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Wythenshawe Hospital, Manchester, UK
Correspondence to: Samantha Decalmer, MD, North West Lung Research Centre, Wythenshawe Hospital, Southmoor Rd, Manchester M23 9LT, UK: e-mail: samdecalmer{at}hotmail.com
To the Editor:
It was with great interest that we read the article by Tutuian et al1 in a recent issue of CHEST (August 2006). The authors investigated temporal relationships between non-acid reflux and cough events in patients with persistent cough, utilizing impedance/pH monitoring and subjective recording of cough events by means of a diary and a self-operated digital data logger.
Examining temporal relationships is important since both cough and reflux are frequent events; hence, coexistence alone does not imply causation. Establishing a reflux-cough association within a given time period may help to determine cause and effect, and may allow better patient selection for surgical intervention. However, there are issues we would like to highlight regarding the identification of cough.
First, the authors reported a total median number of coughs of 8 (interquartile range, 3 to 21 coughs) over the entire 24-h monitoring period. This rate is orders of magnitude lower than those reported by Birring et al2 (mean [± SD] cough rate, 43 ± 8 coughs per hour) and those determined in our own department (median rate, 11.06 cough-seconds per hour; range, 1.06 to 46 cough-seconds per hour).3
Second, the use of the subjective reporting of cough correlates poorly with objectively monitored cough sounds,4 particularly at night.5 The symptom index is a ratio of the number of cough events preceded by reflux to the number of cough events during the monitoring period, expressed as a percentage, and is taken to be positive if 
50%. It seems likely that subjective reporting misses cough events, thus altering this ratio and increasing the chance of an apparently positive relationship.
A validated ambulatory monitor, recording individual cough sounds, could be time-synchronized to the impedance/pH trace. This would enable a more accurate determination of the temporal relationship between cough and reflux.
Footnotes
The authors have reported to the ACCP that no significant conflicts of interest exist with any companies/organizations whose products or services may be discussed in this article.
The author has no conflict of interest to disclose.
References
Division of Gastroenterology and Hepatology, University of Zurich, Zurich, Switzerland Division of Gastroenterology and Hepatology, Medical University of South Carolina, Charleston, SC
Correspondence to: Radu Tutuian, MD, Head, Gastrointestinal Functional Unit, University of Zurich, Division of Gastroenterology and Hepatology, Ramistr. 100, A HOF 109, CH-8091 Zurich, Switzerland; e-mail: radu.tutuian{at}usz.ch
To the Editor:
We would like to thank Decalmer and colleagues for their thoughtful comments on our article1 (August 2006), in particular on the importance of having an objective cough detector when evaluating the association between reflux and cough. We also agree with their comment that detecting a higher number of cough episodes would have decreased the chance of finding a positive symptom association using the symptom index. Still, since we eliminate cough episodes that occur during eating periods due to the difficulty of separating true reflux from frequent swallows using impedance, we might have actually underestimated the total number of cough episodes.
In the same issue of CHEST, Smith et al2 reported on the low-to-moderate level of correlation between objective monitoring and subjective reporting of cough episodes in COPD patients. Having an objective cough detector synchronized with the impedance-pH monitor would not only increase the accuracy of detecting cough events but would also allow differentiating cough-reflux from reflux-cough sequences, the latter making a stronger argument for a cause-effect relationship between reflux and cough. The development of the proposed cough sound detector synchronized to the impedance-pH signal is certainly worthwhile pursuing. Such a system would allow detecting the association (and temporal relationship) between two episodes (cough and reflux) and not only between a reflux episodes and a cough event (subjectively recorded by the patient).
Other investigators have proposed using other cough detectors in evaluating the relationship between reflux and cough. Intraesophageal and intragastric pressure transducers to document cough (as a sudden rise in pressure on both sides of the diaphragm) in combination with impedance-pH monitoring has been previously used by Sifrim et al3 in a group of chronic cough patients who were not receiving acid suppressive medications. In this study,3 the authors noticed similar proportions of reflux-cough and cough-reflux sequences in the instances when a patient-reported cough episode was preceded by reflux within a 2-min time window.
In clinical practice, we regard objective cough detectors as adjuncts to patient diaries. Drawing a parallel to the observation that not all (impedance-) pH-detected gastroesophageal reflux episodes induce symptoms, we dare assuming that patients will not perceive all cough episodes. Having the patient marking a cough event whenever she/he noticed a cough episode helps identifying which cough episodes are symptomatic.
References
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