Expiratory flow‐volume curves in mechanically ventilated patients with chronic obstructive pulmonary disease

Abstract

This study indicates that relaxed expiratory flow-volume curves can be used to assess airflow obstruction in mechanically ventilated patients with COPD. This information can be used to adapt ventilatory settings.

Expiratory Time Constant and Sleep Apnea Severity in the Overlap Syndrome

“…24 The time constant derived from flow decay ideally equals the product of resistance and compliance, similar to the behavior of an electrical RC circuit (resistance and capacitance in series). 2,14,16 While the expiratory pressure decay over time had an excellent exponential fit (R 2 ≥ 0.95) in our data, our assumptions of proportionality of nasal pressure to flow, and of a single compartment model, suggest that our model can best provide an effective time constant rather than one that accurately reflects the product of resistance and compliance. 14 This effective time constant nevertheless reflects lung mechanics given its association with severe sleep apnea.…”

“…2,14,16 While the expiratory pressure decay over time had an excellent exponential fit (R 2 ≥ 0.95) in our data, our assumptions of proportionality of nasal pressure to flow, and of a single compartment model, suggest that our model can best provide an effective time constant rather than one that accurately reflects the product of resistance and compliance. 14 This effective time constant nevertheless reflects lung mechanics given its association with severe sleep apnea. For example, using published estimates for the coefficients of the Rorher model quadratic relationship between pressure and flow during expiration, our technique can be assessed to reflect the product of resistance and compliance but underestimates it by a factor of 1.8.…”

“…22,23 Notably, a single compartment cannot accurately describe passive expiration in intubated and paralyzed patients. 25 However, our patients were in a stable state in the outpatient setting, as opposed to intubated under conditions of respiratory failure, 14,23 or paralysis. 24 The time constant derived from flow decay ideally equals the product of resistance and compliance, similar to the behavior of an electrical RC circuit (resistance and capacitance in series).…”

“…14,22,23 This model consists of a single flow unit with a constant resistance, in series with a single elastic unit of constant compliance. 16 However, there are significant differences in the time constant when it is measured at different lung volumes in subjects with lung disease.…”

“…For example, two groups found a logarithmic correlation between the FEV 1 and the time constant in patients with obstructive lung disease, and the time constant can differentiate between subjects with and those without COPD. 2,14 Moreover, the time constant can be used to accurately calculate the time needed for complete expiration (estimated as being 3 times the time constant and used as an indicator of the severity of COPD) as demonstrated in mechanically ventilated subjects with COPD. 23 Our technique lacks enough sensitivity to detect changes in the time constant with severity of FEV 1 as was shown in other studies.…”