To the Editor:
We read with great interest the paper by Saugel et al.1 reporting that preinduction and lowest intraoperative blood pressure measurements are different than ambulatory measurements made during the daytime and nighttime.
Without entering the debate as to whether intraoperative blood pressure measurements can be used as surrogates for organ perfusion,2 and the fact that there is only an association and not direct causation between intraoperative mean arterial pressures lower than 65 mmHg and adverse clinical outcomes,3 we would like to address some methodologic issues concerning this study.
The two blood pressure monitors used in the study were not calibrated. It is possible that in the same individual there would be differences between simultaneous blood pressure measurements done with the two devices. Additionally, in the ambulatory setting, the blood pressure was measured on the nondominant arm. There is no information on whether this was also done in the operating room. This is an important issue as in many people there are differences in blood pressure measurements between the two arms.4 Moreover, in obese patients (who were not excluded), noninvasive blood pressure measurements are highly inaccurate when compared with invasive measurements.5
The arbitrary exclusion of very low and very high ambulatory blood pressure measurement “artefacts” and measurements done during the retiring and rising period could have skewed the results. Ambulatory blood pressure was measured at 30-min intervals, but then compared with intraoperative measurements made at 3-min intervals and invasively measured blood pressure at 1-min intervals. There were thus at least 10 times more measurements in the operating room than in the ambulatory setting, and it is well possible that the ambulatory measurements do not represent the variability in blood pressure for someone awake or asleep.
Moreover, comparing intraoperative invasive with ambulatory noninvasive blood pressure measurements is problematic. First, these are totally different technologies that cannot be treated as equivalent. In the operating room, during hypotension, noninvasive blood pressure tends to be higher than invasive blood pressure, and during hypertension, the noninvasive blood pressure tends to be lower than invasive blood pressure.6 Second, placing and zeroing of the transducer for invasive blood pressure measurement must be standardized and checked continuously. A height difference of only 5 cm from the left atrium to the transducer results in a 4-mmHg reduction or elevation of the invasive blood pressure measurement. Third, technical issues such as air bubbles (causing “damping” and false lower blood pressure measurements) or amplifying of the signal (causing “overshoot” and false high blood pressure measurements) must be taken into consideration.7
Knowing the “normal” patient’s blood pressure may help to optimize the intraoperative hemodynamics. But with the methods used in this study, the value of the results is questionable since in our opinion, “apples were compared with pears.”
The author declares no competing interests.