Influence of respiratory loading on left‐ventricular function in cervical spinal cord injury

Abstract

Cervical spinal cord injury (C‐SCI) negatively impacts cardiac and respiratory function. As the heart and lungs are linked via the pulmonary circuit these systems are interdependent. Here, we utilized inspiratory and expiratory loading to assess whether augmenting the respiratory pump improves left‐ventricular (LV) filling and output in individuals with motor‐complete C‐SCI. We hypothesized LV end‐diastolic volume (LVEDV) would increase and decrease with inspiratory and expiratory loading, respectively. Participants (C‐SCI: 7M/1F, 35 ± 7 years; able‐bodied: 7M/1F, 32 ± 6 years) were assessed under five conditions during 45° head‐up tilt; unloaded, inspiratory loading with −10 and −20 cmH2O oesophageal pressure (Poes) on inspiration, and expiratory loading with +10 and +20 cmH2O Poes on expiration. An oesophageal balloon catheter monitored Poes, and LV structure and function were assessed by echocardiography. In C‐SCI only, (1) +20 cmH2O reduced LVEDV vs. unloaded (81 ± 15 vs. 88 ± 11 ml, P = 0.006); (2) heart rate was higher during +20 cmH2O compared to unloaded (P = 0.001) and +10 cmH2O (P = 0.002); (3) cardiac output was higher during +20 cmH2O than unloaded (P = 0.002); and (4) end‐expiratory lung volume was higher during +20 cmH2O vs. unloaded (63 ± 10 vs. 55 ± 13% total lung capacity, P = 0.003) but was unaffected by inspiratory loading. In both groups, −10 and −20 cmH2O had no significant effect on LVEDV. These findings suggest greater expiratory positive pressure acutely impairs LV filling in C‐SCI, potentially via impaired venous return, mediastinal constraint and/or direct ventricular interaction subsequent to dynamic hyperinflation. Inspiratory loading did not significantly improve LV function in C‐SCI and neither inspiratory nor expiratory loading affected cardiac function or lung volumes in able‐bodied participants. Key points Cervical spinal cord injury (C‐SCI) alters both the cardiac and the respiratory system, but little is known about how these systems interact following injury. Here, we manipulated inspiratory or expiratory intrathoracic pressure (ITP) to mechanistically test the role of the respiratory pump in circulatory function in highly trained individuals with C‐SCI and an able‐bodied reference group. In individuals with C‐SCI, greater ITP during expiratory loading caused dynamic hyperinflation that was associated with impaired left‐ventricular filling. More negative ITP during inspiratory loading did not significantly alter left‐ventricular volumes in either group. Interventions that prevent dynamic hyperinflation and/or enhance the ability to generate expiratory pressures may help preserve left‐ventricular filling in individuals with C‐SCI.

Breathing new life into the respiratory pump

“…That −20 cmH 2 0 inspiratory pressure loading had no impact on LV end-diastolic volume in either C-SCI or able-bodied participants suggests that even large inspiratory pressure swings are unlikely to significantly impact cardiac filling/function in the healthy heart while in the 45°head-up tilt position. Gee et al (2022), as well as the research participants, are to be commended for completing this difficult study that included intra-thoracic monitoring with oesophageal balloons and determination of operating lung volumes, all the while maintaining end-tidal carbon dioxide values and obtaining cardiac ultrasound images across five breathing conditions. Expiratory loading led to a reduction in LV end-diastolic volume, and the authors rationalize that the reduced end-diastolic volume is likely explained by an increase in right atrial pressure and a reduced gradient for venous return.…”

“…Surprisingly, unlike what is observed in respiratory patients who also show hyperinflation with exercise, individuals with C‐SCI do not appear to have profound expiratory flow limitation during exercise (Taylor et al., 2010) and maintain an appropriate hyperventilatory response up to peak exercise. In a paper in this issue of The Journal of Physiology , Gee and colleagues (2022) hypothesize that by manipulating the intrathoracic pressure swings and enhancing the respiratory pump, left‐ventricular (LV) filling and cardiac output may be increased in C‐SCI, and that this approach could eventually be used as a potential mechano‐therapeutic target to enhance cardiovascular function during exercise in C‐SCI.…”

“…Using inspiratory and expiratory loading, Gee et al. (2022) examined whether augmenting the respiratory pump at rest in the 45° head‐up tilt position altered LV filling and cardiac output in individuals with C‐SCI, and compared the responses to an able‐bodied reference group. Expiratory loading resulted in hyperinflation and reduced LV end‐diastolic volume in C‐SCI, while no changes were observed in the reference group.…”

“…The paper by Gee and colleagues (2022) is an elegant study that not only seeks to understand cardiopulmonary physiology in an under‐studied group, but also provides unique insight into fundamental physiology.…”

“…Gee et al. (2022), as well as the research participants, are to be commended for completing this difficult study that included intra‐thoracic monitoring with oesophageal balloons and determination of operating lung volumes, all the while maintaining end‐tidal carbon dioxide values and obtaining cardiac ultrasound images across five breathing conditions. Expiratory loading led to a reduction in LV end‐diastolic volume, and the authors rationalize that the reduced end‐diastolic volume is likely explained by an increase in right atrial pressure and a reduced gradient for venous return.…”