Measurement of Blood Pressure

Diagnosis and monitoring of treatment of essential hypertension require the accurate repeated measurement of blood pressure. Although on the surface it may seem that measuring blood pressure is simple, many factors need to be considered when obtaining measures of blood pressure for purposes of diagnosing and monitoring essential hypertension. For example, arterial pressure differs depending upon the specific site of the arterial bed from which the measure is obtained; the closer the location is to the heart, the higher the blood pressure. Body position greatly affects blood pressure measurement, as does ingestion of a variety of substances, including alcohol, nicotine, caffeine, and a whole range of prescription and over-the-counter medications. To complicate matters further, blood pressure is a dynamic parameter, forever changing as the organism adapts to altering environmental contexts like noise level, temperature, and presence of interpersonal confrontation; therefore, a single blood pressure assessment will never really provide much useful information. In addition, although numerous manual and automated devices have been developed to measure blood pressures accurately, correspondence of blood pressure values among these devices is not always exact. Let's examine some of the primary methods employed to measure blood pressure.

Methods of Blood Pressure Measurement

Direct Intra-arterial Recording

The first method established for measuring blood pressure, intra-arte-rial recording, was discovered in 1733 when Hales inserted a thin glass tube into a horse's artery during a surgical procedure. The level of the blood rose and fell within the glass tube because of changes in arterial pressure associated with heart action. Experimentation with this method permitted Hales to directly observe changes in blood pressure by measuring the level of the blood in the glass tube. With continued experimentation, blood pressure gauges of this type became standardized so measures of blood pressure could be compared across time and situations as well as across species. Despite its impracticality due to problems associated with blood loss and potential infection, direct in-tra-arterial measures of blood pressure are still considered the 'gold standard' of measurement (Littler and Komsuoglu, 1989). Not only are these measures made directly from catheters positioned in the circulatory system, but they also permit continuous measures of blood pressure on a beat-by-beat basis. Thus, momentary fluctuations in blood pressure in response to various environmental stimuli can easily be detected. But despite the accuracy of intra-arterial methods, their impracticality for clinic use led to the reliance on pulse palpation (sensing variations in the pulse by touch) as a gross estimate of arterial pressure obtained during clinic visits in the 1800s.

Auscultatory Method

Due to the subjectivity associated with pulse palpation and general lack of correspondence between estimates obtained from pulse palpation and intra-arterial measures of blood pressure, considerable effort was focused upon developing an accurate noninvasive method (Cook and Briggs, 1903; Crenner, 1998). With the invention of the blood pressure cuff by Riva-Rocci in 1896, a new tool became available for determining blood pressures without insertion of a catheter. Still the primary method for determining blood pressure today, the occluding cuff is inflated around a limb (arm or leg) until blood flow is entirely blocked. Then, as the air pressure in the cuff is slowly released, audible sounds from the arterial wall can be detected with a stethoscope as the pressure from the cuff drops below SBP and blood flow begins to resume. These audible sounds from the arterial bed eventually fade and disappear as the air pressure of the cuff drops below DBP and blood flow returns to normal in the limb. The onset of these sounds, called Korotkoff sounds after the Russian physician who studied them intensively (Korotkoff, 1905), coincides with SBP, as blood begins to flow into the occluded artery; the muffling (Phase IV) and disappearance (Phase V) of these sounds coincides with DBP Riva-Rocci's invention, in brief, allowed the examiner to make inferences about an individual's blood pressure level by simply monitoring the air pressure in the occluding cuff that corresponded to the appearance and disappearance of Korotkoff sounds.

Traditionally, two types of air pressure gauges have been used in conjunction with occluding cuffs: mercury columns and aneroid manometers. Using the mercury column, an apparatus strangely reminiscent of the original glass tubes used by Hales (1733), involves observing the extent of direct displacement of mercury in a gauged column by air pressure in the occluding cuff. The examiner watches the mercury level decrease in the column as air is released from the cuff and records the values associated with the appearance and disappearance of Korotkoff sounds. The aneroid manometer involves a mechanical device in which air pressure in the cuff causes a display needle to move on a gauged dial. As with the mercury column, the examiner simply records the values from the gauge associated with the appearance and disappearance of Korotkoff sounds.

