Blood pressure is considered a good indicator of the status of the cardio-vascular system. Blood pressure measurement can save many persons from an untimely death by providing warnings of very high blood pressure (hypertension) in time to provide treatment. There are two different methods available today for the measurement of blood pressure namely direct method and indirect method.
In routine clinical tests, blood pressure is usually measured by means of an indirect method using a sphygmomanometer. On the other hand, direct blood pressure measurement do provide a continuous readout or recording of the blood pressure.
I) Indirect Method of Blood Pressure Measurement or Non Invasive Measurement
Usually indirect method uses sphygmomanometer setup for measurements. This method is easy to use and can be automated. But it has a disadvantage that it doesn’t provide a continuous readout or recording of the pressure variations and only systolic and diastolic arterial pressure readings can be obtained with no indication of the details of the pressure waveform. Indirect method fails when the blood pressure is very low (when the patient is in shock).
a) Auscultatory Method
Here stethoscope is used along with sphygmomanometer for measurement. Sphygmomanometer consists of an inflatable pressure cuffs. The cuff consists of a rubber bladder inside an elastic fabric covering that can be wrapped around the upper arm and fastened with either hooks or a Velcro fastener. The cuff is normally inflated (putting pressure) manually with a rubber bulb (green ball) and deflated (removing pressure) through a needle valve.
WORKING
To obtain a blood pressure measurement with a sphygmomanometer and a stethoscope, the pressure cuff (rubber strap) on the upper arm is first inflated to a pressure well above systolic pressure. At this point no sounds can be hearers through the stethoscope, which is placed over the brachial artery, because it have been collapsed by the pressure of the cuff that crushes around the arm. The pressure on the cuff is then reduced gradually. As soon as the cuff pressure falls below systolic pressure, small amount of blood start to flow passing the cuff through the artery and korotkoff sounds starts to hear through the stethoscope. The pressure of the cuff that is indicated on the manometer when the first korotkoff sound is heard is recorded as the systolic blood pressure. As the pressure in the cuff continues to drop, the korotkoff sounds continue until the cuff pressure is no longer sufficient to suppress he vessel (artery) during any part of the cycle. Below this pressure, the korotkoff sounds disappear making the value of diastole pressure. This familiar method of locating systolic and Diastolic pressure by listening to the korotkoff sound is called ausculatory method.
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| Ausculatory Method |
Palpatory Method
Palpatory Method is similar to ausculatory method, except that the physician identifies the flow of blood in the artery by feeling the pulse of the patient from the cuff instead of listening through stethoscope.
b) Oscillometric Method
It is annon invasive or indirect blood pressure technique, measures the amplitude of oscillations that appear in the cuff pressure signal which are created by expansion of the arterial wall each time blood is forced through the artery. The compression cuffs entrained air volume are used to identify blood pressure values. The cuff pressure signal increases within the systolic pressure region, reaching a maximum when the cuff pressure is equal to mean arterial pressure. As the cuff pressure drop below this point, the signal strength decreases proportionally to the cuffs air pressure losing rate.
There is no clear transition in the cuff pressure oscillations to identify diastolic pressure while blood is forced through the artery. But we uses specific proprietary algorithms to estimate and record diastolic pressure also. When the cuff pressure is raised quickly to pressure higher than the systolic pressures, it is observed that the radial pulse disappears, because that much cuff pressure cause the underlying artery to be completely occluded (closed). Pressure oscillations occur in the cuff pressure due to the artery which is pulsating just under the upper edge of cuff with slow cuff pressure reductions, and when cuff pressure is just below systolic pressure, blood quickly flow through artery and the cuff pressure oscillations becomes larger.
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| Oscillometric Method |
c) Ultrasonic non invasive measurement
This is an indirect method for blood pressure measurement. It employs a sensor which detects the motion of the blood vessel walls in various states of occlusion (blocking or partial blocking).
Construction and working
There will be 2 small transmitting and receiving crystals(8 MHz) on the arm. The transmitted signal is focused on the vessel wall and the blood. the reflected signal which is shifted in frequency is detected by the receiving crystal and then decoded. The difference in frequency ranges from 40 to 500 Hz, which is proportional to the velocity of wall motion and the blood velocity. As the cuff pressure is increased above diastolic pressure but below systolic pressure, the blood vessel opens and closes with each heart beat. This opening and closing of vessel is detected by the ultrasonic system. When the applied pressure is further increased, the time between the opening and closing decreases until they coincide. The reading at this point is the systolic pressure. When the pressure in the cuff is reduced, the time between opening and closing increases until the closing signal from one pulse coincide with the opening signal from the next pulse. The reading at this point is diastolic pressure.
II) Direct Blood Pressure Measurement
Direct method provides a continuous readout or recording of the blood pressure waveform, and is considerably more accurate than the indirect method. But in Direct measurements, it is required to puncture (make hole) the blood vessel in order to introduce the sensor. This limits their use to those cases in which the condition of the patient.
Direct measurement of blood pressure usually obtained by 3 methods:
a) Percutaneous insertion
b) Catheterization (vessel cut down)
c) Implantation of a transducer in a vessel or in the heart.
There is another method such as clamping transducer on the intact artery has also been used, but they are not common.
a) Percutaneous insertion
For this method, a local anesthetic is injected near the site of insertion, then the vessel is occluded and a narrow hollow needle is inserted at a slight angle towards the vessel. When he needle is in place, a catheter is fed through the hollow needle with some sort of a guide. When the catheter is secured in place inside the vessel, the needle and guide are withdrawn. Catheter is a long tube that introduced into the heart or major vessels.
b) Catheterization
Apart from obtaining blood pressures in the heart chambers and vessels, this technique is also used to obtain blood samples from the heart for oxygen-content analysis and to detect the location of the abnormal blood flow pathways. Measurement of blood pressure with a catheter can be achieved in two ways:
Introduce a sterile saline solution into the catheter so that the fluid pressure is transmitted to a transducer outside the body.
Pressure measurement is obtained at the source. Here the transducer is introduced into the catheter and pushed to the point at which the pressure is to be measured or he transducer is mounted at the tip of the catheter.
c) Implantation Technique
This technique involves major surgery and thus is normally employed only in research experiments. They have the advantage of keeping the transducer fixed in place in the appropriate vessel for long period of time.