Measurement of Blood Pressure: Principle, Apparatus, Procedure, and Precautions

Principle

Blood pressure can be measured by two methods:

A. Direct Method 

The direct method of recording blood pressure (BP), is in which an artery is punctured with a cannula connected to a manometer. This method is used in animal studies where the transducer is connected to the cannula to record the change in blood pressure on the computer.

B. Indirect Methods 

The direct method is unsuitable for routine clinical procedures. As a result, indirect methods were introduced.

In indirect methods, we can measure blood pressure by

• Palpatory method
• Oscillatory method
• Auscultatory method

Note: for an ideal blood pressure measurement we have to do the first palpatory method followed by the auscultatory method to avoid the auscultatory gap and also to know the amount of pressure to inflate.

PRINCIPLE 

A single artery of sufficient length is chosen in the arm (brachial artery) or the thigh (femoral artery). To begin, the artery is compressed by inflating the cuff of a sphygmomanometer a rubber bag wrapped around the patient’s arm to stop blood flow through the occluded section of the artery. This is confirmed by palpating the radial pulse that disappears when the cuff is raised above arterial pressure. The pressure is then raised further and then gradually released, and the flow of blood at some point through the obstructed segment begins to escape, and the pulse starts reappearing. The nature of the artery is studied using different methods as recorded as blood pressure.

· Palpatory method: feeling the pulse. 

· Auscultatory method: involves listening to the sounds produced in the section of the artery just below the obstructed segment. The detection of the sound when the blood begins to flow in the obstructed artery is taken as systolic blood pressure. The level where the sound disappears is taken as diastolic blood pressure. 

· Oscillometric method: involves observing the oscillations of the mercury column. Detect systolic as well as diastolic blood pressure. 

APPARATUSES 

Stethoscope  

Sphygmomanometer

· Mercury sphygmomanometer: 

· Arenoid sphygmomanometer

· Automatic digital sphygmomanometer 

Mercury sphygmomanometer is replaced in many places with arenoid sphygmomanometer due to mercury toxicity whereas Mercury sphygmomanometer is more accurate than arenoid sphygmomanometer.

PROCEDURES FOR MEASUREMENT OF BLOOD PRESSURE:ds

The subject may be lying down (supine) or sitting but should be mentally and physically relaxed.

Lay the arm bare up to the shoulder and record the blood pressure first with the palpatory method, followed by the auscultatory method. The upper arm on which the BP cuff is to be tied must be at the level of the heart 

In obese subjects, the cuff may be applied on the forearm with the stethoscope placed over the radial artery for the auscultatory method.

A. Palpatory Method 

1. Make the subject sit or lie supine and allow 5 minutes for mental and physical relaxation. 

2. For the mercury sphygmomanometer: Open the lid of the apparatus until you hear the “click”. Release the lock on the mercury reservoir and check that the mercury is at the zero level. If it is above zero, subtract the difference from the final reading. If it is below zero, add the required amount of mercury to bring it to a zero level. 

3. Place the cuff around the upper arm, with the center of the bag lying over the brachial artery, keeping its lower edge about 3 cm above the elbow. Wrap the cloth covering around the arm to cover the rubber bag completely, and to prevent it from bulging out from under the wrapping on inflation. The cuff should neither be too tight nor very loose. 

4. Palpate the radial artery at the wrist and feel its pulsations with the tips of your fingers. Keeping your fingers on the pulse, hold the air bulb in the palm of your other hand and tighten the leak valve screw with your thumb and fingers. 

5. Inflate the cuff slowly until the pulsations disap­pear; note the reading then raise the pressure another 30–40 mm Hg. 

6. Deflate the pressure gradually falls in steps of 2–4 mm Hg per second. Note the reading when the pulse just reappears. The pressure at which the pulse is first felt is the systolic pressure. Deflate the bag quickly to bring the mercury to zero level. 

 The disadvantages of the palpatory method: 

•  This method measures only the systolic pressure; the diastolic pressure cannot be measured. 
•  This method lacks accuracy because the systolic pressure measured by it is lower than the actual by 4–6 mm Hg. 

