These polyneuropathies usually present after days or weeks when patients have been managed in the ICU for a variety of primary illnesses or injuries. It will be noted that there is difficulty in weaning from the ventilator. After cardiac and lung conditions have been excluded, a neuromuscular condition should be suspected if, on attempted discontinuation of ventilation, respiration becomes rapid and there is a rise in blood CO 2. There may be variable signs of abdominal paradox (inward movement of the abdominal wall during inspiration) or respiratory alternans (alternation of rib cage and abdominal movement). Measurements of vital capacity, maximum inspiratory pressure, breathing frequency, and tidal volume may give inconclusive results. Unilateral damage to the phrenic nerve due to operative trauma is often undiagnosed, despite chest radiography and fluoroscopy.
Electrophysiological testing, measurements of creatine phosphokinase, and sometimes muscle biopsy are again necessary to investigate the problem thoroughly. Complications of the sepsis and multiple organ failure syndrome, now termed the systemic inflammatory response syndrome, will be the underlying cause for the neuromuscular condition in most instances (Table.3). This is an important phenomenon, since the incidence of this syndrome in ICUs may be as high as 50 per cent.
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