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 intraarterial measures have yielded very impressive correlations (Pickering and Blank, 1989). This led to gradually increased usage of the auscultatory 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 (Crenner, 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 auscultatory methods.