Invasive ventilation (IV)
Introduction
Indications for ventilation
Early identification of critically ill patients, before the occurrence of significant cardio-respiratory decompensation, is one of the major goals of critical care outreach programmes. Patients who require ventilatory support often develop a common pattern of physiological deterioration including:
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•increasing respiratory rate
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•asynchronous respiratory pattern
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•a change in mentation and level of consciousness
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•frequent oxygen desaturation despite increasing oxygen concentration
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•hypercapnia and respiratory acidosis
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•circulatory problems, including hypotension and atrial dysrhythmias.
Modern ventilators
The modern ventilator is a complex computer-driven tool and a detailed description of its modes and use are beyond the scope of this article (see Further reading). In simple terms, it mixes air under pressure with variable oxygen concentrations to provide inspiration and expiration, each ‘breath’ is characterized by three factors, which can be adjusted by the operator;
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•Trigger: the ventilator can deliver a breath according to a timer that defines a specific set rate (ventilator initiated/mandatory breaths), or as a result of the patient’s own breathing efforts effecting a change in the pressure or flow in the ventilator circuit (patient-initiated/spontaneous breaths).
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•Target: the flow of air into the lung can be to a specific target flow rate (volume control) or pressure (pressure control; pressure support; bi-level)
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•Termination: the signal for the ventilator to finish inspiration and allow expiration (passive) may be the achievement of a specific volume (volume-cycled: volume control), after a specific time (time-cycled: pressure control/bi-level) or following the reduction of inspiratory flow to a preset level (flow-cycled: pressure support)
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