Ambulatory blood pressure monitoring (ABPM) is a technique for measuring blood pressure over a period of 24 hours while subjects go about their normal daily activity. The patient wears a small monitor the size of a Walkman radio (similar in size to a Holter monitor system, but heavier, due to the battery power pack, which is carried on a waist belt), which takes readings automatically every 15 to 30 minutes and stores them in its memory.
The main advantages of this technique are that it gives a more reliable and accurate measure of the blood pressure, for two reasons: first, a large number of readings are taken, and second they are taken outside the artificial environment of the doctor’s office. This gives a better prediction of the risk associated with high blood pressure than the doctor’s readings.
Moreover it is the only way of assessing the diurnal rhythm . It is widely accepted that sleep/wake periods should be based on patients’ actual sleep and wake times. The latter are often based on patients’ diary information. However blood pressure (and continuous ECG recordings) provide more objective data and provide data on the circadian cycle: higher pressures during the day and lower pressures at night.
Also to evaluate the prevalence of hypertension and borderline hypertension (above the limit 140/90 mmHg according to the British Hypertension Society), the 24-h pressure profile and the efficacy of antihypertensive therapy, ABPM remains the method of choice.
Other applications include:
- 1. nocturnal hypertension: the so-called non-dippers. In some patients the blood pressure does not show the usual 10% nocturnal fall during the night. There is evidence that these patients may be at higher risk of complications from their high blood pressure.
- 2. resistant hypertension: in some patients the blood pressure remains high, despite taking many different drugs (3 or more) to lower it. ABPM may show that the blood pressure is lowered outside of the doctor’s office.
- 3. pregnancy: white coat hypertension occurs in about 30% of pregnant women, leading to unnecessary medication.
- 4. even too low blood pressure: in some young people (especially petite women) low blood pressure may cause symptoms of weakness and fatigue and may even lead to fainting.
Ambulatory blood pressure monitoring: methods.
The development of non-invasive ABPM devices has added a new tool to clinical hypertension research. However development of clinically useful and widely accepted devices has been a technological challenge.
Devices based on different principles are available:
- Auscultation: with an arm cuff and based on the detection of the onset and disappearance of Korotkoff sounds by a microphone placed over an artery distal to a deflating compression cuff. Another device uses ultrasound to detect disappearance of blood flow.
- Cuff oscillometry: which relies on the detection of cuff pressure oscillations. Systolic and diastolic pressures correspond to cuff pressures at which oscillations first increase (systolic) and cease to decrease (diastolic). The end points are approximated by analysis with different algorithm according to the manufactures, creating a potential source of variability (Accutracker II, Spacelab).
- Volumetric oscillometry: usually of a finger (Figure 1), with detection of volume pulsations under a cuff. Results are shown in Figure 2 (Portapres).
The two former methods are using arm cuffs. As these relatively large and bulky devices use quit some power for inflating the cuff, they only allow punctual readings every 15 to 30 min. In case of arm cuffs it is important that the inflatable bladder of the sphygmomanometer to encircle at least 80% of the arm and preferably the entire arm, this may be a problem especially in obese patients. The latter method on the other hand, due to its smaller size and less power consumption, allows continuous readings of blood pressure. An example of a continuous ABPM tracing is shown in Figure 2 (top), with the corresponding systogram (bottom) derived from the pressure tracing.
Figure 2. Upper tracing: short recording of blood pressure obtained from finger cuff device, lower tracing: corresponding systogram from a 15 minutes ABPM recording.
Applications of ambulatory blood pressure monitoring.
The importance of ABPM in managing hypertension has been widely acknowledged. More specific reasons to use ABPM can be summarised as follows :
- To exclude «white coat» hypertension.
- Follow-up of borderline hypertension.
- Study the effect on blood pressure of long duration physical exercising (endurance and power training).
- ABPM may be a better predictor of cardiovascular events and mortality than clinical blood pressure readings.
- Control of patients with nocturnal high blood pressure.
- ABPM provides a 24 hour blood pressure profile, allowing assessment of clinic effects, drug effects, work influence, life style changes…
The rationale for the use of ABPM in clinical practice is evidence based. The technique is very specialised and should be reserved to experienced service providers. Physicians using ABPM should receive adequate training for using the devices: assessment and evaluation of the method and calibration testing. Patients should preferentially be monitored on a normal working day, rather than a rest day. It has been reported that ABPM data show a good reproducibility. Ideally an ABPM device should be combined with a continuous ECG monitoring (Holter monitor) system. A combination of the two has a lot to offer for clinical diagnosis purposes. Such a device is under development for space applications.