It is important to select an appropriate cuff and bladder to compress the brachial artery evenly and obtain an accurate blood pressure (BP) measurement in people with large arms [1,2,3,4]. Previous studies have documented that in most obese patients, the upper arms are tronco-conical [3,4,5,6], and recent guidelines recommend that conically shaped cuffs and bladders should be used for these patients [2]. However, in most obese patients, the shape of the upper arm is not tronco-conical but is best depicted by the sum of two truncated cones with different slant angles (Fig. 1). The aim of this study was to investigate the relationship between the shape of the arm and the BP measurement error when a cylindrical cuff is used in very obese subjects with arm circumferences of >40 cm.

Fig. 1
figure 1

Sketch showing two different models of the upper arm in subjects with large arms. In the upper panel, the arm is considered a single truncated cone, and the conical shape is defined by the slant angle of the frustum of the cone (base α). In the lower panel, the upper arm is divided into two truncated cones. The upper α and middle α represent the slant angles for the proximal and distal truncated cones, respectively. The two slant angles are similar in subjects with normal size arms, whereas in subjects with large arms, the middle slant angle is smaller than the upper angle

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Seventy-nine adults aged 46.4 ± 12.6 years (31 men) with upper arm circumferences of >40 cm (obese participants) and 79 control subjects with the same age and sex distribution and upper arm circumferences of <33 cm were enroled. The arm dimensions were measured according to previously published procedures [5, 6]. We considered the upper arm either as a single truncated cone (model 1) or as the sum of two tronco-conical shapes, with the circumferences of the proximal and middle arm serving as the bases, respectively (model 2) (Fig. 1). For model 1, the proximal and distal arm circumferences and the upper arm length were used to calculate the slant angle [5, 6]. The same procedure was used to calculate the slant angles of the two truncated cones for arm model 2, namely, “upper α” and “middle α” (Fig. 1). If the difference between “upper α” and “middle α” was equal to zero, the upper arm could be approximated by a single truncated cone. If the difference was positive (model 2), then the distal half of the arm was more conical than was the proximal half. BP was measured with the standard auscultatory method in the sitting position. For both groups, one tronco-conical and one cylindrical cuff and bladder of an appropriate size were used. In the obese group, the tronco-conical cuff and bladder had an 85.0° base slant angle. For the control subjects, two different tronco-conical and two different cylindrical cuffs and bladders were used depending on whether the middle arm circumference was between 22 and 27.5 cm or between 28 and 32.5 cm [5]. The BP was measured by two observers using the procedures recommended by the 2018 International Protocol for the validation of BP measuring devices [7]. For between-group comparisons, ANOVA was used. Predictors of the between-cuff BP differences were included in the multivariable linear regression analyses.

In all subjects, the upper arm “base α” was <90° and was much smaller in the obese subjects than in the control subjects (p < 0.0001). In the obese group, the “middle α” was smaller than the “upper α”, whereas in the controls, the two angles were similar. Therefore, among the obese subjects, the distal half of the upper arm had a more pronounced conical shape than did the proximal half, whereas among the non-obese participants, the shape of the upper arm could be approximated by a single truncated cone. In the non-obese control subjects, the systolic and diastolic BPs were slightly higher when they were measured with the cylindrical cuff than when they were measured with the tronco-conical cuff (0.9 ± 4.7/0.4 ± 3.9 mmHg, p = 0.002/n.s.). In contrast, in the obese group, with the cylindrical cuff, the systolic and diastolic BPs were overestimated by 4.3 ± 5.4/3.1 ± 4.7 mmHg (both p < 0.0001 also after adjustments for age, sex, and BP). In the whole study sample, the difference between the “upper α” and “middle α” was correlated with the systolic (r = 0.17, p = 0.0008) and diastolic (r = 0.23, p < 0.0001) BP discrepancies between the cylindrical and tronco-conical cuff. In the obese participants with a “base α” of <85°, the “upper α-middle α” difference was 3.7 ± 0.2°, whereas it was 2.4 ± 0.3° in those with a “base α” of ≥85° (p = 0.001). In other words, the more conical the shape of the arm was, the larger the difference between the proximal and distal halves of the arm. In a multivariable linear regression that included sex, age, skinfold thickness, upper arm length, upper arm middle circumference and BP as independent variables, the difference between the “upper α” and “middle α” was an independent predictor of the between-cuff BP differences in men (p = 0.004/0.0007, Table 1). The between-cuff BP discrepancies were significantly larger in the group of men with a “base α” of <85° and an “upper α-middle α” difference of > 2° (N = 15) (8.0 ± 0.8/5.1 ± 0.7 mmHg) than in the remaining men (3.6 ± 0.8/1.9 ± 0.7 mmHg, p = 0.0001/0.0009).

Table 1 Predictors of the between-cuff systolic and diastolic blood pressure discrepancies in 31 obese men
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The present data show that the upper arm can be approximated by a single truncated cone in people with an arm circumference of <33 cm, whereas in obese people, the shape of the arm is formed by two truncated cones of equal height. The measurement error associated with cylindrical cuffs was more pronounced in obese people for whom the distal half of the arm was more conical than the proximal half. In the men with a difference between the slant angles of the two truncated cones of >2° and a base angle of <85°, the BP differences between the two cuffs were 8.0 and 5.0 mmHg. When the slant angles of the frustums of the two truncated cones differ by more than 2°, a cylindrical cuff cannot exert a homogeneous level of pressure over the entire arm because the distal part of the cuff remains loose over the arm, transmitting a lower pressure to the tissues overlying the artery [8, 9]. In the present study, we used a tronco-conical cuff with an 85° slant angle in the obese group on the basis of previous data from our laboratory [6]. This cuff can apply a more uniform pressure on the arm surface than a cylindrical cuff, but in subjects with an arm slant angle of <85°, the large differences between the shapes of the proximal and distal halves of the arm may have even reduced the accuracy of the tronco-conical cuff in this study. It is thus likely that even the single tronco-conical cuff yielded BP values that were higher than the actual values in the highly obese subjects in this study because the entire length of the cuff did not fit their arms. A more anatomical cuff reflecting the actual shape of these arms may likely yield more accurate BP readings. Given the increasing number of people with very large arms, appropriately shaped cuffs should be developed by manufacturers for people with morbid obesity.