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Principles of ventilation

Created: 18/2/2005

Ventilators are used commonly in the operating theatre and in the ICU to deliver mechanical ventilation to the lungs.

In the operating theatre, ventilation is in anaesthetized and often pharmacologically paralysed patients with predominantly normal lungs. These ventilators are relatively simple and are designed to deliver varying concentrations of oxygen, air, nitrous oxide and volatile agents to patients through an anaesthetic circuit.

In the ICU, ventilators provide respiratory support to patients with respiratory failure. Respiratory failure is a state in which the pulmonary oxygen uptake is so severely disturbed that the supply of oxygen to the tissues and/or the removal of carbon dioxide from them is inadequate. Respiratory failure can be caused by relative hypoventilation, characterized by an increase in arterial carbon dioxide tension, or failure of diffusion at the alveolar–capillary membrane, characterized by decreased arterial oxygen tension.

Classification of ventilators

Numerous classifications have been suggested, including those by Ward and Mapleson. Classifications refer to the following elements.


Describes how the ventilator delivers flow to the patient. In volume-controlled ventilators, the flow delivered is constant, the tidal volume is targeted, with a variable pressure delivered relative to compliance of the lung. In pressure-controlled ventilators, the flow decelerates through the breath to maintain the targeted pressure at the peak inspired pressure. The tidal volume delivered is determined by the compliance of the ventilated lung.


Determines how the ventilator switches from inspiration to expiration. Time cycling is used in pressure-controlled ventilation. Flow cycling is used in pressure-support ventilation, where a reduction of the peak inspiratory flow cycles the ventilator into expiration. Volume cycling is used in volume-controlled ventilation. The ventilator cycles to expiration once a set tidal volume has been delivered. If an inspiratory pause is added, the breath is both volume and time cycled.


Is how inspiration is initiated in association with patient breaths. Ventilators may be triggered by changes in pressure, flow or by a preset time interval having elapsed.

 Pressure: the ventilator delivers a breath when the baseline pressure decreases during the patient’s own inspiratory effort.

 Flow: some ICU ventilators deliver a constant background flow throughout the respiratory cycle (flow by). A change in this constant flow, caused by patient inspiration, is sensed at the flow sensor in the expiratory limb. This triggers the ventilator to increase the flow and a breath is delivered to the patient. Flow triggering reduces the work of breathing when compared with pressure triggering because there is always some background gas flow from the patient and no delay in inspiratory valve opening.

 Time: the ventilator cycles at a frequency determined by the respiratory rate or the ratio of inspiration to expiration (I:E).

© 2003 The Medicine Publishing Company Ltd

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