Morphometric study of the human fetal heart I. Arterial segment

Pressure and Flow Relations in the Systemic Arterial Tree Throughout Development From Newborn to Adult

Objective: Distributed models of the arterial tree allow studying the effect of physiological and pathophysiological changes in the vasculature on hemodynamics. For the adult, several models exist; however, a model encompassing the full age range from newborn to adult was until now lacking. Our goal is to describe a complete distributed hemodynamic model for normal development from newborn to adult. Methods: The arterial system was modeled by 121 segments characterized by length, radius, wall thickness, wall stiffness, and wall viscosity. The final segments ended in three-element Windkessels. All parameters were adapted based on body height and weight as a function of age as described in the literature. Results: Pressures and flows are calculated as a function of age at sites along the arterial tree. Central to peripheral transfer functions are given. Our results indicate that peripheral pressure in younger children resembles central pressure. Furthermore, total arterial compliance, inertance and impedance are calculated. Findings indicate that the arterial tree can be simulated by using a three-element Windkessel system. Pulse wave velocity in the aorta was found to increase during development. Conclusions: The arterial system, modeled from newborn to adult bears clinical significance, both for the interpretation of peripheral measured pressure in younger and older children, and for using a Windkessel model to determine flow from pressure measurements.

Morphometric evaluation of the aortic root in stillborns

OBJECTIVE

To describe the dimensions and amount of collagen in the aortic root of autopsied fetuses at different gestational ages.

MATERIAL AND METHODS

40 samples of aortic roots were selected from autopsied fetuses with gestational ages ranging between 20 and 40 weeks. The thickness and the area of the aortic wall were analyzed on slides stained with Hematoxylin and Eosin, and the collagen was quantified on slides stained with Picrosirius, by using an image analyzing system.

RESULTS

A positive correlation was observed between the thickness of the media layer of the aortic wall and the gestational age. There was a positive and significant correlation between the percentage of collagen in the aortic wall with gestational age and fetal weight. The correlation between gestational age and the area of the aortic root circumference was positive and significant. And the correlation between the aortic diameter and the gestational age as well as fetal length was positive and significant.

CONCLUSION

The thickness of the media layer, the amount of collagen in the aortic wall, the area of the aortic root circumference and the aortic diameter rose with the increase of the gestational age. Thus, the morphological analysis of the aortic root may help as a parameter during the follow-up of inter-uterine growth and fetal development.

Aortic isthmus and cardiac monitoring of the growth-restricted fetus

Aortic isthmus acts as an arterial watershed between the cerebral and placental circulations, connecting 2 parallel fetal ventricular pumps. It plays a crucial role in the fetal circulatory dynamics. Information about aortic isthmus blood flow may improve the management of sick fetuses. However, perceived technical difficulties limit the clinical use of aortic isthmus Doppler for fetal hemodynamic monitoring. Changes in aortic isthmus blood flow pattern seem to reflect fetal cardiovascular status accurately and predict perinatal and long-term neurodevelopmental outcome in intrauterine growth restriction. This review evaluates the available scientific information and discusses the role of aortic isthmus in fetal circulation.

Fetal aortic isthmus blood flow and the fraction of cardiac output distributed to the upper body and brain at 11-20 weeks of gestation

OBJECTIVE

To measure serial changes in fetal aortic isthmus (AI) blood flow and estimate the fraction of fetal cardiac output distributed to the upper body, including the brain, at 11-20 weeks of gestation.

METHODS

Using pulsed-wave Doppler and two-dimensional ultrasound, blood flow velocities and inner diameter of the AI, aortic valve (AV) and pulmonary valve (PV) were measured longitudinally in 143 fetuses and volume blood flows (Q) were calculated for each site using the formula: Q (mL/min) = pix (diameter/2)(2) x velocity time integral x heart rate x 60. The sum of Q(av) and Q(pv) constituted the combined cardiac output (CCO) and the fraction (%) of the upper body (including brain) blood flow was calculated as: (Q(av)-Q(ai))x100/CCO.

RESULTS

AI blood velocities as well as the vessel diameter increased with advancing gestation, resulting in a significant increase in Q(ai) from 1.9 to 40.5 mL/min during weeks 11 to 20. The AI peak systolic velocity increased from 29 to 63 cm/s, end-diastolic velocity from 1.2 to 5.2 cm/s, and the time-averaged maximum velocity from 11 to 22 cm/s, resulting in a fairly stable pulsatility index (PI) of 2.4-2.6 and resistance index (RI) of 0.91-0.94. On average, 75% of blood ejected by the left ventricle (which represented about 35% of the CCO) passed through the AI to the descending aorta. The fraction of CCO distributed to the upper body, including the brain, was estimated as approximately 13%.

