LABORATORY FEATURES OF MYOCARDIAL INFARCTION H.A. MWAKYOMA, MD

CARDIAC ENZYMES AND ISCHEMIC HEART DISEASES

Acute Myocardial Infarction Acute myocardial infarction is the rapid development of myocardial necrosis caused by a critical imbalance between the oxygen supply and demand of the myocardium. It is an irreversible myocardial injury from prolonged ischemia.

Acute Myocardial Infarction-cont-- Accurate and early diagnosis is important in minimizing cellular damage and, consequently, in obtaining a successful outcome for the patient, especially since the advent of thrombolytic treatment

The Ideal Biochemical Marker From a clinical point of view, the ideal biochemical marker that detects myocardial injury requires certain properties. T h i s marker should: (a) be present in the myocardium in high concentrations and absent in nonmyocardial tissues; (b) be released rapidly into the blood after myocardial injury so as to achieve optimal sensitivity in the early phase after onset of damage; (c) remain abnormal for several days; (d) be assayed with a rapid turnaround time.

Cardiac Enzymes Enzymes found in plasma are of two types: 1- Those that are normally present and have a functional role in plasma 2- Those released from tissues, The latter can be present normally at very low levels, but have no functional role in the plasma; however they are most important for diagnostic purpose. Normally the plasma content of intracellular enzymes is very low or absent; however, when cells are damaged, the contents can appear in the blood as the cell membrane is damaged.

Isoenzymes Isoenzymes have slightly different structures but catalyze the same reaction • They are encoded by different genes • Oligomeric isoenzymes may have variable subunit composition • Isoenzymes often found in different tissues or different stages of development • Isoenzymes have very different kinetic properties: different Km, Vmax and specific inhibitors. • Isoenzymes allow for regulation of metabolic activity according to necessity in different tissues

AST (SGOT) The activity of this enzyme is high in many tissues, but it is more concentrated in ( heart, liver, skeletal muscles, and kidneys). The first account of the use of a biochemical marker in the study of myocardial injury was published at 1954. They measured serum glutamate oxaloacetic transaminase (SGOT) activity from a few hours to up to 15 days in a group of patients immediately after an acute myocardial infarction (AMI). They reported that enzyme activity increased above the reference range a few hours after AMI, reached a peak after 18 to 24 hs, and then returned to baseline within 4 – 5 days. The peak values of AST activity are roughly proportional to the extent of cardiac damage. It is not routinely used now because it is non specific for MI.

Creatine kinase Creatine phosphate acts as a backup for rapid ATP regeneration in active tissues Creatine phosphate is in energetic equilibrium with ATP Creatine kinase (CK) catalyzes the transfer of phosphate between creatine and ATP/ADP Provides rapid regeneration of ATP when ATP is low Creatine phosphate is regenerated when ATP is abundant ADP ATP CK Cr-P Cr 12

CK isoenzymes Cytoplasmic CK is a dimer, composed of M and/or B subunits, which associate forming CK-MM, CK-MB and CK-BB isoenzymes. CK-MM is the main isoenzyme found in striated muscle CK-MB is found mainly in cardiac muscle CK-BB is the predominant isoenzyme found in brain

CK-MB Isoforms Carboxypeptidases normally present in blood hydrolyze the C-terminal lysine residues from the M subunit of CKMB forming CKMB2 & CKMB1 which are smaller in size than CKMB and appear earlier in the blood after AMI. The CK-MB fractions isoforms 1 and 2 are identified by electrophoretic methodology. The ratio of isoform 2 to 1 (2:1) can provide information about myocardial injury.

CK-MB Isoforms-cont-- An isoform ratio of 1.5 or greater is an excellent indicator for early acute myocardial infarction. CK-MB isoform 2 demonstrates elevation even before CK-MB by laboratory testing. However, the disadvantage of this method is that:- it is skilled labor intensive because electrophoresis is required, and large numbers of samples cannot be run simultaneously nor continuously. False positive results with congestive heart failure and other conditions can occur

Serum total CK activity and CK-MB concentration rise in parallel following myocardial injury, starting to increase 4± 6 h after injury, reaching peak serum concentrations after 12±24 h and returning to baseline after 48±72 h. Serum CK-MB is considerably more specific for myocardial damage than is serum total CK, which may be elevated in many conditions where skeletal muscle is damaged. Consequently, CK should not be used for the diagnosis of myocardial injury unless used in combination with other more specific cardiac markers.

