Sphygmomanometers

Purpose

Automatic electronic sphygmomanometers noninvasively measure and display a patient’s arterial bloodpressure. These devices can help overcome some of the problems associated with manual sphygmomanometry, such as variations in user technique and hearing acuity, and environmental concerns about mercury. Automatic NIBP monitors can display blood pressure continuously or at preset intervals, saving time and allowing clinicians to perform other tasks. In addition, most automatic blood pressure units display pulse rate and sound an alarm if a patient’s blood pressure or pulse exceeds preset limits.Arterial blood pressure measurement is an essential indicator of physiologic condition. As one of themost frequently used diagnostic tests, it is critical to the ongoing management of patients undergoinganesthesia or drug and other therapies to determine the need for blood, a volume substitute (e.g., plasmaexpander), or a change in medication. In addition, periodic measurements are very important during operative procedures in which there can be rapid changesin blood pressure. Although invasive techniques for measuring blood pressure may provide greater accuracy and permit continuous measurement during cardiac and respiratory cycles, noninvasive techniques are most often used because of their low risk and simplicity, and they have proven sufficiently accurate for many clinical applications.

Principles of operation

When the left ventricle of the heart contracts, blood forced into the arteries creates a pressure increase, thepeak of which is called systolic pressure. The first number of a typical blood pressure reading representsthis pressure. The lowest point that the pressure reaches before the next ventricular contraction representsthe diastolic pressure, recorded as the second number in a blood pressure measurement. Most commonly,these pressure values are recorded in millimeters of mercury (mm Hg) — for example, 120/80 mm Hg.Automatic electronic blood pressure monitors use one of four measurement techniques: auscultatory,oscillometric, differential sensor, or plethysmographic. Auscultatory blood pressure monitors rely on thesame principle as most manual sphygmomanometers: the detection of Korotkoff sounds. When an inflated air cuff subjects an artery to pressure that is greater than the systolic pressure, the artery closes and stops the flow of blood through it. As the outside cuff pressure is gradually lowered, it eventually falls below that of the systolic pressure, and some blood does force its way through the artery during the brief period when the arterial blood pressure is higher than the cuff pressure; however, the blood flow is not normal, and theresulting turbulence produces Korotkoff sounds. These sounds persist until the cuff pressure falls below thediastolic pressure and blood flow returns to normal. Thus, the pressure at which Korotkoff sounds firstbegin marks the systolic pressure, while the pressure at which they disappear reflects the diastolic pressure; a microphone positioned against the compressed artery detects the Korotkoff sounds, enablingthe user to directly determine both systolic and diastolic values. Mean arterial pressure (MAP), the meanvalue exerted by flowing blood over the cardiac cycle, is calculated from these values and displayed.

Conversely, in the oscillometric technique of determining blood pressure, when the pressure in the cufffalls, a pressure transducer located in the monitor detects air pressure fluctuations in the cuff instead ofthe Korotkoff sounds. These pressure fluctuations are due to arterial volume changes that occur because ofthe pulsatile flow of blood. The pressure at which the oscillations peak corresponds to the MAP. From theincreasing and decreasing magnitude of these oscillations the device uses algorithms to calculate the systolic and diastolic pressures.

In the differential-sensor method, a cuff inflates around an extremity and then slowly deflates, while a dual-head sensor in the cuff records the relevant signals. As the cuff pressure falls, the side of the sensor against the artery detects Korotkoff, oscillometric, and artifact signals. However, the opposite side of the sensor, which is against the air bladder within the cuff , detects only oscillometric and artifact signals. Themonitor then subtracts the signal received by the side of the sensor against the air bladder from the signalreceived by the side against the artery, leaving only the Korotkoff sounds, which the monitor uses to determine the systolic and diastolic pressures. Theoretically, this process should remove some of the interference caused by unwanted signals. To obtain the MAP, the monitor simply records the pressure corresponding to the peak Korotkoff amplitudes in conjunction with the oscillometric waves.

In addition to the above techniques, some monitors primarily use the auscultatory technique but switch tothe oscillometric method as a backup if the monitor is unable to detect the Korotkoff sounds.

Plethysmographic methods use a light-emitting diode (LED) to detect minute arterial volume changes inconjunction with an electropneumatic servo-controlled valve to regulate pressure in an inflatable cuff bladder.The cuff bladder is slipped over the patient’s finger or thumb and is inflated stepwise under microprocessorcontrol until the signal from the optical sensor determines that the cuff pressure is just sufficient to permitthe artery to remain open; at this point, cuff pressure equals arterial pressure. Initially, this process maytake 15 to 20 seconds but is subsequently quicker and is repeated several times during the first minute untilthe volume of the finger under the cuff stabilizes. (Once stabilization has occurred, the calibration proceduretakes place every minute, in as short a time as three pulse beats.) After the system is calibrated, a minutevolume change causes the optical sensor to send a signal to the servo-controlled valve to increase or decreasethe cuff pressure almost instantaneously. A transducer detects the cuff pressure and produces a corresponding electrical signal that can be displayed as an arterial pressure waveform on the monitor screen or sent to a printer, thus providing a continuous record.

Most blood pressure monitors calculate pulse (heart) rate at each blood pressure reading, and most display systolic, diastolic, and mean arterial pressures as well. While some units’ alarm systems can be set forall four parameters simultaneously, others monitor only one at a time or monitor one parameter only.Many units can also measure other physiologic conditions, such as temperature and oxygen saturation.Some units have a rechargeable battery for portability or for backup in case AC power is interrupted.

Options available from most manufacturers include cuffs of different sizes and a printer. Different-sizecuffs are necessary to allow the monitoring of many types of patients, including neonates and the veryobese; wrist cuffs are also available.


Source: ECRI