Blood Pressure Measurement During Pregnancy: Auscultatory Versus Oscillatory Methods

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1 CLINICAL STUDIES Blood Pressure Measurement During Pregnancy: Auscultatory Versus Oscillatory Methods Linda A. Green, RN, MS, Robin D. Froman, RN, PhD, FAAN Objective: To determine the equivalence of auscultatory and oscillatory blood pressure measurements. Setting: Inner-city prenatal clinic. Participants: Eighty-one women in their 2nd to 9th month of pregnancy. Design: Participants were assessed for systolic and diastolic blood pressures on left and right arms using auscultatory (manual) and oscillatory (electronic) methods. A correlational study design was used. Main outcome measures: Differences in pressures related to arm and method of measurement. Results: The oscillatory method produced consistently higher readings for both systolic (F[ 1,801 = 45.9, p < 0.001) and diastolic (F[1, 801 = 25.79,p < 0.001) pressure readings. Correlations between estimates generally treated as substitutable all fell below the recommended level of 0.80 for measurement equivalence. Conclusions: Results suggest the need for caution when interpreting blood pressure estimates as interchangeable. This is particularly important when patients move from clinic settings, where auscultatory methods predominate, to inpatient settings, where oscillatory methods of measurement are used. JOGNN, 25, ; Accepted: August 1994 tween the two methods are important clinical considerations when screening or treating patients for hypertension. The goal of the current research was to determine the equivalence of the auscultatory and oscillatory methods of measuring blood pressure. Background Because an increase in blood pressure above prepregnant levels is symptomatic of hypertensive disorders in pregnancy, the accurate measurement of blood pressure is a requisite of prenatal care. Pregnancy-induced hypertension may be divided into three,categories: hypertension, preeclampsia, and eclampsia. An increase in the patient's systolic or diastolic pressure may indicate the presence of hypertension. Diagnostic blood pressure criteria are based on the auscultatory system. The popularity of noninvasive oscillatory equipment has created a variation in methods of blood pressure measurement. Researchers have begun to address the issue of the equivalence of the auscultatory versus oscillatory readings. Are auscultatory (manual) and oscillatory (electronic) blood pressure readings equivalent? he measurement of blood pressure is a critical element in detecting and managing hypertension during pregnancy. Auscultatory (manual) methods of blood pressure measurement are used in most outpatient facilities, whereas oscillatory (electronic) equipment is used in most inpatient settings. Researchers have begun to question the accuracy of estimates using oscillatory methods compared with auscultatory methods, as well as the equivalence of measurements from these devices. Possible differences in blood pressure measurement be- Weaver, Park, and Lee (1990) measured the blood pressure of 381 fifth-grade students with the auscultatory method and compared estimates with those obtained by the Dinamap monitor (Critikon, Tampa, FL), an oscillometric device. Two measurements were obtained using each of the two methods. Weaver et al. (1990) found that oscillatory systolic pressures were 5-8 mm Hg higher than the auscultatory values. Oscillatory diastolic pressures were 8-9 mm Hg lower than auscultatory measurements. Correlation co- February 1996 J O C N N 155

2 C L I N I C A L S T U D I E S efficients were r = 0.78 for the systolic blood pressure and r = 0.42 for the diastolic 4th Korotkoff phase. Although statistically significant, the correlations between diastolic readings are low for supposedly equivalent, concurrent measures, suggesting a poor relationship between the two readings. Systolic readings in the present study, though more closely correlated, were borderline for equivalent estimates (Froman & Owen, 1991). The findings of Weaver et al. (1990) suggest that the published, normative levels of auscultatory blood pressure may be inappropriate as standards when blood pressure measurement is obtained by the Dinamap monitor. Marx, Schwalbe, Cho, and Whitty (1993) studied the differences in blood pressure values using both methods and the significance of these differences during labor with epidural analgesia. They compared auscultatory blood pressure measurements for 30 women in labor, 20 full-term pregnant women not in labor, and 