American Journal of Critical Care. 2006;15: 196-205
Copyright © 2006 by the American Association of Critical-Care Nurses.
Clinical Comparison of Automatic, Noninvasive Measurements of Blood Pressure in the Forearm and Upper Arm With the Patient Supine or With the Head of the Bed Raised 45°: A Follow-Up Study
By
Kathleen Schell, RN, DNSc,
Denise Lyons, RN, MSN, APRN, BC,
Elisabeth Bradley, RN, MS, APRN, BC, CCRN,
Linda Bucher, RN, DNSc,
Maureen Seckel, RN, MSN, APRN, BC, CCRN,
Sandra Wakai, RN, BSN, CCRN,
Elizabeth Carson, RNC,
Julie Waterhouse, RN, PhD,
Melanie Chichester, RNC, BSN,
Deborah Bartell, RN, BC, BSN,
Theresa Foraker, RN, BSN and
E. Kathleen Simpson, RNC, BSN.
From
University of Delaware (KS, LB, JW) and Christiana Care Health System (DL, EB, LB, MS, SW, EC, MC, DB, TF, EKS), Newark, Del.
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Abstract
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• Background Noninvasive measurement of blood pressure in the forearm is used when the upper arm is inaccessible and/or when available blood pressure cuffs do not fit a patient’s arm. Evidence supporting this practice is limited.
• Objective To compare noninvasive measurements of blood pressure in the forearm and upper arm of medical-surgical inpatients positioned supine and with the head of the bed raised 45°.
• Methods Cuff size was selected on the basis of forearm and upper arm circumference and manufacturers’ recommendations. With a Welch Allyn Vital Signs 420 Series monitor, blood pressures were measured in the forearm and then in the upper arm of 221 supine patients with their arms resting at their sides. Patients were repositioned with the head of the bed elevated 45° and after 2 minutes, blood pressures were measured in the upper arm and then the forearm. Starting position was alternated on subsequent subjects.
• Results Paired t tests revealed significant differences between systolic and diastolic blood pressures measured in the upper arm and forearm with patients supine and with the head of the bed elevated 45°. The Bland-Altman procedure revealed that the distances between the mean values and the limits of agreement were from 15 to 33 mm Hg for individual subjects.
• Conclusions Noninvasive measurements of blood pressure in the forearm and upper arm cannot be interchanged in medical-surgical patients who are supine or in patients with the head of the bed elevated 45°.
The importance of accurate measurement of blood pressure is well established. 1–4 The American Heart Association provides specific recommendations for the indirect measurement of blood pressure, including recommendations on cuff size and placement.1 Additionally, the National Heart, Lung, and Blood Institute provides guidelines for prevention and treatment of hypertension.5 In critical care and acute care, treatment decisions are based on the stability of a patient’s clinical condition. Assessment of stability includes accurate measurement of blood pressure.6
Automated oscillometric devices for noninvasive monitoring of blood pressure have become the standard, replacing the stethoscope and sphygmomanometer in hospitals.7 The challenge of hearing and recording Korotkoff sounds is eliminated with the use of these devices; however, other sources of human error related to cuff size and positioning of the arm remain. Another challenge to accuracy is evidenced in emergency departments, operating rooms, maternity wards, and other acute care areas where workers are increasingly using the forearm as an alternative site for blood pressure measurement when available cuffs do not fit the patient’s upper arm or when the upper arm is inaccessible.1,8 For example, thigh cuffs may not fit the upper arm of an obese patient or a patient’s forearm may be more easily accessed for blood pressures than the upper arm after the patient has been positioned and draped for surgery. Because of the lack of detailed, valid, and reliable information on measurement of blood pressure in the forearm from experts and manufacturers of oscillometric monitoring devices, further investigation is warranted.
