A patient with acute respiratory failure secondary to a severe acute asthma exacerbation was intubated. The respiratory therapist speaks to you to ask for recommendations regarding initial ventilator settings. Which of the following approaches is recommended?
Maintain a higher respiratory rate (16–20) to decrease CO2, which may have been retained during exacerbation.
+Maintain a higher respiratory rate (16–20) because patient had rapid shallow breaths before requiring intubation.
+Use a tidal volume of 6–8 cc/kg of ideal body weight even if there is mild respiratory acidosis.
+Use a tidal volume of 10–15 cc/kg of ideal body weight.
+Use a pH of 7.40 as your goal to guide future ventilator settings.
Choices A and B urge maintaining an elevated respiratory rate to combat CO2 retention and to match patient’s level of ventilation prior to intubation. In this case, it is advisable to accept some level of hypoventilation to allow the patient to have adequate time to expire because of the airflow obstruction seen during the expiratory phase in acute asthma exacerbation. Ventilator strategies employ increasing airflow during inspiration and lowering respiratory rate. Choice D recommends a higher tidal volume that is not advisable in asthma exacerbation as this may predispose to barotraumas from intrinsic PEEP. Choice E addresses common misconceptions regarding ventilator management in asthmatic patients. In the acute phase of ventilator management for asthmatic patients with respiratory failure, low level of hypoventilation is acceptable and is termed “permissive hypoventilation.” The goal of management should not be based solely on normalizing the arterial pH but evaluating the respiratory parameters such as airflow during expiration, degree of auto-PEEP, and expiratory volume. Many of these parameters are measured or calculated by modern mechanical ventilators and can be ascertained by the respiratory therapist. Choice C is the correct answer and is consistent with a permissive hypoventilation strategy.