Differences in motor activation of voluntary and reflex cough in humans

Dan Lasserson ¹, Kerry Mills ¹, Ramamurthy Arunachalam ¹, Michael Polkey ², John Moxham ¹ and Lalit Kalra ^1*

¹ King's College London School of Medicine, United Kingdom
² Royal Brompton Hospitals NHS Trust, United Kingdom

^* To whom correspondence should be addressed. E-mail: lalit.kalra{at}kcl.ac.uk.

Accepted 12 March 2006

Abstract

Objectives: To study motor activation patterns of voluntary and reflex cough adjusted for cough flow rates.

Methods: Surface electromyogram (EMG) and cough flow rate (CFR) were measured in 10 healthy volunteers. Voluntary cough (VC) was assessed for 20 efforts in each quintile of increasing cough strength. Reflex cough (RC) was assessed for 25 efforts produced by nebulised L-tartaric acid. EMG was recorded over the expiratory (rectus abdominis, obliques, lower intercostals) and accessory (trapezius, pectoralis major, deltoid, latissimus dorsi) muscles. EMG activity, burst duration and onset were compared for each quintile of VC, and between VC and RC matched for CFR.

Results: EMG activity and burst duration of expiratory and accessory muscles during VC increased in proportion to cough strength. Expiratory muscles had longer EMG burst duration (difference 68 [95%C.I. 34 to 102] ms, p<0.01) and earlier onset of EMG activity (difference 44 [95%C.I. 20 to 68] ms, p<0.0001) compared with accessory muscles. EMG activity in all muscles was increased (mean 110.2% v 56.1%, p<0.001) and burst duration (mean 206 v 280 ms, p=0.013) decreased in RC compared with VC of equal intensity. There were no differences in EMG onset (difference 8 [95% C.I. 25 to 9] ms) or burst duration (difference 27 [95% C.I. 58 to 4] ms) between expiratory and accessory muscles.

Interpretation: Functional organisation of motor activity differs between voluntary and reflex cough. VC is characterised by graded and coordinated activation whereas RC by early and simultaneous activation of expiratory and accessory muscles.

Keywords: cough, cough flow rates, EMG, motor activation, respiratory muscles

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