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Review of the Literature
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A detailed review of the literature on use of the forearm as an alternative site for measurement of blood pressure has been published.9 Latman and Latman10 and Emerick11 compared blood pressures measured in the wrist and the upper arm, respectively; therefore their findings cannot be compared with results of an investigation of forearm measurements. Singer et al8 compared sequential, automatic, noninvasive measurements of blood pressure obtained in the forearm versus the upper arm in a convenience sample of 151 patients seated in the emergency department with their left arms positioned at heart level. Mean (SD) systolic blood pressures measured in the forearm and the upper arm were 129.8 (20.7) and 126.2 (17.6) mm Hg, respectively (P = .002). Mean (SD) diastolic blood pressures measured in the forearm and upper arm were 80.7 (14.5) and 76.8 (13.4) mm Hg, respectively (P < .001). Correlations between blood pressures measured in the forearm and in the upper arm were 0.75 for systolic blood pressures and 0.72 for diastolic blood pressures (P < .001). Singer et al8 concluded that it was reasonable to use the forearm to measure blood pressure when the upper arm was not available.
In a replication study with 204 medical patients in stable condition in an emergency department,9 blood pressures were noninvasively and automatically measured in the left upper arm and forearm with subjects seated with their arms at heart level. The Pearson correlation coefficient (R) between systolic blood pressures measured in the upper arm and the forearm was 0.884, and the correlation between diastolic blood pressures measured at those 2 spots was 0.763 (P <.001). Paired t tests revealed significant differences between mean systolic blood pressures measured in the upper arm and the forearm (t=2.07, P=.04). The Bland-Altman procedure indicated that the distances between the mean values and the limits of agreement were from 15 mm Hg (mean arterial pressures) to 18.4 mm Hg (systolic blood pressures), suggesting that noninvasive measurements of blood pressure in the upper arm and forearm are not interchangeable.
The findings from previous studies have contributed important information to the issue of monitoring blood pressure in the forearm; however, the procedures for data collection varied. Netea et al12 described numerous sources of potential errors in blood pressure studies, including failure of the researchers to (1) define cuff placement in relation to the patient’s heart level, (2) describe the precise body and arm positions of patients and the methods used to measure blood pressures, and (3) use consistent measurement devices. Discrepancies in the procedures for data collection and the inconsistent results of studies in which blood pressure measurements in the forearm and upper arm were compared suggest the need for further research.
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Purposes
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The purposes of this study were (1) to compare noninvasive measurements of blood pressure obtained in the forearm versus the upper arm with the patients’ arms resting at their sides and (2) to determine if the patient’s position (supine or with the head of the bed elevated 45°) affects correlations between noninvasive measurements of blood pressure obtained in the forearm and the upper arm.
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Methods
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A descriptive correlational comparison study was conducted in 5 medical-surgical units of a 780-bed acute care facility, Christiana Hospital, Newark, Del, which is part of the Christiana Care Health System. The study was approved by the institutional review board and was implemented in January 2004.
Sampling
A convenience sample of English-speaking inpatients, 18 years or older and in stable condition, was recruited. Potential subjects were excluded if (1) the available noninvasive blood pressure cuffs did not fit the patient’s upper arm or forearm, (2) the patient was unable to expose the arms for blood pressure to be measured, (3) the patient was unable to tolerate either of the body positions required for the study, (4) the patient had a condition that precluded measurement of blood pressure in both arms, (5) the patient was on isolation, and/or (6) the patient was in a specialty bed other than a bariatric bed. A power analysis indicated that a sample size of 203 would have 80% power to detect an effect size of 5 mm Hg (SD 18 mm Hg) at 
= .05. A difference of 5 mm Hg or greater between blood pressures measured in the upper arm and forearm was considered clinically significant.
Instruments
The Welch Allyn Spot Vital Signs 420 Series non-invasive blood pressure monitor (Welch Allyn, Beaverton, Ore) was used for the study. The accuracy of the monitor meets or exceeds SP10-1992 Association for the Advancement of Medical Instrumentation standards (mean error ±5 mm Hg, SD 8 mm Hg). The accuracy of blood pressure measurements was validated for measurements in the upper arm only.13 Four new monitors provided by Welch Allyn were used exclusively for this study. The monitors were calibrated by the hospital system’s clinical engineering department before the start of data collection.
