A Preliminary Evaluation of the Mean Arterial Pressure as Measured by Cuff Oscillometry

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1 articles nature publishing group A Preliminary Evaluation of the Mean Arterial Pressure as Measured by Cuff Oscillometry Harold Smulyan 1, Paul R. Sheehe 2 and Michel E. Safar 3 Background The brachial artery (BA) mean blood pressure (MBP) is now readily available using the oscillometric technique. In contrast to the auscultatory method where MBP is calculated from the systolic (SBP) and diastolic blood pressure (DBP), oscillometric MBP is measured separately from either SBP or DBP. Because the peripheral MBP is free of amplification, it is nearly the same throughout the entire arterial tree and could represent the corresponding aortic pressure. The oscillometric brachial MBP could therefore serve as a surrogate for aortic MBP and might be a valuable non-invasive risk predictor. Methods This study compares the oscillometric BA pressures with simultaneously and directly recorded aortic pressures in 100 patients. The auscultatory method of measuring the blood pressure has been of incalculable value in clinical medicine. Among many other benefits, it provides numerical data for the diagnosis of hypertension and for the assessment of cardiovascular (CV) risk in therapeutic trials and meta analyses. For risk assessment, especially in individuals over the age of 55 years, the diastolic blood pressure (DBP) is less useful because it falls as the individual ages, 1 leaving the systolic blood pressure (SBP), pulse pressure (PP), and the mean blood pressure (MBP) as possible candidates. 2 However, the aortic pressure against which the heart pumps, and not the brachial artery (BA) pressure, is the one that is best suited for the assessment of cardiac risk. Also, the carotid arterial pressure (almost identical to the aortic pressure) would be best suited for assessing the risk of stroke. Unfortunately, the brachial SBP and PP, measured by the auscultatory method, often differ from aortic pressure because of pulse amplification, and are not reliable surrogates for either aortic or carotid pressures. 2 The MBP, by contrast, is not amplified and is known to be very nearly the same in the large elastic and muscular 1 Department of Medicine, Upstate Medical University, State University of New York, Syracuse, New York, USA; 2 Department of Neuroscience and Physiology, Upstate Medical University of New York, Syracuse, New York, USA; 3 Centre de Diagnostic et de Therapeutique, Hopital Hotel Dieu, Paris, France. Correspondence: Harold Smulyan (smulyanh@upstate.edu) Received 20 July 2007; first decision 16 August 2007; accepted 6 November advance online publication 3 January doi: /ajh American Journal of Hypertension, Ltd. Results These results show that, over a wide range of cuff pressures, the oscillometric MBP, whether alone or with age in multiple regression, predicts aortic pressure better than the SBP or DBP do, with a better correlation coefficient (r = +0.91), low aortic cuff MBP difference ( 0.79 mm Hg) and the lowest s.d. of the individual differences (+7.2 mm Hg). Conclusions These results are preliminary and need to be confirmed by larger studies. If confirmed, the predicted aortic pressures should be calculated and displayed by the oscillometric BP devices, the goal being to develop better non-invasive cardiovascular (CV) risk predictors. Am J Hypertens 2008; 21: American Journal of Hypertension, Ltd. arteries. 2 4 If, from a comparison of correlation coefficients, mean differences, and the s.d. of those differences, the brachial MBP could be shown to better represent the aortic MBP, it might prove to be the best brachial pressure of the four for predicting CV risk. This seems plausible given that the brachial MBP has already been described as a CV risk predictor. 5 8 It is possible to calculate the brachial MBP from the SBP and DBP measured by the auscultatory method (DBP + 1/3 PP); however, earlier studies have documented the fact that the onethird rule for calculating the MBP is inaccurate because the aortic and BA pulse shapes change with varying heart rates and with aging. Other, more complex, empirical formulas have therefore been applied to both aortic and BA pressures in an effort to improve MBP accuracy The auscultatory method, using mercury manometry, is being replaced by the automatic oscillometric technique, whose advantages and disadvantages have been recently reviewed. 14 This method measures SBP and DBP, and also measures MBP separately as the lowest cuff pressure at which the oscillations are maximal. Like other BP measurements, this method too has limitations (see Discussion), but still could be a good risk predictor, obviating the need for invasive measurements of aortic pressures or the use of any of the several methods now available for calculating aortic pressures from peripheral pulses The Association for the Advancement of Medical Instrumentation has published standards for the measurement of SBP and DBP, 18 but has not studied the accuracy of the MBP 166 February 2008 VOLUME 21 NUMBER AMERICAN JOURNAL OF HYPERTENSION

