Airway access and mechanical ventilation

Most patients who develop respiratory failure as a consequence of neuromuscular dysfunction will require a tracheostomy. This is usually performed at a relatively early stage, in our unit five to seven days after the institution of mechanical ventilation. It simplifies the management of the patient considerably and allows the withdrawal of all sedation. The tracheostomy tube is well tolerated, gives excellent access for tracheal toilet and chest physiotherapy, and facilitates weaning by virtue of allowing the patient to be placed on and off different modes of respiratory support at will. Percutaneous dilatational tracheostomy is now the technique of choice for the majority of intensive care patients. It can be performed at the bedside, takes less time than a surgical tracheostomy, and is associated with a lower incidence of the most important complications, which are bleeding and infection.51,52 Long term follow up of patients undergoing this procedure has also identified a rate of subglottic stenosis which compares favourably with that of the surgical technique.51,53 It is our impression that the long term cosmetic result is also superior.

In the absence of severe pulmonary aspiration or infection, it is not difficult to achieve adequate pulmonary gas exchange in these patients. Initially, most patients are too weak to generate an adequate negative pressure to "trigger" the ventilator and so require "controlled ventilation". During this phase, tidal volume should be limited to 6-8 ml/kg, plateau pressure to less than 30 cmH2O, and the level of positive end expiratory pressure (PEEP) titrated according to the inspired oxygen concentration (Fio2) in order to prevent ventilator induced lung injury.54 Many modern ventilators, for example the Siemens Servo 300, have a triggering mechanism that requires the patient to change a baseline flow within the machine rather than to reduce a pressure. These machines are much more sensitive to the respiratory efforts of a patient and should theoretically be beneficial in management. In fact, no particular kind of ventilation or ventilator has been shown to be superior in supporting these patients. Most intensivists rely on pressure support or some combination of intermittent mandatory ventilation with pressure support to provide an adequate tidal volume and minute ventilation. Humidification of the inspired gases is essential to avoid the development of mucus plugs. The application of positive end expiratory pressure (with a pressure of 5-15 cmH2O), together with physiotherapy, is used in the often vain attempt to prevent atelectasis.

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