This patient is on pressure support ventilation in NIV. The inspiratory trigger is set at 3 l/min and expiratory trigger sensitivity is set at 50% (of the maximal flow). Why is the inspiratory time so long?
Answer: The inspiratory flow shows a change in slope that corresponds with the relaxation of the inspiratory effort. The inspiratory flow remains high and relatively constant, which means there is a large, unintentional leak.
The leak’s flow rate is higher than the set expiratory trigger sensitivity. This prolongs mechanical insufflation until Ti max - kept at the default value of 2 s – is reached, resulting in a long inspiratory time and delayed cycling.
Therefore, the clinician should try to control leaks by adjusting the interface. As leaks are common in NIV, Ti max should always be set to 0.2 s longer than the patient’s inspiratory time, measured without leak. This helps to synchronize the end of the mechanical breath with the end of the patient’s inspiratory effort and thus prevent discomfort due to delayed cycling.
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