B. Oscillatory Method 

The procedure is the same as that of the palpatory method but instead of palpating the artery the oscillations of the mercury column are noted as blood pressure. The pressure in the cuff is raised and the appearance (systolic pressure) and disappearance (diastolic pressure) of the oscillation are noted while lowering the mercury column. This pressure is the best measure in the arenoid manometer.

 The disadvantage of the oscillatory method: 

• Does not give accurate pressure
• Need adequate experience

 C. Auscultatory method ( best method)

1. Place the cuff over the upper arm as described above, and record the BP by the palpatory method. 

2. Locate the bifurcation of the brachial artery in the cubital space just medial to the tendon of the biceps. 

3. Place the chest piece of the stethoscope on this point and keep it in position with your fingers and thumb of the left hand (if you are right-handed). 

4. Inflate the cuff rapidly, by compressing and releasing the air pump alternately. Raise the pressure to 20 mm Hg above the systolic level as determined by the palpatory method. 

5. Lower the pressure at the rate of 2-4 mm Hg per second gradually until a clear, sharp, tapping sound is heard. Continue to lower the pressure and try to note a change in the character of the sounds. 

These sounds are called Korotkoff sounds ( derived from Russian Scientists) and show the following phases: 

Phase I: This phase starts with a clear, sharp tap when a jet of blood can cross the previously obstructed artery. As the pressure is lowered, the sounds continue as sharp and clear taps. This phase lasts for 10–12 mm Hg fall in pressure 

Phase II: sounds become murmurish in the next 10 mm Hg in pressure.

Phase III: sound changes and louder in the next 15 mm Hg fall in pressure

Phase IV: sound becomes muffled in character during the next 5 mm Hg fall.

Phase V: sound completely disappears.

6. Appearance of the sound is recorded as systolic blood pressure and the disappearance of the sound as diastolic blood pressure. In a person with severe hypertension and children, muffling rather than the disappearance of the sound is taken as diastolic pressure.

7. When measuring the pressure of both arms the higher pressure should be considered as the blood pressure of the subject.

Precautions for an auscultatory method of blood pressure measurement

1. The subject should be physically and mentally relaxed and free from tension and anxiety. He/ she should be assured and rested for 5 minutes. 
2. The arm, with the cuff wrapped around it, should be kept at the level of the heart to avoid the influence of gravity. 
3. The cuff should de-deflate at the rate of 2-4 mm Hg per second

1. The cuff should not be too tight or too loose. The cuff rubber part will be used to cover the brachial arterial part of the arm.
2. The cuff should not be left inflated with high pressures for any length of time, because the discomfort and reflex spasm of the artery and its branches will give falsely high readings. 
3. Do not apply pressure on the artery with the chest piece as this may produce partial obstruction of the artery and a fake low reading. 
4. The palpatory method must always be employed before the auscultatory method. 
5. In suspected and known cases of hypertension, the pressure should always be raised well above 200 mm Hg; or above the level estimated by a palpatory method. 
6. In obese subjects, a cuff that is wider than the standard should be used. Similarly, when measuring the pressures in the thigh, the cuff should be wider.

Automatic digital Blood Pressure instrument. The recorder works on the 2-cuff “oscillometric measuring” principle described above and automatically translates pulse wave oscillations into Systolic Pressure and Diastolic Pressure. The advantage of this method is that it can be easily used by a layperson. 

What is an auscultatory gap?

In severely hypertensive patients, after the appearance of the sounds, occasionally the sounds disappear at a point below 200 mm Hg for a range of 20-50 mm Hg and then reappear and finally disappear at the level of diastolic pressure. The period of the first disappearance of the sound is called the silent gap or auscultatory gap.

If we follow the palpatory method before the auscultatory method we would not miss this auscultatory gap because we will know the actual systolic pressure of the patient. For example, if the systolic blood pressure of the patient is 200 mm Hg. If we directly do the auscultatory method and roughly inflate the cuff to 180 mm Hg thinking that his systolic pressure would not be more than 180 mm Hg, then there is a chance of missing the actual systolic blood pressure of that patient.

Source:

1. Textbook of practical Physiology CL Ghai

2. Textbook of practical Physiology GK Pal