23 March 2009

Pediatric Cardiology #1: Healthy Heart

I recently began working in clinical research again; with the Congenital Heart outcomes group. The study I'm working on has a much wider ranger of diagnoses that my previous work so I'm brushing up on my pediatric cardiology (in all that spare time medical school leaves). I thought it might be an interesting topic to share here, so this post is a primer on normal functioning (I did the drawings).

Unfortunately, most of the diagrams you see in books are functionally correct (ex. above), but not anatomically helpful. If you open up someone's chest (from the front, ie anterior or ventral) you do not see two ventricles sitting side by side. Instead, you see something more like...

You can see immediately that rather than a right-left orientation, the heart is really more front-back (anterior-posterior). Also, the ventricles are not so much "on top" as they are to the right. Which brings me to another point, we name everything by the patient's orientation. Thus, the right ventricle is the patient's right side, not yours. The whole heart sits slightly to the left of midline in the chest and its apex (point) is roughly beneath the left nipple.
The way blood flows through the two interconnected circuits - systemic (body) and pulmonary (lungs) - is frequently disrupted in congenital heart disorders. Normally, blood returns from the body through the superior vena cava (SVC) and inferior vena cava (IVC) into the right atrium (RA). As the right ventricle relaxes (diastole) it fills with blood from the right atrium. This is accomplished through a pressure difference between the ventricle (low) and the atrium (high), which opens the tricuspid valve. When the ventricle is full, the pressure will be higher than the atrium, pushing the valve closed (creating unidirectional flow). The ventricle contracts (systole) and blood enters the pulmonary trunk (through the pulmonary valve). The pulmonary trunk divides into a right and left branch to the right and left lungs. Each of these eventually becomes a capillary bed, which drains into small veins, which drain in to the 4 pulmonary veins. The pulmonary veins drain into the left atrium. As the left ventricle relaxes it fills with blood from the left atrium (same process as right ventricle, except the valve is called the mitral valve). When the left ventricle contracts it sends blood into the aorta (through the aortic valve). From the aorta blood goes all over the body and returns to the heart via the SVC/IVC.

There are two structures unique to babies' hearts: the foramen ovale and the ductus arteriosus. The foramen is a hole in the wall (septum) between the two atria. It's open and birth and normally closes in the first few days of life. Failure to close is called a patent foramen ovale (PFO) and can ultimately lead to congestive heart failure. The ductus is a vessel connecting the pulmonary trunk to the aorta. In fetal life it's used to bypass the lungs and it too usually closes soon after birth. Both of these conduits allow for mixing of blue (deoxygenated) blood and red (oxygenated) blood (which is a normal person is bad).

Summary: SVC/IVC -> R atrium -> (tricuspid) R ventricle -> (pulmonary valve) Pulmonary system -> L atrium (mitral) -> L ventricle -> (aortic valve) Aorta -> systemic circulation -> SVC/IVC

Key Points:
1. Ventricular filling is accomplished (almost entirely) through pressure differentials
2. The valves closing properly is important for unidirectional flow
3. There are two conduits for shunting blood from the right heart to the left: the foramen ovale and the ductus arteriosis
4. The pressures in the right heart are normally much lower than those in the left

If there are any questions, let me know and I will attempt to clarify. I only gets more complex from here!

No comments: