Ambulatory blood pressure monitoring (ABPM) is a method of taking regular blood pressure readings, usually over a 24-hour period, as patients conduct their normal activities. A special, automatic blood pressure monitor is used, and patients are asked to keep a diary or log of their activities during the day.
Ambulatory blood pressure monitoring is usually used when a physician suspects that a patient is suffering from "white coat hypertension." This is a condition in which high blood pressure occurs only in the physician’s office as a result of stress and anxiety.
However, there are a number of other situations that might cause a physician to recommend ambulatory blood pressure monitoring. These include patients who do not respond to medication and patients with symptoms such as fainting (syncope). In addition, ABPM has been found to be a more accurate predictor of patients at high risk of a cardiac event than than other blood pressure monitoring methods.
Ambulatory blood pressure monitoring is distinguished from home blood pressure testing. Home monitoring systems are gaining in popularity. Relatively inexpensive, they allow people to take their own blood pressure at various times throughout the day. Though this can yield valuable information, there are several drawbacks when compared to ambulatory blood pressure monitoring.
ABPM uses specialized equipment to measure blood pressure at regular intervals, 24 hours a day. This information is combined with a written log to aid in the diagnosis and/or treatment of conditions related to blood pressure.
About ABPM
Ambulatory blood pressure monitoring (ABPM) is a method of taking regular blood pressure readings over a 24- or 48-hour period. This is accomplished with a special device that is worn at all times and measures blood pressure automatically, recording the readings.
Studies have shown that ABPM is a valuable tool in the diagnosis of several conditions. It is also beneficial for adjusting doses of medication for high blood pressure and even predicting cardiac events in certain patients.
Blood pressure is a measure of the force, or tension, of the blood against the walls of the arteries. High blood pressure puts an added workload and strain on the heart, whereas low blood pressure (hypotension) can lead to fainting (syncope). High blood pressure (hypertension) is a very common disease in America. It is considered a major risk factor for heart attacks and strokes.
Blood pressure is measured with the use of an arm cuff (sphygmomanometer) and expressed as systolic pressure over diastolic pressure. Systolic pressure is the highest level of the blood’s pressure within the artery walls and corresponds to the contraction of the ventricle. Diastolic pressure is the lowest pressure at which blood stays within the aorta. Both are measured in millimeters of mercury (mmHg).
In most cases, blood pressure is measured in a physician’s office. In recent years, blood pressure measuring equipment has become available that allows people to measure their blood pressure easily and affordably at home. In addition, monitoring equipment is often found in public places, such as grocery stores and pharmacies. However, all of these approaches have certain disadvantages that make ABPM an attractive option for some individuals. For example, in the case of blood pressure measurements at a physician’s office, many people suffer from “white coat hypertension,” or high blood pressure as a result of anxiety connected to the blood pressure test itself. Similarly, the at-home tests and those completed out of a medical setting may be inconsistent and unreliable.
By contrast, the ABPM measures blood pressure at regular intervals throughout the monitoring period, including at night when the patient is asleep. It then records these measurements. Coupled with an activity diary, the ABPM can give a physician a very accurate picture of factors that may be affecting a patient’s blood pressure, such as exercise, eating, medications and certain forms of heart disease.
This information is useful for a variety of reasons. For example, it may help the physician adjust the dosages of antihypertensive medication. Many studies have shown that blood pressure spikes in the morning, right around the time that 24-hour antihypertensives are at their weakest. Thus, heart attacks and strokes may be more likely to occur in the morning. An ABPM could help identify this situation, and the physician could adjust the patient’s medication accordingly. Studies have also shown that ABPM may be a better predictor for certain cardiac events than other methods of measuring blood pressure.
About the ABPM monitor
The special ABPM blood pressure monitor is automatic, lightweight (about 1 pound or less) and quiet. It consists of an arm cuff, a tiny computer and a small compressor to inflate the arm cuff.
The compressor and computer are generally worn on a belt around the waist with a tube leading up to a cuff placed on the upper arm. The monitor is programmed to automatically inflate the cuff at specific intervals during the ABPM period, usually every 15 to 30 minutes. In cases of recurring fainting, measurements may be taken as frequently as every seven to eight minutes.
