Page 286 - Cardiac Nursing

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262 P A R T III / Assessment of Heart Disease
should be taken not to hemolyze the sample. The sample should “glycosylation.” Once glycosylation occurs, it is not easily re-
also be protected from bright light because bilirubin levels are re- versible. HbA 1C is one of three components of HbA 1 and com-
duced after 1 hour of such exposure. 2,3 bines most strongly with glucose. HbA 1C is a specific form of
GHb that has become the most accurate laboratory blood test in
Catecholamines assessing long-term glycemic control in diabetics. 84
Multiple laboratory methods are used to measure the many
Epinephrine and norepinephrine are elevated in pheochromocy- components of GHb. Some assays measure all GHb components
toma, a tumor of the adrenal medulla. Pheochromocytoma is a in a sample, while other assays measure only one or two compo-
cause of high blood pressure. nents. International efforts are being made to standardize the
measurement of GHb and now most assays measure HbA 1C or are
Creatinine calibrated to produce a result equivalent to that measurement.
The American Diabetes Association 94 recommends that laborato-
Creatinine is a waste product formed during muscle protein me- ries use only methods certified as traceable to the Diabetes Con-
tabolism. Serum creatinine is a reflection of the excretory function trol and Complications Trial (DCCT) reference method. Regard-
of the kidneys. It is evaluated in conjunction with BUN, but is a less of the assay method type and specific analyte quantified, all
more sensitive indicator of renal function. People with large mus- results should be reported as “% HbA 1C ” or “% HbA 1C equiva-
cle mass have higher serum creatinine levels than do those with lents.”
less muscle, such as older adults, amputees, and patients with The American Diabetes Association does not recommend us-
muscle disease. 2,3 ing HbA 1C for the diagnosis of diabetes mellitus but for long-term
management of the disease. 84 The advantage of testing HbA 1C
Glucose over plasma glucose testing for long-term diabetes mellitus man-
agement is that the sample can be drawn at any time because it is
Glucose is elevated whenever endogenous epinephrine is mobi- not affected by short-term variations (e.g., food intake, exercise,
lized. Hyperglycemia in the hospital setting is common and may stress, hypoglycemic agents). HbA 1C testing is beneficial for eval-
result from a variety of reasons including stress, administration of uating the success and patient compliance of diabetic treatment,
glucocorticoids and vasopressors, decompensation of diabetes determining the duration of hyperglycemia in patients with newly
mellitus types 1 and 2, chronic renal failure, acute pancreatitis, diagnosed diabetes mellitus, individualizing diabetic control regi-
acute MI, congestive heart failure, extensive surgery, and infec- mens, evaluating the diabetic patient whose glucose levels change
tions. significantly day to day, and in the hospital, differentiating short-
Bedside glucose monitoring is the most common POCT avail- term hyperglycemia in patients who do not have diabetes mellitus
able in the hospital. It is recommended that diabetic patients who (e.g., recent stress from illness or MI) from those that do have di-
are eating have glucose testing before their meals and at bedtime. abetes mellitus (where the glucose has been persistently ele-
Diabetic patients who are not eating should be tested every 4 to vated). 84
6 hours unless they are controlled with IV insulin, which typically In nondiabetic patients, increased levels of HbA 1C may be seen
requires testing every hour until levels are stable and then every with an acute stress response, Cushing’s syndrome, pheochromo-
2 hours. 84 cytoma, corticosteroid therapy, and acromegaly. HbA 1C levels
Studies have shown an association between hyperglycemia and may be decreased in patients with hemolytic anemia, chronic
increased mortality in hospitalized patients with and without dia- blood loss, and chronic renal failure. 2
betes mellitus. A variety of populations, including medical and Prospective randomized clinical trials such as the U.K.
surgical patients, acute MI patients, and cardiac surgery patients, Prospective Diabetes Study 95 and the DCCT 96 have shown that
have been studied demonstrating more intensive glucose control treatment regimens that reduced average HbA 1C to approximately
decreased mortality, reduced length of stay, and decreased infec- 7% (about 1% above the upper limits of normal) were associated
tion rates. 85–91 These studies have led to changes in protocols for with fewer long-term microvascular complications, including rates
the monitoring and treatment of glucose while in the hospital (see of retinopathy, nephropathy, and neuropathy. However, in these
Chapter 39). 84 trials, intensive control was found to increase the risk of severe hy-
poglycemia and weight gain.
93
Glycated Hemoglobin or HbA 1C Rohlfing et al. analyzed DCCT data to determine the relation
between HbA 1C and plasma glucose levels in patients with type 1
Glycated hemoglobin (GHb), also referred to as glycohemoglo- diabetes mellitus. Approximate levels of plasma glucose and corre-
bin, glycosylated hemoglobin, HbA 1C , or HbA 1 are terms used to sponding HbA 1C levels are shown in Table 11-5. The 2007 guide-
describe a series of stable minor Hb components formed slowly lines from the American Diabetes Association 84 recommend that
92
and nonenzymatically from Hb and glucose. The rate at which HbA 1C testing be performed at least two times a year in patients
GHb is formed is proportional to the concentration of blood glu- who are meeting treatment goals (and who have stable glycemic
cose. 93 Because RBCs survive an average of 120 days, the meas- control)and quarterly in patients whose therapy has changed or
urement of GHb provides an index of a person’s average blood who are not meeting glycemic goals (see Chapter 39).
glucose concentration during a 2- to 3-month period. 84
GHb comprises a chemically heterogeneous group of sub- Lactate Dehydrogenase
stances formed by the reaction between sugars and hemoglobin.
In adults, approximately 98% of the Hb in the RBC is hemo- LDH is an enzyme that catalyzes the reversible conversion of lac-
globin A. About 7% of hemoglobin A consists of a type called tate to pyruvate, providing ATP for energy during periods of
HbA 1 that can combine strongly with glucose in a process called anaerobic metabolism. LDH is present in nearly all metabolizing