Respiratory support

In patients presenting with acute weakness the respiratory muscles should be assumed to be involved until proven otherwise. Clinical assessment and simple bedside measures are the best methods of determining the requirement for mechanical ventilation. A vital capacity of less than 1 liter or less than 50 per cent of the predicted value, a respiratory rate of more than 30 breaths/min, a poor cough, or a clinical impression of impending ventilatory failure should lead to an urgent referral to the intensive care unit (ICU). A vital capacity of less than 30 ml/kg leads to reduced capacity to cough, and less than 10 ml/kg causes ventilatory failure. The airway should be secured and mechanical ventilatory support should be instituted if the respiratory function falls below the limits given above. When a decision has been made to institute mechanical ventilation, this should be explained to the patient. It is important to note that succinylcholine (suxamethonium) should never be used where there is muscle damage or denervation. This is because denervated or traumatized muscle releases large amounts of potassium into the circulation when depolarizing agents are used, and this may prove fatal.

Prior to intubation of the trachea, an attempt should be made to ensure that the stomach is empty when respiratory muscle weakness has developed slowly and is not likely to progress with great rapidity. Therefore patients should remain nil by mouth for at least 4 h. In an emergency the stomach may be emptied by inserting a nasogastric tube. The patient should be preoxygenated for approximately 5 min and then anesthesia induced with a suitable induction agent (e.g. propofol, etomidate, thiopental (thiopentone), or midazolam). Muscle relaxation should be ensured with a non-depolarizing muscle relaxant (e.g. atracurium or vecuronium). A cuffed endotracheal tube should be inserted and the cuff inflated while an assistant maintains cricoid pressure, which should only be released once the airway is secured adequately and the endotracheal cuff inflated.

The precise mode of mechanical ventilation depends on the state of the patient. Initially synchronized intermittent mandatory ventilation may be appropriate, and infusions of sedative and analgesic agents may be required to keep the patient comfortable. Initial ventilator settings should include a respiratory rate of between 10 and 12 breaths/min with a tidal volume of about 10 ml/kg. These settings may be titrated in the light of subsequent blood gas analysis. It is usually acceptable to maintain arterial carbon dioxide tension within the normal range with oxygen saturation of over 95 per cent. If the patient is capable of making respiratory effort, pressure support ventilation may be appropriate. For patients with severe weakness which is likely to run a prolonged course (e.g. Guillain- Barré syndrome) it is prudent to perform an early tracheostomy so that the prolonged use of sedative agents may be avoided.

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