Essentials of Ambulatory Blood Pressure Monitoring (ABPM)

Hypertension is diagnosed by either clinic based measurement of blood pressure (Office readings) or by means of out of office blood pressure readings. Out-of-office measurements are highly recommended as an adjunct to office measurements by almost all hypertension associations. The blood pressure measurements could be measured either through Home based self-monitoring (HMBP) or Ambulatory Blood Pressure Monitoring (ABPM). Whenever, HMBP and /or ABPM is used, the various blood pressure targets given for clinic measurements are lowered by 5 mm Hg (eg Hypertension is diagnosed at SBP >135 mm Hg or DBP >85 mm Hg).


Essentials of Ambulatory Blood Pressure Monitoring (ABPM)
Hypertension is the commonest risk factor for the most common cause of deaths in Sri Lanka; coronary heart disease, stroke, heart and renal failure. Nearly one-fifth to one-third of adult Sri Lankans have blood pressure levels that that are above normal and the prevalence is comparable to those in the developed countries 1,2,3 .
Hypertension is diagnosed by either clinic based measurement of blood pressure (Office readings) or by means of out of office  In the absence of a significant night time dip, the patient is labelled a non-dipper.
Dipping patterns are further categorized by the extent of the nocturnal drop in blood pressure relative to the daytime pressure: nondipper, dipper, extreme dipper and reverse dipper. A reverse dipper is a patient whose nocturnal blood pressure is higher than the average day time blood pressure.

Range
Class < 0% Reverse Dipper 0% -10% Non-Dipper 10% -20% Dipper > 20% Extreme Dipper Figure 2. Classification of dipping in blood pressure using systolic blood pressure 8 The assessment of the phenomenon of dipping is useful in defining cardio vascular outcomes and mortality and ABPM could therefore be a better predictor of health outcomes and mortality than clinic measurements 9 . Dippers have significantly lower all-cause mortality than non-dippers or reverse dippers. Furthermore, left ventricular hypertrophy and cardiovascular mortality are high among non-dippers whilst nocturnal hypertension (reverse dipping) is associated with significant target organ damage. ABPM may additionally reveal an excessive morning surge of blood pressure which has been shown to be associated with increased risk of stroke in elderly hypertensives.

Ambulatory pulse pressure
Office pulse pressure is a major predictor of  10 . These data indicate that ambulatory pulse pressure is a more accurate marker than office pulse pressure of increased arterial stiffness or already diseased arteries.

Ambulatory combined with office readings
Combination of office blood pressure with ambulatory blood pressure may identify four different clinical categories of subjects 11 as shown in figure 3.

Pregnancy and ambulatory blood pressure
The key role of ambulatory blood pressure in pregnancy is to identify white coat hypertension; the incidence is around 30% of pregnant that ambulatory measurement may predict pre-eclampsia is not conclusive. However, ambulatory blood pressure correlates better with proteinuria than does conventional blood pressure measurements, and it is a better predictor of complications of hypertension. In addition, women diagnosed with hypertension by ambulatory monitoring have infants with lower birth weights and this association is not found when blood pressure is measured conventionally. Moreover, women with white coat hypertension tend to be more likely to have a caesarean section than women with normal blood pressure, suggesting that if ambulatory measurement was used rather than conventional measurement, some caesarean deliveries could have been avoided 13 .

Measurement of ambulatory blood pressure
Ambulatory blood pressure measurements were initially employed in early 1960s. Since then the equipment and the method have gone through different stages of evolution.
Principally the ambulatory blood pressure monitor measures blood pressure at regular intervals with subjects under taking their usual activities. Emphasis is made on mean day time blood pressure which is a better predictor of cardiovascular risk and target organ damage than clinic readings. And also an ABPM report gives more details and deductions than either a home or clinic reading 14 .
As ambulatory blood pressure is noninvasive, it can be performed in most of the subjects those who need it except for few exceptional circumstances include a noncooperative patient, severe office hypertension with blood pressure levels exceeding 220/120 mm Hg, an arm too big (above 48-50 cm) to wrap the cuff, severe peripheral vascular disease or thrombocytopenia.
As far as the number of readings concerned the following numbers are generally agreed on most of the guidelines. The recommendation is to perform at least 21 readings in the daytime and 7 at night. In more than 70% readings capture success rate should be present.
• Minimum number is 2 per hour.
• Usually readings taken every 20 minutes in the day time and every 30 minutes at night.
• Check the diary to define day or • The subject has to stop and stand still when a reading is being taken (if possible)