Mode of ventilation and level of spontaneous respiration

Control mode

The respiratory rate set on the ventilator is that which the patient receives. The rate should not be reduced to 'encourage' the patient to start breathing, as spontaneous respiration is not possible. A change of mode is required.

Assist-control mode

The patient receives a tidal volume when he or she attempts to take ('trigger') a spontaneous breath. As the patient is triggering every breath, theoretically there is no requirement to set a respiratory rate. However, many ventilators provide back-up ventilation if the patient becomes apneic for a set time period. This back-up rate should be set at two to five breaths less than the spontaneous rate to allow adequate ventilation if the patient remains apneic, and also to allow the opportunity for spontaneous respiration to recommence.

Intermittent mandatory ventilation

A set number of mandatory breaths are delivered each minute. In addition, the possibility of spontaneous breaths is ensured. Synchronized intermittent mandatory ventilation prevents 'stacking' of patient and ventilator breaths on top of each other. With both these modes, the respiratory rate is normally set at a level that would provide full ventilatory support if the patient were making no respiratory effort, and then reduced to a level at which the patient is able to respire spontaneously, appears comfortable, and does not develop a respiratory acidosis. The rate is then gradually reduced as the weaning process is continued, so that the patient is required to take an increasing proportion of spontaneous to mandatory breaths. At less than 6 breaths/min, the work of breathing may increase because the majority of ventilation is spontaneous through the resistance of a demand valve and circuit ( Brown 1994). Therefore the mandatory breath rate is not usually reduced beyond this point.

Pressure support ventilation

Normally, expiration is cycled when the inspiratory flow rate falls below a set level (typically 25 per cent of peak inspiratory flow) and therefore a respiratory rate is not set on the ventilator. However, some ventilators will not allow a lengthening of inspiratory time beyond an inspiration-to-expiration (I:E) ratio of 4:1 (which theoretically could occur if the patient were to inspire at a slow and steady flow rate). Under these circumstances, inspiration is terminated. Therefore setting a high respiratory rate can lead to cycling to expiration before the patient has inspired fully.

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