Measuring lung mechanics of expiratory tidal breathing with non-invasive breath occlusion

Background and objective: Lung mechanics measurements provide clinically useful information about disease progression and lung health. Currently, there are no commonly practiced methods to non-invasively measure both resistive and elastic lung mechanics during tidal breathing, preventing the important information provided by lung mechanics from being utilised. This study presents a novel method to easily assess lung mechanics of spontaneously breathing subjects using a dynamic elastance, single-compartment lung model. Methods: A spirometer with a built-in shutter was used to occlude expiration during tidal breathing, creating exponentially decaying flow when the shutter re-opened. The lung mechanics measured were respiratory system elastance and resistance, separated from the exponentially decaying flow, and interrupter resistance calculated at shutter closure. Progressively increasing resistance was added to the spirometer mouthpiece to simulate upper airway obstruction. The lung mechanics of 17 healthy subjects were successfully measured through spirometry. Results: N = 17 (8 female, 9 male) healthy subjects were recruited. Measured decay rates ranged from 5 to 42/s, subjects with large variation of decay rates showed higher muscular breathing effort. Lung elastance measurements ranged from 3.9 to 21.2 cmH -2 2 O/L, with no clear trend between change in elastance and added resistance. Resistance calculated from decay rate and elastance ranged from 0.15 to 1.95 cmH -2 2 Os/L. These very small resistance values are due to the airflow measured originating from low-resistance areas in the centre of airways. Occlusion resistance measurements were as expected for healthy subjects, and increased as expected as resistance was added. Conclusions: This test was able to identify reasonable dynamic lung elastance and occlusion resistance values, providing new insight into expiratory breathing effort. Clinically, this lung function test could impact current practice. It does not require high levels of cooperation from the subject, allowing a wider cohort of patients to be assessed more easily. Additionally, this test can be simply implemented in a small standalone device, or with standard lung function testing equipment

Non-invasive measurement of tidal breathing lung mechanics using expiratory occlusion

A great amount of research looks at whether information about lung mechanics can be obtained using spirometry, as these mechanics give clinically useful information about lung condition and disease progression. This study uses a time-varying elastance, single compartment lung model to calculate lung mechanics of 15 tidally breathing healthy subjects. A plethysmograph with a built-in shutter was used to induce an exponentially decaying airflow. Lung elastance and respiratory system resistance were separated from the decay rate of flow caused by the shutter. Occlusion resistance was calculated at shutter closure. To simulate upper airway obstruction, progressively larger resistances were added to the plethysmograph mouthpiece. Decay rates measured ranged from 5-42, with large intra-subject variation associated with muscular breathing effort. Measured lung elastance ranged from 3.9-21.2 cmH2O/L and often remained constant as resistance was increased. Resistance calculated from the decay rate was very small, ranging from 0.15-1.95 cmH2Os/L. The low resistance is due to the airflow measured originating from low resistance areas in the centre of airways. Occlusion resistance measurements were as expected for healthy subjects, and followed the expected resistance trend as resistance was increased

Effect of small airways and viscoelasticity on lung mechanics from expiratory occlusion

Monitoring the decay rate of airflow in spirometry may be clinically useful. The decay rate is expected to represent a combination of lung elastance and airway resistance. However, the decay rate calculated using the single compartment lung model is not expected to account for slower lung mechanics, such as small airways resistance and tissue viscoelasticity. This study assesses whether the decay rate is affected by these lung mechanics. An exponentially decaying flow was created using a shutter to occlude airflow during passive expiration for 15 healthy subjects. To approximate small airways resistance and viscoelasticity, the gradient of pressure increase (relaxation gradient) during shutter closure was measured. The occlusion resistance, elastance, and decay rate were also calculated for these breaths. None of these mechanics were found to be correlated with the relaxation gradient. The relaxation gradient was also found to be independent of driving pressure. Conversely, the relaxation gradient was found to depend on lung volume. The results of this study suggest using lung mechanics and decay rate to monitor changes in lung condition over time may miss information about changes in the small airways and viscoelastic lung tissue. Thus, it is useful for monitoring large airways disease, but may be ineffective for small airways disease such as ARDS