Pulmonology

The A-a gradient is the difference between the calculated alveolar oxygen tension and the measured arterial oxygen tension. A gradient that is normal for age suggests hypoventilation or a low inspired oxygen tension (such as altitude), whereas a widened gradient points to V/Q mismatch, right-to-left shunt, or impaired diffusion. The expected upper limit rises with age, roughly (age / 4) + 4 mmHg on room air at sea level.

Five equally-weighted criteria score community-acquired pneumonia severity from 0 to 5, stratifying 30-day mortality risk and informing whether a patient can be managed as an outpatient or needs admission.

Seven weighted clinical criteria estimate the pretest probability of pulmonary embolism. The result can be read three-tier (low, moderate, high) or two-tier (PE unlikely at 4 or less, PE likely above 4), which guides whether D-dimer or direct imaging is the next step.