Studies comparing the auscultatory method of determining blood pressure, initially established by Riva-Rocci and Korotkoff, with intra-arterial measures have yielded very impressive correlations (Pickering and Blank, 1989). This led to gradually increased usage of the ausculta-tory method during the twentieth century, as physicians became trained in this newly validated method rather than relying on the older, less reliable practices of examining the radial pulses. Even during early tests of the auscultatory method in clinical setting, however, there was concern over the accuracy of the blood pressure determinations (Cren-ner, 1998). Indeed, the examiner must attend to several factors to make sure that standard measurement conditions are employed: an occluding cuff of appropriate size, standard arm placement, positioning of the cuff at heart level, having the patient adopt a standard body posture, and assuring use of a calibrated manometer (Pickering et al., 2005). It is also important to obtain blood pressures during periods of silence; not only can the examiner hear the Korotkoff sounds better, but talking during blood pressure determinations has been associated with significantly increased blood pressures of the patient (Le Pailleur et al., 2001). Observer errors are also a source of inaccuracy; foremost among these is a digit preference for numbers ending in a 5 or 0 (Shapiro et al., 1996). These observer errors, however, can be minimized with the use of a random zero sphygmomanometer (Wright and Dore, 1970), a device gauged so that the actual zero point is unknown to the examiner. Additionally, determining DBP by detecting Phase IV Korotkoff sounds (muffling of the Korotkoff sound) typically results in poorer reliability than using Phase V DBP determinations; therefore, Phase V is more commonly used to demarcate DBP (Shapiro et al., 1996).

One strategy for eliminating observer error with the auscultatory method is to use an electronic device that both regulates cuff inflation and deflation and detects Korotkoff sounds using microphone arrays embedded within the occluding cuff. Indeed, a number of such devices are available for both clinical and research use (see Fowler et al., 1991). Because of concerns that many of these devices may not compare favorably with standard intra-arterial measures of blood pressure, the Association for the Advancement of Medical Instrumentation developed a set of standards to evaluate the reliability and validity of electronic blood pressure devices (White et al., 1993). In brief, these guidelines require an adequate number of blood pressure comparisons with either intra-arterial measures or the standard auscultatory method on persons with different arm sizes in a variety of postures (seated, supine, and standing). Blood pressure measures obtained from both the electronic device and standard comparison strategy need to be within ±5 mm Hg in order for the device to be considered acceptable for making accurate determinations (Association for the Advancement of Medical Instrumentation, 1993). Additionally, clinicians or researchers who rely on using electronic auscultatory devices for measuring blood pressure should routinely calibrate their instruments with standard ausculta-tory methods.

Oscillometry Method

A second type of noninvasive blood pressure measurement strategy, the oscillometric method, also employs an occluding cuff. However, in contrast to the auscultatory method, which relies on detection of Korotkoff sounds, the oscillometric method operates by sensing the magnitude of oscillations caused by the blood as it begins to flow again into the limb. Typically, very faint blood flow oscillations begin to be detected as the air pressure in the cuff coincides with SBP. As air pressure is slowly released from the occluding cuff, the amplitude of these pulsatile oscillations increases to a point and then decreases as blood flow to the limb normalizes. Although the oscillation with the greatest amplitude has been shown to correspond reliably with mean arterial pressure (Mauck et al., 1980), determinations of SBP, which are associated with a marked increase in amplitude of oscillations, and DBP, which are associated with the point at which oscillations level off, are often less accurate when compared with auscultatory measures (Fowler et al., 1991). Therefore, while oscillometric methods tend to overestimate SBP and underestimate DBP (Maheswaran et al., 1988; Manolio et al., 1988), they can be useful for determining accurate estimates of mean arterial pressure.

Continuous Blood Pressure Monitoring Methods

Unfortunately, both auscultatory and oscillometric methods of blood pressure assessment are intermittent measures in that a single blood pressure determination can take almost an entire minute to obtain. Additionally, a brief rest period is recommended between measures of blood pressure that require use of an occluding cuff to allow circulation in the limb to return to normal. Therefore, if an investigator is interested in measuring immediate and short-lived alterations in blood pressure, intermittent blood pressure measures would not be a good choice. Two noninvasive approaches for measuring blood pressure continuously have been developed, pulse transit time (or pulse wave velocity) and the vascular unloading method.

