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What is METS? What are the recent guidelines for assessment of functional capacity in a patient with coronary stents in-situ and what is the relevant role of cardiac biomarkers in this patient? [2+4+4]

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METS, Functional Capacity Assessment in Coronary Stent Patients, and Cardiac Biomarkers

Part 1: What is METS? [2 marks]

MET (Metabolic Equivalent of Task) is a unit that expresses the energy cost of a physical activity as a multiple of the resting metabolic rate. One MET is defined as the resting oxygen consumption of a seated individual - approximately 3.5 mL O₂/kg/min (or ~1.2 kcal/min for a 70 kg person).
"One MET is defined as resting metabolic rate in a sitting position, representing the amount of oxygen consumed at rest (approximately 3.5 mL O₂/kg/min or 1.2 kcal/min for a 70-kg person)."
  • Sabiston Textbook of Surgery
The METS (Measurement of Exercise Tolerance before Surgery) is a related validated tool used alongside the DASI (Duke Activity Status Index) for preoperative functional capacity scoring. METs can also be estimated using DASI scores: VO₂ peak (mL/min) = (0.43 × DASI) + 9.6, then METs = VO₂ peak × 0.286.
Clinical significance of MET thresholds:
Metabolic Equivalents of Task (METs)
Figure: MET values for common activities (Sabiston Textbook of Surgery)
MET LevelActivitiesClinical Significance
1 METEating, reading, watching TV, dressingResting baseline
2-3 METsWalking slowly, light houseworkPoor functional capacity
4 METsClimbing stairs, heavy chores, golfThe key clinical threshold
>10 METsHeavy exercise, tennis, soccerExcellent capacity
The 4-MET threshold is the critical perioperative cutoff: patients unable to achieve >4 METs are considered to have poor functional status and are at increased risk for perioperative morbidity and mortality. A meta-analysis showed that each additional MET correlates with a 13% decrease in all-cause mortality and a 15% decrease in long-term cardiovascular disease risk.
  • Sabiston Textbook of Surgery

Part 2: Recent Guidelines for Assessment of Functional Capacity in a Patient with Coronary Stents In-Situ [4 marks]

Preoperative functional capacity assessment in stent patients follows a stepwise approach, informed by the 2014 ACC/AHA guidelines and updated 2022 ESC guidelines. The key concern in stent patients is the intersection of functional status with the risk of stent thrombosis from perioperative hypercoagulability.

Step 1 - Determine Surgical Urgency

Emergency surgery proceeds directly, with postoperative cardiac monitoring and optimization. Non-urgent surgery triggers the full evaluation pathway.

Step 2 - Rule Out Active Cardiac Conditions

Before assessing functional capacity, exclude active cardiac conditions: acute coronary syndrome, decompensated heart failure, severe valvular disease, or significant arrhythmias. The 2014 ACC/AHA guidelines recommend deferring non-urgent surgery until 60 days after a recent myocardial infarction.

Step 3 - Timing Relative to Stent Placement (Stent-Specific Guideline)

This is unique to patients with in-situ coronary stents:
Stent TypeMinimum Delay Before Elective Non-Cardiac Surgery
Balloon angioplasty≥14 days
Bare metal stent (BMS)≥30 days (ACC/AHA); ≥4 weeks (Fuster/Hurst)
Drug-eluting stent (DES)≥6 months (ideal); may consider after 90 days if surgical delay risk > stent thrombosis risk
"The increased risk of perioperative cardiac events following PCI is related to perioperative hypercoagulability associated with surgical stress in the setting of non-endothelialized stent surfaces."
  • Fuster and Hurst's The Heart, 15th Edition

Step 4 - Functional Capacity Assessment Methods

A. Subjective/Unstructured Interview (traditional, but limited):
  • Simply asking patients about their activity level. Sensitivity is only 19% for identifying patients who cannot achieve ≥4 METs on formal testing. Not recommended as the sole method.
  • Miller's Anesthesia, 10e
B. Structured Interview Questions (better than unstructured):
  • Inability to walk 4 blocks or climb 2 flights of stairs predicts increased perioperative complications.
  • Inability to climb ≥3 flights of stairs identifies patients likely unable to achieve ≥4 METs.
C. Validated Questionnaires (preferred over unstructured interview):
  1. DASI (Duke Activity Status Index): 12-item validated questionnaire. Score ≤25 is associated with clinically meaningful increased risk of MI and major complications post-surgery. Optimal threshold for ≥4 METs: DASI score ≥32.
  2. MET-REPAIR (MET: REvaluation for Perioperative cArdIac Risk): Validated 10-item questionnaire with activities scored 1-10 METs. Functional capacity = highest MET level the patient reports being able to perform. Threshold for ≥4 METs on MET-REPAIR: ≥6 METs self-reported.
Both questionnaires are validated against gold-standard exercise testing and outperform unstructured interviews.
  • Miller's Anesthesia, 10e
D. Objective Performance-Based Tests (most accurate):
  • 6-Minute Walk Test (6MWT) and Incremental Shuttle Walk Test: poor performance predicts major postoperative complications.
  • Cardiopulmonary Exercise Testing (CPET): gold standard for measuring true VO₂ max in METs. Particularly useful for high-risk patients or those with ambiguous questionnaire results.
  • Standard Exercise ECG (Bruce Protocol): note that extrapolation from treadmill time on Bruce protocol tends to overestimate true exercise capacity.
  • Resting LVEF is NOT a proxy for functional capacity - exercise capacity varies widely among patients with similar resting LVEF.
E. Pharmacological Stress Testing: Used when patient cannot exercise (e.g., orthopedic limitations). Dobutamine stress echo or nuclear perfusion imaging may be considered in patients with ≥3 RCRI risk factors and poor or unknown functional capacity undergoing intermediate-to-high risk surgery.

