Ventilators modes of ventilation and settings

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The aim of mechanical ventilation is to maintain adequate arterial oxygenation and control acidosis while using specific therapeutic measures to reverse bronchospasm, reduce bronchial epithelium inflammation, mobilize secretions, and suppress airway hyper-reactivity.

The simplest type of control-mode ventilator (either volume- or pressure-cycled) will serve to support an asthmatic patient. However, the current generation of volume-and time-cycled pressure-limited ventilators, capable of several modes of ventilation, provide the flexibility and safety features which make them preferable to the less sophisticated machines. During the initial stages of stabilization of the patient following intubation, control-mode ventilation is ideal. As the patient recovers from sedation, synchronized intermittent mandatory ventilation may be possible. If pressure-limited ventilation is adopted, minimal tidal volume alarms must be set so as to avoid inadvertent (uncontrolled) hypoventilation.

The ventilator settings that must be selected include the tidal volume ( VT), respiratory rate, FiO2, peak inspiratory flow, waveform, and ratio of inspiratory to expiratory time (I:E ratio). Recommended settings are shown in Tabl—. Opposing philosophies have attached different weights to VT selection, the relevance of peak inspiratory pressure and the necessity for its limitation, the rapid normalization of arterial blood gases, and the need for high inspiratory flow rates to maximize expiratory time. The strategy of controlled hypoventilation as a means of limiting complications was proposed by Darioliand Perret. (.1984.). Their priorities were maintaining tissue oxygenation and limiting peak inspiratory pressures to less than 50 cmH 2O; this was achieved by reducing tidal volume, respiratory rate, and inspiratory flow rate.

Present opinion is divided between high and low inspiratory flow rate ( in asthma. Several investigators (Darioli and Perret 1984; Bishop and Hillman 1993)

recommend that a low of 40 l/min should be used as one of the methods to lower peak inspiratory pressure and thus reduce the risk of barotrauma. However,

Tuxen (1994) has shown that reducing from 100 to 40 l/min reduces the peak inspiratory pressure but also decreases expiratory time, which in turn decreases lung emptying. This results in more overinflation of the lung which has been identified as one of the major factors in pulmonary barotrauma. The problem with delivering high flow rates and high peak inspiratory pressures to asthmatic lungs is that the gas would preferentially be applied to the more normal population of alveoli and airways, as the disease process is very heterogeneous.

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