Relationship Between Preoperative Evaluation Blood Pressure and Preinduction Blood Pressure: A Cohort Study in Patients Undergoing General Anesthesia

BACKGROUND: For outcomes research where changes in intraoperative blood pressure are a possible causative factor, it is important to determine an appropriate source for a reference value. We studied to what extent preinduction blood pressure values in the operating room differ from those obtained during preoperative evaluation outside the operating room. METHODS: Cohort study including 4408 patients aged 60 years or older undergoing noncardiac surgery. The outcome was the difference between the preinduction mean blood pressure (MBP) and the MBP obtained during preoperative evaluation. A difference of ≥10 mm Hg was considered clinically relevant. A paired samples t test was used to estimate the difference. Linear regression was used to obtain estimates adjusted for patient characteristics, comorbidity, medications, type of surgery, and preoperative blood pressure. RESULTS: Complete data were available for 3660 (83%) patients. There were 2228 (61%) patients with a difference of ≥10 mm Hg between the preinduction and preoperative MBP. The overall mean difference between both MBPs was 11 mm Hg (95% confidence interval, 10–11) with important variability among individuals. Patients with higher preoperative MBP values had smaller differences. After adjusting for patient characteristics, comorbidity, medications, type of surgery, and preoperative blood pressure, the difference decreased an estimated 5.0 mm Hg (95% confidence interval, 4.7–5.4) for every increase of 10 mm Hg in preoperative MBP. Patient characteristics, comorbidity, type of surgery, or medication were not strongly associated with the difference. CONCLUSIONS: The average preinduction blood pressure was higher than the preoperative blood pressure. This difference between the measurements can be explained by stress-induced effects and regression to the mean. To define an optimal reference value for research purposes or to arrive at a clinical perioperative blood pressure target, one should consider that there is important variability both within and between patients.