Blood Pressure Monitoring Today
Blood pressure may be measured using direct or indirect techniques. Direct measurements use catheters to invasively determine blood pressure, whereas indirect methods utilize a variety of noninvasive techniques.
Direct measurements utilize one of two different types of sensors, in conjunction with a catheter, to determine blood pressure. Extravascular sensors are located outside of the body and use the principle of wave propagation to transmit vascular pressure from the measurement site to the sensor via a fluid-filled catheter. In contrast, intravascular sensors, such as fiber-optic pressure sensors, are positioned on the tip of the catheter and inserted into the artery. Catheter systems are often used for direct, continuous measurement of intra-arterial blood pressure in the aorta, which is considered the "gold standard" for blood pressure measurement.
The need for this type of continuous, highly accurate reading of a patient's central blood pressure is commonly found in critical care surgical settings. However, the invasive nature of catheter procedures prohibits them from being a viable option for routine blood pressure measurement. Instead, physicians use noninvasive methods for casual measurements. Therefore, a major goal in the field of noninvasive blood pressure monitoring has been the development of a method which correlates with "gold standard" catheter measurements.
The noninvasive measurement of blood pressure may be performed in several different ways. Many methods use an air filled cuff to temporarily occlude blood flow through the artery, and then apply a particular technique to obtain blood pressure data while the cuff deflates. The most common indirect technique is the auscultatory method, in which a clinician determines systolic and diastolic pressures by listening to the characteristic Korotkoff sounds of the blood flow during cuff deflation. The Korotkoff sounds have been categorized into five phases (descriptions taken from ANSI/AAMI 1987 guidelines):
The beginning of Phase I corresponds to SBP. The actual point of DBP based on these Korotkoff sounds is less clear. The ANSI/AAMI 1987 report suggests that the pressures at the beginnings of both phases IV and V be recorded, since there may be some debate as to which value best represents DBP. The phase IV values tend to be higher than catheter measurements, while phase V values are usually lower. The DBP criteria is further complicated by the fact that some patients may not have audible phase IV sounds, whereas in others, the beginning of phase V (silence) may be difficult to determine.
Since the auscultatory technique is based on the ability of the human ear to detect and distinguish sounds, there is a possibility for measurement error due to individual levels of auditory acuity and sensitivity. Although a fully-qualified clinician can consistently obtain accurate blood pressure measurements, unqualified or inexperienced personnel may be more susceptible to outside noise interference, or inconsistent assessment of the actual points of Phase I and Phase IV or V.
In an attempt to increase reproducibility, some automated devices have replaced the human ear with a microphone. These devices apply sound-based algorithms to estimate SBP and DBP. In addition to noise-artifact sensitivity, these sound-dependent algorithms may not adequately compensate for patient conditions such as hypotension (i.e. low blood pressure), where the Korotkoff sounds may be muted.
There is no way to determine mean arterial pressure (MAP) solely by the use of Korotkoff sounds, and that is a strong limitation of auscultation. In order to provide an estimation of MAP a formula has been developed, which is quite commonly used in auscultatory devices:
MAP = SBP/3 + 2DBP/3
Oscillometric blood pressure determination is the other common noninvasive method of blood pressure monitoring. The term "oscillometric" refers to any measurement of the oscillations caused by the arterial pulse. These oscillations are the direct result of the coupling of the occlusive cuff to the artery. This technology was originally designed as an alternative to the auscultatory technique, allowing blood pressure measurement of critical care and intensive care unit (ICU) patients whose Korotkoff sounds were inaudible (usually due to hypotension caused by massive hemorrhage or shock).
In oscillometric techniques, the cuff is first inflated until the artery is fully occluded. Then, the monitor takes measurements while the cuff deflates. Most oscillometric devices examine the pulsatile pressure generated by the arterial wall as it expands and contracts against the cuff with each cardiovascular cycle. An electrical signal is generated by the pressure transducer based on the distension of the artery. Over the course of the measurement process, the magnitude of the pulsatile signal increases, reaches a maximum amplitude, and then decreases.
A common algorithm used by many oscillometric devices sets MAP equal to the point of maximum amplitude. SBP and DBP are then determined by the application of predetermined systolic and diastolic ratios. In the height-based approach, for example, ratios of pulse amplitude to maximum amplitude are used. For this type of algorithm, the maximum amplitude is defined as MAP.
An oscillation that satisfies the systolic ratio and occurs before the MAP point, is considered SBP. An oscillation that satisfies the diastolic ratio and occurs after the point of MAP determination, is considered DBP. The determination of both systolic and diastolic ratios are based on the correlation to auscultatory or catheter measurements. Unfortunately, an erroneous determination of MAP, due to any form of artifact, may produce inaccurate values for SBP and DBP.
The majority of the monitors on the market today are either auscultatory or oscillometric in nature, however, there are other types of devices. In addition, some monitors employ both auscultatory and oscillometric methods, using one method as a primary measurement and the other as a secondary measurement for verification.
Not all techniques yield both SBP and DBP. For example, the palpatory method only measures SBP. In this technique, the artery is occluded with a cuff. Then, the cuff is allowed to deflate, and SBP is determined by measuring the cuff pressure at which a radial or finger pulse is first detected. Most techniques, however, provide both SBP and DBP. The infrasound method, for example, attempts to improve on the auscultatory method by detecting low frequency Korotkoff vibrations below about 50 Hz, including sub-audible vibrations.
The ultrasound method is quite different from infrasound, because it measures Doppler shifts to determine blood pressure. Ultrasound waves generated from a transmitter located distal to the cuff are projected towards the artery. These waves contact the arterial wall and are reflected back to the receiver. Distension of the wall causes phase shifts in the reflected waves (known as Doppler shifts). From these phase shifts the opening and closing of the artery can be determined. Systolic pressure is the point at which the artery can remain open with the largest cuff pressure. Diastolic pressure is determined by a similar algorithm.
Another method, known as impedance plethysmography, also measures the volumetric change associated with arterial distension. The volume changes cause changes in the electrical conductivity (impedance) of the measurement site. The pulsatile nature of the volume changes (due to the cardiovascular cycle) is reflected in the measurement of impedance pulses. When graphed over time, these pulses produce a waveform similar to the pressure-generated oscillometric waveform. Pressures are then estimated in a manner similar to the oscillometric technique.
Arterial Tonometry utilizes a very different approach. The artery is flattened and the pressures required to maintain that flat region are measured. This is accomplished by using an array of sensors, each of which measures pressure. This array ensures that the maximal pressure being exerted on the arterial wall by the blood is felt. The result of this method is a waveform similar to catheter measurements, and some type of algorithm must be used to calculate pressures from that waveform.
There are several limitations to tonometry. First, it is a measure of the peripheral circulation, with much different values for pressure than are felt closer to the heart. A second limitation of tonometry is its high sensitivity to sensor location and angle. With each relocation of the sensor, there can be varying values for pressure. Measurement errors may be minimized by experienced, well trained operators. However, inter-operator reproducibility may still be lack consistency. Lastly, tonometry requires a blood pressure measurement for calibration by an independent technique.