2 Oscillometric Mean Blood Pressure articles by any method. Neither has it compared any BP with simultaneous aortic pressures. It was our aim to compare the oscillometric brachial arterial SBP, DBP, PP, and MBP with those in the central aorta in a series of patients representing a wide range of ages and undergoing clinically indicated coronary angiography. Our purpose was to determine whether measurement of the oscillometric MBP is equal or superior to any of the other oscillometric arm pressure measurements as suitable substitutes for aortic pressure in assessing CV risk and, further, to evaluate the influence of age on the prediction of aortic pressures from cuff pressures. Methods Patients. The study group consisted of 100 patients referred for coronary angiography to diagnose or evaluate coronary heart disease. Patient enrollment was prospective but not consecutive and patients were enrolled when the clinical burden in the catheterization laboratory permitted attention to the simultaneity of measurements. Patients with more than mild valvular heart disease, atrial fibrillation and/or frequent premature beats were excluded so as to avoid the variations in the aortic BP induced by changing cycle lengths. There were 49 male patients and 51 female patients, with an average age of 60.4 years and an age range of years. There were 16 patients <50 years of age and 10 patients 75 years of age. The mean body mass index of the patients was 30.7 with an s.d. of 7.8. Twenty-five of the patients were diabetic, 67 had coronary heart disease (defined as a 50% diameter narrowing of at least a single major coronary artery) and 40 patients had an SBP 140 mm Hg. The range of values for SBP was , and for DBP, mm Hg. Eighty-five patients were taking a variety of drugs for the treatment of CV disease. The mean heart rate was 70.7 with an s.d. of 13.2 beats/min. The diversity of the patient cohort makes the data from the study applicable to similar adults with known or suspected coronary heart disease. Institutional Review Board granted permission for the study without individual patient consent allowing only for collection and analysis of routine data. Blood pressure measurement. Brachial artery pressures: Single brachial arterial pressures by oscillometry were measured in the right arm using a previously calibrated (Cuff Link; DNI Nevada, Carson City, NV) Colin Medical Instrument device (San Antonio, TX). Arm cuffs of appropriate widths were used and pressures measured with patients supine, their arms alongside. The oscillometric unit provided a direct readout of the SBP, DBP, and MBP, the last measured separately from the SBP and DBP. However, the exact algorithms used for these measurements are proprietary and unavailable. A second MBP was calculated from the oscillometric SBP and DBP using the standard formula (DBP + 1/3 PP) and this value is hereafter referred to as the calculated MBP. Aortic pressures: Central aortic pressures were recorded through a fluid-filled catheter positioned in the ascending aorta, attached to a strain gauge (Merit Medical, Salt Lake City, UT) set at mid-chest level and amplified by a commercially available catheterization laboratory system (Mennen Medical Horizon 9000 WS, Rehovot, Israel). The performance of the system was tested using both electrical and pressure sine wave generators. At 5.0 and 7.5 Hz there was a 6 and 11% loss of amplitude, respectively, representing a performance adequate for the accurate measurement of the low-frequency aortic SBP and DBP. 2,19 The MBP was obtained by the integrative method. Both phasic and mean aortic pressures were averaged over several respiratory cycles. All pressures were measured simultaneously before injecting contrast media. Statistics. BP values obtained from the arm and from the aorta were compared using univariate least squares regression. The oscillometric brachial MBP s were compared with the aortic values as well as with the MBP s calculated from the SBP and DBP. The comparisons were described using modified Bland Altman analysis 20 as well as multiple regression analysis to measure the relative influence of age and cuff pressures on the prediction of aortic pressures. For purposes of interpretation, the nominal significance of relationships is declared at the two-tailed P < 0.05 level. Results Univariate analyses cuff vs. aorta The best correlation between aorta and cuff was for MBP, with a correlation coefficient of (Figure 1). The plot of correlation shows no distribution differences between patients taking CV medications and those not taking them. Table 1 displays the univariate regression data for each pressure with its corresponding regression equation. These tabulated data are presented with the cuff pressures as the independent variable so that the regression equations can be used for predicting the corresponding aortic pressures. This table shows that the correlation coefficient of for the SBP, is only slightly lower (P = 0.70) than that for the MBP, and that the standard error of estimate about the regression line (SEE) for the MBP (6.8 mm Hg, 6.8% of the mean value) is lower than that for the SBP (12.0 mm Hg, 8.7% of the mean value). Neither the differences in the r values nor the SEE s are statistically significant. Cuff Mean blood pressure y = 0.97x R 2 = 0.83 r = Aorta Figure 1 Scatter plot of oscillometric cuff mean blood pressures vs. aortic mean blood pressures. The regression equations are displayed. R 2 = coefficient of determination, r = correlation coefficient. Patients taking cardiovascular medications are represented by dots and those on no medications are represented by squares. 