Friday, 29 December 2017

Non Invasive Blood Pressure Measurement

Non Invasive Blood Pressure Measurement

Invasive measurement refers to the measurement of BP by placing an intravascular cannula needle in the artery. As the name implies, in indirect measurements, the BP is measured indirectly and here we don’t require any direct contact with the artery. Hence it is called non-invasive measurement. The indirect or non-invasive measurements are simpler and quicker than invasive measurements.

1. Using Sphygmomanometer:

Normally physician measure blood pressure by a device called Sphygmomanometer. This comprises an inflatable cuff to restrict the blood flow and is placed around the upper arm attached to a mercury manometer. The mercury manometer measures the height of the mercury column through we can measure the blood pressure. The word sphygmomanometer comes from Greek works ‘sphygmos’ meaning pulse and ‘manometer’ meaning pressure meter.

Now digital sphygmomanometers are also used for the measurement. Here the process of making the measurement is done electronically and the display shows the result. The procedure for measuring the BP using sphygmomanometer is described below.

1. Firstly the cuff is wrapped around the patient’s upper arm at roughly same vertical height as heart. The cuff is placed over an artery. The physician then places his stethoscope over an artery downstream to the cuff.

2. Then the cuff is inflated so that the pressure inside the inflated bladder increases to a point greater than the expected systolic blood pressure. Since the cuff pressure is greater than the arterial pressure, an occlusion (interruption) to the blood flow occurs. So the blood flow in the vessels is shut off.

3. Then the physician slowly reduces the pressure in the cuff which causes the systolic pressure to increase. When the systolic pressure first exceeds the cuff pressure he can hear some crashing sounds in the stethoscope. These sounds are caused by the first jet of blood pushing through the occlusion when the occlusion is reduced. These sounds are called ‘Korotkoff’ sounds. By using the mercury manometer the physician can note the pressure at the onset of these sounds which will be the systolic pressure. As the physician still reduces the pressure on the cuff these sound disappear. The pressure corresponding to the disappearance of these sounds will be the diastolic pressure. In between systolic and diastolic pressures, we can hear some murmurs. Traditionally systolic pressure is the pressure at which the first Korotkoff sound is heard and diastolic pressure is the pressure at which Korotkoff sound is not audible.

Double Diastolic Pressure:

As the term means, in double diastolic pressure measurements, two diastolic pressures are take. First diastolic pressure is the pressure at which the Korotkoff sounds are very less audible. The second diastolic pressure is the pressure at which the Korotkoff sounds are not at all audible. Double diastolic pressure is indicated as (Systolic pressure/ 1st Diastolic pressure/2nd Diastolic pressure)mm Hg.
example for double diastolic pressure 120/80/77 mm Hg.

2. Ultrasonic blood pressure measurement:

Ultrasonic techniques can be used to measure arterial blood pressure indirectly with the same method as used for flow detection. Here piezoelectric crystals are placed between the patients arm and a blood pressure cuff.

The ultrasonic BP measurements are done with the help of Doppler Effect. The detailed working is explained below. The ultrasonic generator generates pulses and these are made to fall on the brachial artery. Due to the effect of pulses, the blood flow varies. This causes Doppler shift. The Doppler shift is measured by the ultrasonic circuits.

To measure Blood pressure, first the brachial artery is occluded and since there is no relative velocity between the transmitter and the receiver this time, the Doppler shift is zero. But as the Blood Pressure rises so that the arterial pressure is able to overcome the occlusion, a frequency shift is produced (200 – 500 Hz range). This is proportional to systolic pressure. Then as the arterial pressure reaches the diastolic pressure, the frequency shift due to Doppler effect is around 25 – 100 Hz. So by comparing the frequency shift caused due to each heart beat the Blood pressure can be measured by comparing the frequency shifts obtained  each time with the standard reference values.

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