1. You are taking care of a 14 yo M with status asthmaticus. He is currently intubated and sedated and being treated with continuous albuterol, steroids, and a cisatracurium and ketamine infusion. His current ventilator settings show a PIP of 36, PEEP of 5, Vt 6 cc/kg, RR of 12, iTime 1.5, and FiO2 of 45%. A plateau pressure is measured and is 22. His most recent arterial blood gas is 7.08/80/65 and his EtCO2 reads 50. What would be the most appropriate next step in managing this patient?
a) Increase the respiratory rate to 16
b) Call anesthesia to initiate inhaled anesthetic
c) Increase the peak pressures to 40 to achieve higher tidal volumes
d) Decrease the iTime to 1
2. What is generally thought of as the primary determinant of oxygenation when mechanically ventilated ?
a) Mean Airway Pressure
d) Aa gradient
3. Which of the following is NOT an advantage of pressure controlled mechanical ventilation?
a) Can be effective even with significant leaks around the ETT
b) Generally thought of as more comfortable for the patient due to decelerating flow waveform
c) Generally lower peak pressures
d) Guaranteed tidal volume
ANSWERS & EXPLANATIONS
1. D- this patient with status asthmaticus has evidence of severe obstructive lung disease, as evidenced by a large Peak to plateau pressure gradient (36-22=14 cm H20). He also has a large A-E (PaCO2- EtCO2) gradient indicating significant dead space, likely as a result of hyperinflation. Due to this high airway resistance, he needs further time for exhalation. This can be achieved in several ways. One would be to decrease the respiratory rate. By decreasing the RR from 12 to 10, you increase the total cycle time from 5 seconds (12 breaths per min = 1 breath per 5 seconds while 10 breaths per min = 1 breath per 6 seconds), and thus the E time would increase from 3.5 seconds (5-1.5) to 4.5 seconds (6-1.5). This is not an answer choice though. Increasing the respiratory rate, while theoretically improving minute ventilation, would exacerbate the inability to exhale and thus worsen your hypercarbia and acidosis. In addition, you could decrease the iTime to 1 second, now making your E time 5-1 =4 seconds and your I:E ratio 1:4- This is the correct answer choice D. Inhaled anesthetic is a possibility but would likely be used after ventilator adjustments and other adjuvant medications (heliox, terbutaline, magnesium, aminophylline etc.) had been tried. Increasing the peak pressure risks barotrauma (although mitigated by the high airway resistance since the alveolus only ends up seeing the plateau pressure of 22) while your tidal volumes are likely adequate.
2. A- the mean airway pressure is the primary determinant of oxygenation when on mechanical ventilation. Hence, changes that result in increased mean airway pressure generally help to improve oxygenation (i.e. changes in iTime, PEEP or going to HFOV).
3. D- Guaranteed tidal volumes occur with volume controlled ventilation (or PRVC), not pressure controlled ventilation.