The following key words were used to search the databases:
The subject population of the literature review will be limited to studies reported in English concerning pregnant women in the United States.
Literature Selection and Identification
The literature reviewed for this project was conducted using the search terms noted above, in isolation and in combination. Based on the appropriateness of the identified research, the following literature was selected for inclusion in this study. As with many other areas of clinical research, there is a paucity of research concerning the appropriateness of which Korotkoff phase should be used as the endpoint for the measurement of diastolic blood pressure during pregnancy. Further complicating the investigation is the wide range of confounding factors such as age (Mansfield, 1986), race, level of neonatal care and socioeconomic status of the subjects involved (Berry et al., 2000). For instance, pregnant women, especially young, single pregnant women, consistently demonstrate higher levels of prenatal care in neighborhoods with strong social networks, perhaps because these networks put pressure on them to avoid compromising the health of their fetus and also provide them with more information about what constitutes effective prenatal care (Sampson, 1992).
Background and Overview of Blood Pressure Measurement. Generally speaking, cardiac function tests are simply attempts to measure certain variables reflecting the condition of the circulatory system in adjusting to situations of exertion. The variables used most generally include pulse rate and blood pressure, which are recorded under various conditions (Matthews, 1973). Blood pressure refers to the pressure exerted by the blood on the walls of the blood vessels. Typically, systolic and diastolic blood pressure tend to increase during the performance of tasks (Baum, Krantz & Singer, 1983). The blood pressure reaches its highest levels during systole in the left ventricle. This systolic pressure increases during activity and falls during sleep. The diastolic blood pressure is the lowest point to which the pressure drops between beats (Matthews, 1973). When Dr. Nicolai Korotkoff of Leningrad added a stethoscope to Riva-Rocci's technique in order to hear the pulsations of blood in the brachial artery, for the first time both systolic and diastolic blood pressure could be gauged (Lynch, 1985).
The noninvasive methods used for the measurement of blood pressure are known as indirect techniques. Two methods of indirect blood pressure (BP) measurement are currently used for ambulatory blood pressure measurement (ABPM): the auscultatory and oscillometric methods. According to Bonnafoux (1996), the auscultatory method is based on the detection of Korotkoff sounds that are issued from the acoustic transudcer signal. The main advantages of the auscultatory method are (1) similarities with usual clinical measurement of BP; and (2) accurate detection of systolic and diastolic pressures on the appearance and disappearance of sounds. The main disadvantages associated with this method are (1) artefacts due to movements; and (2) difficulties in signal analysis due to physiological variations of the Korotkoff sound patterns or poor signals. The difficulties associated here can be overcome by appropriate signal processing (K2 recognition), noise rejection and/or ECG gating. This may allow relatively accurate BP measurement during mild exercise. With the oscillometric method, air volume variations in the cuff are detected during deflation. The maximum oscillation is related to the mean arterial pressure; the systolic and diastolic BP are determined by an algorithmic interpretation of the shape of oscillometric amplitudes as well as the heart rate. The main advantages in this approach are (1) possibility of BP measurement when the Korotkoff signal is poor; (2) measurement of the mean arterial BP; and (3) there is no need of a microphonic sensor. The main disadvantages of the oscillometric approach are (1) some oscillometric curves are difficult to read accurately; (2) oscillometry is very sensitive to movements due to the bandwidth of the signals, so the arm must be immobile; and (3) the accuracy of the systolic and diastolic BP depends on the algorithm used. "These two methods are complementary and should ideally be associated in the same device" (Bonnafoux, 1996, p. 185). According to John L. Andreassi (2000), the true measurement of blood pressure can only be achieved through the penetration of an artery to insert a sensing device; however, this direct measurement of intra-arterial BP would be a problem in the psychophysiology laboratory using human subjects, because of discomfort for participants and possible medical complications. "The most familiar blood pressure measuring technique involves the use of a sphygmomanometer (from the Greek word sphygmos, meaning "pulse").
