Citric acid cough reflex test: Establishing normative data

Abstract

The citric acid cough reflex test, or inhalation cough challenge, offers potential for improving clinical detection of silent aspiration in dysphagia assessment. For any new technique to be accepted as a clinically relevant tool, established validity, reliability, and normative data are requisite. This study presents a normative data set for the citric acid cough reflex test using a facemask method. Data were collected from 160 healthy participants, equally represented by gender and in two age groups (under 60 years of age and over 60 years of age). They passively inhaled, via facemask, three trials of nebulized citric acid of 10 different concentrations: study A ranging from 0.8 to 2.6 mol/l and study B ranging from 0.1 to 1.2 mol/l, with placebo interspersed. Natural cough thresholds (NCT) and suppressed cough thresholds (SCTs) were reached when subjects produced two successive coughs on at least two out of three trials. While 5% of participants failed to trigger a cough at any concentration, of those that did, the majority of participants (89.5%) reached NCT by 0.8 mol/l, with 95.5% demonstrating SCT by 1.2 mo/l. In study B, using lower concentrations of citric acid, there were significant differences between genders for both NCT (P = 0.001) and SCT (P = 0.004) that were not apparent in study A given the flooring effect. While no difference was found between age groups for NCT (P = 0.361), in Study A, which tested higher concentrations of citric acid, a significant difference between age groups in cough suppression was revealed (P = 0.009) with youngers being able to suppress cough at higher levels.

Measuring voluntary and reflexive cough strength in healthy individuals

“…This outcome might be explained by the protocol of assessing suppressed cough rather than natural cough. It is postulated that suppressed cough is a closer approximation to true reflexive cough with no cortical involvement, as the individual can no longer voluntarily control their response to citric acid (Monroe et al, 2014). However, these findings suggest that although the sensitivity of suppressed coughing is truly reflexive, the individual may retain cortical control of the strength of true reflexive coughing and that in trying not to cough, strength is stifled.…”

“…However, these figures excluded any participants who did not respond to citric acid at any dose, therefore if these numbers were not excluded the percentages would have been 85% and 65% respectively (Monroe et al, 2014). This indicates that some patients may have a pre-morbid disposition to lower cough sensitivity, and that the rate of this is considerably higher when assessing suppressed coughing.…”

“…In order to offset this 'placebo effect', CRT can incorporate the assessment of suppressed coughing, where individuals are instructed 'if you feel the need to cough, try to suppress it'. The rationale for using suppressed coughing as well as, or in lieu of, natural coughing is that there is increased likelihood that any coughs produced are truly reflexive as the person is no longer able to cortically suppress the response (Monroe et al, 2014). The potential drawback of suppressed coughing is that patients with cognitive or communication impairments may not be able to follow instructions to suppress their cough.…”

“…This indicates that either cortical inhibition of RC is possible, or alternatively, that a true RC has not been initiated at all. The potential exists for patients undergoing natural CRT to cough voluntarily -rather than reflexively -during the assessment, because they are aware they are undergoing a cough test (Monroe, Manco, Bennett, & Huckabee, 2014). In order to offset this 'placebo effect', CRT can incorporate the assessment of suppressed coughing, where individuals are instructed 'if you feel the need to cough, try to suppress it'.…”

“…Recent research by Monroe et al (2014) evaluated natural cough threshold and suppressed cough threshold in healthy individuals. They found that 65% of participants triggered a suppressed C2 response by a citric acid dose of 0.8 Mol/L.…”