Comparison between cuff-based and invasive systolic blood pressure amplification

Abstract

Objective: Accurate measurement of central blood pressure (BP) using upper arm cuff-based methods is associated with several factors, including determining the level of systolic BP (SBP) amplification. This study aimed to determine the agreement between cuff-based and invasively measured SBP amplification.

Methods: Patients undergoing coronary angiography had invasive SBP amplification (brachial SBP - central SBP) measured simultaneously with cuff-based SBP amplification using a commercially available central BP device (device 1: Sphygmocor Xcel; n = 171, 70% men, 60 ± 10 years) and a now superseded model of a central BP device (device 2: Uscom BP+; n = 52, 83% men, 62 ± 10 years).

Results: Mean difference (±2SD, limits of agreement) between cuff-based and invasive SBP amplification was 4 mmHg (-12, +20 mmHg, P < 0.001) for device 1 and -2 mmHg (-14, +10 mmHg, P = 0.10) for device 2. Both devices systematically overestimated SBP amplification at lower levels and underestimated at higher levels of invasive SBP amplification, but with stronger bias for device 1 (r = -0.68 vs. r = -0.52; Z = 2.72; P = 0.008). Concordance of cuff-based and invasive SBP amplification across quartiles of invasive SBP amplification was low, particularly in the lowest and highest quartiles. The root mean square errors from regression between cuff-based central SBP and brachial SBP were significantly lower (indicating less variability) than from invasive regression models (P < 0.001).

Conclusions: Irrespective of the difference from invasive measurements, cuff-based estimates of SBP amplification showed evidence of proportional systematic bias and had less individual variability. These observations could provide insights on how to improve the performance of cuff-based central BP.

Figure 1

Bland–Altman plot of difference between estimated SBP and invasive SBP amplification from device 1 (Sphygmocor Xcel, top) and device 2 (Uscom BP+, bottom). Dashed line is the line of best fit. Solid lines are mean difference and ±2 SDs of the difference between estimated and invasive SBP amplification. Bland-Altman plots indicate evidence of systematic bias for greater underestimation of SBP amplification with increasing level of SBP amplification, but with stronger bias observed for device 1 (r = −0.68 vs. r = −0.52; z = 2.72; P = 0.008).

Figure 2

Bar plots (mean ± SE) of estimated SBP amplification (white bars) and invasive SBP amplification (black bars) per invasive SBP amplification quartiles from device 1 (Sphygmocor Xcel, top) and device 2 (Uscom BP+, bottom). From these figures, there was a stepwise increase in mean invasive SBP amplification for each of elevated invasive SBP quartile (Ptrend < 0.001) whilst estimated SBP amplification was overestimated at the first quartiles (P ≤ 0.001, for all) and underestimated at the highest quartile (P < 0.001, for all).