Measuring voluntary and reflexive cough strength in healthy individuals.
Thesis DisciplineSpeech and Language Sciences
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
Introduction: Several studies have evaluated citric acid cough reflex in healthy individuals and neurologically impaired patients. These studies have been instrumental in providing evidence for its use as a validated tool in bedside swallowing evaluations, enabling the identification of patients at risk of silent aspiration. However, inter- and intra-rater reliability for perceptual measurements of strength of coughing is sub-optimal and there are no established objective methods for measuring strength of reflexive coughing. The aim of this study was to objectively evaluate voluntary and reflexive cough strength in healthy individuals. Methods: Fifty-four healthy individuals, aged 50 years and over, participated in this study. Participants performed ‘strong’ and ‘weak’ voluntary coughs and underwent suppressed cough reflex testing using the face-mask method and incremental doses of citric acid at doses of 0.4, 0.8, 1.2 and 1.8 Mol/L. Peak and area under the curve (AUC) measurements were taken for pressure, airflow, and acoustics. Twenty-nine sets of data, where participants produced a C2 response to three doses of citric acid, were included for final analysis. Results: Repeated-measures ANOVA revealed a significant trial effect for reflexive cough, with the second cough in the C2 sequence being smaller than the first, for all 6 measures (pressure: peak p < .01, AUC p < .01; flow: peak p < .01, AUC p < .01; acoustic: peak p = .04 and AUC p < .01). However, there was no significant effect of citric acid dose on strength of reflexive coughing (pressure: peak p = .65, AUC p =.86; flow: peak p = .95, AUC p = .10; acoustic: peak p = .93, AUC p = .93). Repeated-measures ANOVA also revealed a significant effect of type of voluntary cough, with strong coughs having greater values than weak coughs for all 6 measures (p < .01). There was also a significant trial effect of voluntary cough in the C2 sequence, with the second cough being weaker than the first for 4 of the measures (p < .01 for peak and AUC pressure; p < .01 for peak flow and AUC acoustic). Paired t-tests demonstrated that strong voluntary coughs were stronger than reflexive coughs for all measures (p <.01). Conclusions: This research suggests that all six outcome measures were sensitive to measuring changes in coughing strength. The significant difference between strong voluntary coughs and reflexive coughs supports existing research suggesting that assessment of voluntary coughing strength does not provide accurate information about the strength of the reflexive coughing. There was an effect of cough sequence, cough 1 or 2, for both voluntary and reflexive coughing. This might indicate that the first reflexive cough is the most important in terms of airway protection and clearance. There was no effect of concentration of citric acid on strength of suppressed coughing: once the reflex was elicited, coughing strength was stable regardless of stimulus dose. This suggests that either cortical augmentation of voluntary coughing produces a response much greater than the reflexive airway protective response, or that cortical control cannot suppress sensitivity of coughing, but can suppress strength of coughing, at increasing concentrations of citric acid. Alternatively, it may imply that a suppressed cough reflex is an all-or-nothing response, but there is potential that a true reflexive cough, stimulated by aspiration, would show a dose-response effect.