How To Measure Blood Pressure More Accurately
Hypertensive disorders of pregnancy are a leading cause of maternal death. Do midwives really know how to measure blood pressure (BP) accurately, is the question! Sensitive Midwifery Magazine published Prof Justus Hofmeyr’s guidelines, and reprint them here for all midwives to use.
Picking up high blood pressure allows one to intervene and avert dangerous complications such as severe pre-eclampsia, eclampsia and intracranial haemorrhage. However, falsely high blood pressure measurements may result in unnecessary interventions such as admission to hospital and labour induction.
Back to basics
When the heart muscle contracts, blood in the left ventricle is pumped through the aorta and the peripheral arteries. The highest pressure reached during the contraction (systole) is the systolic pressure. The lowest pressure reached during relaxation of the heart muscle (diastole) is the diastolic pressure.
The only way to measure blood pressure directly is with a catheter placed in an artery, which is sometimes done in the intensive care unit. All methods which use an inflatable cuff are indirect estimates of blood pressure and actually measure the pressure in the cuff required to obstruct the blood flow in the artery completely (‘systolic’) or partially (‘diastolic’ pressure). How close the measured cuff pressure is to actual arterial pressure depends on many factors, such as the size of the cuff, the size of the woman’s arm, how well the cuff is secured and how accurately the changes in blood flow in the artery are detected.
The misleading edge
Two main factors may give misleading BP measurement results:
- Physiological changes in actual blood pressure, for instance:
- If a woman is anxious, uncomfortable, cold, has a full bladder, or has recently been active, her blood pressure rises, and what is measured will not be a true reflection of her usual blood pressure.
- If a woman lies flat on her back in late pregnancy, her uterus may press on her inferior vena cava causing reduced blood return to her heart and a fall in blood pressure, or it may press on her aorta causing increased resistance to flow and an increase in blood pressure.
- Technical errors in the estimation of blood pressure, for instance:
- If the cuff is above the level of the woman’s heart, the pressure at the point measured will be lower than her central pressure. If the cuff is below the level of her heart, the pressure will be higher.
- If the cuff is too short (less than 80% of her arm circumference), it will take more pressure to obstruct blood flow, and the measurement will be falsely high. Too large a cuff will give a falsely low reading.
- If the cuff is not securely fastened or the bladder bulges out of the cuff, it will also take more pressure to obstruct the blood flow, with a falsely high measurement
- The point at which blood flow through the artery is interrupted may be detected inaccurately, either by the person auscultating or by the automated system in an automatic blood pressure monitor.
- The pressure sensor in the blood pressure apparatus may be faulty, giving an incorrect pressure measurement.
Improving the accuracy of blood pressure measurements depends on three main factors:
- Tools of the trade
The most reliable method of measuring pressure is the mercury column sphygmomanometer. Provided the mercury level is at zero when not in use, the mercury column will give an accurate pressure measurement without the need for calibration.All other mechanical or electronic methods of measuring pressure can lose accuracy, and need to be calibrated every 3 months.
A manual ‘aneroid’ sphygmomanometer may be checked by connecting the tubing with a ‘Y’ connector to a mercury sphygmomanometer and pumping to various pressure levels
The cuff bladder length should go at least 80% around her arm.
The standard cuff 23x11cm is suitable for a mid-arm circumference up to 29 cm.
A large cuff (bladder 16 x 33 cm) is suitable for a mid-arm circumference of 30 to 42cm.
An extra-large cuff (bladder 17.5 x 35 cm) is suitable for a mid-arm circumference above 42 cm.
The cloth cuff with a long tail which is wound a few times round the arm and tucked in, is the most reliable.
Velcro cuffs are subject to wear and tear and lose their grip with time
If the cuff is not secure, wind a non-stretchable bandage around it.
Check that the earpieces are not blocked, and angled slightly forward.
Select the bell to hear the low-pitched Korotkoff sounds.
- The human factor
The pregnant woman:
Check that she has rested for at least 5-10 minutes.
Her bladder should be empty.
Remove tight clothing from her arm.
Reassure her and explain the procedure to avoid anxiety.
Explain that the cuff will become tight and uncomfortable only for a minute or so.
Seat her comfortably on a straight back chair with her feet flat on the floor.
Support her forearm on a table at a comfortable height
If she is in bed, tilt her abdomen to the side opposite to the cuff arm, and keep her shoulders square so that the cuff is at the same level as her heart.
Applying the cuff
Apply the cuff at least 2-3cm above the elbow with the middle of the bladder centred over the brachial artery which is palpated on the medial side of the arm.
The cuff should be tight enough to allow only 1-2 fingers to be inserted between the cuff and arm.
Check that the cuff is at the same level as the woman’s heart.
- The procedure
Follow these steps:
Avoid keeping the cuff inflated too long.
Close the air release valve on the hand pump.
Palpate the radial artery pulse, pump up the cuff and note the pressure at which the radial pulse disappears (estimated systolic).
Position the stethoscope bell over the brachial artery, palpated in front of the elbow joint, with light pressure so as not to obstruct blood flow.
Continue pumping up the pressure to 30 mmHg above the estimated systolic.
Open the air release valve so that the pressure drops by about 2 mmHg per second.
Note the pressure at the first appearance of a tapping sound (1st Korotkoff sound – systolic pressure) and the last disappearance of the sound (5th Korotkoff sound – diastolic pressure).
If the pressure is halfway between pressure marks, record the higher level.
If you are not sure of the measurement, repeat after a 15 second rest.
In some women the sound appears and disappears twice, so be sure to listen all the way from 30mm above the palpated systolic pressure to 40 mmHg below the disappearance of sound.
In pregnant women, the sound may continue till zero. In that case, take the point at which the character of the sound changes from a sharp sound to a muffled sound (Korotkoff 4) as diastolic.