Ambulatory Blood Pressure Monitoring

Technique. Ambulatory blood pressure monitoring (ABPM) is performed using small fully automatic battery-operated monitors that weigh less than one pound and can be worn on a belt while subjects go about their normal daily activities. The monitor typically contains a pump that inflates the blood pressure cuff at preset intervals, a pressure detection system (usually operating by the oscillometric method), and a memory chip for storing the readings.

The cuff is usually placed on the nondominant arm above the elbow and is connected to the monitor by a thin tube. Subjects are asked to keep their arm still and by their side while a reading is taken. The monitors do not work reliably during vigorous activity (e.g., exercise) or in environments where there is a lot of noise or vibration.

Subjects may be asked to keep a diary describing their activity, mood, and location at the time of each reading. Blood pressure during sleep can also be recorded. At the end of 24 h the subject returns and the readings are downloaded into a computer for analysis. Several types of monitor are available, but not all have been validated for accuracy according to standard protocols.

Advantages of Ambulatory Blood Pressure Monitoring. ABPM has revolutionized the study of the relationships between stress and blood pressure. Previously, reliance was made on blood pressure measured either in an office or in a laboratory setting. In the former case, a very limited number of readings is usually made, and the blood pressure that is recorded may represent the white coat effect rather than the effects of everyday stress. And while reactivity testing in the laboratory permits multiple measurements to be made during exposure to standardized stressors, the situation is artificial, and generalizability to blood pressure measured in real life is very poor.

ABPM can give three different types of information relating to blood pressure: the average level, short-term changes, and the diurnal rhythm. Normally the blood pressure falls by about 10% during the night, but in about 25% of people the changes are smaller than this - the so-called nondipping pattern.

There is now substantial evidence that the average level of blood pressure as measured by ABPM gives a better predictor of cardiovascular risk than conventional clinic measurements. In addition, nondippers have been reported to be at higher risk than dippers. The pathological significance of short-term blood pressure variability is unclear, although it is hypothesized that increased variability may be an independent risk factor.

White Coat Hypertension. One of the important phenomena identified by ABPM is white coat hypertension, which occurs in about 20% of patients with mild hypertension. Such patients usually have less target organ damage than patients with the same level of clinic pressure but whose pressure remains high outside the clinic, and they also appear to be at lower risk of cardiovascular morbidity. The origins of white coat hypertension are unexplained, but it is usually attributed to the stress associated with an encounter with a physician. It has been suggested that the phenomenon may be a conditioned response.

The Influence of Personality and Mood. ABPM can be used to measure blood pressure during normal daily activities, including work. By having subjects keep a diary of their activities and emotions it is possible to correlate behavioral factors and blood pressure. Thus, while anger is the mood that is associated with the greatest increase of blood pressure, blood pressure also increases when subjects report that they are happy as compared to having a neutral mood.

There are also reports of associations between negative moods and ambulatory blood pressure. The analysis of such interactions requires that there is statistical control for other influences such as posture, location, and activity. A number of reports have found that subjects who score high on hostility report more negative moods and have higher blood pressures in association with them than subjects who are less hostile. Hostile subjects also have higher nocturnal blood pressure and show a more rapid increase on waking.

Effects of Acute Stress. Blood pressure is typically highest during the hours of work, which may be the result of both physical and mental activity being greater than at home. In men, blood pressure usually falls when they get home in the evening, but in women it may not do so, particularly if they have small children. The effects of commonly occurring activities on blood pressure are shown in Table 1.

Table 1. Average changes of blood pressure associated with 15 commonly occurring activities

A study of men working in machine shops found a significant correlation between the frequency of somatic complaints such as palpitations, fatigue, and dizziness and ambulatory systolic pressure. Paramedics show 10 mmHg higher blood pressures when they are in an ambulance and 7 mmHg elevations at the scene of an accident than during nonworkday activities.