In morbidly obese patients, oscillometric blood pressure measurements with an upper-arm cuff are often difficult to perform. The alternative method, invasive blood pressure monitoring, can be difficult to place and is associated with risks. A wrist-mounted blood pressure-monitoring device, the Vasotrac, provides accurate blood pressure measurements in lean patients. Even in the obese, wrist morphology remains relatively unchanged. We thus assessed the degree to which blood pressure measurements with the Vasotrac on the wrist and cuff measurements agree with invasive arterial blood pressure monitoring.Methods
We evaluated 22 morbidly obese patients undergoing bariatric surgery lasting 3.8 ± 1.1 h. Intraoperative blood pressure was simultaneously measured using the Vasotrac mounted on one wrist; an arterial catheter was inserted in the opposite radial artery, and an oscillometric cuff was positioned on the upper arm. Preoperative patient comfort was evaluated on a scale from 1 to 10, with 10 being most uncomfortable, just after the first oscillometric cuff inflation. Values from the Vasotrac and arterial catheter were recorded at 5-s intervals. Bias, precision, and clinically acceptable agreement were calculated between the two continuous monitoring devices and between the arterial catheter and the cuff measurements, with the arterial catheter providing the reference value.Results
The patients' age was 44.3 ± 9.5 years (mean ± SD), body mass index was 66.7 ± 13.8 kg/m2, and arm circumference was 48.6 ± 7.5 cm. Patients found the Vasotrac more comfortable than the oscillometric device [1.7 ± 1.8 vs 5.3 ± 0.5 (P = 0.009)]. A total of 40,411 pairs of values from the Vasotrac and arterial catheter were recorded. Lin's concordance correlation coefficient (95% CI) for mean arterial blood pressure measured between the arterial line and the Vasotrac was 0.74 (0.67, 0.82). The bias (mean error) was -0.25 mmHg; however, the Bland-Altman limits where 95% of individual pressure differences are expected to fall was (-20, 20) mmHg. The precisions for diastolic and systolic pressures were even worse.Conclusion
The Vasotrac was more comfortable than an oscillometric device. Although the average accuracy was good, individual mean Vasotrac and noninvasive blood pressure pressures often differed considerably from arterial values. These results suggest that the Vasotrac monitor should not be substituted for an arterial catheter in super-obese patients.