The tf-a value at its first maximum was greater in study participants with one or more than in study participants without cardiovascular risk factor (P < 0.0001; Table 4). Linear regression showed a significant correlation of tad1 with SBPK (P < 0.000.1, r 2 = 019 and P < 0.0001, r 2 = 0.12), but not with DBPK (P = 0.11, r 2 = 0.07 and P = 0.79, r 2 = 0.002), SBPosc (P < 0.055, r 2 = 0.10 and P < 0.099, r 2 = 0.003), DBPosc (P < 0.06, r 2 = 0.098 and P = 0.53, r 2 = 0.012), and MBPosc (P < 0.078, r 2 = 0.089 and 0.89, r 2 = 0.0007), respectively, on the right and the left side. ANOVA showed significant differences in right and left tad1 between the CVRF0, CVRF1, and CVRF>1 groups (P < 0.0001 for all).
The second tf-a maximum was present on one or both sides in 11 out of 53, 40 out of 51, and 34 out of 41 study participants, respectively, without, with one, and with more than one cardiovascular risk factors (P < 0.0001). It was present in six out of 84 study participants under 50 years old and in 47 out of 61 study participants 50 years old or older (P < 0.0001; Fig. 3). It was present in five out of 41 and 27 out of 43 study participants under 45 years old, and in six out of 12 and 47 out of 49 study participants 50 years old or older, respectively, without and with one or more cardiovascular risk (P < 0.0001).
The intraobserver reproducibility study yielded CCC of 0.95 (0.92–0.98) for Pad1 and 0.94 (0.89–0.96) for SBPK.
In ICU patients, SBPIA, SBPK, and Pad1 were normally distributed. Agreement was moderate between Pad1 and SBPK (Fig. 2d), but poor between Pad1 and SBPIA as well as between SBPia and SBPK (Table 5).
Systolic blood pressure measurement
In all study participants and patients, tf-a showed a maximum whose occurrence during cuff deflation closely corresponded to the first Korotkoff sound, allowing fully automated SBP measurement with almost perfect correlation with the auscultatory technique. Moreover, study participants older and/or with cardiovascular risk factors showed not only a prominent first tf-a increase, but also a second increase that was absent in most younger study participants and in study participants without cardiovascular risk factor. When compared with the auscultatory technique, Pad1 yielded much better results than oscillometry.
The arterial wall compliance depends on the transmural pressure, that is, in this setting, the difference between the intra-arterial BP and the pressure of surrounding tissues, which can be approximated to the cuff pressure. The transmural pressure is negative, keeping the artery closed, as long as the cuff pressure remains greater than the intra-arterial pressure. It decreases during cuff deflation and reaches a minimum when the intra-arterial pressure equates the cuff pressure, that is, at the exact time at which the first Korotkoff sounds occur, with the brachial artery compliance at its maximum. It then becomes positive, the PP overcoming the cuff pressure. The fact that tad1 also reaches its maximum at this exact time suggests that it is related to the same arterial wall mechanisms and characteristics as Korotkoff sounds.
Our SFATI technique must be compared with other techniques both from the technical and from the performance point of view.
Conventional oscillometry relies on the OMWE amplitude and shape, considering that its maximum amplitude corresponds to MBP and using coefficients or algorithms (that can be quite different from one device to the other) to estimate SBP and DBP. Approximating the OMWE along line(s) of best fit, or using a probabilistic approach, has been proposed to improve this estimation . Using the OMWE derivative allows avoiding empirical coefficients but is still prone to artifacts. Neural networks have also been used to learn from large datasets to separately estimate SBP and DBP, or to extract the characteristic features of the OMWE . Mathematical models have been built as a basis for new algorithms tracking the effects of cuff pressure on transmural pressure, depending on arterial wall biomechanics and BP. These models allow developing better algorithms but still rely on a limited set of actually measured data .
The oscillometric waveform itself conveys hemodynamic information , whereas the OMWE is prone to artifacts and multiple influences [14–16,19]. Mafi et al. [20,21] looked at the PP waveform modulation and the maximum upslope of the systolic peak to improve NIBP. Some authors investigated the advantages of simultaneously recording ECG. Its first benefit is to get additional information, especially the pulse arrival time (PAT), which is inversely correlated with arterial wall stiffness. Ahmad et al.  showed that PAT follows the same pattern as OMWE during cuff deflation, and can be used conjointly to improve SBP and DBP estimation.
