Fig. 3. Frequency plot of nadir hemoglobin. This graph represents the nadir hemoglobin (lowest recorded hemoglobin in the first 3 postoperative days) in the 2 cohorts (open bars  are control and hatched bars  represent β-blocked patients) before propensity matching (A ) and after matching (B ). Note that β-blocked patients have proportionally more patients with nadir hemoglobin (in the first 3 postoperative days) less than 90 mg/l in (A ) (the unadjusted population). Propensity matching seen in (B ) results in two balanced populations (d = 3.87) A nadir hemoglobin less than 90 mg/l was associated with a troponin measured two times more frequently than nadir hemoglobin more than 90 mg/l (relative risk 1.9; 95% confidence interval [CI] 1.6–2.2). This increased use of troponin testing was seen irrespective of whether the patients were β-blocked or not. In the patients with a nadir hemoglobin less than 90 mg/dl, β-blockade was associated with a 3-fold increase in myocardial infarction (relative risk 2.94; 95% CI 1.4–6.4).