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3 articles Oscillometric Mean Blood Pressure Table 1 Relationships among cuff oscillometric and aortic pressures univariate analysis Mean r Slope Intercept SEE P SBP Cuff <0.001 Aorta 139 DBP Cuff <0.001 Aorta 73 PP Cuff <0.001 Aorta 65 MBP Cuff <0.001 Aorta 99 Calculated MBP Cuff <0.001 Aorta 99 Cuff pressure as the independent variable. DBP, diastolic blood pressure; MBP, mean blood pressure; P, probability; PP, pulse pressure; r, correlation coefficient; SBP, systolic blood pressure; SEE, standard error of estimate. Aorta cuff difference Pulse pressure y = 0.25x 14.0 R 2 = 0.22 r = Aorta Figure 2 Modified Bland Altman scatter plot of aortic oscillometric cuff pulse pressure differences vs. aortic pulse pressures. The regression equations are displayed. R 2 = coefficient of determination, r = correlation coefficient. Univariate analysis cuff/aortic differences Modified Bland Altman plots display the differences between the various aortic pressures and their corresponding cuff pressures as a function of the aortic pressures themselves. Figure 2 shows a significant trend for the PP, with the differences between the aorta and cuff becoming positive at higher PPs (r = +0.47; P < 0.001). However, the shift of differences at higher values does not hold true for the modified Bland Altman plot for MBP, relating the aortic cuff differences to the aortic MBP s (Figure 3). The modified Bland Altman data for all of the pressures are detailed in Table 2. This table shows that, as in the case of the PPs, the aortic SBPs exceed the cuff SBPs at higher aortic pressures, thereby helping to explain the findings displayed in Figure 2. These relationships are not present in the case of the DBP. Table 2 also shows that the standard deviation of the individual differences is descriptively lowest Aorta cuff difference Mean blood pressure y = 0.03x 3.87 R 2 = r = Aorta Figure 3 Modified Bland Altman scatter plot of aortic oscillometric cuff mean blood pressure differences vs. aortic mean pressures. The regression equations are displayed. R 2 = coefficient of determination, r = correlation coefficient. Table 2 Modified Bland Altman plots comparing aortic pressures with aortic/cuff pressure differences Mean s.d. r P SBP DBP PP <0.001 MBP Calculated MBP DBP, diastolic blood pressure; MBP, mean blood pressure; Mean, mean differences; P, probability; PP, pulse pressure; r, correlation coefficient; SBP, systolic blood pressure; s.d., standard deviation of the differences. for the oscillometric MBP; however, this conclusion cannot be directly tested. Separate analyses for male and female patients showed no demonstrable differences. There are weak but significant (P < 0.05) associations between the aortic SBP and MBP and the body mass index (r = for both), similar to those obtaining for brachial SBP and MBP (r = and +0.20, respectively (data not shown). Influence of age Univariate analysis showed that many of the above data are influenced by the age of the patient. Table 3 shows that, as expected, the aortic SBP and PP rise with age while the DBP falls. This relationship is not demonstrable for the cuff SBP and DBP measurements. Also as expected, neither cuff MBPs nor aortic MBPs show a significant relationship to aging. Multiple regression analysis, however, shows that the cuff predictions of aortic MBP, SBP, and PP are all influenced by age, independent of cuff pressures, with significant regression coefficients of 0.33, 0.33, and 0.37, respectively (all P < 0.001). It follows that the prediction of aortic MBP and SBP from the cuff MBP and SBP, respectively, can be improved by the use of the following equations: aortic MBP = (0.90 Cuff MBP) (0.17 age) aortic SBP = (0.88 cuff SBP) + (0.33 age) These equations provide small but significant improvements in aortic MBP and SBP prediction. With age correction, the 168 FEBRUARY 2008 VOLUME 21 NUMBER 2 AMERICAN JOURNAL OF HYPERTENSION

