New guidelines for sickle cell patients with pulmonary hypertension

3/18/2014 04:25:00 PM
Diagram of the heart, with the tricuspid valve and pulmonary artery and vein labeled.
Science Codex is reporting the publication of the first guidelines for treating sickle cell patients with pulmonary hypertension. I consulted on a study of pulmonary hypertension in children with sickle cell a few years back, so this is of interest to me. From the abstract:
"...Target audiences include all clinicians who take care of patients with SCD.

"Results: Mortality risk stratification guides decision making. An increased risk for mortality is defined as a TRVequal to or greater than 2.5m/second, anNT-pro-BNPlevel equal to or greater than 160 pg/ml, or RHC-confirmed PH. For patients identified as having increased mortality risk, we make a strong recommendation for hydroxyurea as first-line therapy and aweak recommendation for chronic transfusions as an alternative therapy. For all patients with SCD with elevated TRV alone or elevated NT-pro-BNP alone, and for patients with SCD with RHC-confirmed PH with elevated pulmonary artery wedge pressure and low pulmonary vascular resistance, we make a strong recommendation against PAH-specific therapy. However, for select patients with SCD with RHC-confirmed PH who have elevated pulmonary vascular resistance and normal pulmonary capillarywedge pressure, we make a weak recommendation for either prostacyclin agonist or endothelin receptor antagonist therapy and a strong recommendation against phosphodiesterase-5 inhibitor therapy..."

Most of my readers will recognize the term "hypertension" meaning high blood pressure. This isn't that. This won't show up when your doctor takes your blood pressure. Pulmonary hypertension refers to high blood pressure specifically in the pulmonary artery, which conveys blood from the heart to the lungs. The pulmonary artery typically has very low blood pressure, and chronically high pressure in this artery is problematic because over time it will damage the sensitive membranes of the lungs. Indeed adults with pulmonary hypertension have substantially shortened lifespans compared to those who do not.

Because it applies specifically to the pulmonary artery and not the whole body, pulmonary hypertension is pretty hard to detect. The only direct way to precisely diagnose it is a highly invasive procedure threading a sensor through a vein in your arm all the way up into the right atrium of the heart. That's neither pleasant nor minimal risk. Fortunately, there are indirect proxies we can use to detect pulmonary hypertension, such as measuring tricuspid regurgitation (TR) jet velocity with an echocardiogram. The basic idea is this: each time the heart beats,blood flows from the right atrium of the heart, which receives all the veins except the pulmonary vein, into the right ventricle, which sends blood to the pulmonary artery. The two are connected by the tricuspid valve that prevents blood from flowing in the opposite direction. However, in all people a tiny bit of fluid "regurgitates" and flows backwards from the right ventricle into the right atrium, which is known as the TR jet, the speed of which we can measure with sensors outside the body, without needing any invasive implants. Some clever scientists have used fluid dynamics to predict the relationship between the blood pressure in the right ventricle where the pulmonary artery starts out and the TR jet velocity, so that we can estimate pulmonary blood pressure as a function of the TR jet velocity. The new guidelines prescribe treatments for patients that present with high TR jet velocities, among other things.

If this sounds like a slam-dunk case, it isn't. Not quite. There are two ways in which this recommendation can go awry: 1) cases where high TR jets arise for reasons other than pulmonary hypertension, and 2) cases where pulmonary hypertension is not a permanent condition (a temporary bout of pulmonary hypertension isn't likely to cause major problems). In adults, these aren't huge issues--if you have pulmonary hypertension as an adult it is very likely to be a life-long, permanent condition that will eventually cause damage, and most cases of adults with high TR jet velocities it is caused by high pressure in the pulmonary artery.

However, when we examined the relationship between TR jet velocity and pulmonary hypertension among kids that had both an echocardiogram and invasive heart catheterization, we discovered that a substantial share those with high TR jet velocities did not have pulmonary hypertension. In these children, the high TR jet velocity was due to other factors such as abnormal left ventricular function which did not cause hypertension in the pulmonary blood vessels. Moreover, it isn't as apparent that pulmonary hypertension in children has the same life-shortening risks as it does in adults. The fact is that kids' hearts are still growing, and pulmonary hypertension observed during childhood could normalize before they become adults, without causing significant health problems. Thus while high TR jets are clearly an important indicator in adults, the same may not be true for children. On the one hand, TR jets aren't as likely to indicate pulmonary hypertension in kids, while on the other hand it isn't clear that pulmonary hypertension is as much a problem in kids, even if the TR jet was a reliable indicator.

My point in all this is to say that some economists do real science the situation is ripe for over-diagnosis: the new recommendations could have doctors administering treatments to a substantial share of children that have no probability of benefiting from them, while the unnecessary treatments carry non-trivial risks of their own. The recommendations themselves do seem reasonable to me, except that I would like to see more differentiation between adults and children with sickle cell, because the named risk factors--TR jets in particular--may not imply the same things for both.