Acute respiratory failure in COPD
LIFE-THREATENING DISORDERIn patients with essentially normal lung tissue, acute respiratory failure (ARF) usually means a partial pressure of arterial carbon dioxide (PaCO2) greater than 50 mm Hg and a partial pressure of arterial oxygen (PaO2) less than 50 mm Hg. These limits, however, don’t apply to patients with chronic obstructive pulmonary disease (COPD), who commonly have a consistently high PaCO2 and low PaO2. In COPD patients, only acute deterioration in arterial blood gas (ABG) levels and corresponding clinical deterioration indicate ARF.
Causes
ARF may develop in COPD patients from any condition that increases the work of breathing and decreases the respiratory drive. Such conditions include respiratory tract infection (such as bronchitis or pneumonia), which is the most common precipitating factor; bronchospasm; or accumulating secretions secondary to cough suppression. Other causes include:
central nervous system (CNS) depression—head trauma or injudicious use of sedatives, narcotics, tranquilizers, or oxygen
cardiovascular disorders—myocardial infarction, heart failure, or pulmonary emboli
airway irritants—smoke or fumes
endocrine and metabolic disorders—myxedema or metabolic alkalosis
thoracic abnormalities—chest trauma, pneumothorax, or thoracic or abdominal surgery.
Signs and symptoms
In COPD patients with ARF, increased ventilation-perfusion mismatching and reduced alveolar ventilation decrease PaO2 (hypoxemia) and increase PaCO2 (hypercapnia). This rise in carbon dioxide tension lowers the pH. The resulting hypoxemia and acidemia affect all body organs, especially the central nervous, respiratory, and cardiovascular systems. Specific symptoms vary with the underlying cause of ARF but can include any of the following:
Respiratory symptoms. The respiratory rate may be increased, decreased, or normal, depending on the cause; respirations may be shallow or deep, or they may alternate between the two; and air hunger may occur. Cyanosis may or may not be present, depending on the hemoglobin (Hb) level and arterial oxygenation. Auscultation of the chest may reveal crackles, rhonchi, wheezes, or diminished breath sounds.
