Critical Care Medicine-Neurologic Disorders>>>>>Mechanical Ventilation
Question 12#

A 22-year-old woman with a history of asthma (previously intubated twice) presents to the emergency department with worsening dyspnea over the past week and is found to have influenza. Her chest radiograph demonstrates bilateral patchy opacities and she is intubated for hypoxemic respiratory failure and receives empiric antibiotics, bronchodilators, and steroids. Ventilator settings are:

On arrival to the ICU, she remains paralyzed after rapid sequence intubation, and on the above settings she is getting a TV of 200 mL (4 mL/kg IBW).

Which of the following statements is MOST accurate regarding her ventilator settings and physiology?

A. While she is paralyzed, her compliance can reliably be calculated from her inspiratory pressure and delivered volume
B. While she is paralyzed, you need to measure a pleural pressure to reliably calculate her pulmonary resistance
C. When her paralytics wear off, you would need to measure pleural pressure to reliably calculate her pulmonary compliance
D. Her pulmonary resistance is best assessed by looking at the difference between her peak inspiratory pressure and a pressure during an end-inspiratory pause

Correct Answer is C


Correct Answer: C

Unlike assist control-volume control, in a pressure-control mode of ventilation, the airway opening pressures are the target variables (ie the variable set on the ventilator). This means that her peak inspiratory pressure will be the same as the pressure that the ventilator has set during an inspiratory pause (in this case, 25 cm H2O) and resistance cannot be calculated from these numbers (answer D is incorrect). Compliance can sometimes be calculated from the set change in pressure at the airway opening and the resulting volume delivered but there are important caveats. If flow does not reach zero at the end of inhalation (for example, in a patient with elevated airways resistance such as this patient or with very short inspiratory times) then you cannot reliably calculate a resistance from the set pressure and the delivered volume (answer A is incorrect). In addition, if the patient is making spontaneous effort in sync with the ventilator then the transpulmonary pressure (the distending pressure of the lung parenchyma) can only be calculated with a measurement of her pleural pressure with an esophageal balloon (answer C is correct). The pleural pressure is not necessary to calculate resistance (answer B is incorrect).