ObjectiveComparison of oscillometric blood pressure measurement with two different devices (M3000A using a new algorithm and M1008A using an established algorithm, both Hewlett Packard) and evaluation of current recommendations concerning the relation between cuff size and upper arm circumference in critically ill patients.DesignProspective data collection.SettingEmergency department in a 2000-bed inner-city hospital.PatientsA total of 30 patients categorized into three groups according to their upper arm circumference (I, 18–25 cm; II, 25.1–33 cm; III, 33.1–47.5 cm) were enrolled in the study protocol.InterventionsIn each patient, two noninvasive blood pressure devices with three different cuff sizes were used to perform oscillometric blood pressure measurement. Invasive mean arterial blood pressure measurement was done by cannulation of the radial artery.Measurement and Main ResultsOverall, 1,011 pairs of simultaneous oscillometric and invasive blood pressure measurements were collected in 30 patients (group I, n = 10; group II, n = 10; group III, n = 10). The overall discrepancy between both methods with the M3000A was −2.4 ± 11.8 mm Hg (p< .0001) and, with the M1008A, −5.3 ± 11.6 mm Hg (p< .0001) if the recommended cuff size according to the upper arm circumference was used (352 measurements). If smaller cuff sizes than recommended were used (308 measurements performed in group II and III), the overall discrepancy between both methods with the M3000A was 1.3 ± 13.4 mm Hg (p< .024) and, with the M1008A, −2.3 ± 11.5 mm Hg (p< .0001).ConclusionThe new algorithm reduced the overall bias of the oscillometric method but still showed a significant discrepancy between both methods of blood pressure measurement, primarily due to the mismatch between upper arm circumference and cuff size. The improvement of the algorithm alone could not result in a sufficient improvement of oscillometric blood pressure measurement. A reevaluation of the recommendations concerning the relation between upper arm circumference and cuff size are urgently required if oscillometric blood pressure measurement should become a reasonable alternative to intra-arterial blood pressure measurement in critically ill patients.