A demographic/clinical data form, based on the form used by Schell et al,9 was modified for this study. Each patient’s medical record number, age, sex, race, and smoking history (within the past 12 months) were obtained when the patient’s blood pressure was measured. Clinical data collected for each patient included circumference of the upper arm and forearm, cuff size, blood pressure, heart rate, order of measurements, which arm was used, position in bed, and time between measurements and repositioning. Patients’ heights, weights, and admitting diagnoses were collected retrospectively through electronic chart review.
Procedure
Before each data collection session, potential subjects were identified, and the stability of their clinical condition was validated by the charge nurse and/or staff nurse. Patients who met the inclusion criteria were approached in their rooms. With privacy ensured, patients were provided a brief, scripted description of the study to determine if they were interested in participating in the study. Patients who were interested were given a formal explanation of the study, and written informed consent was obtained.
Blood pressures were measured in accordance with the 1993 American Heart Association standards14 and the monitor manufacturer’s instructions and recommendations for cuff sizes (Table 1
). For each patient, only one of the patient’s arms was used for all measurements, and blood pressures were measured at least 2 minutes after changes in body position. Patients were asked to remain quiet during these measurements. Figure 1 shows details of the data collection procedure. Error messages displayed by the noninvasive blood pressure monitor were handled according to Welch Allyn guidelines. For each patient, abnormally high or low blood pressures, other unusual signs or symptoms, and changes in condition were communicated to the nurse assigned to the patient.
Before data were collected, 9 data collectors were trained to use the Welch Allyn Spot Vital Signs 420 Series noninvasive blood pressure monitor by a Welch Allyn representative. Interrater reliability was established for measurement of arm circumference and selection of cuff size (mean .97). Proper positioning of the bed was confirmed via the angle indicator on the electric beds. Two researchers abstracted demographic and clinical data from the electronic charts of the subjects; any discrepancies were resolved via electronic mail discussion and agreement among the researchers and 2 co–principal investigators (K.S., D.L.). Intercoder reliability was established at 98% agreement for the 6 data coders before data entry into Microsoft Word files.
Data Analysis
Data were analyzed by using Statistical Analysis System (SAS) Version 8.2 (SAS, Inc, Cary, NC, 1999–2000). Descriptive statistics were calculated by using Proc Means and Proc Freq. Pearson correlation coefficients (Proc Corr) were used to determine the relationship between pressures measured in the upper arm and pressures measured in the forearm. Paired t tests (Proc Means) were used to test for differences between the group measurements. The level was set at .05. Med-Calc for Windows, version 7.4.2.0
[EC]
(MedCalc Software, Mariakerke, Belgium, 2004) was used to conduct Bland-Altman agreement analyses15 to determine the extent of agreement between pressures measured in the upper arm and the forearm of each patient.
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Results
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A total of 225 patients were recruited, but 2 were excluded because the wrong cuff size was selected, and 2 others were excluded because of error codes on the blood pressure monitor. Demographic data for the remaining 221 patients are presented in Table 2. Patients were 18 to 93 years old (mean 61.5 years). More than half (~53%) of the subjects were women, a large majority (~83%) were white, and most (~64%) had body mass indexes indicating they were overweight or obese. Most subjects (~80%) required adult or large adult cuffs. The most frequent admitting diagnoses were related to cardiovascular diseases and respiratory and gastrointestinal disorders.
Mean systolic blood pressures, diastolic blood pressures, and mean arterial pressures measured in the forearm were higher than the corresponding pressures measured in the upper arm, both when patients were supine and when the head of the bed was elevated 45° (Table 3). Results of paired t tests indicated that differences between systolic, diastolic, and mean pressures measured in the upper arm and the forearm were statistically significant regardless of whether patients were supine or elevated 45° (Table 4). The correlations between blood pressures measured in the upper arm and forearm were also statistically significant (Table 5).
