Respiratory muscle oxygen consumption estimated by the diaphragm pressure-time index

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Abstract
The 02 consumption of the respiratory muscles (.ovrhdot.VO2 resp), work of breathing [.ovrhdot.W] and the time integral of the transdiaphragmatic pressure (TTdi) were measured in 4 normal subjects breathing against inspiratory resistance. A total of 39 runs were performed at mean tidal transdiaphragmatic pressures (Pdi) ranging from 15-53 cmH2O respiratory frequencies from 3.5-22 breaths/min and inspiratory time durations (TI) from 32-76% of the total breath duration. Each run was maintained from 8-17 min and the above parameters were kept constant by the subject via visual feed back of Pdi and TI with an oscilloscope. Most of the runs (36 of 39) were performed at TTdi values below those known to produce respiratory muscle fatigue. A strong linear correlation was found between the .ovrhdot.VO2 resp and the TTdi (r = 0.74, P < 0.001) and a weaker correlation between .ovrhdot.VO2 resp and .ovrhdot.W (r = 0.31, P < 0.05). TTdi evidently is a good estimator of .ovrhdot.VO2 resp over a wide range of respiratory patterns during inspiratory resistance breathing. The high variability seen in respiratory muscle efficiency during resistive breathing may be due to .ovrhdot.W not being a good indicator of the energy consumed by the respiratory muscles.