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 degrees and grouped as vertical (+65 degrees to +90 degrees), intermediate (+40 degrees to +60 degrees), and horizontal (< +50 degrees). Chest radiographs established the right diaphragmatic level by posterior rib number or interspace with interspaces designated as "0.5" plus the number of the rib above.

P axes for obstructive vs restrictive disease were different (p < 0.001) as in our previous investigation. In the present series, 19 of 20 electrocardiograms in patients with obstructive disease had vertical P axes between +70 degrees and +90 degrees; in 11 of 19 patients with restrictive disease, P axes were less than +40 degrees (horizontal); 6 were between +40 degrees and +60 degrees (intermediate); and only 2 were vertical. Diaphragm levels were between rib/interspace numbers 10.5 and 12.5 in all patients with obstructive disease. Diaphragm levels among patients with restrictive disease were higher and, like their P axes, more widely distributed: 10 of 19 between rib levels 8.0 and 9.5; only 4 at 10.5 or lower. Thus, vertical P axes corresponded to low (rib/interspace 10.5 to 12.5) and intermediate to horizontal P axes with higher (8.0 to 11.0 rib) diaphragm levels (p < 0.001).

Because the separate P-axis distributions in restrictive and obstructive lung disease parallel the separate diaphragm levels and because the right atrium is necessarily carried by attachments to the right diaphragmatic leaf, it is likely that the consequent positional effects on the right atrium contribute to or cause the significantly different P-axis orientations in restrictive and obstructive pulmonary disease.