Our SFATI technique does not rely on the OMWE and MBP determination but on the time-domain analysis of the pulse waveform for immediate SBP assessment. It appears intrinsically different of all previously published approaches, although it is in line with the works of Forouzanfar et al.  or Mafi et al. , and may be related to PAT and pulse transit time studies .
Comparison with the auscultatory technique is required for validation of oscillometric devices, but results, when published, are generally reported as required to meet the international ISO standard (the mean value of the difference between oscillometric and auscultatory measurements repeated at least three times in each study participants must be within ±5 mmHg with a SD < 8 mmHg; International Standard ISO 81060–2:2013) rather than as CCC. Auscultation is typically performed by two independent observers listening from the same stethoscope bell. Compared with Korotkoff sounds, oscillometry tends to overestimate SBP and underestimate DBP  or yield variable results [8,24,25], but has been reported to overestimate both SBP and DBP in study participants with increased arterial wall stiffness . Landgraf et al.  observed that discrepancies between oscillometry and auscultation were greater in older study participants.
Given the multitude of available oscillometric devices, their constant evolution, and the fact that they use undisclosed algorithm, generalization is impossible and a given study only allows conclusions about the sole devices it compares . We used a widely used and validated oscillometric device, but our comparison results apply only to this specific device and cannot be extrapolated to oscillometry as a whole. Nevertheless, in a relatively large and diverse population sample, our SFATI technique yielded unparalleled correlation with Korotkoff sounds for SBP measurement, particularly in elderly study participants and/or study participants with cardiovascular risk factors or diseases. This is fortunate, as medical devices are expected to provide reliable results not only in healthy study participants but also and above all in patients.
In ICU patients, Pad1 showed poor (minimally better than Korotkoff sounds) correlation with direct intra-arterial measurement. Auscultation as well as oscillometry are known to be poorly correlated with direct intra-arterial BP measurement. Auscultation underestimated SBP and overestimated DBP , whereas oscillometry underestimated SBP and either over or underestimated DBP in lean and overweight critically ill patients [28–30]. In 301 patients, Mireles et al.  reported Pearson r values of 0.68, 0.67, and 0.62 when comparing oscillometric with intra-arterial measurement of SBP, DBP, and MBP, respectively. Comparing auscultation and oscillometry with intra-arterial BP in 50 ICU patients, Ribezzo et al.  also reported poor agreement, especially for SBP, with a Pearson r ranging from 0.82 to 0.88.
Such differences between direct intra-arterial measurement and either auscultation or oscillometry should not be surprising, as the former measures BP itself, whereas the latter only indirectly assess its buckling effects on the arterial wall and the resulting flow disturbance. In other words, noninvasive measurements are mediated by the arterial wall, and, as such, depend on its biomechanics.
Our results in ICU patients did not yield better correlation between oscillometry and intra-arterial measurement than reported in the literature, but Pad1 was still closely correlated to SBPK. This confirms that the first tf-a increase during cuff deflation shares common mechanisms with the production of Korotkoff sounds and arterial wall motion. As such, it should be affected by changes in arterial wall stiffness, which our results indeed suggested.
Systolic peak foot-to-apex time interval and the arterial wall
In our study, older study participants and study participants with cardiovascular risk factors or disease not only showed a prominent first tf-a increase, but also showed a second increase that was absent in most younger study participants and in study participants without cardiovascular risk factor or disease. This second tf-a increase is probably related to distal pulse wave reflection, as it occurs when the brachial artery remains open during a larger part of the cardiac cycle, and is prominent in older study participants and study participants with cardiovascular risk factors in whom distal wave reflection is known to be increased. Cuff inflation reduces the brachial artery transmural pressures, which slows down and dampens the pulse wave. At lower cuff pressure, the pulse wave succeeds reaching the distal part of the cuff and propagates downstream, allowing distal reflection to occur and prolong the systolic peak.