CONCLUSION

We have established longitudinal reference ranges for fetal AI diameter, blood flow velocities, PI, RI and volume blood flow at 11-20 weeks of gestation. The human fetus appears to direct a relatively small fraction (13%) of its CCO to the upper body, including the brain, during this period of pregnancy.

Morphometric study of the ventricular segment of the human fetal heart between 13 and 20 weeks' gestation

Our objective was to determine the normal dimensions of the ventricular segment of the human fetal heart between 13 and 20 weeks' gestation.

STUDY DESIGN

103 hearts obtained by necropsy were dissected and measurements of different portions of ventricles were determined under stereoscopic magnification. In each ventricle were measured anteroposterior and lateral diameters, inlet and outlet length, and thickness of walls at different levels. Our results showed the cardiac apex was constituted by the left ventricle in 68.9% of the hearts. Both ventricles showed linear growth during this period of fetal development. Ranges in median values of external and internal ventricular measurements were determined. The left ventricular wall was thicker than the right, and the right ventricular cavity was larger. This study provides morphometric reference information concerning the dimensions and growth of both ventricles of the fetal heart, which may be useful in pediatric cardiac surgery and echocardiography.

Morphometric study of the ascending aorta in human fetuses

The present study was performed on 128 spontaneously aborted human fetuses, aged 15-34 weeks, to compile normative data for ascending aorta dimensions at varying gestational age. Using anatomical dissection, digital-image analysis (system of Leica QWin Pro 16) and statistical analysis (ANOVA, regression analysis) a range of measurements (Length, original and terminal external diameters, volume) for the ascending aorta during gestation was examined. No significant gender differences were found (P > 0.05). The growth curves of the best fit for the plot for each morphometric feature against gestational age were generated. Both the Length and external diameters of the ascending aorta were found to increase in a linear fashion throughout gestation. The Length ranged from 2.63 +/- 0.42 to 10.80 +/- 1.49mm, according to the linear function y = -4.678 + 0.4647x +/- 0.8447 (r = 0.95). The original external diameter ranged from 2.02 +/- 0.26 to 6.84 +/- 0.63 mm, according to the linear model y = -2.103 + 0.2684x +/- 0.3958 (r = 0.97). The terminal external diameter ranged from 1.73 +/- 0.20 to 6.29 +/- 0.52 mm, with accordance to the linear function y = -2.354 + 0.2567x +/- 0.3826 (r = 0.97). The ascending aorta volume ranged from 7.56 +/- 2.65 to 370.99 +/- 105.42 mm3, according to the quadratic function y = 373.1 - 43.38x + 1.30x(2) +/- 24.51 (R2 = 0.89). The growth curves generated from my data might be useful as a reference for fetal echocardiographers in the detection of some congenital cardiovascular abnormalities.

Abnormal arterial flows by a distributed model of the fetal circulation

Modeling the propagation of blood pressure and flow along the fetoplacental arterial tree may improve interpretation of abnormal flow velocity waveforms in fetuses. The current models, however, either do not include a wide range of gestational ages or do not account for variation in anatomical, vascular, or rheological parameters. We developed a mathematical model of the pulsating fetoumbilical arterial circulation using Womersley's oscillatory flow theory and viscoelastic arterial wall properties. Arterial flow waves are calculated at different arterial locations from which the pulsatility index (PI) can be determined. We varied blood viscosity, placental and brain resistances, placental compliance, heart rate, stiffness of the arterial wall, and length of the umbilical arteries. The PI increases in the umbilical artery and decreases in the cerebral arteries, as a result of increasing placental resistance or decreasing brain resistance. Both changes in resistance decrease the flow through the placenta. An increased arterial stiffness increases the PIs in the entire fetoplacental circulation. Blood viscosity and peripheral bed compliance have limited influence on the flow profiles. Bradycardia and tachycardia increase and decrease the PI in all arteries, respectively. Umbilical arterial length has limited influence on the PI but affects the mean arterial pressure at the placental cord insertion. The model may improve the interpretation of arterial flow pulsations and thus may advance both the understanding of pathophysiological processes and clinical management.