20 nonpregnant volunteers. In the nonlaboring parturients and the nonpregnant controls, systolic and diastolic measurements were considered to show nonsignificant differences. In the laboring parturients, the oscillatory systolic pressures were consistently and significantly higher (p < 0.02) compared with the auscultatory measurements. Diastolic pressures of the parturients were consistently and significantly lower (p < 0.002) when measured by the oscillatory method compared with the auscultatory method. In 13 women, the systolic pressure was >lo mm Hg higher when measured by the oscillatory method compared with the auscultatory method. The diastolic pressure was >10 mm Hg lower with the oscillatory method in nine subjects. Mean arterial pressures did not differ, although the mean difference in arterial pressure was described as moderate (6.2 mm Hg). The researchers hypothesized that there is a factor in the changing pulse pressure of a laboring woman that is not taken into account by the algorithm used by the oscillatory method. The authors found a discrepancy between systolic and diastolic pressures obtained by the oscillatory versus the auscultatory method of measurement and recommended that the diagnoses of hypertension and hypotension be based on the mean arterial pressure rather than on the systolic or diastolic pressure. In 1991, Marx, Sofair, and Winikoff conducted a preliminary study of 12 patients during labor. Their findings revealed that all systolic pressures were higher and diastolic pressures were lower with the Dinamap monitor than with the auscultatory method. Differences ranged from 5-18 mm Hg (M = 10.2 f 3.7 mm Hg) in systolic pressure and from 4-14 mm Hg (M= 9.9 f 3.0 mm Hg) in diastolic pressure. Differences in auscultatory pressures between two observers were 0-2 mm Hg. The same cuffs were used for both oscillatory and auscultatory measurements of blood pressure. An interval of at least 2 minutes separated the measurements, with the auscultatory method preceding the oscillatory. Subjects were in labor, and measurements were taken between contractions and repeated twice within 20 minutes. The purpose of the Marx et al. (1991) study was to determine the sensitivity of the Dinamap monitor in detecting hypotension during and after epidural analgesia. Hypotension is a critical complication of epidural anesthesia. Results demonstrated that the normal definition of hypotension as a systolic pressure below 100 mm Hg, based on auscultatory standards, must be revised when an oscillatory blood pressure device is used, because the systolic pressure was higher than 100 mm Hg with the Dinamap monitor when clinical hypotension was present. The Marx et al. (1991) study, however, was a preliminary study of only 12 patients; thus, the results may not be generalized. Ornstein, Markert, Litchfield, and Zemp (1988) evaluated the Dinamap 8100 (Critikon, Tampa, FL) using the Hawksley Random Zero Sphygmomanometer (Sommerville, NJ) as the standard. Eighty ambulatory patients were studied. Readings were compared in a two-instrument, two-arm, two-investigator study with four pairs of simultaneous measurements. The research design allowed a repeated measures analysis of variance, which assessed the contributions of observer, order, arm, and instrument to blood pressure measurement. Analysis confirmed significant differences in systolic blood pressure determinations between instruments (p < , paired t-test). Dinamap 8100 readings showed higher systolic values. No significant differences were found between observer, order, or arm for systolic measurements. For diastolic measurements, observer differences were barely statistically significant (mean difference 1.O mm Hg, p < 0.05). Order and arm differences were not statistically significant for diastolic measurements, and mean difference in diastolic measurements between devices was not statistically significant. The authors suggested that routine use of the Dinamap 8100 could lead to misclassification errors in routine screening for hypertension, and, given the low predictive values of its readings, they cautioned against using the Dinamap 8100 for the management of hypertension. The Association for the Advancement of Medical Instrumentation (1987) includes standards for the evaluation of oscillatory blood pressure monitor readings compared with standard sphygmomanometers. These standards recommend that mean differences in blood pressure be +5 mm Hg or less, with a standard deviation of 8 mm Hg. Methods A convenience sample of 81 pregnant women was secured from a prenatal clinic population in a large, urban hospital by selecting the first subjects meeting inclusion criteria. Inclusion criteria were that each woman be pregnant, 15 to 42 years of age, and under care at the prenatal clinic of the participating hospital. Subjects were asked to refrain from exercising, eating, and smoking for 1 hour and to rest for 10 minutes with an empty bladder before blood pressure measurement. Permission of the Nursing Research Committee at the study facility was obtained, and all subjects gave written, informed consent to participate in the study. The 156 J O C N N Volume 25, Number 2

