Respiratory time constant

The lung and thorax should return to their resting position at the end of expiration or alveolar pressure will be higher than end-expiratory pressure; this difference is termed 'auto-PEEP'. This increases the risk of ventilator-induced lung injury ( M.aicy a.D.d M.aiiQ! 1991) and may make triggering of the ventilator more difficult (Slutsky.

1994). The time taken for an alveolus to empty is reflected in the 'respiratory time constant', which is equal to resistance multiplied by compliance and is measured in seconds. An increase in resistance (e.g. chronic obstructive pulmonary disease or acute asthma) or compliance (e.g. emphysema) makes the development of auto-PEEP more likely. A reduction in expiratory time, which would occur if the respiratory rate were increased without altering the I:E ratio, also increases the possibility of the development of auto-PEEP.

In patients with conditions that may prolong the time constant, a reduction in respiratory rate to below 8 breaths/min may be required to minimize auto-PEEP

(Kacm§^ek...aQ.d,.yeQegas 1987). Restrictive lung disorders, with low compliance and normal resistance, may be ventilated at rates higher than this (sometimes more than 20 breaths/min) with a low risk of development of auto-PEEP (Kacmarek ainid,..yenegas 1987).

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