Ambulatory Blood Pressure Monitoring: How Important Is It in Estimating Risk or Guiding Therapy?

In recent years numerous reports have described the benefits of 24‐ and 48‐hour ambulatory blood pressure monitoring (ABPM) in hypertensive patients. ¹ , ² Data have indicated that ABPM recordings correlate more closely with organ system involvement than casual blood pressures measured in a clinic or physician's office.

In the early 1970s, intra‐arterial blood pressures were used for continuous recording. At that time, patients were identified whose blood pressures rose in the early morning, remained elevated during the day, and decreased during the night. There were others whose pressures failed to decrease by about 10% during the nighttime hours, from about 1–6 a.m. These patients were believed to have more “fixed” blood pressures. In recent years they have been labeled “non‐dippers.” Non‐dippers have been shown to have more target organ involvement, such as left ventricular hypertrophy, than patients with a decrease in blood pressure at night. ³ Non‐dippers are more commonly found in the black population.

POSSIBLE PROBLEMS WITH ABPM

Although recordings are made over a 24‐hour period at 15‐ to 30‐minute intervals with ABPM, it should be pointed out that the person wearing the monitor must discontinue exercising while a recording is being made. Recordings, therefore, do not include blood pressures during vigorous activity. In addition, blood pressure readings taken at night frequently awaken the patient; these, then, may not actually represent the “sleep blood pressure.” While the technique has improved a great deal in recent years, about 10%–15% of readings must be discarded. Despite these technical and practical difficulties, recordings are generally accurate and give a reasonable picture of blood pressure levels over a 24‐hour period.

POSSIBLE ADVANTAGES AND USES OF ABPM

The addition of ABPM has enabled researchers to more accurately determine the duration of action of various medications. New drugs are now being studied with this technique to demonstrate whether or not they are effective over a 24‐hour period. ⁴ Data indicate that ABPM findings correlate better than casual blood pressures with the presence of urinary protein and the presence of left ventricular hypertrophy. Proteinuria and left ventricular hypertrophy have been identified as definite risk factors for cardiovascular disease. ⁵ Thus, there are some findings that are more accurately predicted with ABPM than by casual office or clinic blood pressure measurement. However, many studies that claim the use of this technique is a more accurate predictor of outcome than casual blood pressures have been poorly controlled, were often retrospective and, in many instances, lacked treatment data and interim blood pressures between the first and last blood pressure recordings. ⁶ Thus far, most data on the long‐term prognostic value of ABPM are not convincing, although there are suggestions that the use of this procedure may be helpful in predicting prognosis.

A major question is when and how often should the physician consider the use of ABPM? Is it necessary to determine prognosis or to guide therapy decisions? In 1996–97, the Joint National Committee on Detection, Evaluation and Treatment of High Blood Pressure did not recommend the use of ABPM in the routine evaluation of the hypertensive patient. ⁷ They did, however, suggest that it might be of use to aid in the diagnosis of “white coat” hypertension, or in the evaluation of so‐called resistant hypertension.

There is a great deal of “literature” pressure to use ABPM as a routine procedure. It is not inexpensive, with a cost between $100 and $300 for a 24‐hour recording, but more importantly: 1) Data on which we base estimates of cardiovascular risk are based on casual blood pressure readings. Individuals arrive at a clinic or physician's office, blood pressure is taken, and those who have higher blood pressures have been found, in long‐term follow‐up studies, to have an increased risk of cardiovascular events. ⁵ None of the well controlled, prospective, epidemiologic studies of hypertension included ABPM in the initial examination and estimation of risk. Recent studies have, however, begun to include this procedure. 2) The data regarding benefits of treatment in the carefully conducted, prospective, randomized trials were obtained with casual blood pressures. None of these trials included ABPM or even home blood pressure monitoring. Patients in the clinical trials were seen only three or four times a year. ⁸ Those with higher blood pressures taken in a clinic or office setting had a poorer prognosis than those with lower pressures. Thus, we have abundant evidence that the casual blood pressure is still a reasonable one to use to estimate risk and monitor outcome.

WHAT ABOUT WHITE COAT HYPERTENSION?

