A study is conducted to assess the effect of beta-blocker therapy in patients with acute myocardial infarction (MI). The study found that 20 patients out of 90 who took a beta-blocker during the week prior to developing an MI had a major arrhythmia event during hospitalization. The study also determined that 30 patients out of 70 who did not take any beta-blocker developed a major arrhythmia. What was the odds ratio of developing major arrhythmia in patients who took beta-blockers compared to those who did not take beta-blockers?

Question

A study is conducted to assess the effect of beta-blocker therapy in patients with acute myocardial infarction (MI).  The study found that 20 patients out of 90 who took a beta-blocker during the week prior to developing an MI had a major arrhythmia event during hospitalization.  The study also determined that 30 patients out of 70 who did not take any beta-blocker developed a major arrhythmia.  What was the odds ratio of developing major arrhythmia in patients who took beta-blockers compared to those who did not take beta-blockers?

Quizplus · Accepted Answer

11ec160b_370e_f0a5_9249_f5b195d80352_MD0001_00 The odds of an event occurring are defined as the probability of that event happening divided by the probability of the event not happening. For instance, when rolling dice, the probability of rolling a one is 1/6, whereas the probability of rolling any other number is 5/6. Therefore, the odds of rolling a one are 1/5. The odds ratio (OR) is a measure of association that compares the odds of an outcome occurring based on exposure status. In this case, it represents the odds that the outcome (eg, major arrhythmia) occurred in the presence of the exposure (eg, beta blocker use) compared with the odds that the outcome occurred in the absence of that exposure. The OR can be calculated using a standard contingency table (with exposures in the rows and outcomes in the columns). 11ec160b_370e_f0a6_9249_99a3a9d123a9_MD0001_00 Odds of developing major arrhythmia in exposed group = 20/70 Odds of developing major arrhythmia in unexposed group = 30/40 (note that the total number of people in the exposed and unexposed groups is not used to calculate odds, unlike calculating probabilities) Therefore, the OR of developing a major arrhythmia in patients who took beta blockers compared with those who did not is as follows: In this case, the OR calculated was the OR of disease (among exposed individuals relative to nonexposed individuals), given by (a/b) / (c/d); in some cases, the OR of exposure (among diseased individuals relative to nondiseased individuals) is calculated, given by (a/c) / (b/d). Both OR definitions are mathematically equivalent and reduce (by a mathematical shortcut) to: OR = ad/bc. Any of these formulas would give the same result. Because the OR is <1, the odds of developing a major arrhythmia is lower for patients who were taking beta blockers compared with those who were not. (Choice E) Relative risk (RR) is the probability of an outcome occurring in the exposed group compared with the probability of the outcome occurring in the unexposed group. It is calculated using the following formula: RR often has more clinical utility than the OR since it gives the change in disease risk based on the presence or absence of risk factors. However, RR cannot be calculated in case-control studies as patients are preselected based on disease status (cases and controls) instead of being sorted into exposure groups and followed over time to assess disease outcomes. Educational objective: The odds ratio (OR) is a measure of the strength of an association between an exposure and an outcome. Specifically, it represents the odds that an outcome occurred in the presence of a particular exposure compared with the odds that the outcome occurred in the absence of that exposure. In a standard contingency table, OR = ad/bc.

A Study Is Conducted to Assess the Effect of Beta-Blocker