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Do cardiac risk stratification indexes accurately estimate perioperative risk in noncardiac surgery patients?

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Neither of the two cardiac risk assessment indexes most commonly used (Table 1)1,2 is completely accurate, nor is one superior to the other. To provide the most accurate assessment of cardiac risk, practitioners need to select the index most applicable to the circumstances of the individual patient.

CARDIAC COMPLICATIONS ARE INCREASING

The Revised Cardiac Risk Index and the National Surgical Quality Improvement Program index
About 5% of patients undergoing noncardiac surgery have a major cardiac complication within the first 30 postoperative days.3,4 This rate has been rising, primarily due to an increasing prevalence of cardiac comorbidities. Thus, accurate preoperative cardiac risk stratification is needed to assess the risk of perioperative major cardiac complications in all patients scheduled for noncardiac surgery. This information helps the perioperative team and patient to better weigh the benefits and risks of surgery and to optimize its timing and location (eg, inpatient vs outpatient surgery center).

CARDIAC RISK ASSESSMENT INDEXES

The 2 risk assessment indexes most often used are:

  • The Revised Cardiac Risk Index (RCRI)1
  • The National Surgical Quality Improvement Program (NSQIP) risk index, also known as the Gupta index.2

Both are endorsed by the American College of Cardiology (ACC) and the American Heart Association (AHA).5 The RCRI, introduced in 1999, is more commonly used, but the NSQIP, introduced in 2011, is based on a larger sample size.

Both indexes consider various factors in estimating the risk, with some overlap. The main outcome assessed in both indexes is the risk of a major cardiac event, ie, myocardial infarction or cardiac arrest. The RCRI outcome also includes ventricular fibrillation, complete heart block, and pulmonary edema, which may be sequelae to cardiac arrest and myocardial infarction. This difference in defined outcomes between the indexes is not likely to account for a significant variation in the prediction of risk; however, this is difficult to prove.

Each index defines myocardial infarction differently. The current clinical definition6 includes detection of a rise or fall of cardiac biomarker values (preferably cardiac troponins) with at least 1 value above the 99th percentile upper reference limit and at least 1 of the following:

  • Symptoms of ischemia
  • New ST-T wave changes or new left bundle branch block
  • New pathologic Q waves
  • Imaging evidence of new loss of viable myocardium tissue or new regional wall- motion abnormality
  • Finding of an intracoronary thrombus.

As seen in Table 1, the definition of myocardial infarction in NSQIP was one of the following: ST-segment elevation, new left bundle branch block, Q waves, or a troponin level greater than 3 times normal. Patients may have mild troponin leak of unknown significance without chest pain after surgery. This suggests that NSQIP may have overdiagnosed myocardial infarction.

USE IN CLINICAL PRACTICE

In clinical practice, which risk index is more accurate? Should clinicians become familiar with one index and keep using it? The 2014 ACC/AHA guidelines5 do not recommend one over the other, nor do they define the clinical situations that could lead to significant underestimation of risk.

The following are cases in which the indexes provide contradictory risk assessments.

Case 1. A 60-year-old man scheduled for surgery has diabetes mellitus, for which he takes insulin, and stable heart failure (left ventricular ejection fraction 40%). His RCRI score is 2, indicating an elevated 7% risk of cardiac complications; however, his NSQIP index is 0.31%. In this case, the NSQIP index probably underestimates the risk, as insulin-dependent diabetes and heart failure are not variables in the NSQIP index.

Case 2. A 60-year-old man who is partially functionally dependent and is on oxygen for severe chronic obstructive pulmonary disease is scheduled for craniotomy. His RCRI score is 0 (low risk), but his NSQIP index score (4.87%) indicates an elevated risk of cardiac complications based on his functional status, symptomatic chronic obstructive pulmonary disease, and high-risk surgery. In this case, the RCRI probably underestimates the risk.

These cases show that practitioners should not rely on just one index, but should rather decide which index to apply case by case. This avoids underestimating the risk. In patients with poor functional status and higher American Society of Anesthesiology class, the NSQIP index may provide a more accurate risk estimation than the RCRI. Patients with cardiomyopathy as well as those with insulin-dependent diabetes may be well assessed by the RCRI.

The following situations require additional caution when using these indexes, to avoid over- and underestimating cardiac risk.

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