4: Common Diagnostic and Laboratory Tests

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Part 4 Common Diagnostic and Laboratory Tests

ARTERIAL BLOOD GAS ANALYSIS

Arterial blood gas (ABG) analysis results are rapidly available and provide a baseline to determine a patient’s current respiratory and metabolic status and needs.

Blood gas interpretation is based on assessing the arterial serum levels of the following variables:

TABLE 4-1 Arterial Blood Gas Values

ABG COMPONENT	NORMAL LEVELS
pH	7.35-7.45
PaCO₂	35-45 mmHg
HCO₃	22-26 mEq/liter
PaO₂	90-110 mmHg

Consistent Approach Is Key

In order to make an interpretation based on the individual ABG values, a consistent sequence of steps should be followed:

1. Evaluate pH to determine the presence of acidosis or alkalosis. The lungs and kidneys regulate the hydrogen ion status within the plasma. Alterations in these systems affect the acid-base balance, causing pH changes that affect multiple body systems.

Within normal limits (WNL) indicates normal or compensated state.

Outside normal limits

<7.35: Acidosis—Acidosis may cause pulmonary vasoconstriction leading to decreased pulmonary blood flow. Acidosis may also cause vasoconstriction to cerebral blood vessels.

>7.45: Alkalosis—Alkalosis may diminish cellular metabolism, depress myocardial function, and dilate pulmonary blood vessels.

2. Evaluate PaCO₂ to assess the alveolar ventilation status. In an uncompensated acidosis or alkalosis, an abnormal PaCO₂ level will generally indicate that the origin of the pH imbalance is respiratory rather than metabolic.

Within normal limits—adequate ventilation

Outside normal limits

>45: Hypercarbia—Hypoventilation leads to an increase in PaCO₂, which in turn lowers the pH, resulting in a respiratory acidosis.

<30: Hypocarbia—Hyperventilation leads to decreased PaCO₂, which in turn raises the pH, resulting in a respiratory alkalosis.

3. Evaluate HCO₃ to assess the effectiveness of renal regulation of blood pH. In an uncompensated acidosis or alkalosis, an abnormal HCO₃ level will generally indicate that the origin of the pH imbalance is metabolic rather than respiratory.

Within normal limits—normal renal function

Outside normal limits

<22: Decreased bicarbonate—Renal mechanisms lead to increased excretion of bicarbonate and a lower serum bicarbonate level. Owing to the absence of normal levels of bicarbonate to buffer serum H⁺ (acid), the pH lowers and metabolic acidosis is the result.

>29: Increased bicarbonate—Renal mechanisms lead to increased retention of bicarbonate. Owing to the higher levels of bicarbonate, more serum H⁺ (acid) is buffered, the pH increases, and metabolic alkalosis is the result.

4. Look for signs of compensation—With prolonged abnormalities in pH, the body tries to return the pH to normal through respiratory compensation (adjusting PaCO₂ levels) or metabolic compensation (adjusting HCO₃ levels). In compensated acidosis or alkalosis, the pH will be normal, but the PaCO₂ and HCO₃ will both be abnormal in the same “direction” (increased or decreased).

The following table may be used to assist with differentiation of respiratory versus metabolic acid-base imbalances, including presence of compensation:

TABLE 4-2 ABG Interpretation

5. Evaluate PaO₂ to assess the oxygenation status. It is important to be aware of a patient’s specific “normal” values. Patients with certain cardiac or pulmonary conditions may have “acceptable” PaO₂ that is below normal limits. Assess each patient’s unique needs, and treat accordingly.

Within normal limits—adequate oxygenation

Outside normal limits

55-85: Mild hypoxemia

40-55: Moderate hypoxemia

<40: Severe hypoxemia

BONE MARROW ASPIRATION AND BIOPSY

Insertion of a needle into the bone marrow to obtain (by aspiration) either (1) bone marrow, or (2) piece of bone (biopsy) for evaluation of cells and bone tissue, respectively. Procedure is performed with child under sedation and with local anesthetic used for dermal penetration. Child should be prepared beforehand. Care after procedure: A small pressure dressing may be used; child may be sore and require analgesic for 24 hours. May be performed on an outpatient basis, and the child may go home once he or she is fully awake and alert. No activity restrictions are required for the test.

BRONCHOSCOPY

BOX 4-1 Bronchoscopy

Bronchoscopy is the direct observation of tracheobronchial tree via bronchoscope.

