Cardiac biomarkers study notes
Cardiac biomarkers study notes
Overview
Heart pumps blood to all tissues; relies on coronary arteries for own blood supply
Ischaemia occurs when blood supply to heart muscle is reduced; may lead to myocardial infarction (MI)
Cardiac biomarkers are blood markers of heart muscle injury or stress
Biomarkers discussed: cardiac troponins (cTnI and cTnT) and creatine kinase (CK); B-type natriuretic peptide (BNP) and NT-proBNP for heart failure
Cardiovascular physiology and disease set
Heart physiology context: cardiac output depends on intact coronary supply; lack of blood causes ischaemia and MI
Heart failure arises when the heart cannot pump enough blood or adequately receives blood from lungs/systemic circulation
Biomarkers detect myocardial injury (ischaemia/necrosis) or myocardial strain (heart failure)
Cardiovascular disease (CVD) framework
CVD encompasses:
Coronary artery disease (CAD) leading to angina and MI
Cerebrovascular disease leading to TIA and stroke
Peripheral vascular disease
Other non-atherosclerotic CVD
Atherosclerosis is the major driver of CVD
Leading causes of death globally (illustrative snapshot)
In 2000 and 2019, the top causes include ischaemic heart disease, stroke, COPD, lower respiratory infections, neonatal conditions, various cancers, dementia, diarrhoeal diseases, diabetes, and kidney diseases
Data sources include WHO Global Health Estimates and AIHW mortality datasets
Atherosclerosis: contributors and pathophysiology
Major contributors: hypercholesterolaemia, hypertension, diabetes, sedentary behaviour, smoking, age, male gender
Pathogenesis concepts:
Endothelial dysfunction increases permeability
LDL modification via enzymatic oxidation drives inflammatory response
Initial triggers include haemodynamic stress and turbulent flow
Inflammation: recruitment of platelets and leukocytes; monocytes become macrophages and foam cells; T-cell activation
Plaque evolution: smooth muscle cell proliferation and fibrous cap formation lead to maturation and stabilization
Plaques can rupture, cause acute occlusion, and precipitate MI
Coronary artery disease (CAD) and myocardial infarction (MI)
CAD occurs when coronary arteries are narrowed due to atherosclerosis → reduced blood flow → chronic or demand ischaemia
Plaque rupture with acute occlusion leads to abrupt loss of blood supply → myocardial infarction
Coronary circulation and pathology visuals (conceptual)
Zones:
Zone of perfusion (area at risk) during acute occlusion
Zone of necrosis if perfusion fails
Cross-sectional infarct patterns depend on which coronary artery is involved (e.g., right coronary, left circumflex, left anterior descending)
Acute coronary total occlusion leads to completed infarct involving most of the area at risk if not promptly treated
Atherosclerosis: initiating and evolution concepts
Initiating events include haemodynamic stress and hypercholesterolaemia
Endothelial dysfunction increases permeability
LDL modification (oxidation/enzymatic changes) propagates inflammation
Inflammation drives plaque growth and instability
Inflammation and plaque biology
Arterial wall inflammation leads to platelet and leukocyte recruitment; inflammatory mediators released
Monocytes become macrophages; ingest oxidized LDL; foam cells form
T cells contribute to inflammatory milieu
Plaque maturation and stability
Smooth muscle cells migrate from the intima to media; proliferate and synthesize collagen
A fibrous cap forms around the plaque; stability increases with cap formation
Pathology references: Robbins & Cotran Pathologic Basis of Disease; Hansson reviews (NEJM 2005)
Acute coronary syndromes and myocardial infarction (ACS/MI) progression
ACS categories:
STEMI: ST-segment elevation criteria on ECG plus symptoms of ACS
NSTEMI: ACS symptoms or ECG changes without ST elevation, with biochemical evidence of myocardial injury
Unstable angina: ACS symptoms with ECG changes but no detectable troponin rise
MI requires biomarkers of myocardial injury (preferably cTn) plus evidence of ischaemia from ECG, imaging, symptoms, or intracoronary thrombus
Ischaemia time is critical: time is muscle; early detection and treatment improve outcomes
Diagnostic electrocardiography and pathology timelines (ACS)
STEMI criteria include characteristic ST elevation on ECG (time-sensitive diagnosis)
NSTEMI/Unstable angina rely on troponin and ECG changes
Time-course of infarction: zone of perfusion at risk evolves into necrosis; earliest viable myocardium is salvageable with timely intervention
Myocardial infarction cross-section concepts
Acute occlusion leads to an evolving infarct; early intervention salvages myocardium
Infarct progresses from reversible to irreversible injury over hours; diagnostic imaging and biomarkers aid in timing and management decisions
Troponin as cardiac biomarkers: structure and rationale
Troponin complex comprises three subunits: Troponin I (cTnI), Troponin T (cTnT), and Troponin C (cTnC)
Roles:
cTnI: binds actin and regulates contraction
cTnT: binds to tropomyosin
cTnC: binds calcium to trigger contraction
Troponin I and T are cardiac-specific isoforms; Troponin C is shared with skeletal muscle
In myocardial injury, cTnI and cTnT are released into plasma and serve as biomarkers
Troponin biology and biomarker use
