Effects of expiratory muscle work on muscle sympathetic nerve activity

Abstract

We hypothesized that contractions of the expiratory muscles carried out to the point of task failure would cause an increase in muscle sympathetic nerve activity (MSNA). We measured MSNA directly in six healthy men during resisted expiration (60% maximal expiratory pressure) leading to task failure with long [breathing frequency (f b) = 15 breaths/min; expiratory time (Te)/total respiratory cycle duration (Tt) = 0.7] and short (f b = 30 breaths/min; Te/Tt = 0.4) Te. Both of these types of expiratory muscle contractions elicited time-dependent increases in MSNA burst frequency that averaged +139 and +239%, respectively, above baseline at end exercise. The increased MSNA coincided with increases in mean arterial pressure (MAP) for both the long-Te (+28 ± 6 mmHg) and short-Te (+22 ± 14 mmHg) trials. Neither MSNA nor MAP changed when the breathing patterns and increased tidal volume of the task failure trials were mimicked without resistance or task failure. Furthermore, very high levels of expiratory motor output (95% maximal expiratory pressure; f b = 12 breaths/min; Te/Tt = 0.35) and high rates of expiratory flow and expiratory muscle shortening without task failure (no resistance; f b = 45 breaths/min; Te/Tt = 0.4; tidal volume = 1.9 × eupnea) had no effect on MSNA or MAP. Within-breath analysis of the short-expiration trials showed augmented MSNA at the onset of and throughout expiration that was consistent with an influence of high levels of central expiratory motor output. Thus high-intensity contractions of expiratory muscles to the point of task failure caused a time-dependent sympathoexcitation; these effects on MSNA were similar in their time dependency to those caused by high-intensity rhythmic contractions of the diaphragm and forearm muscles taken to the point of task failure. The evidence suggests that these effects are mediated primarily via a muscle metaboreflex with a minor, variable contribution from augmented central expiratory motor output.