eMedinewS Editorial

Health Care 13 Comments

Measuring blood pressure (Part 1)

Measuring BP is an expert’s job. It’s not the job of quacks, paramedics or the patients to learn it as they invariably will default. Most often doctors default on this very essential part of examination. Accurate measurement of blood pressure (BP) is important not only to diagnose but treat high BP as well.

Even 2-5 mm variation can make a difference in the treatment. A 5 mm reduction in BP can reduce chances of heart attack by 21%.

Today, I discuss the significant aspects of this very basic but crucial technique of measuring blood pressure, which may be a revision for most of us.

  1. Correct measurement and interpretation of BP is essential.
  2. Proper machine calibration, training of personnel, positioning of patient, and selection of cuff size are essential.
  3. Most doctors do not follow correct steps leading to potential errors in diagnosis and management.
  4. Time of measurement: To diagnose high BP, multiple readings should be taken at various times throughout the waking hours. To monitor treatment, BP should be measured before anti BP drugs are taken. If BP is measured soon after a drug has been taken, it may be normal or even below normal; it will gradually increase to potentially hypertensive levels until the next dose is taken.
  5. Factors that influence BP, such as food intake, strenuous exercise (can lower the BP), smoking and caffeine, should be avoided in the 60 minutes prior to evaluation.
  6. Smoking transiently raises the BP. One may underestimate BP in a heavy smoker who has not smoked for more than 30 minutes before the BP is measured.
  7. Caffeine intake can raise the BP acutely in non habitual coffee drinkers.
  8. Measuring BP in a cool room (12ºC or 54ºF) or while the patient is talking can raise the BP value by 8 to 15 mm Hg.
  9. Type of BP instrument: Mercury sphygmomanometers are the most accurate. Aneroid instruments should be checked against a mercury device since the air gauge may be in error.
  10. Automated oscillometric BP measuring devices are now available. The readings are typically lower than BP obtained with the auscultatory method. The oscillometric method has a somewhat greater inherent error and requires a proper AC atmosphere. Proper timing, patient positioning, cuff size and placement are still necessary, as is evaluation of machine accuracy at periodic intervals.
  11. Cuff size: Use of a proper-sized cuff is essential. If too small a cuff is used, the pressure generated by inflating the cuff may not be fully transmitted to the brachial artery; in this setting, the pressure in the cuff may be considerably higher than the intra arterial pressure, which can lead to overestimation of the upper systolic pressure by as much as 10 to 50 mm Hg in obese patients.
  12. The length of the BP cuff bladder should be 80 %, and the width at least 40 % (46%) of the circumference of the upper arm. This is often difficult to achieve in obese patients.
  13. Appropriate cuff size for a designated arm circumference
    • Arm circumference 22 to 26 cm: ’small adult’ cuff, 12 x 22 cm
    • Arm circumference 27 to 34 cm: ‘adult’ cuff, 16 x 30 cm
    • Arm circumference 35 to 44 cm: ‘large adult’ cuff, 16 x 36 cm
    • Arm circumference 45 to 52 cm: ‘adult thigh’ cuff, 16 x 42 cm
  14. A lack of manufacturing standards also results in designated cuffs differing by several cm in both width and length depending on the manufacturer.
  15. Pseudohypertension: This is a condition of falsely elevated BP and is found in patients with stiff vessels due to marked arterial calcification. Here, compression of the brachial artery requires a cuff pressure greater than systolic. Pseudohypertension is characterized by systolic upper and diastolic lower pressures estimated from the sphygmomanometer that are ≥10 mm Hg above the directly measured intra arterial or oscillometric pressure.
  16. Patient position: The BP should be taken in the sitting position with the back supported. Supine values are different, with the systolic pressure higher by 2 to 3 mmHg and the diastolic pressure lower by a similar degree.
  17. In the elderly, supine and standing measurements should always be taken to detect postural hypotension (fall of BP on standing).
  18. The arm should be supported at the level of the heart.
  19. Allowing the arm to hang down when the patient is sitting or standing will result in the brachial artery being 15 cm below the heart. As a result, the measured BP will be elevated by 10 to 12 mm Hg due to the added hydrostatic pressure induced by gravity. The opposite is true if the arm is above the level of the heart.
  20. The mercury manometer should be visible but does not have to be at the level of the heart.
  21. The patient should sit quietly for five minutes before the BP is measured.

Dr KK Aggarwal
Editor in Chief