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Clinical Laboratory: Cardiac Enzymes


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What are enzymes?
-substances produced by various organs that facilitate chemical reactions
Enzyme measurements are based on ..
activity not quantity
Why do we measure enzymes?
to gain info concerning specific organ function
Comparison in intracellular and plasma enzyme levels?
Intracellular enzyme levels are much higher then plasma levels
Reasons for elevated plasma enzymes levels?
background for Cardiac enzymes
-when cardiac cells die, they release enzymes thus measuring enzymes can assess cell death
-serial measurements are made to assess cardiac damage
-serum measurement is the specimen of choice
When are cardiac enzyme measurement taken?
2-4 hours
6-8 hours
12 hours
Types of cardiac enzymes?
-aspartate aminotransferase (AST) - formerly SGOT
-Lactate dehydrogenase (LDH)
-Creatine Kinase (CK)
-aspartate aminotransferase
-fomerly SGOT
-enzyme used by heart cells
-released after cells are damaged
AST and MI
-AST in elevated 90-95% of acute MI pts
-elevation seen in 8-12 hrs
-peak 24-48 hours
-return to normal 3-8 days
Causes of AST elevation:
-damage to liver, kidney, skeletal muscle, pancreas (good sensitivity, poor specificity)
-certain drugs (demorol, coumadin)
-chornic hypokalemia
-lactate dehydrogenasee
-released with damage to liver, kidney, skeletal muscle, RBCs, WBCs, lung, and tumor tissue
-good sensitivity, poor specificity
-samples are very sensitive to hemolysis
LDH supports dx of unjury or dz involving:
-Skeletal muscle
LDH isoenzymes:
LDH1 17-27% mainly heart

LDH2 27-37% RE system (RBC)

LDH3 18-25% Lungs

LDH4 Kidneys/placenta

LDH5 Liver/muscle
LDH and MI
-elevated in 92-95% of all acute MI pts
-LDH1 and LDH2 ratios are reversed
-elevated 24-48 hours
-peaks 48-72 hours
-normalized 5-10 days
- Ratio appears in 12-24 hours and is 80% specific
-slighly more sensitive than AST
**Rarely used anymore because too late of detection (too late elevation)
Creatine Kinase
-involved in conversion of creatine phosphate to creatine in muscle contraction
Where is creatine Kinase is located?
-found mainly in the heart, skeletal muscle, and brain tissue
-damage to any these tissue will elevate the total CK
Creatine Kinase indications?
-used to support diagnosis of myocardial injury
-may indicate neurological or skeletal muscle damage
CK Isoenzymes:
MB: heart fraction 0%
MM: Skeletal muscle 100%
BB: brain and lung 0%
-elevated in 90-93% of all acute MI pts
-CKMB rises in 3-6 hrs after MI
-CKMB peaks in 12-24 hrs
-CKMB normalizes within 12-48 hrs
CKMB usually rises in..
-unstable angina
-malignant hyperthermia
CKMB does not usually rise in..
-pulmonary embolism
What is used to avoid misdiagnosis of MI?
-ratio of MB: total CK

-a CKMB of 3.0 ng ml with a relative index of >2.5 is highly suggestive of an MI
CKMB is useful in...
-quantifying degree of MI and timing the onset and appropriateness of thrombolytic therapy
What causes a rise in AST and LD but not in CK?
acute liver disease
Total CK rises in...
-moderate strenuous exercise
-distance running
-weight lifting
Total CK and hypothyroidism and hyopkalemia
-elevated in 80% of pts with hypothyroidism or hypokalemia due to muslce damage
Benefits of CK?
-CKMB can help sort out MI versus strained muscle etc
-normalizes in 12-48 hours
-CKMB does not rise with angina, PE, or CHF
-CK does not rise with acute liver damage (unlike AST and LD)
Normal CK value is based on...
patient size and muslce mass
Is single value of CK useful?
single value is of little value; need to look at trend over time
CKMM is elevated in...
-skeletal muslce injury
-IM injection
When should CK measurements be taken?
-4-6 hours
-12 hours
-18 hours
-24 hours
What are Troponins?
-proteins that exist in skeletal and cardiac muscle
-2 types: T and I
What do Troponins do?
-help regulate muscle contraction
How are cardiac specific troponins separated?
-using monoclonal antibodies or ELISA techniques
Troponin values:
TN T <0.2 ng/ml

TN I < 0.3 ng/ml
Troponin elevation, peak, normalization times:
Elevates: 2-6 hours
Peaks: 12-16 hours
TN I normalizes: 7-10 days
TN T normalizes: 10-14 days
-CKMB can be elevated in severe muscle, brain, or lung injury, or renal failure whereas troponins are always normal in non-cardiac injury
-troponins elevate sooner then CKMB (2-6 hours compared to 3-6 hours)
-troponins stay elevated longer (normalize later) than CKMB (7-10 or 10-14 days compared to 24-48 hours)
So if TN I is so good, why bother with CKMB at all?
because TN I remains elevated for 7-10 days whereas CKMB normalizes within 12-48 hours so you can tell if the pt re-infarcts
Disadvantages to Troponin?
Not helpful for reinfarct

cannot tell if elevation is old or new
Indication for Troponin?
-Eval chest pain since they are extraordinary specific for myocardial cell injury
-determine cardiac iscehmia
-differentiate cardiac from non-cardiac chest pain
-eval of pts with unstable angina (divides those with lasting dysfcn from those without)
-may decide if thrombolytic therapy is appropriate
What is Myoglobin?
-oxygen binding protein found in cardiac and skeletal muslce
When is myoglobin test indicated?
-early eval of pt with suspected MI (provides early index of damage to myocardium)
-dx disease or injury to skeletal muscle
when does Myoglobin elevate?
rises: 0-3 hours
peaks: 6-8 hours
normalizes: 20-36 hours
Myoglobin vs CK
-myoglobin is more sensitive than CK isoenzymes but less specific (any trauma or injury may elevate myoglobin)
Cardiac enzymes elevation comparison
Myoglobin (0-3 hr)
Troponin (2-6 hr)
CKMB/ CKtotal (3-6 hr)
AST (8-12 hr)
LDH (24-48 hr)
Cardiac enzymes normalization comparison
CKMB (12-48 hr)
myoglobin (20-36 hrs)
AST (3-8 days)
LDH (5-10 days)
Troponin I (7-10 days)
Cardiac enzymes specificity comparison
troponin (cardiac injury ONLY)
Cardiac enzymes sensitivity comparison
myoglobin (most sensitive, BUT not enzyme)
Cardiac enzymes peak comparions

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