A 65-year-old man is scheduled for hip replacement surgery. He is worried about possible complications of the surgery, particularly the risk of heart attack. A retrospective cohort study identified 98,102 patients who underwent total knee replacement (TKR) or total hip replacement (THR) surgery and compared their rates of myocardial infarction (MI) with those of controls matched by age, sex, and geographic region. The following curves for hazard ratios (HR) were obtained. Based on the study results and assuming statistical significance, which of the following provides the best explanation to the patient?
A)Although there is no risk of heart attack immediately following surgery, the risk increases after hospital discharge.
B)There is an increased risk of heart attack after surgery, which is best explained by risk factors and comorbidities.
C)There is an increased risk of heart attack right after surgery, but it returns to baseline after 4 weeks.
D)There is an increased risk of heart attack up to approximately 8-10 weeks after the surgery.
E)There is no substantial proof that the surgery is associated with a higher risk of heart attack.

Question

A 65-year-old man is scheduled for hip replacement surgery.  He is worried about possible complications of the surgery, particularly the risk of heart attack.  A retrospective cohort study identified 98,102 patients who underwent total knee replacement (TKR) or total hip replacement (THR) surgery and compared their rates of myocardial infarction (MI) with those of controls matched by age, sex, and geographic region.  The following curves for hazard ratios (HR) were obtained.  Based on the study results and assuming statistical significance, which of the following provides the best explanation to the patient?
A)Although there is no risk of heart attack immediately following surgery, the risk increases after hospital discharge. 
B)There is an increased risk of heart attack after surgery, which is best explained by risk factors and comorbidities. 
C)There is an increased risk of heart attack right after surgery, but it returns to baseline after 4 weeks. 
D)There is an increased risk of heart attack up to approximately 8-10 weeks after the surgery. 
E)There is no substantial proof that the surgery is associated with a higher risk of heart attack.

Quizplus · Accepted Answer

Explanation: The graph depicts adjusted hazard ratios (HR) for acute myocardial infarction (MI) (compared to matched controls) following total hip replacement (THR) and total knee replacement (TKR) surgeries. HR is a measure of effect used in survival or time-to-event analysis. It has a similar interpretation to relative risk. The null value for HR is 1 (dotted line), which represents the baseline risk of MI in this situation. For THR and TKR, the HR for MI is elevated immediately following surgery (~38 for TKR and ~32 for THR), but it decreases to baseline (ie, HR of 1 [dotted line]) within subsequent weeks to months. In the case of TKR, the HR decreases to baseline within ~4 weeks, but the decrease takes longer in the case of THR. The risk of MI appears to stay elevated until 8-10 weeks following the surgery, at which time the HR returns to around 1 (baseline). (Choice A) The HR for MI is highest immediately after surgery but then decreases to baseline. (Choice B) The HR curves presented are based on adjusted values; this means that known risk factors and comorbidities have been taken into account in the calculations and do not account for the differences seen. Even without adjustment, the curves alone do not provide sufficient information to attribute the differences in HR to different risk factors or comorbidities. (Choice C) The risk of MI appears to return to baseline in ~4 weeks in the case of TKR (not THR). (Choice E) The HRs are clearly elevated (especially immediately after surgery). Given the assumption of statistically significant results in this case, this means that there is an association between surgery and heart attack risk. Educational objective: Hazard ratios have a similar interpretation to relative risk.

A 65-Year-Old Man Is Scheduled for Hip Replacement Surgery