The Brachial Blood Pressure
The Brachial Blood Pressure
The brachial blood pressure if often performed by others but needs confirmation by the physician at least on the initial visit. A difference of 20 mm Hg in systolic pressure raises the suspicion of subclavian stenosis, an important cause of angina in patients who have had the left internal mammary artery used in coronary artery bypass.
Failure to measure or ask for the blood pressure by the student who is being tested for exam technique is a fatal error! Measuring the brachial blood pressure requires attention to technique as outlined below.
A small cuff in relation to the arm size will artificially increase blood pressure readings. A rough rule of thumb is that the width of the cuff should be about half the circumference of the arm. The standard cuff size for a normal adult will be accurate for arm circumferences of up to 27 cm, but larger cuffs will be required for obese patients.
Ideally the blood pressure should be measured in a seated patient after 5-10 minutes rest, where the cuff is at the level of the heart. If the position is below the heart the pressure will be relatively higher. The converse is also true. Normally the blood pressure remains the same or even increases from the lying to the sitting position, although mean arterial pressure remains the same In patients with postural hypotension the blood pressure will drop on standing by greater than 20mm Hg systolic, and in the elderly may lead to falls and syncope. Common causes include hypovolaemia (due to diuretics, dehydration or bleeding), autonomic dysfunction that may be idiopathic or associated with a systemic illness such as diabetes or Parkinson's disease, or drugs such as L-dopa or vasodilators . Postural hypotension due to hypovolaemia should be associated with a postural tachycardia. Where autonomic dysfunction is causal, this tachycardic response may not be seen.
Place the cuff around the forearm several centimetres above the anti-cubital fossa. Palpate the radial pulse on the same side and then inflate the cuff until the radial pulse can no longer be detected. Then slowly deflate the cuff and note at what point the pulse becomes detectable again. This is the systolic blood pressure as determined “by palpation” or “palp”. Checking the systolic pressure by this means has two advantages. The first is that it is more sensitive in patients with very low blood pressures. The second is that it avoids the problem of the auscultatory gap (see below).
The method for auscultation of the blood pressure was first described by a young surgeon in the Czar’s army, Korotkoff. The five sounds heard during auscultation to determine blood pressure are named in his honour. Fully deflate the cuff. Place the diaphragm of the stethoscope over the brachial pulse, which can be located just medial to the biceps tendon in the antecubital fossa. Inflate the cuff till the pressure is above that of systolic pressure determined by palpation. Slowly deflate the cuff. At about the systolic pressure the artery will allow a small jet of blood through that will be detected by auscultation as a thud. This is referred to as the Korotkoff I sound. The various sounds heard during the further deflation of the cuff are also given the designation Korotkoff and numbers II through V. They are described below. However they are of fairly limited usefulness except for Korotkoff V which is where the sound heard over the brachial artery disappears, and occurs at just below the arterial diastolic pressure. Korotkoff IV, where the sound becomes muffled, is a more accurate approximation of diastolic pressure in severe aortic regurgitation. However, Korotkoff V is far more reproducible and is used to indicate diastolic pressure.
The Korotkoff Sounds
(These actually correspond to blood pressures)
I Pressure at which a sound is first heard during the release of the blood pressure cuff
II Increase in sound intensity as further pressure is released, sound has a blowing quality
III A soft thud as sound decreases in intensity
IV Sound becomes muffled
V Sound disappears
Variation in blood pressure
Blood pressure varies with activity, anxiety, and pain throughout the day, so it is worthwhile repeating the blood pressure measurment at the end of the examination if it was high at the beginning. As a result borderline elevations of blood pressure are often confirmed at a later examination before considering antihypertensive therapy.
The blood pressure in patients with atrial fibrillation varies due to the variability in left ventricular filling and ejection. A simple method is to take the Korotcoff I as the pressure where the majority of beats can be heard, and the Korotcoff V as the pressure where the majority of beats have disappeared. Automatic blood pressure readings tend to fluctuate widely from reading to reading, and are probably best abandoned in favour of manual estimation of blood pressure.
This refers to the loss of Korotkoff sounds during deflation of the cuff from the true systolic reading but at pressures higher than that of the true diastolic pressure. The sounds then re-emerge as the cuff is slowly released, only to disappear as pressure falls below the diastolic. The auscultatory gap can lead to marked underestimation of the true systolic pressure. This can be avoided by checking the systolic pressure by palpation before proceeding to the auscultatory method.
This is the phenomenon of falsely elevated blood pressure due to calcified stiff arteries that resist the external pressure of the inflated cuff. Osler described a technique that allows one to test for this condition. First palpate the radial pulse. Then occlude the artery above this pulse by inflating the cuff of the sphygmomanometer at the antecubital fossa. The pulse will normally no longer be palpable. If the vessel wall is sufficiently stiff though, the artery can be rolled under the finger. A falsely elevated blood pressure reading may well be obtained from such patients.(Osler’s phenomenon)
Aortic dissection and subclavian arterial stenosis
Either of these two conditions can result in a difference in the blood pressure readings between the left and right arms.