The frequency of measurements may be programmed differently during the night to minimize the disturbance to a patient’s sleep. The schedule also adjusts for the fact that changes in blood pressure are less dramatic when the patient is at rest.
There are two basic techniques that can be used with the monitor cuff to read a patient’s blood pressure. Some monitors use one or both of the following techniques:
Auscultation. A microphone in the arm cuff detects the starting and stopping of particular sounds called Korotkoff sounds. Korotkoff sounds are the noises that blood makes as it passes through an artery. These are the same sounds that healthcare professionals listen for with a stethoscope when taking blood pressure readings during an office visit. The pressure at which these sounds start corresponds to systolic blood pressure. The pressure at which the sounds stop is the diastolic blood pressure.
Oscillation. As the blood pressure cuff inflates, vibrations or fluctuations occur in the cuff’s pressure. These vibrations are called oscillations. The point at which the oscillations first increase corresponds to systolic blood pressure and the pressure at which they stop decreasing is the diastolic blood pressure.
Research has not yet determined which of these two methods produces the most reliable results. The microcomputer stores the information during the ABPM, and it is later retrieved and analyzed with special computer software.
No matter which type of monitor is used, it must first be calibrated to each individual patient. A patient may be asked to hold various positions (e.g. lying, reclining, sitting, standing) for a few moments so this can be accomplished.
Although the monitor is designed to be as unobtrusive as possible, patients will be instructed to keep their arm still and away from loud noises or vibrations during the measurements. For instance, if the patient is walking on a treadmill when the cuff begins to inflate, he or she should stop and turn off the treadmill so that its noise and vibrations do not affect the blood pressure measurement. Once the reading is complete, the patient may resume exercising.
Patients are usually asked to keep a diary or log during the ABPM period. In the log, they should record their physical and mental activities, locations, emotions, medications, eating schedule and more. The log will help physicians interpret the results of the ABPM. For instance, some conditions lead to a dramatic decrease in blood pressure after a meal (called postpradial hypotension).
Uses for ABPM
There are several signs and symptoms that may lead a physician to recommend ambulatory blood pressure monitoring (ABPM) for a patient. These include:
Borderline high blood pressure (hypertension) combined with clinical signs of organ damage, such as abnormal kidney function or left ventricular hypertrophy. In these patients, ABPM may reveal elevations in blood pressure during periods of activity or mental stress (at home or in the workplace). Because these triggers generally occur outside the physician’s office, the extent of the patient’s high blood pressure may not have been recognized when the blood pressure reading was obtained during a medical visit. ABPM can therefore help the physician to diagnose high blood pressure in these patients.
Resistant high blood pressure with no signs of organ damage. This is a condition in which a combination of blood pressure medications (antihypertensives) has failed to control a patient’s high blood pressure (as measured in a physician’s office). This condition could be related to the timing of the test in relation to when the medication was taken. Stress-elevated blood pressure readings also can be due to the stress of an office visit or other factors. ABPM is generally recommended if a patient’s blood pressure when measured at home indicates that the condition is responding to the medication, but these results are not seen at a physician’s office. The ABPM profile can help a physician determine if a patient’s high blood pressure is truly resistant to medication or simply appears that way because of other causes.
Episodic high blood pressure. Blood pressure readings that alternate between high and normal may be a sign of pheochromocytoma (a tumor that usually occurs in the adrenal gland) or an anxiety disorder. The results of ABPM can be matched with a patient’s log to help diagnose these and other conditions.
Symptoms of low blood pressure (hypotension) occurring in patients taking medications to lower blood pressure. Temporary episodes of low blood pressure in these patients may indicate the need to change their medications or dosages. Without APBM, this condition is difficult to diagnosis and correct.