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Table 3 Descriptive statistics of arm circumference, cuff size, blood pressure, and heart rate of sample with patients supine and with head of bed elevated 45°
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Table 4 Results of paired t tests for measurements of blood pressure in the forearm and the upper arm with patients supine vs with the head of the bed elevated 45°
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Table 5 Correlations between measurements made in the upper arm and in the forearm with patients supine and with the head of the bed elevated 45°
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Bland-Altman analyses15 were used to determine the agreement between blood pressures measured in the upper arm and forearm with patients supine and with the head of the bed elevated 45°. The computed upper and lower limits of agreement for systolic blood pressures in supine patients suggested that 95% of forearm measurements were between +11 mm Hg and –27.6 mm Hg of upper arm measurements (Figure 2). The upper and lower limits of agreement for systolic blood pressures with the head of the bed elevated 45° showed even greater differences (Figure 3). The limits of agreement for diastolic blood pressures (Figures 4 and 5) and mean arterial pressures (Figures 6 and 7) also indicate large differences among individual patients.
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Discussion
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Despite a lack of statistical support, some clinicians have incorporated measuring blood pressure in patients’ forearms into practice. This study was completed as a follow-up to a previous study9 to include acute care patients. Additionally, for each patient, the arms were positioned at the patient’s side, as typically done at Christina Hospital, rather than at the level of the patient’s heart. The effects of patients’ body mass index and position in bed on differences between forearm and upper arm pressures were explored.
As in previous studies,8–11 we found a strong correlation between measurements of blood pressure in the forearm and in the upper arm. Paired t tests revealed statistically significant differences in systolic, diastolic, and mean arterial blood pressures measured in the forearm and the upper arm with the patient supine and with the head of the bed elevated 45°. These results differ from those of the earlier study,9 in which the only statistically signif icant difference was between mean systolic blood pressures measured in the forearm and upper arm. Possibly, having the patient’s arm positioned at heart level decreased these differences in the earlier study.9
The importance of positioning the patient’s arm at the same level as the patient’s heart when measuring blood pressure has been promoted consistently by experts.1,8,12,16,17 After reviewing the literature on the accuracy of blood pressure measurement in the upper arm, Netea et al12 recommended that, ideally, the arm should be supported passively with the cubital fossa positioned at the approximate level of the right atrium for sitting and standing patients. For supine patients, the cubital fossa should be placed halfway between the bed surface and the sternum. Netea et al emphasized the importance of documenting the position of both the patient’s body and the patient’s arm in research on blood pressure and in practice. We did not find any published recommendations for positioning of the patient’s forearm at heart level.
Netea et al12 also concluded that blood pressures measured in the upper arm with a patient seated should not be considered equivalent to blood pressures measured with the patient supine. Body position was also influential in our study. The group mean differences in blood pressures measured in the forearm versus the upper arm were greater when the head of the bed was elevated 45° than when the patient was supine. Possibly, the 45° elevated position had the greatest variation because patients’ arms rested at the patients’ sides and not at heart level in each body position, increasing the influence of hydrostatic pressure on the values. However, the finding that blood pressures measured at 45° elevation were higher than blood pressures measured with patients supine contradicts the results of Terent and Breig-Asberg16 and Netea et al,17 who found that with patients’ arms at heart level, blood pressures were higher with patients supine than with patients sitting.
Consistent with the results of the earlier study,9 mean differences in blood pressure were unrelated to patients’ clinical diagnoses, sex, cuff size, or order of cuff placement. Race did not influence differences between blood pressures measured in the forearm and the upper arm as in the earlier study9; however, approximately 85% of the participants in the study reported here were white. Finally, differences between blood pressures measured in the forearm and upper arm were not associated with smoking or body mass indexes.
Blood pressures measured in the forearm were consistently higher than blood pressures measured in the upper arm. According to Pickering et al,1 it is generally accepted that systolic blood pressure increases in more distal arteries, whereas diastolic pressure decreases. If one assumes that forearm pressures reflect distal artery readings, a decrease in diastolic blood pressure from upper arm to forearm was not evident in the group data from the current study.
