Background on single ventricle anatomy:
About 5 in 100,000 newborns are born with a single ventricle defect, meaning they are born with only one functional ventricle (2). There are a variety of types of single ventricle defects, such as hypoplastic left heart syndrome (HLHS), tricuspid atresia, and heterotaxy with common AV valve, just to name a few (1). Having only one ventricle is damaging to the heart and oxygen delivery in the body, causing cyanosis, volume overload on the single ventricle, decreased ventricular function, and congestive heart failure (1). Therefore, it is important to diagnose and treat infants with single ventricle anatomy as soon as possible, often performing the first surgery within days after birth (1, 2, 3). For now, the only available treatments for these infants include a heart transplant or the series of the Fontan procedures (1, 3). The Fontan procedures are a creative and impressively successful treatment for single ventricle defect, having a 75% 25-year post surgery survival rate in Australia and New Zealand (3). The Fontan procedures are fairly new in medical practice, thus some of the first patients to receive the Fontan procedures are about 30 years of age today (3).
The fun stuff about surgery and redirecting blood flow (1):
The Fontan procedures are broken into 3 main surgeries, with the first surgery performed only days after birth, and the last surgery performed around 2-4 years of age. The purpose of the Fontan procedures is to redirect blood flow in order to maximize the one ventricle without overloading it. I will walk through the redirecting of blood flow specifically for HLHS, where the left ventricle is underdeveloped, although for many types of single ventricle defects, the operations have very little variations.
An underdeveloped left ventricle seriously decreases systemic flow, along with backing up pulmonary flow. The first surgery, called the Norwood operation, is aimed at establishing sufficient systemic circulation and pulmonary flow. The surgeons do this by widening the atrial septum (the hole between the left and right atria). This allows for mixing of oxygenated and de-oxygenated blood in both atria. They also connect the pulmonary artery to the ascending aorta, along with a shunt between the two. So, the blood (ox and de-ox) in the right ventricle now pumps to the aorta, instead of the pulmonary artery. This is what allows for systemic flow! But, what about pulmonary flow? The ascending aorta is also connected to the pulmonary artery through a shunt. This means that some blood flowing through the aorta is diverted to the pulmonary artery, allowing for blood flow into the lungs. Everything is all great now, right?
Well, the right ventricle is still working extremely hard to pump this blood to the body and the lungs, which is okay for a little while, but long-term, the heart will fail doing this. So, onto step 2! The second surgery, called the bi-directional Glenn shunt procedure, is performed somewhere between 4-9 months of age. The main aim of this procedure is to redirect blood flow to the right pulmonary artery, thus less work for the right ventricle. They do this by connecting the superior vena cava to the right pulmonary artery. So, now the blood returning to the heart from the upper body is now flowing directly and passively to the lungs, instead of to the right atrium. This improves pulmonary flow, without taking away from the redirected systemic flow.
To complete these procedures, the third surgery, called the Fontan procedure, connects the inferior vena cava to the pulmonary artery. Thus, now our blood flow goes from the body to the superior and inferior vena cava to the pulmonary arteries. From here, the blood flows passively to the lungs and the oxygenated blood flows back to the heart to the left atrium, which flows to the right atrium through the hole between them. This blood then goes to the right ventricle and is pumped to the aorta to the body! Thus, blood flow is completely redirected successfully in a way that ensures both pulmonary and systemic flow, while also not overloading the single ventricle.
Sources:
1. O'Brien, P., J. Boisvert. (2001). Current Management of Infants and Children with Single Ventricle
Anatomy. Journal of Pediatric Nursing 16(5): 338-350.
2. "Single Ventricle Defects." Boston Children's Hospital. Retrieved from: http://www.childrenshospital.org/conditions-and-treatments/conditions/s/single-ventricle-defects
3. d'Udekem, Y., et al. (2014). Redefining expectations of long-term survival after the Fontan procedure. Circulation 130(11): 32-38.
I read your post regarding congenital univentricular defect and it reminded me of a patient that I recently saw in the ER. She also was born with a single-working ventricle and what is interesting is that she is living in Colorado, where elevation is high. I was interested on how a patient can live with a single ventricle at high elevation, and so I did some digging and what I found that these patients do have a decease rate of survival at higher elevations (Johnson et al., 2013). These patient also are found to have more risk factors including higher pulmonary artery (PA) pressures, lower oxygen saturation, presence of thrombosis or protein-losing enteropathy (PLE)(Johnson et al., 2013). The recommendation for these patient is pretty much to move to lower elevation. I can't talk more about the patient's case, due to privacy concerns, but it is something to consider that these patient's do have a tougher lifestyle due to their condition, even if they have undergone Fontan procedure.
ReplyDeleteLiving at Altitude Adversely Affects Survival Among Patients With a Fontan Procedure. (2013, February 13). Retrieved from https://www.sciencedirect.com/science/article/pii/S0735109713002556#sec4
That's really interesting. I hadn't thought about how elevation might affect these patients. Of course, the lifestyles of these patients must be altered in many ways, including exercise and extra caution with medications that affect blood pressure, and it is important to take these factors into consideration. As of right now, the only treatment for congenital single ventricle defect is the Fontan procedures or a heart transplant. Both of which have risks and effects on lifestyle for these patients. I wonder if there could be a medication that could be designed to specifically help these patients at high altitude, or some other treatment. Thank you for the article and your input!
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