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 however, 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 is considered a standard method in today’s operating rooms regarding its 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 5 min 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 yet non-invasive. Nevertheless, this option needs to be validated before it can replace the other two methods or can fill 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 greater importance than absolute accuracy and precision is the question if clinical decisions would be identical to those based on Gold Standard IBP if only AT or NIBP were available.
In terms of accuracy and precision, before qualifying a new method, expected target criteria need to be defined. According to the US FDA, interchangeability is granted when a non-invasive method meets the AAMI SP10 or subsequent AAMI 80601 criteria for the 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 criteria were not fully met (Table 4) nevertheless, the fact should be considered that we only compared ABP data during hyper- or hypotension situations.
It is however worth noting that NIBP failed to meet the AAMI criteria even more pronounced in the overall comparison to the IBP (Table 5), 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 in 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 drastically influence overall accuracy and precision. In addition, in our study we enrolled patients during urologic, neuro, thoracic surgeries, and complex general surgeries 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 in such situations by either manual or automated recalibration. Nevertheless, during such moments there is a potential for artifacts or – in the case of recalibration – a phase of “no valid data” for 1-2 minutes. With the NIBP method on the other hand, even if cuff intervals are reduced from 5 minutes (as chosen in this study) to 1-2 minutes, such “blind spots” occur routinely and more often.
It can be said 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. 15 mm Hg or 30 mm Hg, is of less importance, because too high is too high and too low is too low and requires 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, our results compare very well to those of previous literature.
The most important and innovative part of this study, however, is the question if clinical decision making is impacted by AT? So far, all studies 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 uses 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 require 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 had no statistical difference compared to IBP, NIBP however, it 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 rapid changes of blood pressure. Our study found, that although AT could not achieve complete coincidence with IBP, the discrepancy does not negatively affect the anesthesiologists ability to make efficient and appropriate clinical decisions. Of course, with approximately 77% of captured IBP decision-making moments (IBP n=513, AT n=394, Table 2), AT may not yet be the perfect alternative to IBP. But compared to only 45% of captured IBP decision-making moments of the cuff (IBP n=513, NIBP n=233, Table 2), AT seems to be a reasonable and clinically acceptable alternative.
During initial evaluation, the absolute number of decision making moments with – on average 2.1 per surgery – seems to be low, this however can be explained by several factors: We decided to exclude intubation and extubation period because during this period in our OR, the draping, undraping, and positioning of the patient is happening in tandem. During this process arm and body positions are constantly changing which would have required frequent repositioning or restarting of the non-invasive devices. As the latter would potentially have falsified the results, we justified that excluding this period was appropriate for a first study of this kind. Moreover, if an event did last for several minutes, we did not count it as a new event, but we only counted the first decision-making moment. If e.g. hypotension was detected as decision-moment, but after the first clinical action the blood pressure did not increase satisfactorily thus requiring a second clinical action, this was not counted as additional decision-making moment.
To our best knowledge, this study is the first one to record the moments of ABP-related clinical decisions as well as the ABP values at these decision-making 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 are not, the latter having missed 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. Considering this, our accuracy comparisons of a potentially 5-minute-old NIPB signal to an actual IBP or AT signal must be interpreted with caution. On the other hand, our approach reflects a 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 the possibility that we may have missed some data points. Applying the NIBP to the same arm as the IBP however would not have been a solution, as then we would have repeatedly lost the Gold Standard signal thus also resulting in “missing data”.
This last limitation especially calls for another study with a 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 AT or NIBP data only where only their consequent action, based on such information, is recorded. Once such a study has been performed with positive results, a final comparative outcome study would also be warranted.
However, although radial arterial tonometry is an intriguing approach, its susceptibility to 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. The latter method would seem to be the best choice if highest safety for the patient is indicated. 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.