Clinical diagnosis of neuromuscular respiratory failure

The danger of respiratory failure should be considered in every patient with progressive weakness, especially if the upper limbs and bulbar muscles are involved. The patient complains of weakness and fatigue but, unlike a patient with parenchymal lung disease or airway obstruction, does not appear wheezy or cyanosed. Instead, the patient prefers to sit or lie still in bed, becomes breathless on talking or swallowing, and uses the accessory muscles of respiration (pectoral, scalene, sternocleidomastoid, and levators of the nostrils). Diaphragm weakness may be detected by indrawing of the abdominal wall during inspiration, that is, paradoxical abdominal movement. Although the respiratory rate may be rapid and shallow, and the observation chart may show an increase in heart and respiratory rate over the previous few hours, this is not invariable and some patients present with ventilatory failure and a normal or reduced respiratory rate. All such patients should be monitored from the outset, especially during sleep, by pulse oximetry for the early detection of arterial desaturation. Clinical assessment, however, is better than blood gas analysis in assessing the need for ventilatory support. As respiratory failure worsens, the patient becomes increasingly anxious and, though exhausted, may be unable to sleep. Additional bulbar weakness or insensitivity with the attendant danger of inhalation is particularly hazardous.

Although the decision to intubate and start artificial ventilation depends primarily on the overall clinical assessment, measurement of vital capacity provides a useful quantitative assessment of respiratory muscle strength and helps to determine when intubation is necessary. In general, a steady fall in vital capacity over several consecutive hours usually predicts the need for ventilation. Impaired clearance of secretions occurs at values below 30 ml/kg and frank respiratory failure at less than 10 ml/kg. In practice, mechanical ventilation is usually instituted when vital capacity is approximately 15 ml/kg.3 Whilst measurement of vital capacity is a useful guide to the timing of intubation in patients with GBS, it is less so in patients with myasthenia gravis, perhaps due to the fluctuating nature of this disease.4 Respiratory rate should also be monitored closely, since the development of rapid shallow breathing indicates that the patient is likely to require intubation within the next eight hours.3 Arterial Pco2 measurements are not a reliable guide to the timing of intubation, and indeed bulbar function may have deteriorated to the point of frank pulmonary aspiration before any rise in Paco2 is observed.

Maximal static respiratory pressures (maximum inspiratory pressure, PImax, measured at residual volume; maximum expiratory pressure, PEmax, measured at total lung capacity) obtained while breathing against an occluded mouthpiece are said to be more sensitive indicators of respiratory muscle weakness.5 A PEmax of less than 40 cmH2O (adult normal = 100 cmH2O) is associated with an inability to cough and clear secretions adequately, whereas a PImax of less than - 20 cmH20 (adult normal of more than -70 cmH2O) precludes effective ventilation and maintenance of a normal arterial CO2 tension. In patients who find this manoeuvre difficult to perform, measurement of sniff nasal pressure (SNIP) is an alternative. This is done by placing a tight plug, connected to a pressure transducer, into one nostril and asking the patient to inhale deeply through the unobstructed nostril.6

Respiratory muscle strength may also be assessed by recording transdiaphragmatic pressure during tidal breathing

Box 11.1 Central nervous system disorders causing respiratory failure

Sedative drugs

Secondary effects of metabolic disorders Central transtentorial herniation Brain stem lesions Infarction Haemorrhage Extrinsic compression Intrinsic tumour Encephalitis Multiple sclerosis Motor neuron disease Central pontine myelinolysis Spinal cord lesions Cord compression Motor neuron disease Intrinsic tumour Multiple sclerosis Transverse myelitis Poliomyelitis Rabies and on maximal inspiration (± measurement of the diaphragmatic EMG). Alternatively magnetic stimulation of the phrenic nerves may be used to assess diaphragmatic contractility. However, neither of these tests is routinely performed outside research centres.

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