The sphygmomanometer is comprised of an inflatable rubber cuff, which is wrapped around the upper arm and which is connected to an apparatus that records pressure, usually in terms of the height of a column of mercury or on a dial. A blood pressure reading consists of two numbers, which are typically recorded as x/y. The x is the systolic pressure, and y is the diastolic. The term, systole, refers to the contraction period of the heart, when it forces blood from the heart into the circulating system, and diastole refers to the resting period, when the heart expands and receives another supply of blood. At each heartbeat, blood pressure is raised to the systolic level, and, between beats, it drops to the diastolic level. As the cuff is inflated with air, a stethoscope is placed against the skin at the crook of the arm. As the air is released, the first sound heard marks the systolic pressure; as the release continues, a dribbling noise is heard; this sound represents the diastolic pressure (Sphygmomanometer, 2004).
One method of measuring blood pressure involves the use of a pressure cuff, a rubber bulb, a mercury (Hg) manometer, and a stethoscope (Andreassi, 2000, p. 305). In this method, the pressure cuff is wrapped around the upper arm and inflated to a level well above the expected systolic pressure (for instance, 175 mm Hg). The stethoscope, which has been placed over the brachial artery, is able to discern no sound at this level, because the artery has been collapsed by the cuff pressure. The cuff pressure is then very gradually reduced, until sounds are heard. Gradual reductions of about 2 mm Hg per sec on the mercury manometer provides relatively accurate measurements of BP (Andreassi, 2000).
Korotkoff Sounds. The sounds produced by small amounts of blood passing through the cuff are called Korotkoff sounds, after the physician who first used this method in the early 1900s (Andreassi, 2000). Five Korotkoff phases are described in adults (O'Sullivan & Murray, 2001) as shown in Figure 1 below:
Figure 1. Five Korotkoff Phases [Source: Gedney & Sorenson, 2000].
The Korotkoff sounds are divided into five phases based on the loudness and quality of the sounds. According to Allen et al. (2004), the sounds that are associated with the five classical Korotkoff phases are clinically important for measuring systolic and diastolic blood pressures.
The frequency ranges of the sounds have been described by simply using the overall peak frequencies within each phase by Fourier methods; however, Allen et al. suggest that such analysis may be missing potentially useful clinical information.
Phase 1: Loud clear tapping or snapping sounds are heard. They grow louder as the cuff is deflated.
Phase 2: A succession of murmurs is heard. Sounds may disappear during this phase if the cuff is deflated too slowly.
Phase 3: The sounds become louder and have thumping quality similar to phase 1.
Phase 4: The thumping sounds of phase 3 are abruptly replaced by a muffled sound.
Phase 5: All sounds disappear. This phase is absent in some people (Gedney & Sorenson, 2000).
The pressure on the manometer is recorded when the first sound is heard with each pulsation. This is the systolic pressure (SBP) and, for a normal adult, ranges between 95 and 140 mm Hg, with 120 mm Hg being average (Andreassi, 2000). The pressure in the cuff is then reduced further, until the sounds are no longer heard. When the sounds disappear, the manometer reading at that point indicates diastolic pressure (DBP). Normal diastolic pressure ranges between 60 and 89 mm Hg for the adult.
According to Andreassi, the Korotkoff sounds are thought to be caused by blood jetting through the partly collapsed artery. "The jet causes turbulence in the open artery beyond the cuff, and this sets up the vibrations heard in the stethoscope" (Andreassi, 2000, p. 305). This technique is known as the auscultatory method of obtaining blood pressure, and is considered to be adequate for the clinician who is mainly concerned that patients fall within a normal range; however, for psychophysiological research, it is necessary to have automated, accurate techniques that enable frequent measurements of blood pressure. According to Sebald, Bahr, and Kahn (2002), auscultatory blood pressure measurement uses the presence and absence of acoustic pulses that are generated by an artery (i.e., the so-called Korotkoff sounds), which are detected with a stethoscope or a sensitive microphone, to noninvasively estimate systolic and diastolic pressures. "Unfortunately, in high…