14.5: Equipments Used To Measure Blood Pressure

Direct Method

This invasive approach involves cannulating a peripheral artery. During each cardiac contraction, pressure generates mechanical motion within the catheter, transmitted through rigid, fluid-filled tubing to a transducer. This transducer converts mechanical motion into electrical signals displayed as waveforms on a monitor. An automatic flushing system prevents blood backflow. Due to the potential risk of unexpected arterial blood loss, this method is primarily used in intensive care settings. Regularly calibrating and maintaining the transducer to ensure accurate readings is crucial.

Indirect Methods:

The indirect measurement methods are non-invasive and use devices such as sphygmomanometers, Doppler ultrasound devices, and a stethoscope.

Mercury Sphygmomanometer

This device consists of an inflatable cuff, a mercury manometer, a bulb, and a valve. The cuff is inflated to a pressure higher than the anticipated systolic pressure to measure blood pressure. Pressure is then slowly released while listening to Korotkoff sounds. Systolic pressure is identified at the first appearance of these sounds, and diastolic pressure is noted when the sounds disappear.

Manual Aneroid Sphygmomanometer

It comprises a cuff, bulb, valve, and manometer. This device requires correct cuff sizing for accurate readings. The cuff should be twenty percent wider than the upper arm diameter or forty percent of the arm’s circumference and should cover two-thirds of the arm’s length. It includes two tubes: one connected to the manometer and the other to the inflation bulb. The manometer, encased in glass, displays pressure in millimeters of mercury and should be calibrated to zero before use. Aneroid sphygmomanometers require biannual calibration checks and regular maintenance of the release valve.

Automated Blood Pressure Monitors

These monitors use oscillometric or auscultatory methods to measure blood pressure automatically without manual intervention. They detect arterial wall pressure, converting it into digital readouts. These devices facilitate regular blood pressure monitoring across various healthcare settings.

Doppler Ultrasound Device

This device emits high-frequency sound waves that reflect off circulating blood cells, producing an audible signal. It is particularly useful in conditions where pulse sounds are compromised, such as in patients with heart failure or coronary artery disease. The Doppler enhances and audibly represents these signals, aiding in accurate measurement under challenging conditions.

There are two methods for determining blood pressure.

The direct method is invasive and requires peripheral artery cannulation. 

The indirect method is non-invasive and requires a specific device such as a mercury sphygmomanometer, an aneroid sphygmomanometer, an electronic BP monitor, or a Doppler ultrasound device.

The mercury sphygmomanometer has a manually inflatable cuff attached to the mercury-infused column. Many nations have banned this device because mercury is a toxic substance.

A manual aneroid sphygmomanometer has a bulb, valve, manometer, cloth, or disposable vinyl compression cuff and has an inflatable bladder.

Two tubes are attached to the bladder within the cuff. One connects to a manometer, and the other is to a bladder inflation bulb.

The manometer has a glass-enclosed circular gauge containing a needle. It indicates the air pressure applied to the cuff in millimeters of mercury or mmHg.

Electronic or automated BP monitors sense blood flow pressure within the artery, record the pressure readings, and display them in digital numbers.

The Doppler ultrasound device contains an ultrasound probe and is used in conjunction with a BP cuff. It amplifies faint pulse sounds to measure blood pressure.