Pulse Transit Time

Pulse transit time reflects the time it takes the pulse wave to travel from the heart to a site in the peripheral circulation, typically the finger or earlobe. It is commonly assessed by measuring the duration of time (in ms) between the initiation of the cardiac contraction from the electrocardiogram (ECG) and the arrival of the pulse wave at the peripheral site, typically measured using photoplethysmography. Presumably, as arterial pressure increases, the pulse wave travels more quickly to the peripheral site (lower pulse transit time); conversely, as arterial pressure declines, pulse transit time lengthens (Gribbin, Steptoe, and Sleight, 1976). Although studies comparing changes in pulse transit time with blood pressure change have yielded significant inverse correlations, these correlations have been more commonly observed between measures of pulse transit time and SBP than between pulse transit time and DBP (Newlin, 1981; Obrist et al., 1979). Furthermore, researchers who employed measures of pulse transit time have disagreed as to whether the continuous temporal parameter actually represented an index of blood pressure, as there was considerable evidence suggesting it was more strongly linked to beta-adrenergic cardiac activity than to blood pressure (Newlin, 1981; Obrist et al., 1979). Because of these equivocal findings linking changes in pulse transit time to alterations in blood pressure, this method has not been recommended as a surrogate measure of blood pressure (Shapiro et al., 1996).

Vascular Unloading Method

The vascular unloading method, initially described by Penaz (1973), involves obtaining estimates of blood pressure from a small pressurized cuff positioned over a finger in conjunction with a photoplethysmo-graph. One such apparatus, called the Finger Arterial Pressure System or finapres™, monitors blood flow into the finger and provides continuous information to a mechanism that automatically adjusts air pressure in the cuff to maintain a stable partial blood flow through the artery in the finger. Blood flow oscillations are sensed by the encircling finger cuff and translated into beat-by-beat estimates of blood pressure. Although this device may become uncomfortable during extended measurement periods, it can be used to measure blood pressure continuously for a few hours, and the ambulatory version, which alternates blood pressure determinations between two integrated finger cuffs, has been used for periods as long as 24 hours (Imholz et al., 1993).

Naturally, whenever a new method for assessing blood pressure is developed, it is important to validate it with established measurement strategies. In some studies of this type, blood pressure values obtained from the devices using the vascular unloading principle have been shown to compare favorably with intra-arterial measures of blood pressure (Imholz et al., 1990; Parati et al., 1989) as well as intermittent noninvasive measures (Dorlas et al., 1985; Pace and East, 1991). However, other studies have reported that indices of blood pressure from these devices either overestimated (Epstein et al., 1989; Kurki et al., 1989) or underestimated blood pressure (Imholz et al., 1988; van Egmond, Hasenbos, and Crul, 1985). It appears that some of this lack of correspondence between methods of blood pressure determination is unique to the vascular structure of individuals. With the device positioned at the recommended heart level, some individuals display estimates of blood pressure that are over 20 mm Hg higher than their corresponding oscillometric or auscultatory values, while others display estimates that are over 20 mm Hg lower (Larkin et al., 1995). Calibration of recordings by adjusting arm position above or below heart level improves correspondence between measures, but this effect appears to be only temporary (Larkin et al., 1995).

Regardless of whether the absolute measures of blood pressure obtained from devices employing the vascular unloading principle accurately portray an individual's resting blood pressure level, the continuous finger arterial recordings provide a reliable index of change in blood pressure in response to acute environmental stimuli (Parati et al., 1989). Therefore, perhaps the true utility of this instrument is to assist in providing accurate measures of acute blood pressure responses to stress, particularly responses to short-term stressors that may be missed if an intermittent noninvasive measurement is employed. In this usage, Gerin, Pieper, and Pickering (1993), indeed, demonstrated that the FiNAPRES™-derived measures of blood pressure were more reliable than blood pressures obtained using an intermittent blood pressure measurement device.

Clinic Measurement of Blood Pressure

Although significant technological advances have permitted the development of several valid ways to measure blood pressure, most clinic de-

Table 2.1. Auscultatory Blood Pressures Measured during Franklin's Three Clinic Visits

First Clinic Visit

Table 2.1. Auscultatory Blood Pressures Measured during Franklin's Three Clinic Visits

First Clinic Visit

10 min resting

152/100

15 min resting

158/108

20 min resting

160/104

Mean: FirstVisit

158/104

Second Clinic Visit

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