Dyspnoea, Hyperventilation and Functional Cough: a Guide to Which Tests Help Sort Them Out

Andrew Robson [email protected] @CoughingAndyR NHS Lothian Respiratory Physiology Service, Western General Hospital, Edinburgh, UK. Dyspnoea, hyperventilation and functional cough: a guide to which tests help sort them out Cite as: Robson A. Dyspnoea, hyperventilation and Physiology masterclass functional cough: which tests help to sort them out? Breathe 2017; 13: 45–50. Dyspnoea is a multifactorial symptom, defined been investigated and eliminated, a potential by the American Thoracic Society as “a subjective functional diagnosis can be considered. experience of breathing discomfort that consists Much of the published literature surrounding of qualitatively distinct sensations that vary in the investigation of either acute or chronic intensity” [1]. Dyspnoea is caused by a wide range hyperventilation has centred on patients with of conditions, ranging from asthma to pulmonary anxiety, depression and panic disorder but embolism, and including such nonrespiratory patients with a pre-existing respiratory condition factors such as diabetic ketoacidosis. A patient with may also experience periods of acute or chronic asthma who is aware of an increase in the muscular hyperventilation. Although most published data work of breathing during an attack, another patient refer to asthma [2], other conditions such as COPD with a pneumothorax experiencing increased and pulmonary fibrosis can also be associated with efferent nerve stimulation from pulmonary hyperventilation. stretch receptors and a third patient with type 1 This article will briefly describe methods respiratory failure would all describe themselves that can be used to investigate dyspnoea in a as “breathless”, but for very different physiological newly referred patient. Methods of assessing reasons. Identifying the exact source of the patient’s functional breathlessness and cough when other symptoms can be a lengthy process. investigations have been found to be normal, or Cough and breathlessness are two of the most where the degree of symptoms reported by the commonly reported respiratory symptoms with patient is out of proportion to any abnormalities many possible causes that may require significant found, will also be discussed. No mention will be investigation before the cause is identified. After made of the treatment of either functional cough potential organic diseases such as asthma, chronic or breathlessness. obstructive pulmonary disease (COPD), interstitial It is assumed that readers will have access lung disease and pulmonary vascular disease have to a clinical respiratory physiology laboratory @ ERSpublications Methods that can be used to investigate dyspnoea in a newly referred patient http://ow.ly/EsQW307U9hd http://doi.org/10.1183/20734735.019716 Breathe | March 2017 | Volume 13 | No 1 45 Dyspnoea, hyperventilation and functional cough that is equipped to perform spirometry, make immediately after exercise, then tests to investigate measurements of transfer factor and lung exercise-induced asthma should be considered. volumes, and carry out exercise tests, preferably a The patient should have spirometry performed cardiopulmonary exercise test. before exercise and then at regular intervals (usually every 5 min) for 20–30 min after the completion of exercise, with the first post-exercise measurements Investigation of dyspnoea made as soon as possible after completion. Any readers requiring further information on Before a diagnosis of functional breathlessness can the use of laboratory tests in the investigation be considered, other potential causes of dyspnoea of dyspnoea are referred to the publications by must be investigated and either excluded or Hughes [3] and Gibson [4] who explore this topic quantified. Spirometry can identify patients with in more detail. obstructive or restrictive disorders and assessing a patient’s response to a short-acting β2-agonist, such as terbutaline or salbutamol, can help to identify Investigation of functional patients with asthma. Flow–volume loops can also cough help to identify the location of airflow obstruction (intrathoracic or extrathoracic). Functional cough is one of the most problematic Although airflow obstruction can be identified symptoms to investigate in a general respiratory with confidence from spirometry, a restrictive clinic. The American College of Chest Physicians defect is best identified from measurement of produced guidelines in 2006 based on a literature lung volumes, either by inert gas dilution or body review of the diagnosis and management of plethysmography. Measurement of lung volumes functional cough [5]. This guideline was updated can also identify significant air trapping, which may in 2015 [6] but both papers highlighted the lack of be found in patients