Step 5 - Clinical Decision Following Assessment

  • Good functional capacity (≥4 METs): Proceed to planned surgery, continue cardiac medications perioperatively.
  • Poor functional capacity (<4 METs) or unknown: Consider further cardiac evaluation, specialist consultation (cardiology), and optimization before elective surgery.

Antiplatelet Therapy Management (stent-specific)

Dual antiplatelet therapy (DAPT) must be managed carefully in stent patients:
  • Continue DAPT within the first 4 weeks after BMS and 6 months after DES unless surgical bleeding risk clearly outweighs stent thrombosis risk.
  • If P2Y12 inhibitor (clopidogrel, prasugrel, ticagrelor) must be stopped, continue aspirin and restart P2Y12 inhibitor as soon as possible postoperatively.
  • Decision requires consensus between surgeon, anesthesiologist, and cardiologist.
  • Sabiston Textbook of Surgery; Fuster and Hurst's The Heart, 15th Edition

Part 3: Relevant Role of Cardiac Biomarkers in This Patient [4 marks]

In a patient with coronary stents in-situ, cardiac biomarkers serve distinct roles at different timepoints: preoperative risk stratification, intraoperative/perioperative monitoring, and postoperative surveillance.

1. B-Type Natriuretic Peptide (BNP) and NT-proBNP

Role in preoperative risk stratification:
  • BNP/NT-proBNP reflect ventricular wall stress and are prognostically useful for perioperative cardiovascular risk assessment.
  • A meta-analysis of 18 studies showed that BNP ≥92 ng/L is associated with increased risk of cardiac death or non-fatal MI in non-cardiac surgery.
  • AHA/ACC position: Routine BNP measurement is NOT recommended as part of standard preoperative assessment because biomarker-based management strategies have not yet been shown to reduce cardiovascular risk.
  • ESC 2022 guideline position: BNP/NT-proBNP may be helpful when the decision to proceed to further preoperative cardiac testing is unclear. A low BNP/NT-proBNP indicates relatively low risk, while an elevated level warrants further workup, discussion, and shared decision-making.
  • Sabiston Textbook of Surgery; Fuster and Hurst's The Heart, 15th Edition

2. Cardiac Troponin (cTn)

Role in preoperative assessment:
  • AHA/ACC: Do not recommend routine preoperative troponin in asymptomatic patients. Troponin should be measured when signs or symptoms suggest myocardial ischemia or infarction.
  • ESC 2022 guidelines: Recommend obtaining preoperative troponin in patients >65 years old or those with cardiac risk factors undergoing intermediate- to high-risk surgery. Elevated preoperative troponin identifies patients at higher perioperative cardiac risk.
  • Sabiston Textbook of Surgery; Fuster and Hurst's The Heart, 15th Edition
Role in stent-specific context - Type 4 MI:
  • Type 4a MI (PCI-related): Defined by cardiac troponin elevation above the 99th percentile upper reference limit within 48 hours post-PCI, with either new ECG changes, new LBBB, new regional wall motion abnormality, or imaging confirmation of new loss of viable myocardium. Troponin must be >5× 99th percentile URL for this diagnosis (WHO/ESC universal MI definition).
  • Type 4b MI (stent thrombosis-related): Troponin elevation with documented stent thrombosis on angiography or at autopsy.
  • Type 4c MI: Linked to in-stent restenosis.
  • Sabiston Textbook of Surgery; Rosen's Emergency Medicine
Role in postoperative surveillance:
  • Perioperative Myocardial Injury (PMI): Defined as troponin elevation within 30 days of surgery. Even asymptomatic troponin rises (>99th percentile URL) in the postoperative period carry prognostic significance.
  • In stent patients specifically, postoperative troponin elevation may signal stent thrombosis, acute in-stent restenosis, or type 1 plaque rupture MI - all requiring urgent angiography.
  • Monitoring troponin postoperatively (at 24 and 48 hours) is particularly important in stent patients undergoing intermediate-to-high risk surgery, per ESC 2022 recommendations.

3. Natriuretic Peptides for Heart Failure Monitoring

In stent patients who also have reduced LV function (e.g., prior large MI), BNP/NT-proBNP also serve to:
  • Guide perioperative fluid management
  • Identify decompensated heart failure requiring optimization before surgery
  • Track response to treatment in the postoperative period

Summary Table

BiomarkerPreoperativePerioperativePostoperative
BNP/NT-proBNPRisk stratification (unclear decision); low value = reassuringGuide fluid managementMonitor for HF decompensation
Troponin (cTnI/cTnT)Routine NOT recommended (AHA/ACC); YES if >65y or cardiac risk + intermediate/high risk surgery (ESC 2022)If symptoms of ischemiaDetect PMI, stent thrombosis, Type 4 MI
"While the AHA/ACC guidelines recommend perioperative measurement of cardiac troponin levels whenever signs or symptoms are suggestive of myocardial ischemia or infarction, a routine screening of cardiac troponin levels in asymptomatic patients is not recommended."
  • Fuster and Hurst's The Heart, 15th Edition
Sources:
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