Preoperative BNP: Epidemiological Patterns, Management Strategies and Cost Analysis of Care

Major cardiac complications are responsible for at least a third of perioperative deaths and are associated with significant morbidity, prolonged hospitalization and increased costs. This number is expected to rise as more surgeries are undertaken in older and frailer patients. The term myocardial injury after noncardiac surgery (MINS) is used to describe all myocardial injury due to ischemia occurring in the first 30 days after surgery. MINS includes:

• myocardial infarction (cardiac troponin I ≥ 0.04 ug/L) with at least one of: symptoms of ischemia, new ST segment / T wave changes or new left bundle branch block, pathological Q waves, new loss of viable myocardium or regional wall motion abnormality on echocardiogram, intracoronary thrombus identified on angiogram.
• myocardial injury without MI (elevated troponin without the additional criteria needed to diagnose MI).

The Canadian Cardiovascular Society (CCS) has developed a set of guidelines to help stratify which patients are at higher risk for cardiac mortality. In the latest revision of these guidelines, preoperative B-type natriuretic peptide (BNP) has been identified as a powerful independent predictor of perioperative cardiovascular complications. BNP is a protein released from ventricular cardiomyocytes. It is released in limited amounts during physiological conditions but this is increased significantly in ventricular failure, inflammation, fibrosis, ischemia and hypoxia. Rodseth et al. conducted a systematic review of the prognostic value of preoperative BNP and postoperative BNP in noncardiac surgery and showed that elevated preoperative BNP (i.e. ≥ 92 ng/l) carries a significant risk of 30-day mortality or nonfatal MI (21.8%). This is a significantly lower BNP value compared to what is typically seen in congestive cardiac failure patients. This study has been used to support the most recent CCS guidelines which add preoperative BNP as an important preoperative cardiac risk stratification tool. According to the CCS guidelines, patients with an elevated preoperative BNP should have daily troponins measured for 48 - 72 hours after surgery.

Troponins are components of the myocardial cell involved in contractility, specific subtypes I and T are expressed almost exclusively in the heart. An elevated troponin reflects necrosis of myocardial cells and is extremely useful in the diagnosis of a myocardial infarction. Even a slight elevation in postoperative troponins (TnTi ≥ 0.03ng/ml) without ischemic clinical symptoms can lead to a higher mortality, nonfatal cardiac arrest, nonfatal congestive heart failure, and nonfatal stroke.

While the value of screening for high risk patients through the use of preoperative BNP been demonstrated, the management of postoperative ischemia is less clear. At present, CCS guidelines recommend optimizing aspirin and statin therapy in patients with MINS, but no clear strategy has been elucidated. Evidence to support the use of aspirin and statins comes from the POISE trial. The MANAGE trial recommends starting patients who are not at risk of bleeding on the oral anticoagulant dabigatran for two years. Patients with ST-elevation MI carry a high mortality risk without reperfusion therapy (percutaneous coronary intervention or coronary artery bypass graft) and a high risk of bleeding with it. Patients with non ST-elevation MI may also require revascularization and subsequent dual antiplatelet therapy. It is evident from this brief overview that management of MINS varies from the non-invasive (aspirin and statin administration) to the invasive (PCI or CABG) depending on the extent of myocardial injury, hemodynamic stability and risk of bleeding secondary to the primary surgical procedure.

Screening for patients at risk of MINS by measuring BNP may have a significant impact on health care resources. 80% of patients with MINS are asymptomatic and go unrecognized. Screening for patients who are at risk of postoperative myocardial ischemia will ostensibly uncover a new population of patients i.e. those who, prior to BNP screening, experienced subclinical postoperative ischemia which went undiagnosed until a later juncture by which time potentially worse and irreversible myocardial injury had occurred.

In this study the objectives are to investigate:

• incidence of qualification for BNP measurement as determined by CCS guidelines
• incidence of patients with BNP ≥ 92 ng/L
• incidence of MINS (myocardial infarction and myocardial injury without MI) at 30 days and incidence of myocardial infarction at 1 year after surgery
• evaluate the management of patients with positive troponin after surgery
• identify a matched cohort group from the SPOR database
• compare the incidence and timing of MINS in the BNP group versus that in a matched cohort group who qualified for BNP screening but did not have it done
• determination of cost of screening, diagnosis and management of MINS and MI for up to 1 year after surgery when BNP is used as a screening tool

In this study, elective patients deemed at risk of a postoperative cardiac event will be monitored, as determined by preoperative BNP measurement, up to 1 year postoperatively. BNP positive patients will be monitored for myocardial injury postoperatively by measuring troponin daily for 72 hours. The incidence of MINS (MI and myocardial injury without MI) in BNP positive patients will be determined. Patients with troponin I ≥ 0.04 ug/L will be referred to the cardiology consult service for further evaluation and management. Subsequent management will be categorized as 1) monitor 2) medical management and 3) revascularization. A cost analysis of the management of all three groups will be performed.

Inclusion criteria

• All adult patients:

• ≥ 65 years of age
• ≥ 18 years of age with a revised cardiac risk index (RCRI) ≥ 1
• between 45 and 64 years of age with significant cardiovascular disease
• Elective non-cardiac surgery with an overnight hospital stay (minimum 1 day)

Exclusion criteria

• Subjects < 18 years of age
• Patient refusal to participate
• Patients undergoing cardiac surgery
• Patients undergoing day surgery (< 1day hospital admission)