Ambulatory blood pressure monitors periodically measure patient blood pressure over a predetermined length of time (typically 24 hours) as the patients go about their normal daily routines. The purpose of ambulatory blood pressure monitoring (ABPM) is to obtain data that reflects the cardiovascular state of the patient under conditions more representative of their normal everyday lifestyle than those inherent in a clinical environment.
The need for an ambulatory device was first recognized by Dr. Maurice Sokolow, head of the hypertension section of the San Francisco General Hospital during the 1950's. He conducted a study demonstrating that there were persons with highly elevated blood pressures who lived a normal life span, whereas others with modest hypertension died at an early age due to cardiovascular complications. From these findings, Dr. Sokolow theorized that the clinic pressures of patients were not always representative of their everyday pressures.
In order to monitor patients outside of the clinical environment, Dr. Sokolow designed and constructed an ambulatory device. By using the ambulatory monitor, he was able to show that blood pressure varied throughout the day, and that "clinic pressures tended to be higher than ambulatory pressures in the majority of patients". Since the 1950's, there has been a great deal of research in the area of ABPM. As a clinical procedure, ABPM has played a major role in the discoveries of white coat syndrome and the circadian rhythm of blood pressure, as well as the development of hypertension-relieving chronotherapeutics. ABPM has also been found to provide good correlative information regarding end-organ damage, such as left ventricular hypertrophy.
White coat hypertension is generally defined as "a persistently elevated clinic blood pressure and a normal pressure at other times," The elevation in blood pressure may be due to several factors, including nervousness, stress, or increased sympathetic activity associated with the clinical environment. It should be noted that it is not unusual for patients to exhibit high blood pressure in a new clinical environment. Therefore, elevated blood pressure may constitute a more significant symptom of hypertension once the patient has become acclimated to the clinical site.
Although the general description of white coat hypertension is agreed upon, the exact definition varies. Somewhere between 20% to 40% of patients with mild to moderate hypertension in a clinical setting may actually be white coat hypertensive. There are two salient issues that arise from white coat hypertension: the effects of anti-hypertensive drugs on normotensive individuals, and also the cost of administering those drugs. In order to avoid administering unnecessary hypertension management therapy to white coat hypertensives, these "white coat hypertensives" must be identified. Therefore, an increasing number of physicians are implementing ABPM into their hypertension diagnosis procedures.
There are three major mechanisms to explain white coat hypertension. The first is that it characterizes an exaggerated alerting response and, therefore, a heightened response to stressful stimuli. The second explanation is that white coat hypertension is a learned, or conditioned, response. The third theory is that it is a precursor of sustained hypertension. Since the mechanisms responsible for white coat hypertension are not completely agreed upon, the question of whether the condition is benign or malevolent is also a matter of some debate. Currently, research is being conducted to determine whether white coat hypertensives should be classified for treatment purposes as normotensives, hypertensives, or as a completely new category with separate risks, characteristics, and treatment guidelines.
In addition to identifying "white coat hypertensives," ABPM is also extremely helpful in the development of chronotherapeutics (time-released medication) for hypertension management. Chronotherapy varies drug delivery rates throughout the day, either by automated delivery devices or by manual regulation. This can be used to account for periodic variations in patient conditions caused by circadian rhythms and periodically conducted activities. By examining individual time intervals, the physician is able to determine how much medication is required for each interval. Chronotherapeutics are then prescribed, allowing the patient to be treated with a minimum of medication. ABPM may prove to be extremely helpful in identifying the time periods when medication is required for chronotherapeutic treatment of hypertension. Once treatment has been initiated, subsequent ambulatory sessions would allow the physician to optimize the treatment program.
ABPM has also directly led to the discovery of the circadian rhythm of blood pressure: a decrease in blood pressure levels from periods of wakefulness to periods of sleep (for convenience, daytime and nighttime will be used for wakefulness and sleep, respectively). Many people exhibit this circadian rhythm, which consists of a blood pressure decrease of approximately 15%-25% during the evening, with increases to daytime levels again in the morning. Clinical studies have shown that people may be classified as either "dippers" or "non-dippers", depending on whether their blood pressure exhibits the circadian rhythm or remains close to its daytime level during nighttime hours. Since hypertensive "dippers" may be normotensive during nighttime hours, ABPM may prove helpful in optimizing treatment programs for hypertension.
An important characteristic of the circadian rhythm is a sharp increase in blood pressure early in the morning. This is accompanied by increases in heart rate, catecholamine levels, a-adrenergic receptors, corticosteroids, and platelet aggregation. Together, these sudden changes may result in early morning myocardial ischemia/infarction, stroke, and sudden death. ABPM may be a key factor in discovering the causes of such early morning cardiovascular complications.
ABPM data provides the best correlation between blood pressure and left ventricular hypertrophy (LVH), a measure of cardiac end-organ damage. White et. al. found a correlation between average daily blood pressure obtained by ABPM and LVH, whereas no relationship could be found between clinically obtained blood pressure data and LVH. This correlation has already been used in several important discoveries. For example, Verdecchia et. al. found that non-dippers have significantly higher degrees of LVH than dippers. This led to support for the hypothesis that non-dippers, when compared with dippers, also have a significantly greater number of strokes.
Clinical research in the field of ABPM has led to the application of additional analysis techniques that may allow the physician to obtain a clearer assessment of a patient's hypertensive condition. For example, the large volume of data collected from one ambulatory session allows the calculation of blood pressure and heart rate averages based on an entire day, as opposed to measurements taken during the limited time span of a visit to a clinic. This may give the physician additional information regarding a patient's overall cardiovascular condition.
Another technique now used with ABPM is load analysis. Load analysis quantifies the degree of patient hypertension by determining the total duration of patient blood pressure elevation as a percentage of the ambulatory period. Physicians may also graphically determine when during the ambulatory period a patient's blood pressure was elevated, to validate alternative explanations to hypertension for blood pressure elevation (food consumption, exercise, stressful situations, etc.). Load analysis is a potentially powerful tool that enables the physician to more specifically assess the severity and the nature of hypertension, and has already been used to quantitatively characterize the likelihood of LVH occurrence.
An important factor in any medical procedure is cost. ABPM may be cost effective by reducing the number of patients who are mislabeled as hypertensive and subsequently undergo hypertension management therapy. ABPM is also useful for long-term patient tracking, to allow earlier detection of cardiovascular changes. In addition, one twenty-four hour ABPM session using Pulse Dynamic technology may provide reliable information regarding blood pressure, arterial compliance, peripheral resistance, white coat classification, dipper/non-dipper classification, and cardiac end-organ damage evaluation. This decreases the need for exhaustive testing and allows quicker, easier diagnosis and treatment program development.