The Role of Cardiac Imaging in Congenital Heart Defects

The Role of Cardiac Imaging in Congenital Heart Defects

mukesh kumar 22-09-2020

We live in the era of non-invasive imaging in medicine. The top rankers in the nation-wide competitive exam for post-graduate medical education invariably choose radiology as their specialization of choice. 

What is Cardiac Imaging?

Imaging is the main pillar of congenital heart disease (CHD) diagnosis and congenital heart disease treatment. A century earlier CHD was suspected clinically and diagnosed on an autopsy performed after the child died. 

Even four decades earlier, CHD was diagnosed by cardiac catheterization and angiography. This was an invasive procedure which carried a small but definite risk of adverse events. Thankfully, advances in non-invasive imaging have allowed a more accurate and safe diagnosis of CHD in neonates, infants and young children.

The three modalities which we use most commonly in clinical practice are the echocardiogram, the computerized tomogram (CT) and cardiac magnetic resonance imaging (CMR). 

There is a peculiar challenge in imaging the heart unlike other organs of the body. The brain, liver, kidneys and gut remain static for the most part. However, the heart is a dynamic organ which first relaxes to receive blood (a phase termed diastole) and then contracts to push the blood out (a phase termed systole). This process is termed the cardiac cycle which comprises of both systole and diastole. Each of the four chambers of the heart (the two upper chamber called atria and the two lower chambers called ventricles) go through this cardiac cycle tens of thousands of time every day. 

In addition to the three standard dimensions of length, breadth and depth, the heart needs to be studied in time as well to understand the changes happening during each phase of the cardiac cycle. An ideal imaging modality should provide accurate information about the various parts of the heart in the traditional dimensions (termed spatial resolution – a modality which provides high quality images of the cardiac chambers is said to have good spatial resolution) and also be capable of tracking the changes in the chambers with time (termed temporal resolution). It suffices to say that no single modality available today offers superior spatial and temporal resolution. Pediatric cardiologists, hence, resort to a combination of imaging modalities to obtain all the information required before subjecting a child with CHD to surgery.

Echocardiogram: The Most Common Method

The echocardiogram is the most commonly performed investigation in the pediatric cardiology and may be considered “the workhorse” of a busy pediatric cardiac practice. The echocardiogram uses ultrasound waves to image the heart. 

Our ear is attuned to hearing sounds of frequencies between 20 to 20000 hertz (20 Hz to 20 kHz). Sounds of frequencies above this value are used by bats to move (a process called echolocation). When the sound waves hit a barrier, they are reflected back. The bats analyze the reflected waves to understand the location of barriers and avoid them while flying. Medical ultrasound analyses the sound waves reflected from the various organs of the body and reconstructs them using a computer algorithm to provide images of the organs in a digital screen. The sound waves used in cardiac imaging vary from 1000000 to 14000000 (1MHz to 14 MHz). Two-dimensional ultrasound has excellent spatial resolution and temporal resolution. It is readily available and relatively inexpensive compared to other imaging modalities. It is hence the first imaging modality of choice in all children with suspected or known CHD.

The echocardiogram is performed by a pediatric cardiologist or a trained pediatric cardiac sonographer. A typical echo study takes between 5 to 30 minutes. The room is kept relatively dark to enable the cardiologist to see the screen better. This is comparable to switching off the lights in a cinema hall. 

Making Echocardiogram Comfortable for Children

The children are made to lie on a comfortable couch. Most children above 4 years of age are co-operative during a study. This process can be further enabled by talking to the child, putting the child at ease and continuing to engage the child in conversation during the study. 

However, it is often a challenge to obtain a study in younger children. Toddlers become intimidated by the darkness in the room. The cardiologist uses a gel to act as an interface between the echo probe and the body. This enables them to see the structures better. The gel is often cold and uncomfortable for younger children. Cardiologists have worked out a number of techniques to facilitate this process. Children younger than 6 months of age often go to sleep immediately after a feed. Hence the parents are encouraged to feed the baby. The sleeping baby is then wrapped in a warm blanket to make him or her comfortable. A soother in the mouth also helps to relax the baby. 

Older infants and toddlers can be distracted by making them watch their favorite cartoons. Sometimes children are more confident while lying in their mother’s lap and the study can then be performed in this fashion. 

However frequently, children of this age group will need to be put to sleep by administering medications. In most centers, the medicines used are Tricoflos, chloral hydrate or melatonin. The use of this medicine is safe and will not harm of the child. However, the child’s breathing and heart rate will need to be monitored for a short while. Children go to sleep within half an hour of administering the medicine and remain asleep for 1 – 2 hours. 

It helps when the parents are explained about the process of congenital heart disease treatment and their confidence is gained. It allows them to be actively involved in calming the baby and makes the job easier for everyone involved.