Voluntary Activation of the Inspiratory Muscles Determined by Spinal Root Magnetic Stimulation During Graded Respiratory Efforts
Authors List
Prajnadewie, C.1,2, Luu, B.L. 1, Mathew, A.J.1,2, Finn, H.T.1,2, Gandevia, S.C.1,2, Hudson, A.L.1,2,3, & Butler, J.E.1.
1Neuroscience Research Australia, 2University of New South Wales, 3Flinders University, Australia
Introduction and Rationale
Voluntary activation of the diaphragm can be assessed with the twitch interpolation method. Historically, the phrenic nerves have been maximally activated bilaterally using electrical stimuli delivered via wires inserted under the skin (e.g. 1). Magnetic stimulation is a non-invasive method used to activate the human diaphragm and can be delivered over the phrenic nerves in the neck bilaterally or cervically to activate the phrenic nerve roots (for review see 2). Here, we assessed the reliability of phrenic nerve root magnetic stimulation as a method to calculate voluntary activation of the diaphragm.
Methods
Nine healthy able-bodied human participants (age, 18-40 years) performed quasiisometric voluntary inspiratory efforts (5 sets of contractions at 100%, 75%, 50% and 25% maximal inspiratory efforts (PImax)) at end-tidal volume. Oesophageal pressure and gastric pressure were recorded using a nasogastric catheter and were used to calculate the transdiaphragmatic pressure (Pdi). Magnetic stimulation of the phrenic nerve roots bilaterally
between C3-C5 was delivered using a 13-centimetre diameter round coil placed over the cervical spine (Magstim, UK, Bistimulation module, 0 ms time interval). First, single stimuli (intensity, 30-100%) were used to evoke resting pressure twitches for a recruitment curve at end-tidal volume. Second, single stimuli were delivered at 100% magnetic stimulator output during each voluntary inspiratory effort to produce a superimposed twitch, and again ~5 s
later at rest from which voluntary activation was calculated.
Results
The recruitment curve did not plateau in any of the participants, suggesting that supramaximal stimulation was not achieved. Despite this, potentiated resting twitch pressures for Pdi reached 30.2(11.1) cmH2O (mean(SD)) and for each targeted contraction strength (100%, 75%, 50% and 25%), voluntary activation was 92(9)%, 82(18)%, 62(27)% and 33(29)%, respectively. In 7 of the 9 participants, voluntary activation was linearly related to contraction intensity and the superimposed Pdi twitch was inversely related to contraction intensity. For the two other participants, voluntary activation reached 100% at prematurely low contraction intensities (52-66% PImax) and superimposed Pdi twitches were not visible during higher contractions.
Conclusions
Phrenic nerve root magnetic stimulation may be a useful technique to measure voluntary activation of the diaphragm in a majority of individuals. It provides a less invasive technique than percutaneous electrical stimulation to measure inspiratory muscle activity that could be utilised in future research and clinical studies. However, there are limitations to this technique that must be considered, particularly its inability to evoke truly maximal contractions.
Research Funding Source: The study was supported by the National Health and Medical Research Council of Australia and the University of New South Wales.
References:
1. Bellemare F, Bigland-Ritchie B (1984). Assessment of human diaphragm strength and activation using phrenic nerve stimulation. Respir Physiol 58:263–77.
Prajnadewie, C.1,2, Luu, B.L. 1, Mathew, A.J.1,2, Finn, H.T.1,2, Gandevia, S.C.1,2, Hudson, A.L.1,2,3, & Butler, J.E.1.
1Neuroscience Research Australia, 2University of New South Wales, 3Flinders University, Australia
Introduction and Rationale
Voluntary activation of the diaphragm can be assessed with the twitch interpolation method. Historically, the phrenic nerves have been maximally activated bilaterally using electrical stimuli delivered via wires inserted under the skin (e.g. 1). Magnetic stimulation is a non-invasive method used to activate the human diaphragm and can be delivered over the phrenic nerves in the neck bilaterally or cervically to activate the phrenic nerve roots (for review see 2). Here, we assessed the reliability of phrenic nerve root magnetic stimulation as a method to calculate voluntary activation of the diaphragm.
Methods
Nine healthy able-bodied human participants (age, 18-40 years) performed quasiisometric voluntary inspiratory efforts (5 sets of contractions at 100%, 75%, 50% and 25% maximal inspiratory efforts (PImax)) at end-tidal volume. Oesophageal pressure and gastric pressure were recorded using a nasogastric catheter and were used to calculate the transdiaphragmatic pressure (Pdi). Magnetic stimulation of the phrenic nerve roots bilaterally
between C3-C5 was delivered using a 13-centimetre diameter round coil placed over the cervical spine (Magstim, UK, Bistimulation module, 0 ms time interval). First, single stimuli (intensity, 30-100%) were used to evoke resting pressure twitches for a recruitment curve at end-tidal volume. Second, single stimuli were delivered at 100% magnetic stimulator output during each voluntary inspiratory effort to produce a superimposed twitch, and again ~5 s
later at rest from which voluntary activation was calculated.
Results
The recruitment curve did not plateau in any of the participants, suggesting that supramaximal stimulation was not achieved. Despite this, potentiated resting twitch pressures for Pdi reached 30.2(11.1) cmH2O (mean(SD)) and for each targeted contraction strength (100%, 75%, 50% and 25%), voluntary activation was 92(9)%, 82(18)%, 62(27)% and 33(29)%, respectively. In 7 of the 9 participants, voluntary activation was linearly related to contraction intensity and the superimposed Pdi twitch was inversely related to contraction intensity. For the two other participants, voluntary activation reached 100% at prematurely low contraction intensities (52-66% PImax) and superimposed Pdi twitches were not visible during higher contractions.
Conclusions
Phrenic nerve root magnetic stimulation may be a useful technique to measure voluntary activation of the diaphragm in a majority of individuals. It provides a less invasive technique than percutaneous electrical stimulation to measure inspiratory muscle activity that could be utilised in future research and clinical studies. However, there are limitations to this technique that must be considered, particularly its inability to evoke truly maximal contractions.
Research Funding Source: The study was supported by the National Health and Medical Research Council of Australia and the University of New South Wales.
References:
1. Bellemare F, Bigland-Ritchie B (1984). Assessment of human diaphragm strength and activation using phrenic nerve stimulation. Respir Physiol 58:263–77.
