Pulmonary System
Core concepts
Separate obstructive from restrictive physiology first. Obstruction (asthma, COPD, bronchiectasis) shows a reduced FEV1/FVC ratio below 0.70; restriction shows a reduced FVC with a normal or increased ratio. Asthma is reversible, with at least 12 percent and 200 mL improvement in FEV1 after a bronchodilator.
Read arterial blood gases in a fixed order: pH first, then PaCO2 to identify the respiratory direction, then bicarbonate to identify metabolic compensation and chronicity. Hypoxemia with a normal alveolar-arterial gradient suggests hypoventilation; a widened gradient suggests ventilation-perfusion mismatch, shunt, or diffusion impairment.
Pulmonary embolism: risk stratify with a clinical prediction tool, use D-dimer only in low or intermediate probability patients, and image with CT pulmonary angiography when probability is high or D-dimer is positive. A hemodynamically unstable patient with suspected embolism needs bedside echocardiography and consideration of thrombolysis, not a trip to the scanner.
Pneumonia severity drives disposition: confusion, elevated blood urea nitrogen, respiratory rate of 30 or more, low blood pressure, and age 65 or older. Pleural fluid is exudative when the protein ratio exceeds 0.5, the lactate dehydrogenase ratio exceeds 0.6, or fluid LDH exceeds two-thirds the upper limit of normal.
Traps
- Reassurance from a silent chest in a severe asthma attack; it signals poor air movement, not improvement.
- Waiting for imaging in a tension pneumothorax. There is no indication to delay needle decompression.
- Attributing dyspnea to COPD in a patient with new unilateral leg swelling.
Rule of thumb
Expected alveolar-arterial gradient equals age divided by four, plus four. Use it to decide whether hypoxemia is a lung problem or a breathing problem.