Intraoperative Blood Pressure Measurement Modalities Are Separate and Not Equal

To the Editor: We read with great interest the recent publication by Wax et al. .1The authors argue that because noninvasively detected blood pressure tends to be higher than that measured invasively in hypotensive patients and lower in hypertensive patients, some patients monitored with invasive arterial lines alone may undergo potentially harmful vasoactive and transfusion therapy if hemodynamic instability is not confirmed with noninvasive means. However, in the absence of outcome studies following these cohorts (patients whose treatment was guided by invasive arterial lines alone vs. those in whom such therapy was withheld based on noninvasive readings), one cannot make the authors' claim: it remains entirely possible that the reliance on confirmatory noninvasive reading leads to potentially harmful undertreatment.In reality, invasive and noninvasive modalities of blood pressure measurement use vastly different physical phenomena. Whereas an invasive line displays a direct, electronically processed, beat-to-beat intravascular pressure waveform, the noninvasive tool used in the study is an indirect oscillometric device. Oscillometric parameters are derived, not measured, from the superimposition of a pulse oscillogram envelope on a cuff pressure curve, with mean blood pressure determined at the point of maximal oscillations. Proprietary software algorithms, specific to individual manufacturers, are utilized in analyzing the slopes of scillograms to derive systolic and diastolic readings. These manufacturer-specific algorithms are generally not standardized for measurement accuracy,2,3and, in addition to numerous other technical (i.e. , the rate of cuff “bleed”) and clinical (i.e. , size and location of the cuff not recorded in the authors' database, and elastic properties of the vasculature) variables, affect the reliability of indirect oscillographic determinations. Therefore, in critically ill patients, noninvasive blood pressure measurements are generally considered unreliable.4,5We applaud Wax et al. for their confirmation of a significant discrepancy between the direct and indirect modalities of blood pressure measurement, but would caution against withholding therapy based on nonuniformly standardized, and possibly inaccurate, confirmatory data.