The respiratory muscles eventually become weak and uncoordinated as they perform no work during conventional mechanical ventilation. Moreover, there is usually some persisting abnormality of lung function. Thus, in patients who have been artificially ventilated for any length of time, spontaneous respiration usually has to be resumed gradually.
Critical illness neuropathy
This recently recognized acquired polyneuropathy has most often been described in association with persistent sepsis and MOF. It is characterized by a primary axonal degeneration involving both motor and sensory nerves. Clinically the initial manifestation is often difficulty in weaning the patient from respiratory support. There is muscle wasting, the limbs are weak and flaccid and deep tendon reflexes are reduced or absent. Cranial nerves are relatively spared.
Nerve conduction studies confirm axonal damage. The cerebrospinal fluid (CSF) protein concentration is normal or minimally elevated. These findings differentiate critical illness neuropathy from Cuillain-Barre syndrome in which nerve conduction studies show evidence of demyelination and CSF protein is usually high.
The cause of critical illness neuropathy is not known and there is no specific treatment. With resolution of the underlying critical illness, complete recovery can be anticipated between 1 and 6 months, although weaning from respiratory support and rehabilitation are likely to be prolonged.
Criteria for weaning patients from artificial ventilation
Clinical assessment is of paramount importance when deciding whether a patient can be weaned from the ventilator. The patient’s conscious level, psychological state, metabolic function, the effects of drugs, cardiovascular performance and mechanical factors must all be taken into account. Objective criteria are based on an assessment of pulmonary gas exchange (blood gas analysis), lung mechanics and muscular strength.
Techniques for weaning
Patients who have received artificial ventilation for less than 24 hours, e.g. elective IPPV after major surgery, can usually resume spontaneous respiration immediately and no weaning process is required. This procedure can also be adopted for those who have been ventilated for longer periods but who clearly fulfil the objective criteria for weaning.
The traditional method of weaning in difficult cases is to allow the patient to breathe entirely spontaneously for a short time, following which [PPV is reinstituted. The periods of spontaneous breathing are gradually increased and the periods of IPPV are reduced. Initially it is usually advisable to ventilate the patient throughout the night. This method can be stressful and tiring both for patients and staff, although some patients do not tolerate IMV (see below) and the traditional method of weaning may then be necessary. SIMV can be used to provide a smoother, more controlled method of weaning; it may also enable weaning to commence at an earlier stage than is possible using the conventional method. There is no evidence, however, that SIMV enables patients who could not be weaned using conventional methods to resume spontaneous respiration, and in some cases the weaning process may be unnecessarily prolonged.
The application of CPAP can prevent the alveolar collapse, hypoxaemia and fall in compliance that might otherwis occur when patients start to breathe spontaneously. It is therefore often used during weaning with [MV and in spontaneously breathing patients prior to extubation, particularly when they were previously receiving IPPV with PEEP.
Extubation
his should not be considered until patients can cough, swallow, protect their own airway and are sufficiently alert to be cooperative. Patients are assessed on their ability to breathe spontaneously via the endotracheal tube over a period of time. In those who have undergone prolonged artificial ventilation, this period may need to be 24-48 hours, or even longer, while patients ventilated for less than 12-24 hours can often be extubated within 10- 15 min. During this ‘trial of spontaneous respiration’ the patient should be closely observed for any signs of respiratory distress.