3 Blood Pressure Meusurement Table 1. Descriptive Statistics (N = 81) Vartable Name Mean SD Range Age Weight (Ibs) Height (in) Gestational age (days) Arm (cm) circumference Systolic blood pressure" (manual) Diastolic blood pressure" (manual) Systolic blood pressure" (Electronic) Diastolic blood pressuren (Electronic) Ethnic status Black Latino White Asian Readings averaged across arms n = 28 n = 38 n= 12 n= purpose of the study, as well as the risks and benefits, were delineated in the permission form. Confidentiality of information was assured by the use of assigned identification numbers for collecting data, conducting analysis, and reporting results. Blood pressure was measured by one of the authors (LAG) in a quiet room with the patient seated at a table and her arm supported at heart level. Left arm measurements were taken first. Blood pressure measurements on the same subject were taken with the same size cuff. The cuff was 40-50% of the circumference of the upper arm, as recommended by the American Heart Association (1987). The 5th Korotkoff phase was used as the diastolic standard because the fourth sound was not heard consistently. Subjects' blood pressure was measured by the same author for all measurements. The auscultatory method was used first to prevent observer bias as an intervening variable because recording an electronic readout could create an expectancy about a manual reading. Two blood pressure measurements were made with each instrument on each arm at intervals of at least 2 minutes until venous congestion was resolved, as recommended by the American Heart Association (1987). The Dinamap model 1846SX (Critikon, Tampa, FL) was the oscillometric study instrument. A mercury gravity sphygmomanometer was the standard auscultatory device. Proficiency and accuracy in the use of the mercury sphygmomanometer and the Dinamap 1846SX were demonstrated by adherence to the standards of the American Heart Association (1987) and the Association for the Advancement of Medical Instrumentation (1987). Both Blood pressure readings were higher with the oscillatory method. instruments were tested for accuracy by the institution's biomedical engineering department using manufacturer's recommended guidelines. Data from systolic and diastolic readings for each arm were recorded on data collection sheets. Demographic information recorded for each subject included identification number, age, ethnicity, weight, height, arm circumference, and weeks of gestation. Analysis and Results Data were analyzed in three steps. First, descriptive statistics were calculated on the demographic variables to assess representativeness of the sample. These statistics are provided in Table 1. The second step of data analysis used repeated measures analysis of variance (R-ANOVA) to test for arm and method differences in systolic and diastolic readings. Two within-group variables, arm and method (oscillatory versus auscultatory), were used in the analysis. Separate R-ANOVAs were conducted on systolic and diastolic data. Results of the R-ANOVA are presented in Tables 2 and 3. Significant differences (F[1, 801 = 45.9, p < 0.001) were found between auscultatory and oscillatory systolic readings. Systolic pressures were significantly higher for the oscillatory method. Average systolic blood pressures, using the oscillatory method, were 115 and 112 for right and left arms, respectively. Average auscultatory systolic pressures were 108 and 105 for right and left arms, respectively. The test for differences in systolic pressures between right and left arms showed significant differences (F[l, 801 = 9.70, p < 0.005). Right arm readings were higher than left arm readings. The R-ANOVA comparing diastolic readings produced similar results. Oscillatory readings were significantly higher than auscultatory readings (F [I, 801 = 25.79, p < 0.001). Average diastolic pressures using the oscillatory method were 64 and 65 for right and left arms, Table2. Analysis of Variance on Systolic Readings Source DF Mean square F Method 1, * Arm 1, t Interaction Arm X Method 1, * p < tp< February 1996 JOG" 157

