Diaphragm Levels as Determinants of P Axis in Restrictive vs Obstructive Pulmonary Disease

Verticalized P axes in adults with obstructive lung disease have long been appreciated as characteristic of emphysema. After demonstrating P axes in restrictive lung disease to have a significantly different orientation (intermediate to horizontal), it was hypothesized that opposite effects on diaphragm level by obstructive disease (low diaphragm) and by restrictive disease (high diaphragm) could explain the axis differences, because the right atrium is attached via the inferior vena cava and adjacent pericardium to the right leaf of the diaphragm. Electrocardiograms and chest radiographs were analyzed independently in a new series of 20 consecutive patients with purely obstructive and 19 consecutive patients with purely restrictive pulmonary disease. P axes were calculated to the nearest 5° and grouped as vertical (+65° to +90°), intermediate (+40° to +60°), and horizontal (