Invasive mechanical ventilation in adults with acute exacerbations of asthma
- Carlos A Camargo, Jr, MD, DrPH
Carlos A Camargo, Jr, MD, DrPH
- Professor of Emergency Medicine and Medicine
- Harvard Medical School
- Professor of Epidemiology
- Harvard T.H. Chan School of Public Health
- Jerry A Krishnan, MD, PhD
Jerry A Krishnan, MD, PhD
- Professor of Medicine and Public Health
- Section of Pulmonary, Critical Care, Sleep, and Allergy
- Division of Epidemiology and Biostatistics
- University of Illinois at Chicago
Intensive therapy with inhaled bronchodilators and systemic glucocorticoids is usually sufficient to reduce airflow obstruction and ameliorate symptoms in patients with acute asthma exacerbations. However, approximately 5 percent of all patients hospitalized for acute asthma develop respiratory failure and require invasive mechanical ventilation [1-3]. Although potentially life-saving, mechanical ventilation and its associated interventions (eg, sedatives, paralytics) can also cause morbidity and mortality [4-8].
In this topic review, the indications, management, and adverse effects of invasive mechanical ventilation in patients with severe acute asthma (eg, status asthmaticus) will be reviewed. The pharmacologic treatment of acute exacerbations of asthma and the role of noninvasive positive pressure ventilation are discussed separately. (See "Treatment of acute exacerbations of asthma in adults" and "Noninvasive ventilation in acute respiratory failure in adults".)
The primary indication for mechanical ventilation in an acute asthma exacerbation is acute respiratory failure (ie, insufficient oxygenation or alveolar ventilation). The decision to initiate mechanical ventilation should be based on serial clinical evaluations that consider the severity of airflow limitation (eg, peak expiratory flow), degree of respiratory difficulty (eg, respiratory rate >40/minute, inability to talk), clinical findings (eg, accessory muscle use, intercostal retractions, fatigue, somnolence), hypoxemia, hypercapnia (elevated arterial tension of carbon dioxide [PaCO2]), and response to therapy . Bronchoconstriction can worsen abruptly after placement of an endotracheal tube, so the need for ventilatory support must be weighed against the potential for initial worsening of ventilation. Nonetheless, intubation and mechanical ventilation should not be delayed until the need becomes emergent.
Generally, acute asthma exacerbations are associated with mild hyperventilation and a low PaCO2. However, with worsening airflow limitation, the high work of breathing leads to fatigue, a resultant decrease in the minute ventilation, and an increase in PaCO2. Thus, during an acute asthma exacerbation, a PaCO2 of 42 mmHg or greater, while technically “normal,” may suggest incipient respiratory failure. On the other hand, hypercapnia alone is not an indication for mechanical ventilation in the absence of decreased mental status or exhaustion.
While the optimal role of noninvasive positive pressure ventilation (NPPV) in acute asthma exacerbations is unclear, a brief trial of NPPV may be reasonable in selected patients prior to intubation and mechanical ventilation . Failure of NPPV to improve oxygenation would be an indication for invasive mechanical ventilation. (See "Noninvasive ventilation in acute respiratory failure in adults", section on 'Asthma'.)
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- GENERAL APPROACH
- Induction agents
- Ventilator mode
- Initial ventilator settings
- Administering inhaled bronchodilator
- TROUBLESHOOTING HIGH PEAK PRESSURES
- Differentiating airway and lung parenchymal causes of high pressures
- Dynamic hyperinflation
- - Assessment
- - Adverse effects of dynamic hyperinflation
- - Adjustments to decrease dynamic hyperinflation
- - Adding extrinsic PEEP to offset intrinsic PEEP
- Permissive hypercapnia
- TROUBLESHOOTING HYPOXEMIA
- ADDITIONAL AND UNCONVENTIONAL THERAPIES
- General anesthesia
- Extracorporeal life support
- SUMMARY AND RECOMMENDATIONS