The Accuracy of the CNAP® Device Compared with Invasive Radial Artery Measurements for Providing Continuous Noninvasive Arterial Blood Pressure Readings at a Medical Intensive Care Unit: A Method-Comparison Study

BACKGROUND: In cases of intensive care unit (ICU) patients without an arterial line, noninvasive continuous blood pressure monitoring could be very beneficial. The CNAP((R)) monitor (CNSystems Medizintechnik AG) provides noninvasive, beat-to-beat blood pressure (BP) estimates using the volume clamp method to measure finger arterial pressure calibrated to brachial pressure values. The aim of this study was to compare noninvasive BP estimates of the CNAP monitor with invasive blood pressure (IBP) measurements obtained via a radial arterial catheter in unselected medical ICU patients under routine clinical conditions. METHODS: In 40 adult patients, IBP and noninvasive CNAP blood pressure (CBP) were measured simultaneously for 30 minutes. Bland-Altman analysis accounting for repeated measurements revealed accuracy and precision of CBP toward IBP. Percentage errors were calculated using the summary measures method and tested for interchangeability. Trending analysis was assessed using 4-quadrant plots and polar plots, whereby each reported statistical calculation used the sample size of n = 40 patients. RESULTS: A total of 7200 measurement pairs of CBP and IBP were analyzed. For mean arterial pressure, accuracy precision resulted in 4.6 +/- 6.7 mm Hg (limits of agreement -8.7 to 17.8 mm Hg) with a percentage error of 6.77% (95% confidence interval [CI], 6.57%-6.97%). Trending analysis of 3-minute intervals showed a concordance rate of 94.6% (95% CI, 94.0%-95.2%; exclusion zone 10%) and a polar concordance rate of 99.50% (95% CI, 99.48%-99.52%) for changes lying within 10% limits. CONCLUSIONS: The CNAP device provided feasible estimates of BP in unselected medical ICU patients under routine clinical conditions. Mean arterial pressure met interchangeability criteria for accuracy toward radial arterial pressure, as well as for percentage error, and showed good trending capabilities according to the Critchley predefined criteria.