Old age does not influence ventilatory transdiaphragmatic pressures following unilateral phrenicotomy

Aging has a major effect on the function of the diaphragm muscle (DIAm), with reduced maximum force generation and atrophy of type IIx/b fibers, consistent with functional reductions in expulsive behaviors that lead to increased incidences of aspiration pneumonia. This diminished capacity of DIAm function is underscored by a loss of phrenic motor neurons (PhMN) and denervation of DIAm fibers. Collectively, this aging-related pathophysiology selectively impacts fast fatigable DIAm motor units. It remains unknown if the ventilatory function of the DIAm is compromised in old age under circumstances where the reserve capacity is diminished by a reduction in maximum force. Here, we used an assessment of transdiaphragmatic pressure (Pdi) to evaluate ventilatory behaviors (eupnea, sigh, occlusion) before and after unilateral phrenicotomy in young and old female and male rats (6- [n = 7], and 24-months old [n = 6]). We show that ventilatory Pdi was affected by phrenicotomy, but these effects were not exacerbated by old age. These results underscore that DIAm deficits are exclusive to non-ventilatory behaviors in old age. Deficits in the breath with aging are a result of other non-DIAm factors, including neuromotor dysfunctions of the upper airway, increased lung compliance, stiffening of the chest wall, and pulmonary fibrosis/remodeling.