Neurosurgery notes/Other specialities/ITU/Respiratory/Respiratory failure

Respiratory failure

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Respiratory

Types

  • Lung failure → Type 1 hypoxaemic respiratory failure
  • Pump failure → Type 2 hypercapnic respiratory failure

Restrictive respiratory failure

  • Neuromuscular disorders may cause weakness of the diaphragm (resulting in hypoventilation) and weakness of the oropharyngeal and upper airway muscles (leading to aspiration or obstruction).
  • Patients may present with respiratory failure due to
    • Progression of a chronic progressive neuromuscular condition or
    • As an early manifestation of acute neuromuscular weakness (e.g. GBS, botulism, myasthenic crisis).
  • Signs and symptoms
    • Sleep associated hypoventilation (diaphragmatic weakness),
    • “Nasal” speech (palate weakness),
    • Dysphonia (larynx),
    • A weak cough (epiglottis)
    • Accessory muscle use and tachycardia.
  • Investigation
    • Oxygen saturation,
    • Forced Vital Capacity (FVC)
        • Assesses composite inspiratory and expiratory force and lung volume.
        • Should be measured at least 4 hourly in the acute setting.
        • FVC falls below 15-20 ml/kg (i.e. 1.0 l in a 70 kg individual)
          • Respiratory failure is likely
        • Technique
          • Sealing their lips tightly over the tube, the patient is asked to inhale as deeply as possible
          • Then, the patient exhales as forcefully as they can
        • Rough estimate of FVC
          • Patient to count to 20 after taking a single breath: inability to do so corresponds to a greatly reduced vital capacity in the order of 15-18 ml/kg (normal values are 65-75 ml/kg).
        Lung volumes
        Lung volumes
    • Other measurements
      • Peak inspiratory force
        • < 25 cmH₂O is typically associated with impending respiratory failure
      • FEV1 (the volume expired in 1 s of forced expiration) and peak expiratory flow rate (PEFR) offer composite indices of expiratory muscle force only and airway resistance and they will be NORMAL in GBS.
    • ABG
    • Arterial blood gases are an extremely unreliable indicator of respiratory strength or the need for mechanical ventilation in neuromuscular disorders.
    • Patients with rapid shallow breathing should be closely monitored even if blood gases are normal.
  • Treatment
    • Indications for mechanical ventilations in the patient with acute/subacute weakness are
      • FVC < 15 ml/kg,
      • Oropharyngeal weakness,
      • Choking on secretions,
      • Intermittent aspiration.