Thoracic reflexes stabilizing loaded ventilation in normal and cord-injured man

Area of Science:

• Respiratory Physiology
• Neurology
• Pulmonary Medicine

Background:

• Tidal volume regulation is essential for maintaining adequate gas exchange during breathing.
• Cervical cord injury can disrupt neural control of respiration, potentially affecting breathing mechanics.
• The role of thoracic sensory feedback in volitional breathing control under load is not fully understood.

Purpose of the Study:

• To quantify the ability to regulate tidal volume in patients with cervical cord injury.
• To investigate the relationship between respiratory loading, tidal volume compensation, and phrenic motoneuron output.
• To test the hypothesis that thoracic sensory receptors maintain phrenic motoneuron output under load.

Main Methods:

• Measured passive respiratory elastance (Ers) in 24 cervical cord-injured patients with relaxed muscles.
• Quantified "effective" elastance (E'rs) during single-breath elastic loading.
• Compared E'rs to Ers to assess tidal volume compensation and analyzed inspiratory duration.

Main Results:

• Most patients (13/24) failed to compensate for elastic loads (E'rs ≤ Ers), exhibiting impaired tidal volume defense.
• This impairment was linked to shortened inspiratory duration and premature phrenic motoneuron output cessation.
• Patients with preserved compensation (E'rs ≥ 2Ers) perceived loading as obstruction and prolonged inspiratory duration.

Conclusions:

• Cervical cord injury significantly impairs the ability to regulate tidal volume during elastic loading.
• This deficit is associated with altered phrenic motoneuron output timing, not diaphragmatic paralysis or instability.
• Findings support the role of thoracic sensory receptors in maintaining phrenic motoneuron output and tidal volume stability in intact subjects.