Myocardial Infarction - Diagnostics

Electrocardiography: When blood flow to the heart is decreased, ischemia and necrosis of the heart muscle occur. These conditions are reflected in altered Q wave, ST segment, and T wave on the 12 – lead ECG. The Q wave change is significant; normally the Q wave is small or absent. Ischemic tissue products an elevation in the ST segment and a peaked T wave or inversion of the T wave. ST segment elevation is considered significant if greater than 1 mm. Through the course of an MI, changes occur first in the ST segment, then the T wave, and finally the Q wave. As the myocardium heals, the ST segment and T waves return to normal, but the Q-wave changes persist. However, an ECG can be completely normal in a client with AMI, especially in the early hours following infarct.

Laboratory Tests: CK-MB. Serum levels of CK-MB (an isoenzyme of CK found primarily in cardiac muscle) increase 3 to 6 hours after the onset of chest pain, peak in 12 to 18 hours, and return to normal levels in 3 to 4 days.

Myoglobin: Myoglobin is a heme protein found in striated muscle fibers. Myoglobin is rapidly released when myocardial muscle tissue is damaged. Because of the rapid release, it can be detected within 2 hours after AMI.

Troponin: The cardiac troponin complex is a basic component of the myocardium that is involved in the contraction of the myocardial muscle. Cardiac troponin T and I are more sensitive to cardiac muscle damage than cardiac troponin C.

Cardiac troponin T is similar to CK-MB with regard to sensitivity, and levels increase within 3 to 6 hours after pain has started. Levels remain elevated for 14 to 21 days.

Cardiac I levels increase 7 to 14 hours after AMI. This is a very specific and sensitive indicator of AMI and is not affected by any other disease or injury to any other muscle except cardiac muscle. Elevation persists for 5 to 7 days.

LDH. The LDH subunit is plentiful in heart muscle and is released into the serum when myocardial damage occurs. Serum levels of LDH elevate 14 to 24 hours after onset of myocardial damage, peak within 48 to 72 hours, and slowly return to normal over the next 7 to 14 days.

AST:Serum levels of AST increase within several hours after the onset of chest pain, peak within 12 to 18 hours, and return to normal within 3to 4 days.

Leukocytes. Leukocytosis (10,000 to 20,000 mm) appears on the second day after AMI and disappears in 1 week. Myeloperoxidase, a leukocyte enzyme, was recently shown to be predictive of AMI even in clients without elevations in troponin T.

Imaging studies:

Radionuclide imaging studies provide information on the presence of coronary artery disease as well as the location of ischemic and infracted tissue..

Positron Emission tomography: Positron emission to mography (PET) is used to evaluate cardiac metabolism and to assess tissue perfusion. It can also be used to detect CHD, assess coronary artery flow reserve, measure absolute myocardial blood flow, detect AMI, and differentiate ischemic from nonischemic cardiomyopathy. It may also be used to assess myocardial viability to determine which clients can benefit from CABG.

Magnetic Resonance Imaging: Magnetic resonance imaging (MRI) helps identify the site and extent of an MI, assess the effects of reperfusion therapy, and differentiate reversible and irreversible tissue injury.

Echocardiography: Echocardiography is useful in assessing the ability of the heart walls to contract and relax. The transducer is placed on the chest, and images are relayed to a monitor screen. Wall motion is abnormal in ischemic or infracted areas.

Tran esophageal Echocardiography. Tran esophageal echocardiography (TEE) is an imaging technique in which the transducer is placed against the wall of the eaophagus. The image of the myocardium is clearer when the esophageal site is used because no air is between the transducer and the heart. This technique is particularly useful for viewing the posterior wall of the heart.

Category: Medicine Notes