Characterization of Ventilatory Drive in Community-Dwelling Adults with Chronic Spinal Cord Injury

Abstract

Objective: To advance characterization of ventilatory drive in community-dwelling adults with chronic spinal cord injury (SCI).

Design: Cross-sectional analysis of data from a parent randomized clinical trial. Relationships between ventilatory drive and the following variables were assessed: (1) age, (2) sex, (3) height, (4) body mass index, (5) American Spinal Injury Association Impairment scale, (6) neurologic level of injury, (7) time since injury, (8) maximal inspiratory pressure, (9) the ratio of ventilatory drive to maximal inspiratory pressure, (10) tidal volume, (11) breathing frequency, (12) minute ventilation, (13) body mass-normalized tidal volume and minute ventilation, and (14) end-tidal carbon dioxide.

Setting: Clinical research laboratory in a rehabilitation hospital.

Participants: Volunteer sample of community-dwelling, independently breathing adult participants (N=29) (median age, 34y; range, 18-67; 27 men) with chronic (median, 2.25y postinjury; range, 1.1-30y) complete or incomplete SCI from the first cervical to sixth thoracic level.

Interventions: Not applicable.

Main outcome measures: Mouth occlusion pressure (P0.1) as an indicator of ventilatory drive.

Results: Participants presented with reduced pulmonary function (percent predicted forced vital capacity median, 75%; interquartile range, 64%-84%). P0.1 values were consistent with those in the general population (median, 1.00 cmH₂O; interquartile range, 0.80-1.26). P0.1 was strongly correlated with body mass index (r_s=.56, P<.01) and significantly correlated with body mass (r_s=.48, P<.01), breathing frequency (r_s=.41, P=.03), body mass-normalized tidal volume (r_s=-.39, P=.04) and minute ventilation (r_s=0.40, P=.03).

Conclusions: Independently breathing adults with chronic SCI have P0.1 values within normal range. This suggests their intrinsic resting ventilatory drive recovers from potentially elevated states in the acute phase of injury and is sufficient to sustain ventilation. The significant correlation between P0.1 and body mass has not been reported in the general population and suggests that body mass, P0.1, and ventilatory drive are tightly coupled in adults with SCI despite independent breathing ability. The potential for increased body mass-induced strain on the respiratory pump after SCI may have implications for breathing during daily activities and exercise which should be explored in future research.

Keywords: Mouth occlusion pressure; Rehabilitation; Spinal cord injuries; Ventilation.

Fig 1

Boxplot of P0.1 results for the full sample of participants. Participant 03 is represented by the gray square.

Fig 2

Scatterplots of significant relationships of P0.1 with body mass (Panel A; r_s=.475, P=.009) and body mass index (Panel B; r_s=.561, P=.002). Participant 03 is represented by the gray square.

Fig 3

Scatterplot of significant relationship between P0.1 and breathing frequency (r_s=0.41, P=.03). Participant 03 is represented by the gray square.

Fig 4

Scatterplot of significant relationship between P0.1 and minute ventilation (r_s=.4, P=.03). If participant 03, represented by the gray square is removed for having an extreme P0.1 value relative to the rest of the sample, the relationship becomes nonsignificant (r_s=.33, P=.09).

Fig 5

Scatterplot of significant relationship between P0.1 and body mass-normalized tidal volume (r_s=–.39, P=.04). If participant 03, represented by the gray square is removed for having an extreme P0.1 value relative to the rest of the sample, the relationship becomes nonsignificant (r_s=–.32, P=.09).