Increased diaphragm motor unit discharge frequencies during quiet breathing in people with chronic tetraplegia

Abstract

Key pointsr Respiratory muscle strength is compromised in people with tetraplegia, which may be compensated for by an increase in neural drive to the diaphragm.r We found that the discharge frequencies of diaphragm motor units are higher in people with chronic tetraplegia compared with able-bodied people during quiet breathing.r Furthermore, we found that the area of single motor unit potentials was increased in people with tetraplegia.r These results suggest an increased motoneurone output to the diaphragm and remodelling of diaphragm motor units to maintain ventilation in tetraplegia.Abstract People with tetraplegia have reduced inspiratory muscle strength, ß40% of able-bodied individuals. Paralysed or partially paralysed respiratory muscles as a result of tetraplegia compromise lung function, increase the incidence of respiratory infections and can cause dyspnoea. We hypothesised that reduced inspiratory muscle strength in tetraplegia may increase neural drive to the inspiratory muscles to maintain ventilation. We recorded the discharge properties of single motor units from the diaphragm in participants with chronic tetraplegia (8 males, 42-78 years, C3-C6 injury, AIS A-C) and able-bodied control participants (6 males matched for age and body mass index). In each group, 117 and 166 single motor units, respectively, were discriminated from recordings in the costal diaphragm using a monopolar electrode. A linear mixed-effects model analysis showed higher peak discharge frequencies of motor units during quiet breathing in tetraplegia (17.8 ± 4.9 Hz; mean ± SD) compared with controls (12.4 ± 2.2 Hz) (P < 0.001). There were no differences in tidal volume, inspiratory time or mean air flow between groups. Motor unit potentials in tetraplegia, compared with controls, were larger in amplitude (1.1 ± 0.7 mV and 0.5 ± 0.3 mV, respectively, P = 0.007) and area (1.83 ± 1.49 µV D. A. T. Nguyen and others J Physiol 598.11 ms and 0.69 ± 0.52 µV ms, respectively, P = 0.003). The findings indicate that diaphragm motor unit remodelling is likely to have occurred in people with chronic tetraplegia and that there is an increase in diaphragm motor unit discharge rates during quiet breathing. These neural changes ensure that ventilation is maintained in people with chronic tetraplegia.

Causes of, and countermeasures to, increased ventilatory drive in tetraplegia

“…In a recent issue of The Journal of Physiology , Nguyen et al . (2020) examined diaphragm motor unit properties following cervical spinal cord injury (C‐SCI) (Nguyen et al . 2020).…”

“…(2020) examined diaphragm motor unit properties following cervical spinal cord injury (C‐SCI) (Nguyen et al . 2020). To test their hypotheses that (a) diaphragm motor unit discharge frequencies would be higher and (b) diaphragm motor unit potentials would be larger following C‐SCI they examined eight males with SCI between the third and sixth cervical level (56 ± 11 years).…”

“…Nguyen et al . (2020) conducted a well‐designed, hypothesis‐driven study. Respiratory physiology changes following C‐SCI have important clinical implications, as respiratory dysfunction is the leading cause of death following SCI (Savic et al .…”

Diaphragm remodelling following cervical spinal cord injury: Can intrinsic neural plasticity be harnessed to improve respiratory motor function?

“…2019). Recently, in The Journal of Physiology , Nguyen et al (2020) extended this line of enquiry in an important study, which addressed whether neural drive to the primary muscle of breathing is increased following cervical spinal cord injury.…”