Quantitative assessment of respiratory function following contusion injury of the cervical spinal cord

In this study, we describe a new method for quantitative assessment of phrenic inspiratory motor activity in two models of cervical spinal cord contusion injury. Anesthetized rats received contusion injury either to the descending bulbospinal respiratory pathway on one side of the spinal cord alone (C2 lateralized contusion) or to both the descending pathway, as well as the phrenic motoneuron pool bilaterally (C4/C5 midline contusion). Following injury, respiratory-associated phrenic nerve motor activity was recorded under standardized and then asphyxic conditions. Phrenic nerve efferent activity was rectified, integrated, and quantitated by determining the mean area under the integrated neurograms. The mean integrated area of the four inspiratory bursts recorded just before turning off the ventilator (to induce asphyxia) was determined and divided by the integrated area under the single largest respiratory burst recorded during asphyxia. This latter value was taken as the maximal inspiratory motor response that the rat was capable of generating during respiratory stress. Thus, a percentage of the maximal inspiratory motor drive was established for breathing in control and injured rats under standardized conditions. The results indicate that noninjured rats use 52 +/- 1.8% of maximal inspiratory motor drive under standardized conditions. In C2-contused rats, the results showed that while the percentage of maximal inspiratory motor drive on the noncontused side was similar to the control (55 +/- 4.1%), it was increased on the contused side (78 +/- 2.6%). In C4/5 lesions, the results indicate that this percentage was increased on both sides (77 +/- 4.4%). The results show the feasibility for performing quantitative evaluation of respiratory dysfunction in an animal model of cervical contusion injury. These findings lend to further development of this model for investigations of neuroplasticity and/or therapeutic interventions directed at ameliorating respiratory compromise following cervical spinal cord trauma.