The results from the Bland-Altman procedure indicated further variation for individual subjects. Differences in forearm and upper arm blood pressures were between approximately 7 and 28 mm Hg in supine patients and between 3 and 33 mm Hg in the patients elevated 45°. Overall, the limits of agreement for measurements of systolic, diastolic, and mean blood pressure exceeded the 5 mm Hg difference that was considered clinically significant. These data indicate that blood pressures measured in the upper arm and in the forearm are not interchangeable for subjects positioned supine or with the head of the bed elevated 45°. These findings are consistent with those of the earlier study9; however, it remains difficult to determine in which patients these differences are clinically significant.
Limitations
This study has several limitations. Noninvasive blood pressure monitors found in hospitals are not specifically designed for measuring blood pressures in the forearm and may introduce measurement error. Bias is possible with the use of convenience sampling but was controlled through collection of demographic data and clinical diagnoses. Coding of clinical diagnoses was challenging at times because of subjectivity; however, coders discussed questionable diagnoses with the principal investigators to obtain consensus.
The findings in this study can be generalized only to hospitalized patients whose arms are positioned at their side while blood pressure is being measured. The ability to generalize findings to patients who (1) are nonwhite, (2) smoke, and/or (3) require extra-large blood pressure cuffs is also limited because of the small number of such subjects in the study.
Future Research
Investigation of the relationship between blood pressures measured in the forearm and in the upper arm in studies in which a predictive design is used could improve interpretation of measurements obtained in the forearm. Although body mass index did not influence differences between blood pressures measured in the forearm and in the upper arm in this study, focusing only on overweight subjects might have different results. Exploration of anatomical structures such as subcutaneous tissue and vessel size and diameter may also provide insight into which subjects will experience differences in blood pressures measured in the forearm versus the upper arm. Finally, future studies are indicated in patients who are pregnant, are critically ill, or smoke tobacco.
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Conclusions and Clinical Implications
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These findings indicate that despite strict attention to correct cuff size, in the acute care setting, noninvasive measurements of blood pressure in the upper arm and forearm are not interchangeable in adult patients positioned supine or in patients with the head of the bed elevated 45°. Measuring blood pressure in the forearm is not recommended; however, situations in which the upper arm is inaccessible or in which the proper-sized cuff is not available will continue to occur. If the forearm must be used, selection of the correct cuff size, placement of the forearm at the level of the patient’s heart, documentation of the forearm location, and use of the same site for further measurements of blood pressure are crucial to accurate and meaningful measurements.1,10,16 Institutional procedures for measuring blood pressure should include these recommendations if use of the forearm to measure blood pressure persists. Clinicians who manage patients must realize that blood pressures measured in the forearm may differ from blood pressures measured in the upper arm by up to 33 mm Hg and that blood pressures measured in the forearm may be higher than blood pressures measured in the upper arm.
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ACKNOWLEDGMENTS
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This research was funded in part by a grant from the Southeastern Pennsylvania Chapter of the American Association of Critical-Care Nurses, Wynnewood, Pa. Blood pressure monitors and cuffs were provided by Welch Allyn. We thank the administration and staff of the medical-surgical units at Christiana Care Health System who supported this study. Finally, we are especially grateful to the medical-surgical patients who participated in the study.
To purchase electronic or print reprints, contact The InnoVision Group, 101 Columbia, Aliso Viejo, CA 92656. Phone, (800) 809-2273 or (949) 362-2050 (ext 532); fax, (949) 362-2049; e-mail, reprints{at}aacn.org.
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- Joint National Committee on Detection, Evaluation, and Treatment of High Blood Pressure. The Seventh Report of the Joint National Committee on Detection, Evaluation, and Treatment of High Blood Pressure. Bethesda, Md: National Institutes of Health; 2004. NIH publication No. 04-5230.
- American Heart Association. ACLS Provider Manual. Dallas, Tex: American Heart Association; 2001.
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Abstract
Review of the Literature
