REDUCED VOLUNTARY DRIVE TO THE HUMAN DIAPHRAGM AT LOW LUNG-VOLUMES

Maximal inspiratory and transdiaphragmatic pressures vary with lung vo lume but the possibility that some of this variability reflects variab le voluntary drive to the diaphragm has not been investigated systemat ically. We assessed the influence of lung volume on the ability to act ivate the diaphragm with voluntary effort during maximal Mueller manoe uvres. Voluntary activation of the diaphragm was assessed using twitch interpolation with bilateral phrenic nerve stimulation in 6 subjects. Each performed 10 maximal efforts at lung volumes around functional r esidual capacity (FRC) and additional efforts at volumes above and bel ow FRC. Voluntary activation of the diaphragm was higher at lung volum es above FRC (>60% TLC; activation 98.3+/-2.6%) than at lung volumes a round FRC (45-60% TLC; activation 95.5+/-3.5%) and below FRC (<45% TLC ; activation 83.3+/-15.8%; p < 0.05). Submaximal diaphragm activation at low lung volumes may reflect differences in the length-tension rela tionships of the various inspiratory muscles and/or reflex inhibition of phrenic motoneurones at low lung volumes.