Cardiac troponin complexes become detectable in plasma after myocardial injury
Specificity: cTnI and cTnT are cardiac isoforms; cTnC is common to skeletal muscle (hence not used alone as specific marker)
Troponin release pattern: rise and/or fall in concentration indicates acute injury (AMI) per the universal definitions
4th Universal Definition of AMI (2018)
Myocardial injury definition: at least one cTn value above the 99th percentile upper reference limit; considered acute if there is a rise and/or fall
Acute myocardial infarction (AMI): acute myocardial injury with rise and/or fall in cTn values AND evidence of myocardial ischaemia such as:
Consistent ECG changes
Imaging evidence of new loss of viable myocardium
Symptoms of ischaemia
Autopsy or imaging identification of intracoronary thrombus
Formal biomarker criterion is the biochemical rise/fall above the 99th percentile with supportive clinical evidence
Measuring troponins: assay design and potential interferences
Automated immunoassays are used (examples: Architect by Abbott; two-site chemiluminescent immunoassay)
Assay design in brief:
Antibody 1 bound to a solid carrier (paramagnetic microparticle)
Antibody 2 labeled with acridinium (light-producing label)
The light emitted is proportional to troponin concentration in the sample
Possible false positives: heterophile antibody interference, rheumatoid factor, macrotroponin
Macrotroponin and heterophile interference (clinical interference concepts)
Macrotroponin is an immunoglobulin-troponin complex that can cause persistent elevated troponin without true myocardial injury
Macrotroponin may yield false-positive troponin results
Heterophile antibodies can bridge capture and detection antibodies, leading to false positives
Visual schematic (described): Capture antibody binds analyte; label antibody binds analyte; in presence of interfering antibodies, signal may be generated without analyte
Demonstrations and studies show these interferences can mislead interpretation
Laboratory evaluation for macrotroponin and interference detection
Strategies to evaluate macrotroponin:
Run on different troponin assays to see discrepancies between platforms
PEG precipitation to remove immunoglobulins and assess signal reduction
Chromatographic studies to characterize the interfering species
Practical approach: use multiple assays and sample treatments to confirm true elevation
Troponin criteria and high-sensitivity assays
From the 4th universal definition: cTn rise and/or fall with at least one value above the 99th percentile upper reference limit; CV (coefficient of variation) at the 99th percentile should be ≤ 10%
Rise/fall interpretation depends on assay precision and biological variation; time intervals (e.g., 2 hours) used to detect significant change
High-sensitivity troponin (hs-Tn):
Coefficient of variation at the 99th percentile ≤ 10%
Capable of assigning a numeric value to >50% of the reference population
Greater diagnostic performance than older-generation assays, especially within the first hours after onset
Prior-generation troponin assays often had many values below the reporting limit (e.g., “< 0.04 mcg/L”)
Assay performance and diagnostic accuracy (hs-Tn vs standard)
Comparative performance studies show hs-Tn assays outperform standard sensitive troponin assays in both overall presentation and early presenters (e.g., within 3 hours of chest pain onset)
Diagnostic performance is commonly summarized by Receiver Operating Characteristic (ROC) curves and Area Under the Curve (AUC)
Conceptual takeaway: within the same patient cohort, hs-Tn assays typically yield higher AUCs and better discrimination for acute MI, particularly early after symptom onset
Example framing: four sensitive troponin assays vs standard assay; hs assays tend to show superior sensitivity without compromising specificity in many settings
Time course and interpretation pathway for troponin (clinical workflow)
Example protocol (illustrative):
Presenting symptoms suggestive of myocardial ischaemia
Initial ECG assessment (ST-elevation or not)
If STEMI: treat immediately; do not wait for troponin results
For non-ST-elevation presentations: baseline troponin and ECG
Recheck troponin at 2 hours (and 6 hours if risk factors present)
MI is likely if baseline troponin is normal with a subsequent rise of ≥ 50% at 2 hours; if baseline is elevated, a rise of ≥ 20% at 2 hours may indicate progression
Causes and interpretation of elevated plasma cTnI (Troponin I)
Causes of troponin elevation beyond MI include:
Myocardial injury, dysfunction, or strain due to: MI, myocarditis, cardiac contusion, heart failure, pulmonary embolism, arrhythmia
Renal failure
Neurological conditions (e.g., stroke, subarachnoid haemorrhage)
Endurance exercise or extreme athletic exertion (controversial and context-dependent)
Interpretation nuance: elevated troponin indicates injury but not exclusive to MI; clinical context and corroborating evidence are essential
Prognostic value: troponin elevation carries prognostic information in various settings beyond diagnosis
Heart failure: physiology, diagnosis, and biomarkers
Definition: Heart failure is when the heart cannot pump enough blood to meet tissue needs, or does so at elevated filling pressures
