Page 276 - Cardiac Nursing

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252 P A R T III / Assessment of Heart Disease
normal CK level should never be used to make a decision about higher risk of mortality in the immediate to long-term postoper-
discharge from the emergency department, or to withhold throm- ative period. With troponin evaluation becoming more common,
bolytic therapy. Januzzi et al. 39 determined troponin-T levels offered a superior
predictor of complications from cardiac surgery than CK-MB.
Myoglobin Kathiresan et al. 40 and Croal et al. 41 found similar results, the
higher the troponin, I or T, the increased risk of mortality after
Myoglobin is a low-molecular-weight, oxygen-binding protein CABG surgery. These studies demonstrate that biomarker evalua-
found in the myocardium and skeletal muscle. Myoglobin is re- tion postoperatively also is important and should not be consid-
leased into the circulation after damage to the heart or skeletal mus- ered inconsequential.
2
cle. Because of its release from other muscle tissues, troponin rather
than myoglobin is the biomarker of choice for diagnosing MI.
After MI, myoglobin levels increase in 2 to 3 hours, peak in 6 to BLOOD LIPIDS
9 hours, and return to normal (undetectable) as early as 12 hours
but more typically after 24 to 36 hours (see Table 11-2). 3 An accumulation of lipids within the arterial wall is considered a
Elevated myoglobin levels are seen after MI, reinfarction, co- part of the process of atherogenesis. Alteration of blood lipid lev-
caine use, skeletal muscle injury, trauma, exercise, IM injections, els has been identified as a coronary heart disease (CHD) risk fac-
severe burns, electrical shock, polymyositis, alcoholic myopathy, tor. Certain lipoproteinemias have been identified as contributing
delirium tremens, metabolic disorders (e.g., myxedema), malig- to total plasma cholesterol levels. Plasma normally contains insol-
nant hyperthermia, systemic lupus erythematosus, muscular dys- uble lipid elements: free fatty acids; exogenous triglycerides; en-
trophy, rhabdomyolysis, and seizures. 2,3 Myoglobin may not be dogenous triglycerides, which are manufactured in the liver; cho-
excreted in renal failure, so caution should be used when inter- lesterol; and phospholipids. To be transported, each is attached to
2
preting results. Further, very high levels of myoglobin are toxic to a protein. Distinguishing lipoprotein abnormalities is useful be-
the kidneys and thus, careful monitoring of renal function is war- cause therapy is based on an understanding of the origin of the
ranted. problem.
Biochemical Marker Activity after PCI
Blood Lipid Laboratory Measurement
After elective PCI for stable angina, biomarker elevation is fairly Elevated lipid levels are considered a risk factor for cardiovascular
common. CK and CK-MB elevation occurs in 5% to 30% of pa- disease. Cholesterol and the protein components of high-density
tients. These elevations have been associated with increased risk of lipid (HDL), low-density lipid (LDL), and triglycerides are evalu-
death, MI, and need for repeat revascularization. 34 Prasad et al. 35 ated by electrophoresis when hyperlipoproteinemia is suspected. 42
found that troponin elevation is frequent after elective PCI. Of See Table 11-3 for recommended levels of cholesterol and its
the patients in their study, 19% had an elevated troponin level. components. In most people, the cholesterol values remain con-
These patients had more complex angiographic characteristics and stant over 24 hours; a nonfasting blood sample for measurement of
had undergone multivessel PCI. They found that an elevated tro- total blood cholesterol is acceptable. However, a nonfasting sample
ponin level was associated with increased morbidity and mortal- for HDL, LDL, and triglyceride levels is of less value. National
ity. Miller et al. 29 measured troponin before (baseline) and after Cholesterol Education Program 2001 guidelines on cholesterol
PCI and found that prognosis was most often related to the base- screening recommend everyone over age 20 have a fasting lipopro-
line troponin level and not the biomarker response after the pro- tein profile (total cholesterol, LDL, HDL, and triglycerides) every
cedure. Nallamothu et al. 36 analyzed 1,157 patients who under- 5 years. 13 Lipoprotein electrophoresis is necessary to evaluate
went elective PCI and found that troponin-I elevation was serum for hyperlipoproteinemia. LDL is more difficult to isolate
common after the procedure (29%), and that large troponin ele- and measure. Therefore, if LDL is not measured in a screening
vations, up to eight times normal, were associated with decreased lipoprotein test, it may be calculated using the Friedewald for-
long-term survival. Taken together, these studies demonstrate that mula. The Friedewald formula is inaccurate if the triglycerides are
continued use and evaluation of biochemical markers is essential greater than 400 mg/dL (see Display 11-2). 43
after coronary intervention.
It is recommended that lipid profile tests should be performed
after a 12-to-14 hour fast and having a stable diet for 2 to 3 weeks
Biochemical Marker Activity after prior to testing. It is also recommended that testing occur in the
Cardiac Surgery absence of acute illnesses including stroke, trauma, surgery, acute
infection, weight loss, and pregnancy. These conditions often re-
All types of cardiac surgery involve considerable injury to the my- sult in values that are not representative of the person’s usual
ocardium. However, differentiating between ischemic alterations level. 13
associated with surgery and peri-operative MI may be difficult. Current National Cholesterol Education Program guidelines
The evaluation of troponin and cardiac enzymes is common after do recommend patients admitted to the hospital for acute coro-
cardiovascular surgery. Researchers have focused on the prognos- nary syndromes have lipid measurements taken on admission or
tic implication of elevated troponin and cardiac enzymes meas- within 24 hours. 13 Values obtained during this acute phase may
37
38
urements after surgery. Klatte et al. and Costa et al. found that provide guidance for initiating lipid-lowering therapy. LDL cho-
elevated levels of CK-MB in serial measurements after coronary lesterol levels begin to decline in the first few hours after a coro-
artery bypass graft (CABG) surgery were associated with in- nary event and are significantly decreased by 24 to 48 hours and
creased mortality, and that the higher the level of CK-MB, the may remain low for many weeks. Thus, the initial LDL cholesterol