4 C L I N I C A L STUDIES Table3. Analysis of Variance on Diastolic Readings Source DF Mean square F Method 1, * Arm 1, Interaction Method X Arm 1, p < Discussion and Implications Readings from an oscillatory device may be misleading if measurements are used to diagnose and treat pregnancyinduced hypertension. Existing standards for screening and referral are based on data collected using the manual, auscultatory method of blood pressure estimation. These standards may not be appropriate cut-off values for oscillatory methods of blood pressure measurement. respectively. Average auscultatory diastolic pressures were 60 and 61 for right and left arms, respectively. The test for differences between right and left arms was nonsignificant. The final analysis used correlation to estimate the equivalence of readings across the two methods. The R- ANOVAs were useful in noting gross differences between readings by method but could not specify whether rank ordering within each method was equivalent. Correlational analysis was used to determine whether application of a regression formula or addition of a constant would allow auscultatory readings to be transformed into equivalent oscillatory estimates. If a minimum correlation of 0.80 was found between any pair of readings, development of a regression formula would be indicated (Froman & Owen, 1991). Table 4 presents the correlations between all estimates. Correlations between auscultatory and oscillatory measurements assumed to be equivalent ranged from a low of 0.58 to a high of These correlations are less than the recommended 0.80 alternate methods reliability generally accepted for the substitution of scores (Nunnally, 1978). This implies that there are differences between oscillatory and auscultatory readings, as shown by the R-ANOVA, and that these differences cannot be adequately adjusted with the use of a regression formula or the addition of a constant. Caution must be taken in interpreting auscultatory and oscillatory blood pressure readings as equivalent. If women are screened for hypertension at an outpatient clinic with the auscultatory method and subsequently are treated using oscillatory readings, discrepancies between readings may occur resulting from measurement differences inherent in the two devices. These differences may interfere with interpreting the effectiveness of treatment. If patients are diagnosed with hypertension in the prenatal clinic and are sent to the labor and delivery unit for further evaluation, placing them in a side-lying position for awhile may affect their blood pressure even though the measurements are taken in a sitting position. In such a situation, differences in.position and activity level rather than in equipment may be affecting the readings. Another danger lies in not detecting hypertension because of possible falsely low readings from use of the auscultatory method. If oscillatory methods are more objective, accurate estimates of blood pressure, borderline hypertension may be missed at early screenings if the auscultatory method is used. Table 4. Correlation Matrix for Arm and Method Equivalence Manual Manual Manual Manual Electronic Electronic Electronic Diastolic Diastolic Systolic Systolic Dtastoltc Diastoltc Systolic Electronic Systolic Left Arm Right Arm Left Arm Right Arm Left Arm Right Arm Left Arm Right Arm Manual diastolic right arm Manual diastolic left arm 0.61 Manual systolic right arm Manual systolic left arm Electronic diastolic right arm 0.61" Electronic diastolic left arm " Electronic systolic right arm " Electronic systolic left arm a a Alternate methods equivalence estimates. Note. All values reoresent uroduct moment correlations 158 JOGNN Volume 25, Number 2

