Dyspnoea is defined as “a subjective experience of breathing discomfort that consists of qualitatively distinct sensations that vary in intensity” 5,6. BPD was determined by either Physiotherapist assessment or by using The Brompton Breathing Pattern Assessment Tool (BPAT). A BPAT > 4 was seen as positive7.
26 subjects (13 ID, 13 BPD) underwent cardiopulmonary exercise test as part of routine clinical care. Table 1 shows the primary diagnoses for the ID cohort.
This study was approved by the Respiratory Medicine Research Committee at St Barts NHS Trust.
Table 1: Idiopathic dyspnoea primary diagnoses
|
|
Primary Diagnosis
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Number of Subjects
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Asthma
|
1
|
Chronic Cough
|
1
|
Gastro-Oesophageal Reflux Disease
|
2
|
Obstructive Sleep Apnoea
|
1
|
Pulmonary Embolism
|
3
|
Shortness Of Breath On Exertion
|
2
|
Unexplained Breathlessness
|
3
|
|
|
Time-domain variability analysis of data was applied retrospectively. Root mean square successive difference (RMSSD) (Figure 1) is widely used in cardiology to quantify heart-rate variability and is appropriate for this subject group 8,9,10.
Study Protocol
This was a retrospective, single centre study, all subjects performed a maximal cardiopulmonary exercise test using an electrically-braked cycle ergometer (VIAsprint 150 cycle ergometer, Vyaire Medical Products Ltd, Basingstoke, UK) following an individualised incremental ramp protocol up to exhaustion. All tests achieved at least two of the following criteria to be considered maximum: Post-exercise blood lactate levels ≥8 mmol·L−1, VEpeak / MVV % ≥85, or a respiratory exchange ratio ≥ 1.05.6
Cardiopulmonary exercise tests were performed with gas exchange and ventilatory variables being analysed breath-by-breath using a calibrated computer-based exercise system (Vyntus CPX Metabolic Cart, Vyaire Medical Products Ltd, Basingstoke, UK). The O2 and CO2 analysers were calibrated before each test using a reference gas (12% O2; 5% CO2; nitrogen balance). The pneumotachograph used was also calibrated, with an automatic differential flow profile.
During each cardiopulmonary exercise test, a 12-lead electrocardiogram was continuously recorded (Vyntus ECG 12-lead PC-ECG) and heart rate automatically derived. Heart Rate (HR), Breathing Frequency (BF), Minute Ventilation (VE) Oxygen Uptake (VO2) and ventilatory parameters of Inspiratory Time/Total Inspiratory and Expiratory Time Ratio (ti/ttot), Inspiratory And Expiratory Time Ratio (ti/te), Inspiratory Tidal Volume Time Ratio (VTin/ti) and Breathing Frequency Tidal Volume Ratio (BF/VTex) were measured by breath-by-breath.
All breath-by-breath results were exported to an Excel spreadsheet (Microsoft Corporation, USA), where root mean square successive difference (RMSSD) of minute ventilation during exercise test was calculated for each subject. Data was normalised to reduce the risk of observation bias.
We hypothesised that time-domain ventilatory variability during exercise would be greater in Idiopathic Dyspnoea subjects than in Dysfunctional Breathing subjects due to a lower ventilatory threshold14. Therefore the present study was designed to compare time-domain minute-ventilation variability of these groups during a progressive maximal exercise test.
Statistical Analysis
Statistical analysis was performed using the software Graphpad 7.0 (Graphpad Software Inc, USA). Variables from the cardiopulmonary exercise tests showed normal distribution when analysed by the Shapiro Wilk’s test. Exercise variables in both groups were compared by paired two-tailed Student T test. Significance was set at p < 0.05. Results are presented as mean ± standard deviation. Based on similar studies a sample size of 26 individuals (13 in each group) would be needed to provide an >80% power with a 2-sided alpha of 0.05 7.