In this study, ABP was monitored in patients undergoing elective surgery using 3 different methods. ABP monitoring with IBP is considered the Gold Standard with regards to accuracy and reliability although this method is associated with potential complications related to intravascular catheter placement such as bleeding, limb ischemia, and catheter-related bloodstream infections 15, 16].
NIBP may be considered as a standard method in today’s operating rooms regarding practicability, quick and easy employment and appropriateness for the needs of a majority of physicians and patients not requiring blood gas analyses. Most anesthetists believe to be able to cope with the drawbacks of NIBP like wrong cuff size and the ABP blind window of 5min or longer, although there exists evidence that both can have a negative impact on accuracy or outcome 17–20).The third method, radial applanation tonometry using AT, is an alternative that combines the best of both worlds, being continuous but being still non-invasive. Nevertheless, this offer needs to be validated before it either can be a replacement of the other two methods or can be filling the gap in between. One part of such validation is comparing accuracy and precision of absolute values of ABP signals by AT with the Gold Standard IBP, which has been done already in the past and was part of this study as well. However, of much higher importance than absolute accuracy and precision is the question if clinical decisions would be identical to that based on Gold Standard IBP if only AT or NIBP would be available.
In terms of accuracy and precision, before qualifying a new method, expected target criteria need to be defined. According to US FDA, interchangeability is given when a non-invasive method meets the AAMI SP10 criteria for non-invasive ABP monitoring 21]. With regards to bias between IBP and AT this is the case for our results in the mean, although—and this is very important—we only compared ABP values in hyper- or hypotensive situations.
Looking at hyper- or hypotensive data alone, AAMI SP10 bias criteria are not fully met (Table 4). Regarding limits of agreement, AT data of our study do not completely meet AAMI SP10 criteria (Table 4), because bias is required to be lower than 5mmHg and SD ≤8mmHg, which translated into ≤16mmHg for 2xSD in Bland-Altman plots. Here again, the fact must be considered that we only compared ABP data during situations of hyper- or hypotension.
It is however remarkable that NIBP failed to meet the AAMI SP10 criteria even more pronounced in the overall comparison (Table 5) to the IBP, showing that NIBP is clearly not only inferior to IBP but also to AT.
Comparing our results to previous literature, where accuracy and precision of AT was evaluated in the OR or at the ICU 10–12, 14, 22, 23], we also found a comparable bias but higher limits of agreement in our study, which underlines the already mentioned reasoning that only looking at hyper- and hypotonic situations where ABP fluctuates more remarkably can have drastic influence on overall accuracy and precision. In addition, in our study we enrolled patients during urologic, thoracic surgery, and complex general surgery where occasional signal loss due to repositioning or external pressure from surgeons on the AT bracelet may occur more often than in quiet ICU situations. Generally, the AT device can compensate such situations by either manual or automated recalibration, nevertheless during such moments there is a potential for artifacts or—in case of recalibration—a phase of no valid data for 1–2 minutes.
One may say that, especially in the more extreme ABP situations accuracy and precision is of higher importance as during normal conditions, on the other hand, one could argue that whether ABP is too high or too low by e.g. 15mmHg or 30mmHg, is of less importance, because to high is too high or too low is to low and requests for action anyway.
This is why not only absolute accuracy and precision is important but also trending capabilities. Looking into trending capabilities, the results of our 4-quadrant plots (Figure 3) show that with concordance rates of 0.75 to 0.91 AT shows good results and can be considered as a replacement to IBP from this perspective. Here our results compare very good to that of previous literature.
The most important question and most innovative part of this study, however, is the question if clinical decision making is impacted by AT. All studies so far have only compared measured ABP values, but not investigated if AT could really replace IBP with regards to clinical decision making. A Gold Standard method to evaluate this, would be an outcome study where in one group only IBP and in the other group only AT would be available for the anesthetists. For ethical and safety reasons we preferred to go a different route by defining ABP criteria based on IBP data which would evoke action by the treating physician and then evaluating if those criteria are met at the same statistically significant frequency by AT and NIBP.
We found that the frequency of ABP decision moments based on AT has no statistical difference compared to IBP, NIBP however clearly showed significantly lower frequency (Table 2). As NIBP does not allow beat-to-beat monitoring, it may be blind to rapid ABP changes or short episodes of hemodynamic instability. Thus, NIBP is not applicable in patients undergoing surgeries with risk of massive hemorrhage or fast changes of blood pressure. Our study found, that although AT could not achieve complete coincidence with IBP, the small discrepancy does not negatively affect the anesthesiologists to make efficient and right clinical decisions.
To our best knowledge, this study is the first one to record the moments of ABP related clinical decisions and the ABP values at these decision moments. As such, for the first time we could show that IBP and AT are clinically equivalent for decision making but not IBP and NIBP, the latter missing several moments of hypotension or hypertension (1, IQR 0–2). Given also the inherent risks on patient outcome associated with blind moments of the NIBP technology 17, 19, 20] AT is clearly superior to NIBP.
In addition, we found that neither age, BMI, nor length of surgery affects the comparison of decision moments, making AT the device of choice for surgeries with no absolute necessity of IBP. A timelier monitoring of ABP compared to use of NIBP may have positive impact on patients’ prognosis and outcome.
Our study has some limitations. Firstly, as we collected ABP data only during hyper- or hypotensive periods, we were not able to compare ABP data of the investigated methods over the full spectrum of ABP. This however was not the primary scope of this study anyway, as it was not the primary intenion to compare NIBP to AT, which would have required that NIBP is triggered exactly when a decision moment had occurred. Under this aspect, our accuracy comparisons of a potentially almost 5 minute old NIPB signal to a actual IBP or AT signal must be interpreted with caution. On the other hand, our approach reflects a realistic real world scenario. Secondly, our patient setting of applying AT on the same arm as NIBP may have reduced the frequency of simultaneous AT vs. IBP measurements and thus could have led to some “AT missing data” in the statistical review. NIBP inflates in regular intervals, causing a short “black out” of any downstream-applied blood pressure measurement device. However, as the AT device has an automated compensation mechanism for such short moments of NIBP inflation, we consider the number of potentially missed AT decision moments as low, but we cannot completely rule out that we may have missed some data points. Applying the NIBP at the same arm as the IBP however would not have been a solution, as then we would have lost the Gold Standard signal repeatedly and also have “missing data”.
Especially the last limitation calls for another study with larger sample size, in which not only frequency of decision moments is investigated, but—same as in this study- at predefined clinical situations based on Gold Standard IBP—physicians are confronted with only AT or NIBP data only and their consequent action based on such data alone is recorded. Once such study has been performed with positive results, as a final consequence a comparative outcome study is warranted.
However, although radial arterial tonometry is an intriguing approach, its susceptibility to the external interference preclude the use of the device as a single source of BP information in unstable patients or surgeries with high risk. In such situations either IBP is needed or the blind spots of NIBP should be “closed” by shorter NIBP intervals or by continuous AT monitoring. Especially the latter seems to be the method of choice if highest security for the patient is aimed for. In types of surgery where frequent blood-gas analyses are needed, IBP should still be the method of choice.
In conclusion, we found that ABP measurement based on AT technology is feasible in anesthetized patients and in contrast to NIBP does not show statistically significant difference regarding timely information delivery for clinical decision making compared to IBP.