with more severe degrees of research in this area regarding the diagnosis and airflow obstruction. Measurements of carbon management of functional cough. monoxide transfer factor are an effective (and The European Respiratory Society produced an noninvasive) method of assessing the efficiency of excellent guideline outlining the detailed assessment the pulmonary circulation, although an abnormally of cough via specific cough challenges [7]. A cough low result does not help to identify the possible challenge can identify patients with an exceptionally cause of the impairment. sensitive cough reflex, which should be absent in a When a patient complains of dyspnoea on patient with a purely functional cough. exercise, some form of field exercise test should be As not all centres may have the facilities to carry carried out to investigate. In many patients, a walking out specific cough challenges (using citric acid or test such as a 6-min distance test or a shuttle walking hypertonic saline, for example) a suitable alternative test is the most appropriate test to use. Where a may be a bronchial challenge test, as mentioned more intense workload is required to replicate the above. All three challenge agents are known to conditions when the patient becomes symptomatic, provoke cough as well as bronchoconstriction a bicycle exercise test may be more appropriate. in patients and repeated bronchial challenges In all field exercise tests, the main factors to be could be performed in a patient to assess the investigated are oxygen desaturation and heart rate effectiveness of therapeutic interventions on a response. Any patient showing clinically significant patient’s cough. This type of investigation should desaturation (traditionally defined as a fall in oxygen be considered inferior to a formal cough challenge. saturation measured by pulse oximetry 3%) > Ambulatory cough monitors have become more requires further investigation to identify the reason common in clinical practice and they can provide for the desaturation. information on the frequency of cough, but are less Additional information can be obtained if valuable in assessing the intensity of the cough [8]. patients are asked to rate their breathlessness Cough monitoring may be useful to identify if there at rest and at the end of exercise, using either a is a periodicity to the cough or any environmental Borg score or visual analogue scale. Information triggers. Questionnaires investigating the impact on the patient’s perception of breathlessness can of cough on quality of life have been developed and be especially useful in serial exercise tests, where can provide insight into a patient’s perception of a patient reports an increase in symptoms but their cough but they are not intended to specifically with no reduction in exercise capacity or increase identify patients with functional cough [9]. in desaturation. When a patient complains of cough and dyspnoea, one potential cause could be asthma. Tests of bronchial hyperreactivity, using challenge agents such Investigation of as histamine, methacholine or mannitol, can identify hyperventilation patients with an enhanced bronchoconstrictor response, which is one of the hallmarks of asthma. An average adult at rest has a respiratory rate of If the patient only experiences symptoms during or 8–14 breaths per minute and a mean tidal volume 46 Breathe | March 2017 | Volume 13 | No 1 Dyspnoea, hyperventilation and functional cough of 500 mL per breath. This maintains the arterial Many patients who have a confirmed diagnosis carbon dioxide tension (PaCO2) between 4.6 and of asthma can score highly on the Nijmegen 6.0 kPa. Hyperventilation is traditionally defined as questionnaire [2, 14], which may relate to poor breathing more than the metabolic requirements asthma control. While this reduces the utility of of the body [10] and could be further defined as an the questionnaire, it is still a very useful way of increase in alveolar ventilation that is more than the finding out about the frequency of symptoms the level required to maintain blood gas homeostasis, patient experiences, but this must only be part of the resulting in a fall in PaCO2 and the development investigation process rather than the whole content. of respiratory alkalosis. Hyperventilation is not synonymous with tachypnoea, as patients may have an increased respiratory rate but a low tidal The hyperventilation volume, thus maintaining a normal PaCO2. provocation test The mechanisms behind hyperventilation are Investigation of hyperventilation requires some controversial, often considered to either be the method of recording respiratory rate and ETCO . result of anxiety or hypochondria [10], or due to a 2 In a patient with normal lung function, ETCO resetting of

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