Submitted on October 9, 2003
Accepted on February 16, 2004
DEPRESSION OF DIAPHRAGM MOTOR CORTEX EXCITABILITY DURING MECHANICAL VENTILATION
Tarek Sharshar1, Ewen Ross2, Nicholas S Hopkinson2, Raphael Porcher3, Annabel H Nickol2, Sophie Jonville2, Mark J Dayer2, Nicholas Hart2, John Moxham4, Frederic Lofaso5, and Michael S Polkey2*
1 Respiratory Muscle Laboratory, Royal Brompton and Harefield NHS Trust, London, SW3 6NP, United Kingdom; Departement de Biostatistique et Informatique, Hopital Saint-Louis, AP-HP, Paris, 75010, France
2 Respiratory Muscle Laboratory, Royal Brompton and Harefield NHS Trust, London, SW3 6NP, United Kingdom
3 Department of Respiratory Medicine, King's College Hospital, London, SE5 9PJ, United Kingdom
4 Departement de Biostatistique et Informatique, Hopital Saint-Louis, AP-HP, Paris, 75010, France
5 Service d'Explorations Fonctionnelles, Hopital Raymond Poincare, AP-HP, Garches, 92380, France
* To whom correspondence should be addressed. E-mail: M.Polkey{at}rbh.nthames.nhs.uk.
The effect of mechanical ventilation on the diaphragm motor cortex remains unknown. We assessed the effect of mechanical ventilation on diaphragm motor cortex excitability by measuring the costal and crural diaphragm motor evoked potential (MEP) elicited by single and paired transcranial magnetic stimulation (TMS). In six healthy subjects, MEP recruitment curves of the costal and crural diaphragms were assessed at relaxed end-expiration during spontaneous breathing (VTbaseline) and isocapnic volume cycled ventilation delivered non-invasively (NIV) at three different levels of tidal volume (VTbaseline, VTbaseline + 5ml/kg L and VTbaseline + 10 ml/kg L). The costal and crural diaphragm response to peripheral stimulation of the right phrenic nerve was not reduced by NIV. NIV reduced the costal and crural MEP amplitude during NIV (P<0.0001) with the maximal reduction at VTbaseline + 5ml/kg. Response to paired TMS showed that NIV (VTbaseline + 5ml/kg) significantly increased the sensitivity of the cortical motor neurons to facilitatory (>9 ms) interstimulus intervals (P=0.002), suggesting that the diaphragm MEP amplitude depression during NIV is related to neuromechanical inhibition at the level of motor cortex. Our results demonstrate that mechanical ventilation directly inhibits central projections to the diaphragm.