5 Blood Pressure Measuremerit A difference of 7 mm Hg in systolic readings between the two methods exceeds the standard of +5 mm Hg established by the Association for the Advancement of Medical Instrumentation (1987). No quality control method exists for routine comparison of oscillatory and auscultatory blood pressure readings. Current findings reiterate discrepancies shown in other studies comparing the oscillatory and auscultatory methods of blood pressure measurement. Some studies have found that systolic pressures are significantly higher with the oscillatory compared with the auscultatory method (Marx et al., 1993). However, other reports show diastolic readings are significantly lower with the oscillatory devices than with the auscultatory methods (Ornstein et al., 1987; Weaver et al., 1990). The current study found systolic and diastolic pressures were significantly higher when measured with the oscillometric method. Differences in blood pressure measurements found in the current study and other studies may be linked to the differences in devices or the differences in clinical profiles of the different populations. Additionally, the current study was limited by the assessment of only one instrument, the Dinamap 1846 SX. That instrument may have idiosyncratic problems not detected at the time of calibration. The Dinamap is available in several models, and other manufacturers also produce oscillatory blood pressure recorders. Data obtained in the current study are applicable only to a gravid population. The current study did not provide information as to which method more accurately reflects directly measured arterial pressure. Marx et al. (1993) recommend the use of mean arterial pressure to diagnose hypertension rather than systolic or diastolic blood pressure. To assess the related criteria and predictive validity of each method of blood pressure measurement, a longitudinal study would help to determine in which subjects hypertension actually developed during pregnancy. Auscultatory and oscillatory methods measure blood pressure in different ways. The 5th Korotkoff phase was used in the current study for auscultatory diastolic readings. Dinamap measures blood pressure through an oscillometric technique. A cuff is inflated to a pressure above the systolic pressure. As this cuff deflates, oscillations in the pressure within the cuff are sensed, stored, and analyzed electronically. Systolic pressure is the point of rapid increase in the amplitude of oscillations. Diastolic pressure is the point of rapid decrease in oscillation after maximum oscillation (Marx et al., 1993). Therefore, using the fifth sound for the auscultatory method and the fourth sound for the oscillatory method may have contributed to the differences in readings. Readings from oscillatory devices may be misleading in the clinical area when they are used to diagnose and treat hypertension of pregnancy. Existing standards for hypertension are based on data collected using the auscultatory method of blood pressure measurement. These manual standards of measurement may not be appropriate cut-off values for electronic blood pressure measurements, as indicated by results from the ANOVAs and correlational analysis conducted for the current study. Correlational analysis demonstrated that the differences between the oscillatory and auscultatory readings could not be considered directly equivalent, nor could they be corrected by a formula or constant. The ANOVAs documented significant differences within individual readings across methods. To diagnose hypertension, the current study supports the importance of evaluating the patient within the context of her clinical symptoms and not by blood pressure changes alone. References American Heart Association. (1987). Recommendations for hzcman blood pressure determination by sphygmomanometers. Dallas: Author. Association for the Advancement of Medical Instrumentation. (1987). American nationalstandard for electronic sphygmomanometers. Arlington, VA: Author. Froman, R., & Owen, S. (1991). Teaching reliabilityandvalidity: Fun with classroom applicdtion.journal of Continuing Education in Nursing, 22, Marx, G., Schwalbe, S., Cho, E., & Whitty, J. (1993). Automated BP measurement in laboring women: Are they reliable? American Journal of Obstetrics and Gjinecology, 168, Marx, G., Sofair, D., & Winikoff, S. (1991). BP values in parturients: Auscultatory vs. oscillatory values. Anestbesiu Anulgesia, 72, Nunnally, J. (1978). Psychometric theory. New York: McGraw- Hill. Ornstein, S., Markert, G., Litchfield, L., & Zemp, L. (1988). Evaluation of the DINAMAP blood pressure monitor in an ambulatory primary setting. Journal of Family Practice, 26, Weaver, M., Park, M., & Lee, D. (1990). Differences in blood pressure levels obtained by auscultatory and oscillometric methods. American Journal of Disease Control, 144, Address for correspondence: Linda A. Green, RN, MS, 26 Yeomans Road, Columbia, CT Linda A. Green ts a staff nurse at St. Francis Hospttal and Medical Center, Hartford, CT. Robtn D. Froman is a professor and director of the Center for Nurstng Research at the School of Nursing, Untversity of Connecticut, Storrs. Februuy 199G J O C N N 159