How to measure BP: The correct way
BP measured at home and clinics of doctors vary with the time of day or night, physical activity and methods of measuring. To reach a diagnosis of hypertension, various precautions are required. The following should be kept in mind while measuring BP.
Methods used for measuring BP.
- Mercury sphygmomanometers: Traditional and accurate. Requires correct training. Now not used due to a ban on the use of mercury due to environmental pollution.
- Aneroid sphygmomanometers: The cuff may be inflated manually. With a stethoscope, sounds are heard over the brachial artery.
- Automated oscillometric BP measurement: The most popular method nowadays. It has a battery operated system for inflating the cuff. The cuff deflates on its own and BP is displayed digitally.
- Ambulatory BP monitoring(ABPM): Measures BP every 15 mins in the day and every 30 to 60 mins at night. The readings can be recorded and later displayed digitally via a computer.
ABPM is the best and close to the ideal way of measuring BP. Not used often as the instrument is not widely available, is costly and not many people including health personnel are aware of it. An average ABPM recording is usually lower than other methods of measurement and hypertension is diagnosed when average readings are > 135/85 mm Hg.
Cuff size: The length of bladder cuff should be 80% of arm circumference and its width 50%. Smaller size cuffs overestimate BP and bigger ones underestimate.
Cuff position: The centre of the cuff (may be marked with an arrow) should be over the middle of arm artery(brachial). It should be 2-3 cms above the elbow crease in auscultatory methods.
The position of the person whose BP is being measured:
It should in most cases be sitting without crossing the legs and back should be supported. The arm should be at the level of the heart, relaxed and supported on the table etc. It should not be hanging on the side of the chest or raised higher.
The timing of BP measurement: BP should be measured thrice over a week in clinics before a diagnosis of hypertension unless it is very high and targets organ damage or other evidence is clear. It should be measured at different times of the day, morning and evening etc.
It should not be measured within ½ hour of coffee, smoking, exercise or food intake.
Both the health personnel and the person whose BP is being measured should be sitting quietly without talking or moving.
BP is measured repeatedly with a gap of about 2 mins till the readings are almost similar or difference is < 5 mm. The lower readings are usually closer to truer BP as compared with ABPM.
White Coat Hypertension: About 20 to 25% of the population may have higher recordings of BP when measured by doctors. The repeated measurement may record lower BP readings. Nurses BP recording may be lower. With time in some cases, this effect disappears.
In follow up cases
In clinics, it should be measured at the same time of day during subsequent visits. It is best measured just before the BP medicines are due so that BP is recorded when the medicine concentration in the body is lowest.