We thank Dr. Dueck1 for his thoughtful reading of our article.2 Dr. Dueck’s first comment was also made by Saugel et al. in the accompanying editorial to our article.3 Therefore, we take the opportunity to comment on the issues raised and to discuss our statistical approach as detailed in the supplements.2
We clearly stated in the Methods section of our article that we planned to calculate the differences of measurements between the two methods according to Bland–Altman for clustered observations.4 Despite this improved statistical approach, we assumed that the high variance in the number of measurements per patient (5 to 74) and the dominance of values in the normotonic range of blood pressure would confound the analysis. For anesthesiologists, the extreme blood pressure values, which naturally are rarely measured, are particularly important. Supplemental Digital Content 1 of our article (https://links.lww.com/ALN/C442) shows data of descriptive analyses and Supplemental Digital Content 2 (https://links.lww.com/ALN/C443) the analyses proposed by Dr. Dueck; Pearson’s r for mean arterial pressure was 0.95 compared to 0.96, the systematic error was identical (−0.3 mmHg), and the limits of agreement were slightly wider (6.8 vs. 6.4 mmHg and −7.4 vs. −7.0 mmHg) when all 1,887 measurements were included in the analysis.2 In principle, our assumption was justified, even if the effects were less than we had expected. The reason we stayed with our first statistical analysis in the main article is simply that we made the statistical analysis plan prospectively.
The repeatability of measurements at very short time intervals was not the goal of this clinical evaluation. We focused on a wide range of blood pressures and the ability of the high-fidelity upper arm cuff to correctly track changes in blood pressure, as this is clinically important to anesthesiologists. We did not observe clinical signs of venous congestion distal to the high-fidelity upper arm cuff, even during major abdominal surgery with high volumes of fluids given.
We acknowledge that the duration of a single noninvasive measurement is of clinical importance. Blood pressure measurements in quick succession may be necessary in emergency situations. The blood pressure measurements with the new high-fidelity cuff exhibited a mean actuator pressurization time of 64±10 s (mean ± SD), which is comparable to many oscillometric devices. The reason behind the “slow” inflation is that blood pressure swings of at least three mechanical ventilation cycles can be captured in this way, and from that, we intend to validate the measurement of the fluid responsiveness parameter pulse pressure variation as well. The new method of hydraulic coupling, however, also enables a “fast mode” shortening of mean actuator time to 37±5.3 s (mean ± SD). The agreement with invasive blood pressure was also high (see table 2 of our article).2 Hydraulic coupling offers sufficient information for an exact calculation of the blood pressure when an actuator inflation is stopped at a pressure of 85% of the systolic blood pressure. This prevents the blood flow from stopping in the arm, causes less physical stress to the arm, and enables measurements to be taken in fast sequence (e.g., 1-min intervals).
Dr. Pfeiffer reports former UP-MED GmbH having received payments in 2011 and 2012 through German governmental grant “Zentrales Innovationsprogramm Mittelstand (ZIM)” KF2664502AK0. Further Dr. Pfeiffer, as former owner of UP-MED GmbH, which was merged with Philips Medizin Systeme Böblingen GmbH in 2018, had and continues to have financial interest in the technology. Philips Medizin Systeme Böblingen GmbH is also Dr. Pfeiffer’s current employer. In addition, Dr. Pfeiffer is also the founder of Pulsion Company, which is today a part of GETINGE group AB. Dr. Briegel declares no competing interests.