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Computer simulation: a guideline in ventilator setting in severe lung disease

Mechanics and gas exchange can in be studied with a computer-controlled ventilator. The physiological profile obtained describes the Pel/V diagram, inspiratory and expiratory resistance versus volume and the expired volume of CO2 versus tidal volume. When setting PEEP, frequency. I : E ratio, and minute ventilation or inspiratory pressure the physician needs to assimilate the information of the physiological profile and all clinical information to assure an adequate gas exchange at a non-traumatic ventilation. In ALI/ARDS harm can be caused both by ventilation at too low lung volumes and by ventilation at high volumes. In COPD the task is to ventilate at the lowest possible volume and airway pressure.

The complexity of the physiology and ventilator settings makes it impossible to figure out what is the ideal pattern of ventilation in order to reach the immediate therapeutic goals defined by the physician. However, on the basis of an adequate mathematical physiological profile, a computer can by simulation prognosticate what would be the consequences of alternative modes of ventilation . Through repeated simulations the physician can search a mode of ventilation that leads to his goals.

Computer simulation can be used to: a) increase the understanding of various patterns of ventilation in disease, b) predict the consequences of alternative settings in a particular patient.

In left diagram the total pressure in the ventilator (Pvent), the tracheal pressure (Ptr) and the alveolar, i.e. the elastic pressure, Pel, is shown for a patient with critical obstructive lung disease. Under current setting (volume control 10 l/min, Ti = 25%, Tpause = 10%, RR = 16, PEEP = 4 cmH20) the PaCO2 of 9.2 kPa was deemed acceptable. The pressures were, however, very high. By repeated simulations it was possible to identify a setting which dramatically would reduce the pressures without changing PaCO2.

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Uttman, L., Sigurdsson, S. & Jonson, B. Computer simulation: a guideline in ventilator setting in severe lung disease. Crit Care 3 (Suppl 1), P037 (2000).

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