Accuracy of ankle versus arm oscillometric non-invasive blood pressure in detecting hypotension during shoulder surgery in the beach-chair position: A prospective observational study

Background and Aims: Accurate blood pressure monitoring is crucial in the beach-chair position (BCP), which poses a risk of cerebral hypoperfusion. This study compared arm and ankle non-invasive blood pressure (NIBP) with invasive arterial blood pressure (IBP) for detecting hypotension. Methods: This prospective observational study included 36 adults undergoing shoulder surgery in the BCP after obtaining written informed consent. The primary objective was to evaluate the diagnostic performance of arm and ankle NIBP in detecting hypotension at IBP. Secondary objectives assessed agreement with IBP using Bland–Altman analysis (BAA) and clinical risk using error grid analysis (EGA). Results: A total of 1088 paired NIBP–IBP readings were analysed, of which 326 were hypotensive. Arm mean arterial pressure (MAP) identified hypotension more accurately than ankle MAP, with a higher area under the curve of 0.83, 95% confidence interval (CI) 0.81–0.86 versus 0.72, 95% CI 0.70–0.76, P < 0.001. At arm, an MAP threshold of 75 mm of Hg could detect hypotension with 77% sensitivity, 76% specificity, and 77% overall accuracy. At the ankle, a threshold of 76 mmHg had 62% sensitivity, 72% specificity, and 69% accuracy. BAA showed that both arm and ankle NIBP overestimated invasive MAP, with a bias of + 4.6 mmHg for the arm (95% limits of agreement: −37.9 to + 47.2) and + 5.9 mmHg for the ankle (95% limits of agreement: −10.4 to + 22). On EGA, 83% of arm readings and 75% of ankle readings were in Zone A, whereas high-risk errors in Zones D–E were more common with ankle measurements than with arm measurements (5.4% vs 3.6%). Conclusion: In the beach-chair position, both ankle and arm NIBP are unreliable for detecting hypotension and invasive arterial monitoring should be preferred when accurate blood pressure assessment is vital.