Work of breathing influences muscle sympathetic nerve activity during semi-recumbent cycle exercise.

UNLABELLED

Reducing the work of breathing during exercise improves locomotor muscle blood flow and reduces diaphragm and locomotor muscle fatigue and is thought to be the result of a sympathetically mediated reflex.

AIM

The aim of this study was to assess muscle sympathetic nerve activity (MSNA) when the work of breathing is experimentally lowered during dynamic exercise.

METHODS

Healthy subjects (n = 12; age = 29 ± 9 years) performed semi-recumbent cycling trials at 40%, 60%, and 80% of peak workload. Exercise trials consisted of spontaneous breathing, reduced work of breathing (proportional assist ventilator), followed by further spontaneous breathing (post-ventilator). MSNA was recorded from the median nerve.

RESULTS

There was no difference in work of breathing between PAV and post-PAV at 40% peak work. At 60% peak work, the ventilator significantly (P < 0.05) reduced work of breathing (103 ± 39 vs 144 ± 47 J min ), sympathetic nerve activity (35 ± 5 vs 42 ± 8 burst min ), and (2.4 ± 0.5 vs 2.6 ± 0.5 L min ) without influencing ventilation (86 ± 9 vs 82 ± 10 L min ; P > 0.05), for PAV and post-PAV respectively. During 80% peak work (n = 8), the ventilator significantly (P < 0.05) reduced work of breathing (235 ± 110 vs. 361 ± 150 J min ), MSNA (48 ± 7 vs 54 ± 11 burst min ), and (2.9 ± 0.6 vs 3.2 ± 0.7 L min ) but not ventilation (121 ± 20 vs 123 ± 20 L min ; P > 0.05), for PAV and post-PAV respectively. There was a significant relationship between MSNA and (P < 0.0001) with a significant interaction due to the ventilator (P < 0.05).

CONCLUSION

Lowering the normally occurring work of breathing during exercise results in commensurate reductions in MSNA. Our findings provide evidence of a sympathetically mediated vasoconstrictor effect emanating from respiratory muscles during exercise.