Page 284 - Cardiac Nursing

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260 P A R T III / Assessment of Heart Disease
of acid–base balance (by combining with chloride or bicarbonate amount bound to protein, the ionized calcium may be found to
ions), and transmission of nerve impulses by the sodium pump. be normal. The formula for the computation of ionized calcium
Sodium balance is regulated by aldosterone, atrial natriuretic hor- is shown in Display 11-2. Decreased serum sodium ( 120
mone, and antidiuretic hormone (ADH). Aldosterone causes mEq/L) increases protein-bound calcium and consequently in-
sodium conservation (and water retention) by stimulating the kid- creases the total calcium; the opposite is true of increased serum
neys to reabsorb sodium. Aldosterone is secreted in response to low sodium. 2,3
extracellular sodium levels, an increase in intracellular potassium,
low blood volume or cardiac output, and physical or emotional Magnesium
stress. When serum sodium levels are too high, atrial natriuretic Magnesium is essential for over 300 enzymatic activities involving
hormone is secreted from the atrium and acts as an antagonist to lipid, carbohydrate, and protein metabolism. It is the second most
renin and aldosterone. ADH, secreted by the posterior pituitary predominant intracellular cation. Most of the body’s magnesium
gland, controls serum sodium by regulation of the amount of in- is stored in the bones in an insoluble state; one third is bound to
tracellular fluid reabsorbed at the distal tubules. 2,3 protein, and approximately 1% is found in the serum. Because of
its importance in phosphorylation of ATP, magnesium is seen as a
Potassium critical component of almost all metabolic processes. Its impor-
Potassium is the major intracellular cation in concentrations of tance in the care of patients with cardiac disease stems from its
approximately 150 mEq/L. It is regulated in a very tight range in role in neuromuscular regulation. 2,3
the extracellular fluid. Potassium plays a crucial role in initiating Ventricular arrhythmias after MI have been associated with
and sustaining cardiac and skeletal muscle contraction. It is also magnesium deficiency. Magnesium sulfate 1 to 2 g IV should be
important for acid—base balance and maintenance of oncotic considered in ventricular fibrillation or ventricular tachycardia for
pressure. patients who have alcoholism or malnutrition with suspected low
Maintenance of potassium within the normal range is crucial levels of magnesium (hypomagnesemia). It is recommended that
in the care of a patient with cardiac disease. Failure to do so results magnesium sulfate should be administered to patients with ven-
in dangerous sequelae for the patient. In general, potassium levels tricular fibrillation or ventricular tachycardia with a torsades de
in patients with cardiac disease are maintained above 4.0 mEq/L. pointes pattern. 79
Special care should be taken in patients with cardiac disease re- Hypomagnesemia may precipitate cardiac arrhythmias, in-
ceiving potassium-sparing diuretics or angiotensin-converting en- cluding atrial fibrillation, because of enhanced myocardium ex-
zyme inhibitors, especially in light of decreased renal blood flow. citability. Hypomagnesemia is very common after cardiovascular
Potassium levels are falsely elevated by analysis of hemolyzed spec- surgery and has been found to be an independent risk factor for
imens. Prolonged use of a tourniquet, having the patient clench atrial fibrillation following cardiac surgery, most likely due to he-
and unclench a fist before blood draw, or delayed processing of the modilution, elevated epinephrine levels, increased loss through
specimen all may cause hemolysis. 2,3 the urine, or due to the use of diuretics. Atrial fibrillation is a com-
mon complication after cardiovascular surgery occurring in ap-
Chloride proximately 20% to 50% of cases. 80 Research on the administra-
Chloride is the major extracellular anion. It helps to maintain tion of magnesium sulfate before and/or after cardiovascular
electrical neutrality and acts as an acid–base buffer. The rise and surgery has shown mixed results. However, a recent meta-analy-
fall of chloride levels follows sodium and bicarbonate shifts. When sis 81 demonstrated a significant reduction in postoperative atrial
carbon dioxide increases, chloride shifts to the intracellular space fibrillation from 28% in the control group to 18% in the treat-
as bicarbonate goes extracellular. Along with sodium, chloride also ment group without a significant change in length of hospital stay.
helps to maintain osmotic pressure. Found primarily in hy- Administration of magnesium sulfate after CABG surgery is con-
drochloric acid in stomach secretions, chloride also provides the sidered a frontline strategy in the prevention of atrial fibrillation
acid medium for digestion and enzyme activation. 2,3 according to the American College of Cardiology/American Heart
Association 2004 Guidelines. 82
Calcium Hypermagnesemia results in depressed neuromuscular con-
Calcium is found mainly in the bones and teeth, with only ap- duction, and consequent slowing of conduction in the heart. The
proximately 10% found in the blood. Calcium is essential for the most common cause of hypermagnesemia is renal failure.
formation of bones and for blood coagulation. Calcium ions af-
fect neuromuscular excitability and cellular and capillary perme- Carbon Dioxide
ability. It is essential for nerve transmission and cardiac and skele- Measurement of carbon dioxide assists the clinician in evaluation
tal muscle contraction. Calcium also contributes to anion–cation of electrolyte status and acid–base balance. Because approximately
balance. Calcium can be found ionized (free) in the serum or 80% of carbon dioxide is found as bicarbonate, it is a good re-
bound to serum albumin. The ionized calcium, which is approxi- flection of the bicarbonate level. The carbon dioxide level should
mately one-half of the total calcium, is the fraction important to not be confused with the PCO 2 obtained from blood gas read-
cardiac and neuromuscular excitability. In acidosis, more calcium ings. 2,3
appears in the ionized form; in alkalotic environments, most of
the calcium remains protein bound. Anion Gap
Calcium levels in the blood follow a diurnal variation, with the The anion gap measures the normal balance between positive and
lowest values occurring in the early morning, and highest values negative electrolytes in the serum. It describes the relation between
occurring at mid-evening. Ionized calcium is difficult to measure, serum sodium (a cation) and bicarbonate and chloride (anions). A
so total calcium is reported in most hospitals. In some situations, normal anion gap is 12 mEq/L. A value greater than 12 mEq/L is
the measured calcium level may be low, but by estimating the considered abnormal. This test is useful in determining whether