Localizes abnormalities in major airways

Provides access to (1) remove aspirated foreign bodies from major airways, (2) remove obstructive mucous plugs, (3) perform bronchial lavage, (4) directly observe and assess anatomic structures of airway for any pathology.

Procedure requires sedation and monitoring of vital signs during and after procedure until child is awake and alert.

CARDIAC CATHETERIZATION AND COMMON CARDIOLOGY TESTS

TABLE 4-3 Cardiac Catheterization and Common Cardiology Tests

PROCEDURE	DESCRIPTIVE
Chest radiograph (X-ray)	Produces images of internal structures of chest, including air-filled lungs, airways, vascular markings, heart, and great vessels; shows heart size
Electrocardiography (ECG)	Graphic measure of electrical activity of heart
Holter monitor	24-hour continuous ECG recording used to assess dysrhythmias
Echocardiography	Use of high-frequency sound waves obtained by a transducer to produce an image of cardiac structures
Transthoracic	Performed with transducer on chest
M-mode	One-dimensional graphic view used to estimate ventricular size and function
Two-dimensional (2-D)	Real-time, cross-sectional views of heart used to identify cardiac structures and cardiac anatomy
Doppler	Identifies blood flow patterns and pressure gradients across structures
Fetal	Imaging fetal heart in utero
Transesophageal (TEE)	Transducer placed in esophagus behind heart to obtain images of posterior heart structures or in patients with poor images from chest approach
Cardiac catheterization	Imaging study using radiopaque catheters placed in a peripheral blood vessel and advanced into heart to measure pressures and oxygen levels in heart chambers and visualize heart structures and blood flow patterns
Hemodynamics	Measures pressures and oxygen saturations in heart chambers
Angiography	Use of contrast material to illuminate heart structures and blood flow patterns
Biopsy	Use of special catheter to remove tiny samples of heart muscle for microscopic evaluation; used in assessing infection, inflammation, or muscle dysfunction disorders; also used to evaluate for rejection after heart transplant
Electrophysiology (EPS)	Special catheters with electrodes employed to record electrical activity from within heart; used to diagnose rhythm disturbances
Exercise stress test	Monitoring of heart rate, blood pressure, electrocardiogram (ECG), and oxygen consumption at rest and during progressive exercise on a treadmill or bicycle
Cardiac magnetic resonance imaging (MRI)	Noninvasive imaging technique; used in evaluation of vascular anatomy outside of heart (e.g., coarctation of the aorta, vascular rings), estimates of ventricular mass and volume; uses for MRI are expanding

COMPLETE BLOOD CELL COUNT

TABLE 4-4 Complete Blood Cell Count

TEST (AVERAGE VALUE)	DESCRIPTION/COMMENTS
Red blood cell (RBC) count (4.5-5.5 million/mm³)	Number of RBCs/mm³ of blood Indirectly estimates Hgb content of blood Reflects function of bone marrow

Hemoglobin (Hgb) determination (11.5-15.5 g/dl)^*

Amount of Hgb (g)/dl of whole blood

Total blood Hgb primarily depends on number of circulating RBCs but also on amount of Hgb in each cell

Hematocrit (Hct) (35%-45%)

Percent volume of packed RBCs in whole blood

Indirectly measures Hgb content

Is approximately three times Hgb content

RBC indexes Mean corpuscular volume (MCV) (77-95 fl)

Average or mean volume (size) of a single RBC

MCV values are expressed as femtoliters (fl) or cubic microns (μm³)

Mean corpuscular hemoglobin (MCH) (25-33 pg/cell)

Average or mean quantity (weight) of Hgb in a single RBC

MCH values are expressed as picograms (pg) or micromicrograms (μmcg)

MCV and MCH depend on accurate counts of RBCs, whereas MCHC does not; therefore, MCHC is often more reliable.

All indexes depend on average cell measurements and do not show individual RBC variations (anisocytosis).

Mean corpuscular hemoglobin concentration (MCHC) (31%-37% Hgb [g]/dl RBC)

Average concentration of Hgb in a single RBC

MCHC values are expressed as percent Hgb (g)/cell or Hgb (g)/dl RBC.

RBC volume distribution width (RDW) (13.4% ± 1.2%)

Average size of RBCs

Differentiates some types of anemia

Reticulocyte count (0.5%-1.5% erythrocytes)

Percent reticulocytes in RBCs

Index of production of mature RBCs by bone marrow

Decreased count indicates depressed bone marrow function.

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