[Pathophysiological basis of mechanical ventilation]

D Köhler, M Pfeifer, C Criée
Pneumologie 2006, 60 (2): 100-10
Mechanical ventilation is required if ventilatory insufficiency is present. This is typically indicated by hypercapnea. Hypoxemia occurs secondary to hypoventilation. Usually overload of the respiratory muscles (ventilatory pump) will be the underlying mechanism, for the most part caused by acute or chronic disease. In case of sole hypoxemia mechanical ventilation will only be indicated if the oxygen-content (equals oxygen saturation x haemoglobin x 1.39) drops below a critical threshold or if ventilatory pump failure is imminent on account of the underlying disease (eg. pneumonia). The background of our recommendations is to avoid potential damage caused by mechanical ventilation. Especially high inspiratory pressures and oxygen concentrations can be harmful to the lung. Therefore every case has to evaluated for individual target parameters of ventilation. The use of the oxygen-content instead of the arterial oxygen pressure as the target parameter will usually lead to a more careful ventilation. Cardiogenic pulmonary oedema is an exception to this rule since inspiratory positive pressure and PEEP will result in improved diffusion as well as reduction of preload and work of breathing. In recent years progress has been made on the field of ventilation access especially in severe and acute cases. Non-invasive ventilation is superior to invasive ventilation in patients with exacerbated COPD since it improves outcome effectively. This is being caused by a decline in ventilator associated pneumonias, most likely because non-invasive ventilation allows patients to clear their secretions by coughing, resulting in improved lung clearance. Controlled ventilation allows optimal unloading of the respiratory muscles which have been overloaded by the underlying disease. Application of a controlled ventilation mode in acute disease will usually require some kind of sedation. Assisted ventilation will result in improved gas exchange but only incomplete unloading of respiratory muscles and therefore delayed restitution. Permanent controlled ventilation under sedation for a prolonged period (days) requires intermittent periods of assisted- or spontaneous breathing in order to avoid atrophy of the respiratory muscles. This review summarizes background information on the nature of the derangement, the relation between oxygen supply and consumption under special consideration of respiratory muscle insufficiency and impact of different ventilation modes.

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