The literature is confusing. Several researchers have reported that approximately 20% of patients have elevated blood pressure in a doctor's office, e.g., 150–155/90–95 mm Hg, with a more normal or normal blood pressure, e.g., 130–135/80–85 mm Hg at home—the so‐called white coat syndrome. They have concluded that patients with white coat hypertension are at little risk of developing cardiovascular disease and should simply be followed. Other investigators indicate that people with higher blood pressures in the office and more normal pressures at home have physiologic changes that are indicative of the hypertensive state, i.e., peripheral vascular resistance is increased, there is some evidence of diastolic dysfunction, and the metabolic changes often found in hypertensive patients, such as hyperlipidemia and insulin resistance, are present. ⁹ Thus, for some investigators white coat hypertension is not a benign phenomenon and office blood pressures should be used to determine the initiation of therapy and ongoing treatment. ¹⁰

At present, the balance of data suggests that the casual or office pressure should remain the blood pressure that is used to monitor patients. If blood pressure is consistently elevated to levels of > 140/90 mm Hg in a physician's office or a clinic, treatment should be instituted whether or not left ventricular hypertrophy or proteinuria is present. Obviously, if these are present, treatment should be pursued more vigorously. Some may argue that in patients at low risk, with no cardiovascular risk factors other than elevated blood pressure, especially those with stage 1 or grade I hypertension (140–160/90–100 mm Hg), ABPM is useful in determining whether or not hypertension should be treated. That is, if ABPM pressures are <130/80 mm Hg, medication may not be indicated. There are few data to back up this approach. While it is possible that some patients may be treated unnecessarily if office blood pressures are used exclusively to make a diagnosis, the numbers can be minimized in the low‐risk patients by repeated recordings over several months, while lifestyle modifications are initiated. The vast majority of patients with stage 1 or grade I blood pressure readings (140–160/90–100 mm Hg) who will become normotensive on follow‐up will do so within 3–6 months. One important point: if therapy posed a risk or was complicated or expensive, the benefit/risk ratio in the “low‐risk” stage 1 patient might not justify treatment. Therapy, however, is generally simple, quite safe, and relatively inexpensive. ¹¹

HOME BLOOD PRESSURE RECORDING

What about home blood pressure recordings? There are some patients who must know their blood pressures. In these cases, the purchase and use of a relatively inexpensive home blood pressure monitor is reasonable. Taking blood pressures at home helps to reinforce the message that control is important, and if blood pressures are not controlled, serve to motivate the patient to get his/her doctor to do a better job. Most of the available anaeroid or electronic machines have been shown to be accurate and reliable. It is true that some patients will record and show their physicians only the good readings and omit readings that are high. There is one way to circumvent this: have the blood pressure machine connected to a central monitoring computer so that all recordings are registered. This approach might be useful for a few months until blood pressure control is achieved. Cost is not increased significantly.

How else might home blood pressure readings be useful? Guidelines have been published. ¹² 1) In a patient who has symptoms, such as headaches or dizziness, it is helpful to determine whether blood pressures are high or unusually low when the patient experiences these. 2) When multiple drugs are being titrated, it is useful to judge the direction of blood pressure readings. It is far more useful to obtain an ongoing record of blood pressures taken at different times of the day over weeks than to depend on just one 24‐hour monitoring. This approach is certainly less costly. Hundreds of millions of dollars would be spent if ABPM were used in only five million of the 40+ million hypertensive patients. ¹¹ Use of ABPM is limited at present by the lack of universal reimbursement. Many do not believe that this procedure should be reimbursable. Home blood pressure monitoring by the patient or a member of the family should remain the approach of choice if recordings outside the doctor's office are deemed necessary.

CONCLUSIONS

At present, casual or office blood pressure recordings should probably continue to guide treatment decisions in the management of hypertension. Although ABPM is helpful in determining the duration of action of medications and appears to correlate better with target organ involvement than casual pressures, these advantages do not appear to be important enough to warrant routine use of this procedure in hypertensive individuals. If casual blood pressures are consistently >140/90 mm Hg, the patient should be treated whether or not there is evidence of target organ damage.