External negative pressure ventilation techniques.

Abstract

External ventilation was introduced at the beginning of the century to treat acute and chronic respiratory failure. Long-term negative pressure ventilation (NPV) has proved useful in patients with respiratory failure, secondary to a restrictive impairment of neuromusculoskeletal origin. Although NPV may be successfully used in acute-on-chronic respiratory failure in patients with chronic airflow obstruction (CAO), its use in the long-term management of this type of patient seems much less promising. The various types of ventilators, iron lung, pneumo-wrap and cuirass are discussed. All these prostheses, except for the pneumo-wrap, are connected to a pump able to generate negative and positive pressure. NPV must be a controlled ventilation, because an effective trigger is not yet available. The most important feature of the ventilator must be the ability to vary the ventilatory pattern to adapt the machine drive to the patient drive. The goal of NPV is not only to normalize gas exchange, but also to restore an effective spontaneous ventilation. This kind of ventilation has no negative effects on haemodynamics, because it does not vary physiological gradients of transthoracic pressure. However, due to lack of coordination between the respirator and the pharyngeal muscles of the patient, obstructive apnoeas may occur during NPV in sleeping patients.