Common etiologies include: CAD, chronic hypertension, valvular disease, dilated cardiomyopathy, infiltrative diseases, arrhythmias
Compensatory mechanisms in heart failure
Frank-Starling mechanism: increased preload enhances contractility via increased actin-myosin cross-bridging
Cardiac hypertrophy: compensatory initially but can become maladaptive over time
Neurohormonal activation: renin-angiotensin-aldosterone system (RAAS) activation aids short-term perfusion but is maladaptive long-term
Natriuretic peptide system: atrial natriuretic peptide (ANP) and B-type natriuretic peptide (BNP) are released in response to myocardial stretch; promote natriuresis and vasodilation; RAAS inhibition
BNP and NT-proBNP in heart failure assessment
BNP is synthesized and released by atria and ventricles in response to stretch; biologically active peptide with natriuretic and vasodilatory effects; also inhibits RAAS
NT-proBNP is the inactive N-terminal cleavage product of the prohormone proBNP after processing; both BNP and NT-proBNP can be measured for diagnosis and monitoring of HF; clinical utility similar but cut-offs differ
Diagnostic utility: BNP/NT-proBNP improve diagnostic accuracy for HF vs relying on clinical judgment alone
Key study (McCullough et al., Circulation 2002): BNP or NT-proBNP improved diagnostic accuracy over clinical judgment; AUCs around high 0.8–0.9 range; combined BNP+clinical assessment reached higher discriminative performance
Diagnostic cut-offs: not fully universal; BNP < 100 ng/L makes HF unlikely in acute dyspnea settings (varies by assay and context)
NT-proBNP vs BNP: NT-proBNP levels tend to rise with age and higher BMI; there are age-specific cut-offs for NT-proBNP; assay standardisation issues exist for both markers
Renal impairment elevates both BNP and NT-proBNP, reducing their specificity for HF without careful interpretation
Sacubitril (neprilysin inhibitor) in heart failure treatment: neprilysin degrades BNP; thus BNP may be less useful for monitoring progress in patients on sacubitril; NT-proBNP is preferred in that setting
Measurement issues and assay limitations for BNP/NT-proBNP
Assay standardisation issues: BNP glycosylation variability; NT-proBNP can reflect different N-terminal cleavage products depending on the manufacturer
Renal impairment complicates interpretation due to reduced clearance of natriuretic peptides
Drug interactions: neprilysin inhibition (sacubitril) alters BNP metabolism, affecting BNP utility; consider NT-proBNP in these patients
Practical and clinical takeaways
Troponin I and Troponin T are central to AMI diagnosis; treat STEMI based on ECG/clinical criteria without waiting for troponin
Turnaround time for troponin results is clinically important to expedite management
BNP and NT-proBNP are useful adjuncts for diagnosis and monitoring of heart failure in appropriate clinical settings
In practice, combine biomarker information with clinical assessment, imaging, and ECG to reach a diagnosis
Exam-focused topics and example questions
Define CAD, atherosclerosis, ACS, AMI, STEMI, NSTEMI, and CHF; explain how they relate and differ
Describe the 4th Universal Definition of AMI in terms of required biomarker changes and accompanying evidence
Explain the design of troponin immunoassays and how they detect troponin in plasma
List and describe two major causes of positive interference in troponin assays and three ways to detect interference
List five potential causes of increased plasma cardiac troponin concentration beyond MI
Describe how sacubitril affects BNP and NT-proBNP, and indicate which assay should be used in patients on this drug for monitoring
Quick reference formulas and thresholds (high level)
AMI biomarker criterion: at least one cTn value above the 99th percentile upper reference limit plus evidence of myocardial ischaemia
Evidence examples: consistent ECG changes, imaging loss of viable myocardium, clinical symptoms, intracoronary thrombus on imaging/autopsy
High-sensitivity troponin performance target: CV_{99 ext{th percentile}} \leq 10\%
Rise/fall interpretation (illustrative):
Baseline cTn normal → subsequent rise of \Delta cTn \geq 50\% \text{at 2 hours} is suggestive of MI
Baseline cTn elevated → subsequent rise of \geq 20\% \text{at 2 hours} may indicate ongoing injury
BNP/NT-proBNP cut-offs: not universal; in acute dyspnea with a strong clinical focus, BNP < 100 ng/L makes HF unlikely (context-dependent)
Time-is-muscle concept: early intervention preserves salvageable myocardium; ongoing infarct evolves over hours
Appendix: terminologies and quick references
Cardiac troponin I (cTnI) and Troponin T (cTnT): cardiac-specific biomarkers for myocardial injury
Troponin C (cTnC): shared with skeletal muscle; not used as a standalone cardiac biomarker
Macrotroponin: macrocomplex of troponin with immunoglobulin causing false-positive results; requires confirmatory testing
Heterophile antibodies: potential interference causing falsely elevated troponin readings
hs-Tn: high-sensitivity troponin assays offering earlier and more reliable detection of myocardial injury
BNP: active natriuretic peptide; NT-proBNP: inactive fragment; both useful in HF diagnosis/monitoring but differ in metabolism and assay characteristics
Sacubitril: neprilysin inhibitor used in HF; affects BNP metabolism but not NT-proBNP suitability for monitoring