Autonomic dysfunction. A person’s autonomic nervous system controls involuntary body systems, such as heart rate, breathing and sweating. Patients with disorders of the autonomic nervous system will have a different ABPM profile than healthy patients. Signs of autonomic dysfunction that may be determined by ABPM include:
Low blood pressure during waking hours
High blood pressure during sleep
Episodes of low blood pressure during the day, especially when rising from a seated or lying position (orthostatic hypotension)
Abrupt lowering of blood pressure after meals
Little or no variations of heart rate occurring along with the episodes of lower blood pressure
Carotid sinus syncope and pacemaker syndrome are two conditions more easily diagnosed with a combination of ABPM and a Holter monitor or other type of continuous electrocardiogram (EKG). Carotid sinus syncope is a condition in which fainting occurs due to overactivity of the carotid sinus (a section of the carotid artery in the neck). It may be caused by pressure on the carotid artery. Pacemaker syndrome can involve dizziness, fatigue and possibly fainting. It occurs when an artificial pacemaker is no longer synchronized with the patient's own heart rhythm. As a result, the heart attempts to pump blood through a closed valve.
White coat hypertension or office hypertension. This is a condition in which a patient’s blood pressure is elevated during physician office visits, but not in other situations. It may persist for months, even years, despite the patient’s becoming more familiar with the particular office and healthcare professionals. Curiously, other locations of recurrent stress (such as the workplace) do not seem to produce elevated blood pressure levels in patients with this condition.
Angina pain, shortness of breath or pulmonary congestion that occurs primarily overnight. An increase in blood pressure occurring immediately before these episodes may indicate a higher risk of a cardiovascular event. EKG readings are often taken in conjunction with ABPM when assessing these patients.
In addition, research has indicated that ABPM may be a better predictor of first heart attacks in patients with high blood pressure than more traditional methods of obtaining blood pressure measurements.
Limitations and risks of ABPM
Ambulatory blood pressure monitoring (ABPM) is generally not accurate in patients with certain forms of abnormal heart rhythms (arrhythmias), such as atrial fibrillation, frequent extrasystoles or frequent episodes of supraventricular tachycardiasupraventricular tachycardia. Readings may be less accurate for elderly patients or those with very high blood pressure. These problems with accuracy are usually detected when the monitor is being placed on the patient because calibration may be difficult or impossible.
It is important to note that ABPM readings have not been as carefully researched and documented as the use of clinic blood pressure readings. For instance, medications that have been proven to reduce blood pressure and death rates from hypertension-related illnesses are usually officially evaluated with the use of clinic blood pressure readings. Though ABPM is becoming increasingly popular as a method of evaluating medications and other treatment options during clinical trials, the majority of information available applies to clinic blood pressure readings.
There are very few safety issues associated with ABPM, and complications are rare. Bruising or swelling of the arm around the cuff area may occur in some patients, especially those with impaired function of blood platelets (substances that help the blood to clot in response to injury). Skin inflammation or rash may also develop. In very rare cases, the repeated pressure on the arm can cause palsy (or paralysis) of the ulnar nerve, which affects movement and sensation in the wrist and hand.
Normal ABPM results
There are no set numbers that indicate a normal result with ambulatory blood pressure monitoring (ABPM). Analysis of an ambulatory blood pressure profile is complex and relies on the interpretation of not only the raw numbers provided by the monitor, but also the circumstances surrounding the patient at the time of the test (as recorded in the patient log).
In general, a person’s blood pressure peaks during the daytime hours and falls to its lowest point overnight. In the early morning, blood pressure usually rises from the patient’s overnight low to the daytime level very quickly. This is one reason that this time period is considered riskier for cardiovascular events, such as a heart attack or stroke.
In addition to this daily cycle, healthy individuals tend to have:
Higher blood pressure at work than at home
Lowest blood pressure during sleep
Higher systolic blood pressure during exercise
Any deviation from this general pattern can assist physicians in interpreting the ABPM results and making a diagnosis. For instance, a patient whose blood pressure readings do not dip at night might have a disorder of the autonomic nervous system, the body system that controls “automatic” functions such as heartbeat and breathing.
Questions for your doctor about ABPM
Preparing questions in advance can help patients have more meaningful discussions with their physicians regarding their conditions. Patients may wish to ask their doctor or healthcare professional the following questions about APBM:
How does an ABPM work?
Why should I use ABPM?
What can it tell you about my blood pressure problems?
Where will I get the device?
How long will I have to wear it?
What should be recorded in my daily log?
Are there any restrictions when I am using ABPM?
How will I know if the device is working properly?
Who will evaluate and explain the results of my readings?
