Oscillometric brachial mean artery pressures are higher than intra-radial mean artery pressures in intensive care unit patients receiving norepinephrine

Background: Norepinephrine is frequently used in intensive care unit (ICU) patients to raise and maintain an adequate mean arterial pressure (MAP). Conflict arises as to which MAP should guide vasopressor dose, as oscillometric MAP in the brachial artery often diverges from intra‐radial MAP. We wanted to estimate the magnitude of this difference in ICU patients receiving norepinephrine, and to see whether the patient age, norepinephrine dose, age of radial catheter, ventilation treatment or severity of illness influenced the difference. Methods: Prospective observational study comparing oscillometric MAP and intra‐radial MAP performed in a central hospital‐based surgical ICU during the period from February 2002 through to October 2003. Sixty‐eight consecutive patients who received a norepinephrine infusion to maintain MAP above 70 mmHg were included. The mean arterial blood pressure was measured both oscillometrically and in the ipsilateral radial artery. Results: Oscillometric MAP was on average 6.6 mmHg higher (95% CI 5.3–7.9) than intra‐radial MAP in the 134 sets of measurements (P < 0.001). The patient’s age (P = 0.319), norepinephrine dosage (P = 0.959) the age of the radial catheter (P = 0.992) were not significant. The difference in MAP was marginally greater in ventilated patients (P = 0.061) and significantly greater as the simplified acute physiology score (SAPS) score increased (P = 0.022). Conclusions: MAP measured oscillometrically over the brachial artery directly in the radial artery gave different results in an ICU population receiving norepinephrine. In more than one of four patients this difference is so large, and increases with illness severity, that the selection of any one method would influence treatment.