4 Oscillometric Mean Blood Pressure articles Table 3 Relationships of cuff and aortic pressures Pressure/site r P Slope Intercept SEE Relationships of cuff and aortic pressures to age SBP/aorta SBP/cuff DBP/aorta 0.27 < DBP/cuff PP/aorta PP/cuff MBP/aorta MBP/cuff Relationships of aortic/cuff pressure differences to age SBP DBP PP MBP DBP, diastolic blood pressure; MBP, mean blood pressure; P, probability; PP, pulse pressure; r = correlation coefficient; SBP, systolic blood pressure; SEE = standard error of estimate. Calculated cuff y = 0.15x R 2 = 0.10 r = 0.32 Mean blood pressure Age Figure 4 Scatter plot of differences between calculated and oscillometric cuff brachial mean blood pressures (MBPs) vs. age. The regression equations are displayed. R 2 = coefficient of determination, r = correlation coefficient. correlation coefficient relating cuff to aortic MBP increases from to and the SEE decreases from ±6.8 to ±6.5 mm Hg (P = for both), while the r value relating cuff to aortic SBP increases from to and the SEE falls from to (P = for both). Additional analyses for aortic MBP and SBP showed that neither diabetes mellitus (P = 0.27; 0.14) nor body mass index (P = 0.15; 0.14) contributed to the prediction of either pressure. Table 3 details the relationships between aortic/cuff pressure differences and age. There are positive relationships between these differences and aging for SBP and PP, because of the greater increase in aortic SBP and PP with aging when compared with arm pressures. The opposite results are obtained when the aortic/cuff MBP differences are related to age. This negative relationship is because of the slight rise in cuff MBP and the slight fall in aortic MBP with age, magnified when they are presented as differences. Calculated vs. cuff MBP As expected, the correlation between the calculated MBP and the measured cuff MBP is excellent with a correlation coefficient of (calculated MBP = cuff MBP data not shown). The differences between the values again relates to age. The plot shown in Figure 4 displays a significant relationship (r = 0.32, P < 0.001), with the calculated/cuff differences growing greater with aging, as the measured cuff MBPs become higher than the calculated values. Discussion Limitations of auscultatory BP measurements: The data published by the Association for the Advancement of Medical Instrumentation for The American National Standard for Blood Pressure Measurement, show that the auscultatory SBP and DBP represent intra-atrial pressures only poorly. 18 The Association for the Advancement of Medical Instrumentation compared the two methods in five studies totaling 197 patients, and found the average difference between arterial and cuff SBP ranged from 0.9 to 12.3 mm Hg with s.d. ranging from 1.3 to 13.0 mm Hg. Similar comparisons for DBP showed average differences from 8.3 to 18.0 with s.d. ranging from 1.1 to 9.3 mm Hg. These inconsistencies are necessarily passed along to the MBP and PP when they are calculated from the auscultatory SBP and DBP. Furthermore, depending upon the age of the patient, the brachial SBP and PP will differ from aorta pressures, making these cuff pressures unreliable surrogates for aortic pressures. There are now several techniques available to calculate aortic pressures from non-invasive recordings of peripheral pulses, in an effort to circumvent both the liabilities of the cuff method and the requirement for vascular invasion However, these techniques require additional equipment and expertise and, more importantly, are each dependent on calibration of the external pulses by the unreliable arm cuff methods. Oscillometric BA pressures: Conceptually, the oscillometric brachial MBP offers a partial solution, because the oscillometric method measures MBP separately from the SBP and DBP. Unfortunately, the Association for the Advancement of Medical Instrumentation has never studied the accuracy or precision of the MBP by any method. Not all manufacturers of oscillometric devices measure or display the MBP, probably because the oscillometric MBP measurement has its own problems. In 1969, Posey et al., 21 showed that the lowest compression chamber pressure at its point of maximum oscillation was closely related to the arterial MBP, the latter determined by electronic damping. Subsequent studies have demonstrated that the compression pressure at maximum oscillations is, unfortunately, influenced by other factors in addition to the MBP. These factors include the nature of the compressing substance (fluid vs. air), the volume of air in the compression chamber (cuff), the distensibility of the enclosed artery, the PP, the heart rate vs. the rate of cuff deflation and even the shape of the arterial pulse AMERICAN JOURNAL OF HYPERTENSION VOLUME 21 NUMBER 2 FEBRUARY

