Intersubject comparison of the crural diaphragm electromyogram, as measured
by an esophageal electrode, requires a reliable means for normalizing the
signal. The present study set out 1) to evaluate which voluntary respirator
y maneuvers provide high and reproducible diaphragm electromyogram root-mea
n-square (RMS) values and 2) to determine the relative diaphragm activation
and mechanical and ventilatory outputs during breathing at rest in healthy
subjects (n = 5), in patients with severe chronic obstructive pulmonary di
sease (COPD, n = 5), and in restrictive patients with prior polio infection
(PPI, n = 6). In all groups, mean voluntary maximal RMS values were higher
during inspiration to total lung capacity than during sniff inhalation thr
ough the nose (P = 0.035, ANOVA). The RMS (percentage of voluntary maximal
RMS) during quiet breathing was 8% in healthy subjects, 43% in COPD patient
s, and 45% in PPI patients. Despite the large difference in relative RMS (P
= 0.012), there were no differences in mean transdiaphragmatic pressure (P
= 0.977) and tidal volumes (P = 0.426). We conclude that voluntary maximal
RMS is reliably obtained during an inspiration to total lung capacity but
a sniff inhalation could be a useful complementary maneuver. Severe COPD an
d PPI patients breathing at rest are characterized by increased diaphragm a
ctivation with no change in diaphragm pressure generation.