Determination of arterial blood pressure
Arterial blood pressure helps in diagnosing the defects of heart and circulatory system. It can be measured by two means- The direct and The indirect (clinical) methods.
Direct method
Experimentally the carotid artery can be connected to any of the following devices such as, the mercury manometer, membrane manometer or the optical manometer to record BP. Mercury manometer is a `U’ glass tube containing mercury in one limb and 10% sodium citrate in the opposite limb to balance the mercury.
The limb with sodium citrate is connected to carotid artery through a tube with a cannula at its end. The float over the mercury column will record the BP over the kymograph.
Indirect method
Clinically, BP is measured by sphygmomanometer (mercury manometer). It has an inflatable cuff connected to the manometer through a rubber tube. By another tube, the cuff is connected to an inflating bulb. A deflating valve is attached to the inflating bulb to deflate the air from the cuff.
The cuff is placed in the arm (brachial artery) in human, thigh region in dog (femoral artery), tail region in cattle (middle coccygeal artery) which may be inflated by the inflating bulb. When the pressure in the cuff exceeds the pressure of the artery, the blood vessel collapses and no blood flow through it. If the stethoscope is placed below the level of the cuff there won’t be hearing of any sound of blood flow. The pulse also completely disappears.
When the pressure in the cuff is released gradually by the deflating valve the pressure of the cuff slowly drops. When the cuff pressure is less than the pressure of the blood vessel, the blood begins to flow through the partially opened artery producing the sounds that are called Korotkoff sounds. The pulse and sound reappear.
The pressure indicated by the manometer when the first sound is heard represents the systolic pressure. As the cuff is further deflated, more blood returns to the artery the sound gradually reduces (muffles) and disappears, the normal pulse is re-established. The pressure at muffling or at disappearance of sound indicates diastolic pressure.
When the BP is measured using stethoscope, the method is called as auscultatory method. In palpatory method pulse is used to find systolic pressure only.
Capillary pressure can be measured by introducing a micropipette into the capillary. In human, the capillary pressure at arteriolar and venous end is 45 and 22 mm H2O, respectively.
Venous pressure is very low and can be estimated by inserting a needle into the vein. Saline is allowed to flow into the vein from a long vertical graduated tube.
The height of the vertical column is adjusted so that the flow ceases, the height of the top of the saline column above the heart is the venous pressure in mm H2O. It is very low in great veins and negative at atria. BP determination helps in diagnosing disorders of heart and circulatory system.
Ultrasound Method
In this method the Korotkoff sounds are amplified by using piezoelectric microphones mounted within or below the cuff. The electric signal obtained is amplified to increase the audibility.
Microphone Method
In Microphone method ultrasound is used to detect arterial wall movement as pressure is decreased in the blood pressure cuff.
Determination of capillary pressure
Capillary pressure can be measured by introducing a micropipette into the capillary. In human, the capillary pressure at arteriolar and venous end is 45 and 22 mm H2O, respectively.
Determination of venous pressure
Venous pressure is very low and can be estimated by inserting a needle into the vein. Saline is allowed to flow into the vein from a long vertical graduated tube.
The height of the vertical column is adjusted so that the flow ceases, the height of the top of the saline column above the heart is the venous pressure in mm H2O. It is very low in great veins and negative at atria. BP determination helps in diagnosing disorders of heart and circulatory system.