Diagnostic tests and procedures
If the patient’s history and physical examination reveal evidence of pulmonary dysfunction, diagnostic tests will help identify and evaluate the dysfunction. These tests include blood, sputum, and pleural studies; endoscopic and imaging tests; biopsies; and various other tests, including pulmonary function tests, pulse oximetry, and thoracentesis.
BLOOD STUDIES
Blood studies used to diagnose respiratory disorders include arterial blood gas (ABG) analysis, arterial-to-alveolar oxygen ratio, white blood cell (WBC) count, and WBC differential.
Arterial blood gas analysis
ABG analysis is one of the first tests ordered to assess respiratory status because it helps evaluate gas exchange in the lungs by measuring:
pH—an indication of hydrogen ion concentration in the blood, which shows the blood’s acidity or alkalinity (see Balancing pH)
partial pressure of arterial carbon dioxide (PaCO2)—known as the respiratory parameter, reflects the adequacy of the lungs’ ventilation and carbon dioxide (CO2) elimination
partial pressure of arterial oxygen (PaO2)—reflects the body’s ability to pick up oxygen from the lungs
bicarbonate level (HCO3−)—known as the metabolic parameter, reflects the kidneys’ ability to retain and excrete bicarbonate. (See Understanding acid-base disorders, pages 66 and 67.)
BALANCING PH
To measure the acidity or alkalinity of a solution, chemists use a pH scale of 1 to 15 that measures hydrogen ion concentrations. As hydrogen ions and acidity increase, pH falls below 7.0, which is neutral. Conversely, when hydrogen ions decrease, pH and alkalinity increase. Acid-base balance, or homeostasis of hydrogen ions, is necessary if the body’s enzyme systems are to work properly.
The slightest change in ionic hydrogen concentration alters the rate of cellular chemical reactions; a sufficiently severe change can be fatal. To maintain a normal blood pH—generally between 7.35 and 7.45—the body relies on three mechanisms.
Buffers
Chemically composed of two substances, buffers prevent radical pH changes by replacing strong acids added to a solution (such as blood) with weaker ones. For example, strong acids capable of yielding many hydrogen ions are replaced by weaker ones that yield fewer hydrogen ions. Because of the principal buffer coupling of bicarbonate and carbonic acid—normally in a ratio of 20:1—the plasma acid-base level rarely fluctuates. Increased bicarbonate, however, indicates alkalosis, whereas decreased bicarbonate points to acidosis. Increased carbonic acid indicates acidosis, and decreased carbonic acid indicates alkalosis.
Respiration
Respiration is important in maintaining blood pH. The lungs convert carbonic acid to carbon dioxide and water. With every expiration, carbon dioxide and water leave the body, decreasing the carbonic acid content of the blood. Consequently, fewer hydrogen ions are formed, and blood pH increases. When the blood’s hydrogen ion or carbonic acid content increases, neurons in the respiratory center stimulate respiration.
Hyperventilation eliminates carbon dioxide and hence carbonic acid from the body, reduces hydrogen ion formation, and increases pH. Conversely, increased blood pH from alkalosis—decreased hydrogen ion concentration—causes hypoventilation, which restores blood pH to its normal level by retaining carbon dioxide and thus increasing hydrogen ion formation.
Urinary excretion
The third factor in acid-base balance is urine excretion. Because the kidneys excrete varying amounts of acids and bases, they control urine pH, which in turn affects blood pH. For example, when blood pH is decreased, the distal and collecting tubules remove excessive hydrogen ions (carbonic acid forms in the tubular cells and dissociates into hydrogen and bicarbonate) and displaces them in urine, thereby eliminating hydrogen from the body. In exchange, basic ions in the urine—usually sodium—diffuse into the tubular cells, where they combine with bicarbonate. This sodium bicarbonate is then reabsorbed in the blood, resulting in decreased urine pH and, more important, increased blood pH.
UNDERSTANDING ACID-BASE DISORDERS
This table outlines acid-base disorders, along with laboratory values associated with the disorder.
DISORDERS AND | POSSIBLE CAUSES | SIGNS AND |
|---|---|---|
RESPIRATORY ACIDOSIS (EXCESS CO2 RETENTION) | ||
pH < 7.35 (SI, < 7.35) HCO3− > 26 mEq/L (SI, > 26 mmol/L) (if compensating) PaCO2 > 45 mm Hg (SI, > 5.3 kPa) | • Central nervous system depression from drugs, injury, or disease • Asphyxia • Hypoventilation due to pulmonary, cardiac, musculoskeletal, or neuromuscular disease • Obesity • Postoperative pain • Abdominal distention | Diaphoresis, headache, tachycardia, confusion, restlessness, apprehension |
RESPIRATORY ALKALOSIS (EXCESS CO2 EXCRETION) | ||
pH > 7.45 (SI, > 7.45) HCO3− < 22 mEq/L (SI, < 22 mmol/L) (if compensating) PaCO2 < 35 mm Hg (SI, < 4.7 kPa) | • Hyperventilation due to anxiety, pain, or improper ventilator settings • Respiratory stimulation caused by drugs, disease, hypoxia, fever, or high room temperature • Gram-negative bacteremia • Compensation for metabolic acidosis (chronic renal failure) | Rapid, deep breathing; paresthesia; lightheadedness; twitching; anxiety; fear |
METABOLIC ACIDOSIS (HCO3− LOSS, ACID RETENTION) | ||
pH < 7.35 (SI, < 7.35) HCO3− < 22 mEq/L (SI, < 22 mmol/L) PaCO2  Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
Get Clinical Tree app for offline access
| ||