation of left ventricular filling pressure in sinus tachycardia. A new application of tissue doppler imaging. Circulation 1998 ; 98 : 1644 – 50 .
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12. Wilson AD , Rao PS , Aeschlimann S . Normal fetal foramen flap and transatrial Doppler velocity pattern. J Am Soc Echocardiogr 1990 ; 3 : 491 – 4 .
13. Sutton MS , Groves A , MacNeill A , Sharland G, Allan L . Assessment of changes in blood flow through the lungs and foramen ovale in the normal human fetus with gesta-tional age: a prospective Doppler echocardiographic study.
Br Heart J 1994 ; 71 : 232 – 7 . output estimated by Doppler echocardiography during mid- and late gestation. Am J Cardiol 1987 ; 60 : 338 – 42 . 18. Allan LD , Chita SK , Al-Ghazali W , Crawford DC, Tynan M .
Doppler echocardiographic evaluation of the normal human fetal heart. Br Heart J 1987 ; 57 : 528 – 33 .
19. Kenny J , Plappert T , Doubilet P, Salzman D, Sutton MG . Effects of heart rate on ventricular size, stroke volume, and output in the normal human fetus: a prospective Doppler echocardiographic study. Circulation 1987 ; 76 : 52 – 8 . 20. Kiserud T , Rasmussen S , Skulstad S . Blood flow and the
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22. Tonge HM , Wladimiroff JW , Noordam MJ , Stewart PA . Fetal cardiac arrhythmias and their effect on volume blood flow in descending aorta of human fetus. J Clin Ultrasound 1986 ; 14 : 607 – 12 .
23. Reed KL , Sahn DJ , Marx GR, Anderson CF, Shenker L . Cardiac Doppler flows during fetal arrhythmias: physio-logic consequences. Obstet Gynecol 1987 ; 70 : 1 – 6 . progressive drop in the pulsatility indexes and presumably
also blood flow resistance in the brachiocephalic circula-tion. At the same time, from 20 weeks onwards pulsatility indexes in the ductus arteriosus and the descending aorta remain relatively constant. This divergent development of ventricular afterload results in physiological redistribution of the cardiac output in favor of the left ventricle and therefore the upper body. In the vascular beds distal to the descending aorta, development of Doppler indexes is also divergent in pregnancy. While vascular resistance in the limbs and therefore the external iliac arteries increases, there is a steady decrease in the umbilical artery and there-fore the internal iliac pulsatility index. Owing to this development, the major proportion of descending aorta blood flow is distributed to the placental vascular bed for oxygenation. The combined cardiac output increases steadily toward term. The increased contractility and preload and decrease in afterload are associated with increased peak blood flow velocities and an exponential increase of cardiac output during gestation. After 18 weeks ’ gestation the relationship between cardiac output and fetal body weight remains relatively constant at 450 ml/kg per minute. Although placental blood flow increases from 115 ml/minute from 20 weeks to 415 ml/minute at term, it decreases in relation to fetal body weight, resulting in relative placental insufficiency at term. 126
Improved resolution of modern ultrasound machines allows the study of small-caliber vessels. Studies of human fetuses with growth restriction suggest that the cerebral, adrenal, and coronary circulations are capable of autoreg-ulation in the second trimester. 113 , 139 – 142 In addition, the liver and spleen appear to be preferentially perfused organ systems. 115 , 117 With delivery of the fetus, the fetal circula-tion gathers its postnatal funccircula-tionality by closure of the central shunts 143 ( Figure 11.7 ).
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