What are the indications for mechanical ventilation in severe asthma (status asthmaticus)?

What are the indications for mechanical ventilation in severe asthma (status asthmaticus)? What are the specific concerns?

The indications for tracheal intubation and mechanical ventilation in severe asthma are the absence of response to treatment, worsening physiologic variables such as tachypnea, hypoxemia, hypercapnia and obtundation, and physical exhaustion in face of maximal therapy.

The main concerns regarding the ventilation of asth-matic patients are increased airway resistance with high peak insufflation pressures and prolonged expiration with the risk of auto-PEEP and “stacking” of mechanical breaths. It is unclear, in these patients with high airway resistance, whether high peak pressures measured at the ventilator level actually correspond to high alveolar pres-sures. The mainstay of mechanical ventilation in patients with obstructive disease is to use, for a given minute venti-lation, an elevated tidal volume (10–12 mL/kg or more), slow respiratory rates (6–8 breaths/minute), prolonged expiratory times, high inspiratory peak flows (80–100 L/ min), and a low I/E ratio (e.g., 1:4 to 1:6.) Most modern respirators allow auto-PEEP to be measured by triggering an expiratory pause (provided the patient is sedated enough to prevent spontaneous breathing). Occasionally, in patients with extreme bronchospasm, neuromuscular blockade might be used to increase chest wall compliance and make ventilation somewhat less difficult.

A strategy of permissive hypercapnia might reduce the risk of barotrauma, but is not supported by studies as it is in patients with acute respiratory distress syndrome.

A concern when ventilating a patient with bronchospasm in the operating room is the inability of most anesthesia ventilators to maintain flows at high impedance. Moreover, the high compliance of the anesthesia circuit may result in as much as 7–10 mL/cm H₂O of the delivered tidal volume lost in the circuit. This would ultimately result in decreasing the effectiveness of patient ventilation.