An ABPM device consists of a blood pressure cuff that is worn on the arm, and which is attached to a recording device (roughly the size of a CD player) that is worn on a belt. The ABPM device is worn for either 24 or 48 hours, and it records your blood pressure periodically (usually at 15-minute or 30-minute intervals) throughout that period. So the ABPM provides your doctor with a complete record of your blood pressure for a one- or two-day period.
The information the ABPM provides is fundamentally different from the information the doctor gets by taking your blood pressure in the office. The office blood pressure recording is a single value that is meant to reflect your blood pressure during quiet rest (which explains why, given the hectic environment of most doctors offices these days, the readings may not always be entirely accurate).
The ABPM, in contrast, gives the blood pressures that are obtained through a wide range of situations and activities -- from running to catch a bus to sleeping. And it is normal for a person's blood pressure to fluctuate tremendously during a day's activities. So, unlike the blood pressure you get in the doctor's office, the ABPM does not yield a single value for systolic and diastolic blood pressure, but a range of (often) widely variable values.
Using the ABPM to diagnose hypertension, then, requires a different approach.
The technique most commonly used for evaluating the results of ABPM is to average the patient's systolic and diastolic blood pressures for a full 24 hour period, and also for the hours that the patient is awake and asleep.
The "average" values that are used to diagnose hypertension are as follows:
- 24 hour average: systolic above 129 mmHg, OR diastolic above 80 mmHg.
- Average for "awake" hours: systolic above 134 mmHg, OR diastolic above 85 mmHg.
- Average for "asleep" hours: systolic above 124 mmHg, OR diastolic above 75 mmHg.
When Is ABPM Used?
ABPM has been helpful in assessing patients with white coat hypertension, allowing their doctors to decide whether their in-office blood pressure elevations reflect a "non-resting" state (that is, a state of anxiety), or whether the high in-office readings actually indicate that true hypertension is present.ABPM has also been useful in situations where it has been difficult to assess the effectiveness of an anti-hypertensive treatment regimen, or when a patient is suspected to have abnormally wide fluctuations in blood pressure that make diagnosing and treating hypertension difficult. ABPM can also help in the diagnosis and treatment of some forms of dysautonomia, especially when episodes of very low blood pressure are suspected.
An argument could even be made that ABPM ought to be the standard for diagnosing and treating hypertension, since getting truly accurate resting blood pressure measurements in the doctor's office can be problematic. However, measuring ABPM is much more cumbersome than simple office recordings of blood pressure, and is currently quite expensive (costing several hundred dollars for a one-or-two day assessment), so its use remains limited.
Sources:
Myers, MG. Ambulatory blood pressure monitoring for routine clinical practice. Hypertension 2005; 45:483.