Forouzanfar et al.  mathematically modeled the pulse transit time as a function of the arterial lumen changes under the cuff and showed that it can be used for a coefficient-free assessment of SBP, MBP, and DBP. Also using a mathematical model, Liu et al.  demonstrated that calculating SBP and DBP from the oscillometric envelope with fixed ratios measurement produces errors that increase when the arterial wall stiffens and/or the PP increases. Differences between oscillometric and auscultatory BP measurements are indeed greater in study participants with increased arterial wall stiffness , and have been proposed as an indicator of arterial stiffness, predictive of coronary lesions . It is therefore all the more interesting that our technique yielded its best correlation with Korotkoff sounds in older study participants and in study participants with cardiovascular risk factors.
The tf-a changes we observed were significantly related to age and cardiovascular risk factors. This is another clue pointing at the arterial wall biomechanics as involved in both Korotkoff sounds and tf-a changes. The relationship between tf-a and arterial wall stiffness deserves further clinical investigation.
Although we referred to the ISO validation procedure for oscillometric devices, we did not fulfill all of its requirements, especially regarding the distribution of limb circumference in the population sample. On the other hand, our sample was greater than the required number of study participants (180 vs. 85). Instead of asking two independent observers to listen to Korotkoff sounds, we used an electronic stethoscope, thus allowing automatic reading and providing objective records. Cuff deflation was manually controlled, at a 2–3 mmHg/s rate, and should be automatic and linear for greater convenience in routine clinical practice. We used a validated, widely available automatic oscillometric device for comparison, but the SFATI and oscillometric measurements, although performed during the same session, were not strictly simultaneous, which may partly explain the differences we observed, because of BP variability. Nevertheless, repeated Pad1 as well as SBPK measurements during the same session showed substantial reproducibility. Anyhow, as current oscillometric devices often use undisclosed algorithms and are not standardized, we cannot generalize our findings .
In ICU patients, we performed NIBP measurement at the arm, whereas intra-arterial measurement was performed via a radial artery catheter on the contralateral side, which may also explain a difference. Nevertheless, this would have resulted in a systematic bias, which was not apparent in our study.
In conclusion, using time-domain analysis of the PP waveform instead of the amplitude of the oscillometric envelope, we designed SFATI, an innovative, straightforward, fully automated method for the measurement (rather than estimation) of SBP, obtaining almost perfect correlation with Korotkoff sounds. This easily implemented SFATI algorithm can be used as a complement of the algorithms currently used for MBP assessment and would overcome the main limitation of current oscillometric devices by providing accurate SBP results and allowing their long awaited standardization. We are now looking forward for the independent replication of our study and further investigation of its potential interest for the assessment of the arterial wall biomechanics.
A.M.B. received doctoral scholarship #1262 from the European Union ERASMUS-Mundus METALIC program.
ORCID number: A.M.B., 0000 0002 8785 293X; A.P-M.; 0000 0002 8527 7783; S.C., 0000 0002 6110 2583; I.S., 0000 0002 8735 4308; I.A., 0000 0002 1892 7256; J.L., 0000 0002 1853 2721; F.B-R., 0000 0002 5788 6055; M.D., 0000 0002 9496 3857.
Conflicts of interest
There are no conflicts of interest.
Reviewers’ Summary Evaluation
Systolic blood pressure (SBP) was measured by means of a novel oscillometric technique and was found to have high correlation coefficient with SBP reading, measured with electronic stethoscope.
Because the presently available oscillometric devices have low accuracy, the development of an accurate automatic SBP measurement technique has great significance. However, high correlation coefficient does not assure a small measurement error. It is still necessary to validate the novel oscillometry using generally accepted practices: comparing it to the auscultatory sphygmomanometry, the common gold-reference, and applying a common criterion for the mean and standard-deviation of the deviations between the two techniques.
The strength and novelty of the paper lie in the finding that the air pressure variation during cuff deflation, widely used for BP determination by the oscillometric method, contains identifiable “events” that mark the Korotkoff sounds used for detecting systolic BP. As a result, this fully automated method is free of assumptions associated with the oscillometric method. The weakness of the study lies in the lack of an attempt to add vascular measures, e.g., arterial stiffness, that might explain the reduced correlation between these two methods observed in some cohorts. A parallel search for “events” that mark the diastolic BP could be a great challenge.
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Keywords:Copyright © 2017 Wolters Kluwer Health, Inc. All rights reserved.
auscultation; automation; Korotkoff sounds; noninvasive measurement; oscillometry; SBP