status asthmaticus

Severe airway obstruction that does not improve after vigorous treatment with bronchodilators. Pathology: 1) patients with near fatal asthma a) impaired perception of dyspnea b) diminished ventilatory response to hypoxia 2) recent increase in use of beta-2 adrenergic agonists & diminished responsivenss to them 3) auto-PEEP contributes to increased work of breathing during exacerbations of obstructive lung disease Clinical manifestations: 1) rapid respirations 2) upright breathing posture 3) use of accessory muscles of respiration 4) pulsus paradoxus > 12 mm Hg Laboratory: 1) arterial blood gas (ABG) a) indicated when FEV1 < 1 L b) respiratory alkalosis is most common c) respiratory acidosis portends acute respiratory failure 4) a slighly elevated or even normal pCO₂ may portend acute respiratory failure 2) chem-7 a) non anion gap metabolic acidosis (compensatory) b) lactic acidosis is uncommon Special laboratory: - electrocardiogram: a) sinus tachycardia b) right axis deviation may be present (right heart strain) which disappears with treatment - pulmonary function testing a) FEV1 b) peak expiratory flow Differential diagnosis: - the need for large amounts of supplemental oxygen suggests diagnosis other than obstructive lung disease (COPD, asthma) Management: 1) identify triggers of status asthmaticus a) pharmaceutical agents 1] aspirin 2] beta-blockers b) indoor allergens (pets) c) occupational agents 2) oxygen a) 1-3 L/min by nasal cannula to keep O₂ sat > 90% b) non re-breather face mask c) heliox 3) mechanical ventilation a) indications 1] respiratory fatigue 2] acidemia: pH < 7.30 b) ventilator settings 1] low tidal volumes of 8-10 mL/kg 2] respiratory rate of 11-14/min 3] high inspiratory flow rates of 80-100 L/min b) complications: 1] increased thoracic pressure can compromise venous return & cardiac output 2] high risk for barotrauma a] pneumothorax b] pneumomediastinum c] keep peak airway pressures under 50 cm H20 d] permissive hypercapnia 4) pharmaceutical agents a) glucocorticoids 1] methylprednisolone IV 60-125 mg every 6 hours - 1 mg/kg if pregnant 2] prednisone: 40-60 mg every 4-6 hours b) albuterol nebulizer 2.5 mg in 2.5 mL normal saline 1] every 20 min for 1st hour 2] every 4h & every 2 hours PRN c) subcutaneous adrenergic agonists 1] patients refractory to albuterol 2] avoid in patient > 40 years of age, especially with history of CAD c) aminophylline 1] 5 mg/kg IV loading dose over 30 min (if not receiving theophylline) 2] infusion 0.4 mg/kg/hour 3] therapeutic monitoring: 8-12 ug/mL d) ipratropium bromide 0.5 mg by nebulizer every hour e) magnesium sulfate: 1-2 g IV over 20 min a) 50% of patients with status asthmaticus present with hypomagnesemia b) controlled studies have failed to show benefit

References

- Medical Knowledge Self Assessment Program (MKSAP) 11, 15, American College of Physicians, Philadelphia 1998, 2009