Macro CKs Type 1. Complex of a CK isoenzyme (most frequently CK-BB) with immunoglobulins. Frequently occurs in women above 50, (In severe gastrointestinal and vascular diseases, adenomas, and carcinomas) Type 2. Oligomeric mitochondrial CK. Mostly in severe malignant or liver diseases

Total CK can be elevated AMI • i.m. injection • Traumatic damage of skeletal muscle • Hypothermia • Exercise • Intoxication • Dose-related side effect in statin treatment (Statin-related increases in CK mainly affect MM isoenzyme)

Limitations of CKMB Elevated CKMB Levels can be observed in: • Skeletal Muscle Involvement • Duchenne Muscular Dystrophy • Polymyositis • Alcohol Myopathy • Thermal or Electrical Burn Patients • Carcinomas (Colon, Lung, Prostate, Endometrial..) athlets (e.g. Marathon runners), when the increase in CK-MB attributes to a high propotion of regenerating skeletal muscle fibers, which, like fetal skeletal muscle, are relatively rich in CK-MB

MB Index MB Index = (CKMB /total CK) x 100 Rationale for using MB Index Using CKMB alone (RI <4.7 ng/mL) often yields FP results Combined use with MB Index helps to rule-out patients with skeletal muscle injury

LDH (Lactate Dehydrogenase) It is present in the cytoplasm of different tissues (Liver, kidney, cardiac muscle, skeletal muscle, RBCs, pancreas, spleen, lung……) So Serum total LDH is increased in a wide variety of diseases. There are at least 5 isoenzymes for LDH Normal range: LDH 1: 19 ~ 31 % LDH 2: 30 ~ 39 % LDH 3: 17 ~ 27 % LDH 4: 5 ~ 13 % LDH 5: 6 ~ 13 % LDH 2 > LDH 1 > LDH 3 > LDH 5 > LDH 4

LDH-1 ￪ : MI, Hemolytic anemia, testicular tumor LDH-2 ￪ : Pneumonia, CHF, lymphoma LDH-3 ￪ : Lung diseases LDH-4 ￪ : Pancreatitis LDH-5 ￪ : Liver diseases, Intestinal diseases, Muscle disease

LD1/LD2 POST AMI Increase 8-12 HRS LD1/LD2 > 1 48-72 HRS Return to Normal 8-14 DAYS

Conditions Causing Flipped LD1/LD2 Without Acute Myocardial Infarction Hemolysis • Megoblastic & Pernicious Anemia • Renal Cortex Infarction • Testicular Germ Cell Tumors • Small Cell Lung Carcinoma • Adenocarcinoma of the Ovary • Acute Coronary Insufficiency (Unstable Angina) • Exercise Induced Myocardial Ischemia • Polymyositis • Muscular Dystrophies • Well Trained Athletes • Rhabdomyolysis

Troponins Troponin is a regulatory complex of 3 protein subunits located on the thin filament of the myocardial contractile apparatus. Its function is the regulation of striated and cardiac muscle contraction. The complex regulates the calcium-modulated interaction between actin and myosin on the thin filament. Troponin C (18 kd) • Calcium-binding subunit • No cardiac specificity

Troponins-cont-- • Troponin I (26.5 kd) Actomyosin-ATP-inhibiting subunit • Cardiac-specific form • Troponin T (39 kd) • Anchors troponin complex to theTropomyosin strand

In the absence of calcium ions, tropomyosin blocks access to the mysosin binding site of actin. When calcium binds to troponin, the positions of troponin and tropomyosin are altered on the thin filament and myosin then has access to its binding site on actin.. When the calcium level decreases, troponin locks tropomyosin in the blocking position and the thin filament slides back to the resting state.

Tissue Specificity of Troponin Subunits Troponin C is the same in all muscle tissues • Troponins I and T have cardiac-specific forms, cTnI and cTnT • Circulating concentrations of cTnI and cTnT are very low • cTnI and cTnT remain elevated for several days. Hence, Troponins would seem to have better specificity than CK-MB, and the long-term sensitivity of LD-1

Troponin Release Kinetics Detectable in blood 4-12 h, similar to CKMB Peaks 12-38 h Remains elevated for 5-10 days

Glycogen Phosphorylase BB Glycogen phosphorylase (GP) catalyses the breakdown of glycogen in the sarcoplasmic reticulum. It is a dimeric enzyme. Three isoenzymes have been identified: BB, found in brain and heart; MM, found in skeletal muscle; and LL, found in the liver. It has been suggested that, after glycogenolysis in ischaemic tissue, GP-BB is released from the sarcoplasmic reticulum into the cytoplasm, and then into the circulation through the damaged cell membrane. GP-BB appears to be released into the circulation 2±4 h after myocardial injury, returning to normal within 36 h of damage occurring.

AMI cardiac markers Myoglobin CK-MB Troponin-I