Influence of breathholding on forced expiratory flow
When inhaling to the level of the TLC lungs and airways are stretched; if the distending pressure is maintained the tissues give way somewhat, a phenomenon called stress relaxation. A rapid inspiration leads to higher effective elastic lung recoil, unlike a slow inhalation followed by a few seconds pause at TLC, a maneuver that causes stress relaxation. A different time course of inspiration and differences in airway and lung hysteresis affect forced expiratory flows. This phenomenon is most pronounced in patients with airway obstruction. On that account their peak expiratory flow, but also other expiratory flows and the FEV1, may be appreciably larger if patients exhale forcibly immediately after completing a rapid deep breath, than if they pause for a few seconds at the level of TLC. It is best to standardize for this phenomenon by instructing the patient to start the FVC maneuver without a pause at TLC level.
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