Educational aims
The reader will come to appreciate the:
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Excellent correlation between peak cough flow [PCF] and peak expiratory flow derived from using the same spirometer.
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Benefits of repeated testing to achieve reproducible spirometry to better inform and guide clinical care.
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PCF testing can be utilised for children where performing technically acceptable spirometry is not possible.
Abstract
Spirometry and peak cough flow testing (PCF) are commonly used in the respiratory assessment of children with a neuromuscular disorder (NMD). Testing uses two different machines, increases laboratory time, costs and resource utilisation. No studies have assessed the correlation between peak expiratory flow (PEF) obtained from spirometry and PCF in children with NMD using one device. An audit of children with a NMD managed at the Children’s Hospital at Westmead in 2022–2024 aged < 20 years who performed spirometry and PCF testing on the same device (Vyaire Body Box TM , Ultrasonic flow meter-based, or Vyaire Pneumotachograph TM , Pneumotach flow meter-based; Germany) was conducted to assess the correlation between PCF and PEF. Fifty-one sets of testing were identified, and 40 subjects (9F) had reproducible testing and were included. Median (range) age was 14.95 (7.20–19.00) years. Median PEF (L/min) was 4.05 (1.22–10.26) and median PCF (L/min) was 4.29 (1.69–10.82). PEF and PCF had a strong Pearson’s correlation coefficient, (R = 0.97, p = 0.03). The coefficient of determination was 0.93. If laboratory resources permit, spirometry should be the test of choice for children with NMD. On average, spirometry required multiple practices to achieve reproducibility to meet ATS/ERS standards. PCF testing can be utilised for children where performing technically acceptable spirometry is not possible.
Introduction
Regular, ongoing pulmonary function testing (PFT) is critical for children with neuromuscular diseases (NMD) to monitor lung function in the light of respiratory muscle strength, restrictive lung disease and progression of thoracic scoliosis. There are many different NMDs, all with different patterns of disease progression and severity . Respiratory muscle weakness is one of the leading causes of mortality in NMDs . Respiratory muscles can be divided into inspiratory and expiratory muscles and assessed separately. The diaphragm and external intercostal muscles are the main inspiratory muscles, and the internal intercostals and the abdominal muscles are the expiratory muscles. Peak cough flow (PCF) is the maximum flow rate produced from a cough manoeuvre . PCF testing assesses cough strength and is therefore a measure of respiratory muscle strength. It can be used to determine if cough assistance is required . Peak flow values < 160 L/min are suggestive of an ineffective cough . Spirometry assesses lung function and is recognised for its longitudinal accuracy and reliability . A recommendation from the British Thoracic Society for respiratory management of children with NMD is that all children able to perform spirometry correctly, should measure their vital capacity (VC) as part of their patient management . Peak expiratory flow (PEF) is the maximum flow rate produced during a forced manoeuvre from total lung capacity (TLC), which can be measured while performing spirometry. See ( Table 1 ).
| Neuromuscular Diagnosis | Total Number (%) |
|---|---|
| Congenital Myopathy | 4 (10.0) |
| Duchenne Muscular Dystrophy | 13 (32.5) |
| Emery-Dreifuss Muscular Dystrophy | 2 (5.0) |
| LMNA Muscular Dystrophy | 2 (5.0) |
| Merosin Deficient Muscular Dystrophy | 5 (12.5) |
| Myotonic Dystrophy | 1 (2.5) |
| Nemaline Myopathy | 2 (5.0) |
| SMA 2 | 6 (15.0) |
| Ullrich’s Muscular Dystrophy | 5 (12.5) |
Previous studies have reported the coefficient of determination between PCF and PEF in children with a NMD to be modest (0.53 and 0.65 . Both studies measured PCF and PEF on different devices. Children with an ineffective cough have higher PCF values when recorded on a pneumotachograph in comparison to a portable peak flow meter . The aim of this study was to investigate the correlation between PCF and PEF in children with NMD using the same device. If PCF and PEF are demonstrated to be similar on the same device, busy laboratories can offer clinically more relevant information using spirometry in children with neuromuscular conditions in a time efficient manner and at less cost.
Methods
Participants less than 20 years of age with a confirmed neuromuscular disease diagnosis were included in this study.
Patients’ standing height was measured, or an estimation was obtained by measuring the ulna length for non-ambulant participants . Spirometry was performed first, followed by peak cough. Spirometry was performed in accordance with the 2019 Standardisation of Spirometry Update by the American Thoracic Society (ATS) and the European Respiratory Society (ERS) . There are no technical standards for performing PCF testing. Unassisted PCF was performed from total lung capacity (TLC). The highest PCF value was recorded from a minimum of 3 repeatable tests. All trials were performed on laboratory equipment (Vyaire Body Box TM , Ultrasonic flow meter-based, or Vyaire Pneumotachograh TM , Pneumotach flow meter-based; Germany). Both spirometry and PCF were performed on the same device as two separate manoeuvres. Participants were required to have acceptable spirometry reported as Grade A, B or U according to the ATS/ERS guidelines . Spirometry gradings were crosschecked by two respiratory scientists.
Statistical analysis was performed using SPSS statistical software (version 25) to generate a correlation coefficient value, scatter graph and Bland-Altman Plot. The Shapiro-Wilk test was performed to determine the distribution of data, due to the small sample size.
Sydney Children’s Hospital Network Ethics committee approval for the audit was obtained (2024/ETH00516).
Results
There were 51 sets of data (9F) and after review, 40 participants (9F) had acceptable results for analysis [ Fig. 1 ]. The 11 excluded participants (0F) had a median (range) age of 12.00 (4.70–17.60) years. The 8 subjects with unsatisfactory spirometry data included the following conditions and ages at testing: Myotonic dystrophy aged 17, DMD aged 11, DMD aged 12, DMD aged 16, DMD aged 10, DMD aged 15 and DMD aged 14. The 3 subjects with inadequate PCF performance included: DMD aged 14, SMA 1 aged 8 and DMD aged 10. Median (range) age of the 40 participants was 14.95 (7.20–19.00) years [ Table 2 ].
| Variable | Median (range) |
|---|---|
| Age | 14.95 (7.20–19.00) |
| FEV 1 (L/min) | 1.69 (0.40–4.45) |
| FEV 1 z-score | −2.72 (-6.63–1.41) |
| FVC (L/min) | 1.89 (0.40–5.02) |
| FVC z-score | −3.10 (-8.03–1.20) |
| PEF (L/min) | 4.05 (1.22–10.26) |
| PCF (L/min) | 4.29 (1.69–10.82) |
The formula for average difference was calculated as:
Difference=(PEF-PCF)PEF
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