The lab finding which would indicate myocardial damage in a patient with suspected myocardial infarction?

RUHS, Jaipur, PB B.Sc Nursing Entrance-2018

Medical Surgical Nursing

Easy

Appeared in: RUHS, Jaipur, PB B.Sc Nursing Entrance-2018

Explanation

CPK-MB (Creatine Phosphokinase-Myocardial Band) is an isoenzyme of creatine kinase that is found almost exclusively in heart muscle cells.
Following injury to the myocardium, as seen in a myocardial infarction, these cells die and release their contents, including CPK-MB, into the bloodstream.
The elevation of CPK-MB in the blood is a highly specific indicator of cardiac muscle damage, making it a key diagnostic marker for MI among the given choices.
Its levels typically rise within 3-6 hours of an MI, peak around 24 hours, and normalize within 2-3 days.

Option A: Elevated SGOT (AST) and LDH are not specific to the heart. These enzymes are present in many other tissues, such as the liver, skeletal muscle, and red blood cells. Their levels can be elevated due to various non-cardiac conditions, making them unreliable as primary indicators of MI.
Option B: Elevated total CPK is not specific for myocardial damage. Total CPK includes isoenzymes from skeletal muscle (CK-MM) and the brain (CK-BB) in addition to the heart (CK-MB). Strenuous exercise, muscle injury, or a cerebrovascular accident can all cause elevated total CPK levels.
Option C: Myocardial damage leads to the release of enzymes from damaged cells, causing their levels in the blood to increase, not decrease. Decreased levels of CPK and SGOT are not associated with acute myocardial infarction.

Clinical Relevance

Nursing practice connection: Use the key finding related to Cardiac Biomarkers for Myocardial Infarction Diagnosis to guide bedside assessment, documentation, and the next nursing action.
In a suspected MI, nurses play a crucial role in obtaining blood samples for serial cardiac marker analysis, typically at presentation, and then at 3-6 hour intervals to track the trend.
While waiting for lab results, the nurse's priority is to perform a 12-lead ECG (within 10 minutes of arrival), administer oxygen, ensure IV access, and manage pain with nitroglycerin or morphine as prescribed (MONA protocol).
The current gold standard for diagnosing MI is cardiac Troponin (cTnI or cTnT) due to its superior specificity and sensitivity. It remains elevated for a longer duration, allowing for a wider diagnostic window.

How to Approach the Question

First, understand the question is asking for the most specific lab indicator of heart muscle (myocardial) damage from the given options.
Recall or deduce the source of each lab marker. CPK, SGOT, and LDH are enzymes found in the body.
Analyze the specificity of each marker. SGOT and LDH are found in many tissues (liver, muscle, etc.), so they are not specific to the heart.
Compare total CPK with CPK-MB. Total CPK comes from the brain, skeletal muscle, and heart. CPK-MB is the 'myocardial band' or heart-specific isoenzyme.
Conclude that an elevated level of the heart-specific isoenzyme (CPK-MB) is a much better indicator of myocardial damage than total CPK or other non-specific enzymes.
Eliminate the option suggesting decreased levels, as tissue damage causes a release and thus an increase of intracellular enzymes into the blood.

Concept Tested & Keywords

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E6 textbook of Applied Biochemistry and Nutrition & DieteticsHarbans Lal (pp 26-464 of 479) pp. 140-142, 139-141

E6 PATHOLOGY QUICK REVIEWBased on Harsh Mohan Textbook of PATHOLOGY p. 196-198