Inspiratory Mouth Pressures And Motor Evoked Potentials Following Non-fatiguing Inspiratory Loading

Abstract

We have shown previously that maximal inspiratory pressure (MIP) increases by 10% following prior inspiratory loaded breathing at 40% MIP (ILB) and that this increase is accompanied by increases in surface EMG of the diaphragm and intercostals. PURPOSE To examine the effect of an acute bout of ILB on the facilitation of cortically evoked motor evoked potentials (MEPs) during voluntary activation of the inspiratory muscles. METHODS Surface EMG from the chest wall was used to record diaphragm and intercostal EMG activity during an inspiratory effort of 50% MIP (MIP50), as well as MEPs evoked in the contralateral diaphragm and intercostals by transcranial magnetic stimulation of the motor cortex. MEPs were recorded during relaxation, MIP50 and maximal MIP (MIP 100) in 11 volunteers who gave informed consent, following Ethics committee approval. Mouth pressure was measured from an occluded airway. After baseline measurements, subjects performed ILB (two × 30 inspiratory efforts at 40% of initial MIP). Measurements were taken immediately after the first and second sets and 15 min after completion of the ILB. Data presented as means ± SEM., and compared using paired t-tests. RESULTS Values are expressed as percentage of baseline measures. RMS, root mean square;*, P = <0.05 Maximal MIP increased to 111+5%* of baseline after the first set of ILB, 115±5%* after the second set, and 107±4% 15 min after cessation of the breathing task.TableCONCLUSION The increase in MIP and RMS EMG after ILB confirm an increased activation and synergistic co ordination of the diaphragm and intercostal muscles during inspiratory efforts. The increase in MEPs during relaxed and MIP50 suggest that ILB increases the level of excitation in the inspiratory motoneurone pool. The decrease in MEPs during a MIP100 is difficult to interpret, but may indicate that with increased excitation and faster firing rates, some motor units become refractory. The changes in EMG and MEPs are maintained 15min following cessation of ILB indicating these modulations are not transient.