Although pressure support may be used as a total ventilatory assistance method, it should be remembered that neither tidal volume nor respiratory rate are preset in the ventilator. Consequently, pressure support ventilation requires an intact spontaneous inspiratory activity. To the extent that the volume delivered to the patient depends not only on the preset level of pressure but also on the mechanical characteristics of the respiratory system, the efficacy of this method may not be optimal in certain unstable patients. A level of pressure support which may be clinically appropriate for a period of time may be inefficient later if changes in inspiratory demands or respiratory system mechanics occur. As a consequence, close monitoring of respiratory rate and minute ventilation is advised when this method is used in critically ill patients.
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