Cardiac Biomarkers and Biochemical Profiles

Myocardium

The middle muscular layer of the heart wall containing striated muscle fibers made of contractile proteins (actin and myosin), regulatory troponins, and enzymes such as creatine kinase (CK) and lactate dehydrogenase (LDH).

Myocardial Infarction Classification by Size

Myocardial necrosis categorized into microscopic (focal necrosis), small ($< 10\%$ of the left ventricular myocardium), moderate ($10\%\text{--}30\%$ of the left ventricular myocardium), and large ($> 30\%$ of the left ventricular myocardium).

Cardiac Leakage Markers

Intracellular proteins and enzymes released into circulation following cardiac cell injury and loss of membrane integrity, including Troponin I, Troponin T, Troponin C, Myoglobin, Creatine Kinase (CK), LDH, AST, and Myosin light and heavy chains.

Cardiac Functional Markers

Biomarkers reflecting neurohormonal and hemodynamic responses to cardiac stress, including B-type natriuretic peptide (BNP), Atrial natriuretic peptide (ANP), C-reactive protein, Endothelin-1, Epinephrine, Norepinephrine, Renin, Angiotensin II, Aldosterone, and Arginine vasopressin.

Ideal Cardiac Marker Criteria

Properties defining an optimal biomarker: high specificity for myocardial damage over skeletal injury, high sensitivity for minor damage, ability to differentiate reversible from irreversible injury, prognostic utility, rapid and cost-effective testing, and complete absence in healthy individuals.

Creatine Kinase (CK) Isoenzymes

Dimeric cytosolic enzymes composed of B and M subunits: CK-BB (CK-1, brain origin), CK-MB (CK-2, predominantly specific for the myocardium), and CK-MM (CK-3, found primarily in skeletal muscle and heart).

CK-MB Kinetics

Serum levels begin to rise within $4\text{--}6\,\text{hours}$ after the onset of chest pain, reach peak concentration at $12\text{--}24\,\text{hours}$, and return to normal baseline within $2\text{--}3\,\text{days}$.

CK-MB Relative Index

A calculated ratio defined as $(\text{CK-MB mass} / \text{total CK}) \times 100$ that distinguishes myocardial damage from skeletal muscle or neural injury; an index exceeding $3$ is indicative of acute myocardial infarction.

CPK/GOT Ratio

A diagnostic ratio used to distinguish tissue damage: a ratio of $< 10$ indicates myocardial infarction, whereas a ratio of $> 10$ indicates muscular damage.

Lactate Dehydrogenase (LDH) Isoenzymes

Five tetrameric isoenzymes composed of H and M subunits: LDH-1 ($\text{H}_4$, heart and RBCs), LDH-2 ($\text{H}_3\text{M}$, heart and reticuloendothelial system), LDH-3 ($\text{H}_2\text{M}_2$, lungs), LDH-4 ($\text{HM}_3$, kidneys, placenta, pancreas), and LDH-5 ($\text{M}_4$, liver and striated muscle).

Flipped LDH Pattern

A diagnostic reversal where serum LDH-1 levels exceed LDH-2 levels, which is suggestive of acute myocardial infarction (contrasting with normal physiology where LDH-2 is higher than LDH-1).