The Application of High-Resolution Ultrasound for Assessment of Cardiac Structure and Function Associated with Developmental Toxicity

Congenital heart defects (CHD) are the most prevalent anomaly both clinically and in laboratory animal species. Historically, it was difficult to assess the longitudinal progression or repair of such anomalies because assessment methodologies were too invasive (gross exams and/or histology). Recently, technological advances in the field of diagnostic imaging have led to the manufacture of high-resolution ultrasound (HRUS), capable of characterizing both embryonic and maternal cardiovascular structure and function in small animals (rat and mouse). HRUS is relatively noninvasive, facilitating the longitudinal assessment of heart development throughout gestation and postnatally, providing a comprehensive evaluation of changes in cardiovascular performance following toxicant exposure.Described herein is a brief overview of important theoretical and practical considerations when applying HRUS to understand the impact of perturbations on the fetal heart. Examples are given from our own work to help the reader interpret their own HRUS images and more readily identify anomalies in utero. In addition to embryonic assessment, maternal pathologies may adversely affect the cardiovascular performance of the conceptus indirectly. Umbilical blood flow is particularly vulnerable to such effects and procedures to assess this endpoint are described. Neonatal rats, born with CHD, may respond pathologically to cardiovascular challenges as they mature, and we outline the use of HRUS to evaluate cardiac performance over the lifetime of the animal. Some of the caveats related to HRUS are discussed, particularly with the emphasis on how this may impact experimental design.

Murine fetal echocardiography

Transgenic mice displaying abnormalities in cardiac development and function represent a powerful tool for the understanding the molecular mechanisms underlying both normal cardiovascular function and the pathophysiological basis of human cardiovascular disease. Fetal and perinatal death is a common feature when studying genetic alterations affecting cardiac development. In order to study the role of genetic or pharmacologic alterations in the early development of cardiac function, ultrasound imaging of the live fetus has become an important tool for early recognition of abnormalities and longitudinal follow-up. Noninvasive ultrasound imaging is an ideal method for detecting and studying congenital malformations and the impact on cardiac function prior to death. It allows early recognition of abnormalities in the living fetus and the progression of disease can be followed in utero with longitudinal studies. Until recently, imaging of fetal mouse hearts frequently involved invasive methods. The fetus had to be sacrificed to perform magnetic resonance microscopy and electron microscopy or surgically delivered for transillumination microscopy. An application of high-frequency probes with conventional 2-D and pulsed-wave Doppler imaging has been shown to provide measurements of cardiac contraction and heart rates during embryonic development with databases of normal developmental changes now available. M-mode imaging further provides important functional data, although, the proper imaging planes are often difficult to obtain. High-frequency ultrasound imaging of the fetus has improved 2-D resolution and can provide excellent information on the early development of cardiac structures.