5 articles Oscillometric Mean Blood Pressure The question is whether these extraneous factors significantly impair the usefulness of oscillometric cuff brachial MBP measurements as a clinical tool. An early clinical study compared oscillometric MBPs with auscultatory MBPs, 25 and the European Society of Hypertension has published an extensive review of such comparisons carried out among a large number of oscillometer models from various manufacturers. 26 These studies show a wide spectrum of results, partly because of the differences in the various manufacturers models and their individual proprietary algorithms. Oscillometric MBPs have been compared with direct recordings of radial arterial pressures in patients with clinically indicated indwelling arterial lines, and it was shown that there was good agreement between the cuff and direct arterial pressures. Although useful, these reports did not compare the oscillometric cuff MBP with that of the central aorta. Such a comparison was first reported in 1982 by Borow 30 in 30 patients during cardiac catheterization. The correlation coefficient of the values obtained by using the two methods was +0.89, the mean difference between the MBP measurements was 3.5 mm Hg and the s.d. of the errors was 7% of the aortic MBP. Twenty of the 30 patients had an average error of <5%. No further studies of this sort were reported until 2006 when Umana et al., 31 reported a persistent underestimation of the aortic SBP when using the brachial cuff technique and overestimation of the DBP in both normal and overweight patients. However, MBP comparisons were not reported and no analysis was made of the effects of the patient s age on pressure values. Oscillometric MBP as a surrogate for aortic MBP effects of aging: Our data showed that the best cuff/aorta correlations were for MBP, with SBP close behind. As expected, aging was associated with increasing aortic SBP and PP, with little effect on aortic MBP. However, the differences between cuff and aortic SBP, PP, and MBP were also affected by aging. Our regression line relating age to the differences between aorta SBP and cuff SBP crossed the zero difference point at age 60. The differences between oscillometric MBP s and calculated MBP s also grew larger with aging, supporting the view that, with increasing age of the patient, the calculated MBP became less accurate because of the changes in arterial pulse shape. When age was included as a factor in multiple regression analyses, it was found to be a small but significant factor in the prediction of aortic MBP and SBP from their respective cuff pressures. Using the multiple regression equations, aortic MBP and SBP, predicted and age-corrected, can be calculated, and the difference between them can be used for calibrating the vertical distance between these two pressure values on an applanated carotid pulse trace. This would permit the further, non-invasive calculation of DBP and PP. MBP and PP should be the two best aortic pressures for risk prediction, given that MBP reflects the status of the peripheral resistance and PP that of aortic stiffness. Of the four oscillometric cuff pressures, the age-corrected MBP appears overall to be the most suitable for predicting the aortic pressure in this population. The MBP has, descriptively though not to a significant extent, the smallest average difference between cuff and aorta pressure values and the smallest s.d. of differences. It also has the highest correlation coefficient and a low SEE. The age-corrected cuff SBP appears to be the next best. Limitations of the study: The cohort of 100 is small, allowing its number and variability to heavily influence the analysis. Some of the variability is a result of the comparison of a single cuff reading to multiple aortic pressures averaged over several respirations. The respiration-driven variations in aortic phasic and mean pressures can be large in patients who are apprehensive and mildly sedated while undergoing coronary angiography. Although fluid-filled catheters were used for measuring aortic pressures, the tested frequency characteristics of the system were satisfactory for recording SBP, DBP, and MBP. Our study population was diverse with regard to age and comprised patients of both genders but only 40% were hypertensive, and therefore this sample was unlikely to adequately represent the vast number of individuals being evaluated for hypertension. Our data may not be exactly replicated by oscillometric cuff BPs measured using models made by other manufacturers, because the algorithms used for pressure detection are not standardized and may differ. Oscillometric devices are now in frequent use, but estimates of their accuracy and precision are limited, and the results are therefore difficult to validate. More studies are needed in order to compare oscillometric with intra-arterial pressures, but these studies are problematic, and data measured using different devices are difficult to interpret. Given the ongoing competition among manufacturers, it seems unlikely that standardized algorithms for all units will be available any time soon, but MBP should, at least, be a standard display in all models. In summary, this initial study suggests that the oscillometric MBP (and possibly the SBP) together with age, can usefully predict aortic pressures and have potential as non-invasive risk predictors in future epidemiologic studies. Clearly, larger studies are needed to confirm these findings. Acknowledgments: The authors acknowledge the support and cooperation of Hani Kozman MD, Director of the Cardiac Catheterization Laboratory at the Upstate Medical University as well as of the other physicians and nurses of the laboratory who willingly helped with the acquisition of the data. 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