Treatment of Hypertension

Individualizing the Treatment of Hypertension


Impedance Cardiography - A new way of tailoring blood pressure therapy
By DrRich

The "best" way to treat hypertension (high blood pressure) has engendered tremendous controversy over the years, at least among hypertension specialists. Part of the problem is that a huge array of drugs are available to treat hypertension, each with its own profile of effectiveness and side effects, each with its own faction of supporters among the experts, and (seemingly) each with its own large randomized clinical trial "proving" that it ought to be among the first drugs used in the hypertensive patient.

Subsequently, the unfortunate primary care doctors - the ones who do most of the actual heavy lifting in treating patients with high blood pressure - often are left in a state of high confusion. At best, they have adopted an organized trial-and-error approach to treating hypertension.
More commonly they bounce around from approach to approach depending either on the latest randomized trial, or on the latest "key opinion leader" to have been brought in (usually by a drug company) to deliver a lecture at Grand Rounds.

A study published in the April issue of Hypertension may help lead the way toward a more logical approach to treating high blood pressure. Doctors from the Wake Forest University School of Medicine used a relatively new non-invasive tool (impedance cardiography) that measures various aspects of patients' cardiovascular status - such as fluid volume, the amount of blood being pumped by the heart (the cardiac output), and the amount of tension present in the lining of the blood vessels (the vascular resistance) to tailor blood pressure therapy for individuals. These impedance cardiography measures allowed doctors to more accurately assess exactly why the blood pressure was elevated in specific individuals, and thus tailor drug therapy to address the specific underlying cause. When they randomized therapy in 164 patients to treatment based either on impedance cardiography or on standard "empiric" methods, patients whose blood pressure was managed according to the results of impedance cardiography measurements achieved significantly better blood pressure control. The authors, as well as the author of an accompanying editorial in the same edition of Hypertension, believe that more widespread usage of impedance cardiography in managing hypertension will take the guesswork out of designing therapy for individuals, and will result in improved blood pressure control for the population.

DrRich Comments:

Trying to improve blood pressure has some similarities with trying to improve miles per gallon. There are many factors that affect miles per gallon, including (among many others) tire wear and pressure, patterns of driving (such as acceleration and deceleration patterns), total weight of the car, and engine maintenance. So if you did a randomized automotive trial to see whether optimizing tire pressure resulted in an improvement in miles per gallon, you would probably find that it did. However, in applying the results of that randomized trial to individuals, you would find that in some individuals optimizing tire pressure would make a big difference, while in others (whose tire pressure is already pretty good) it would not. Maybe in that latter group, advising the driver and his spouse to each lose 75 pounds would be the best way to pick up an extra 2 miles per gallon.

Blood pressure is similar to miles per gallon in that it is the product of many, many determining factors, including some of the factors assessed by impedance cardiography. So if a doctor can assess whether the hypertensive patient's vascular resistance is elevated (so that a vasodilator like an ACE inhibitor or calcium channel blocker might be a logical first-line therapy), or that the patient's cardiac output is depressed (so that a beta blocker would probably best be avoided), it just makes sense that relying on such information is likely to give better results than relying on a study that measured the average response to a particular drug of a large group of patients. Impedance cardiography is one way (there are probably others) of individualizing the approach to blood pressure based on which factors are most likely causing the hypertension in the individual patients.

Note: Impedance cardiography is NOT something most doctors have access to, and most will look at you cross-eyed if you ask for such a thing. Before the individualization of blood pressure therapy becomes routine, even though it makes sense and even though this early research looks very promising, decades of tradition and scores of exalted academic careers favoring population-based therapy will have to be overcome.

Sources:

Smith RD, Levy P, Ferrario CM, et al. Value of noninvasive hemodynamics to achieve blood pressure control in hypertensive subjects. Hypertension 2006; 47:769-775.

Flack JM. Noninvasive hemodynamic measurements. An important advance in individualizing drug therapies for hypertensive patients. Hypertension 2006; 47:646-647.