status asthmaticus

Introduction

Severe airway obstruction that does not improve after vigorous treatment with bronchodilators.

Pathology

• patients with near fatal asthma
  • impaired perception of dyspnea
  • diminished ventilatory response to hypoxia
• recent increase in use of beta-2 adrenergic agonists & diminished responsivenss to them
• auto-PEEP contributes to increased work of breathing during exacerbations of obstructive lung disease

Clinical manifestations

• rapid respirations
• upright breathing posture
• use of accessory muscles of respiration
• pulsus paradoxus > 12 mm Hg

Laboratory

• arterial blood gas (ABG)
  • indicated when FEV1 < 1 L
  • respiratory alkalosis is most common
  • respiratory acidosis portends acute respiratory failure
• a slighly elevated or even normal pCO2 may portend acute respiratory failure
• chem-7
  • non anion gap metabolic acidosis (compensatory)
  • lactic acidosis is uncommon

Diagnostic procedures

• electrocardiogram:
  • sinus tachycardia
  • right axis deviation may be present (right heart strain) which disappears with treatment
• pulmonary function testing
  • FEV1
  • peak expiratory flow

Differential diagnosis

• the need for large amounts of supplemental oxygen suggests diagnosis other than obstructive lung disease (COPD, asthma)

Management

• identify triggers of status asthmaticus
  • pharmaceutical agents
    • aspirin
    • beta-blockers
  • indoor allergens (pets)
  • occupational agents
• oxygen
  • 1-3 L/min by nasal cannula to keep O2 sat > 90%
  • non re-breather face mask
  • heliox
• mechanical ventilation
  • indications
    • respiratory fatigue
    • acidemia: pH < 7.30
  • ventilator settings
    • low tidal volumes of 8-10 mL/kg
    • respiratory rate of 11-14/min
    • high inspiratory flow rates of 80-100 L/min
  • complications:
    • increased thoracic pressure can compromise venous return & cardiac output
    • high risk for barotrauma
      • pneumothorax
      • pneumomediastinum
      • keep peak airway pressures under 50 cm H20
      • permissive hypercapnia
• pharmaceutical agents
  • glucocorticoids
    • methylprednisolone IV 60-125 mg every 6 hours
      • 1 mg/kg if pregnant
    • prednisone: 40-60 mg every 4-6 hours
  • albuterol nebulizer 2.5 mg in 2.5 mL normal saline
    • every 20 min for 1st hour
    • every 4h & every 2 hours PRN
  • subcutaneous adrenergic agonists
    • patients refractory to albuterol
    • avoid in patient > 40 years of age, especially with history of CAD
  • aminophylline
    • 5 mg/kg IV loading dose over 30 min (if not receiving theophylline)
    • infusion 0.4 mg/kg/hour
    • therapeutic monitoring: 8-12 ug/mL
  • ipratropium bromide 0.5 mg by nebulizer every hour
  • magnesium sulfate: 1-2 g IV over 20 min
  • 50% of patients with status asthmaticus present with hypomagnesemia
  • controlled studies have failed to show benefit

More general terms

• acute asthma; asthma exacerbation

Additional terms

• assessment of severity of acute asthma