Proceedings of The Physiological Society
University of Manchester (2010) Proc Physiol Soc 19, PC23
Differential modulation of respiration in response to percutaneous electrical stimulation delivered during inspiration in man
A. Riza-Khan1, D. A. Green1
1. Applied biomedical Research, King's College London, London, United Kingdom.
Percutaneous electrical stimulation at 100 and 125% of the pre-determined pain threshold (PT) delivered at the onset (200ms) of inspiration (but not expiration), evoked significant shortening of inspiratory time and an increase in tidal volume, thereby elevating mean inspiratory, but not peak flow (Green et al., 2008). Such findings suggest a specific noxious within-a-breath response. We sought to investigate whether differential (dose-dependent) responses are evoked when stimulation varying from 50% (innocuous) to 150% (noxious) of the PT is delivered at 400ms into inspiration - a time when ramp neurons that determine inspiratory magnitude are believed to be active. 14 healthy subjects (5 male and 9 female; mean age 29.5 ± 3.7 yrs) seated in a reclined position with the stimulated leg horizontal, gave written informed consent to participate in the study, which received local research ethical committee approval. The PT was determined by varying delivered current in 5mA steps using a 21 point (0-20) numerical rating scale where, 0 indicated no sensation, 10 the PT, and 20 maximum pain tolerance. Pulse packet (200ms) stimulation (10 pulses; 50µs pulse width; 50Hz) time-locked to inspiration were delivered to the skin overlying the tibia via adhesive surface electrodes connected to a constant current stimulator (Digitimer). Each PT intensity was pseudo-randomly delivered on 8 occasions, hence 88 total stimuli. Breath-by-breath parameters using an oro-nasal mask (Hans Rudolph) were recorded via pneumatachography (Validyne) with a 1401 ADC (200Hz) and Spike2 software (CED). Repeated measures one-way ANOVA and Bonferroni post-hoc t-tests investigated effect of stimulation intensity (SPSS v17). 50-100%PT evoked significant inspiratory and subsequent expiratory time (s) shortening. Inspiratory mask pressure (mmHg) decreased in response to supra (>100%PT) stimulation, whilst subsequent expiratory pressure increased. Peak inspiratory flow (L/s) and tidal volume (L. BTPS) increased with all supra PT stimulation. Peak expiratory flow increased for 100, 110 and 120%PT whilst tidal volume decreased. Both mean inspiratory, and expiratory flow (except 140%) increased in response to supra PT stimuli. The current (mA) required to evoke the PT was significantly (p<0.001) greater POST (270.1 ± 78.0) vs. PRE (262.9 ± 78.4) suggesting sensory habituation. Noxious stimulation appears to negate phase shortening and evoke augmentation of inspiratory flow (and hence volume) despite a fall in mask pressure. In contrast to previous work, subsequent expiratory phase mask pressure/flow augmentation was noted in the absence of significant volume increments. Thus, within-a-breath modulation of breathing appears complex and contingent upon concurrent respiratory neural activity.
Where applicable, experiments conform with Society ethical requirements