Critical Care Medicine-Neurologic Disorders>>>>>Mechanical Ventilation
Question 13#

An 84-year-old man with severe chronic obstructive pulmonary disease (FEV1 20% predicted, on 4 L/min home O2 ) is admitted with severe hypoxemic respiratory failure due to a Streptococcus pneumoniae infection. He is intubated and placed on:

When the paralytic used for intubation wears off, the patient is noted to be triggering additional spontaneous breaths with a total respiratory rate of 36 breaths per minute, and his exhaled TVs vary from 100 to 800 mL. During an end-expiratory pause, his airway pressure is 18 cm H2O.

Which of the following is the MOST accurate statement regarding his ventilator settings?

A. Because the patient is on volume control, he is receiving lungprotective TVs
B. The patient’s intrinsic PEEP is likely overestimated by the measured end-expiratory airway pressure
C. Changing the patient to pressure control will improve the patient’s intrinsic PEEP
D. The patient’s intrinsic PEEP is likely making it more difficult for him to trigger spontaneous breaths
E. Changing the patient to pressure control will provide more lung- protective TVs

Correct Answer is D

Comment:

Correct Answer: D

This patient is on a volume-control mode of ventilation but is asynchronous with variable exhaled TVs, which indicates “breath stacking,” a type of cycle asynchrony in which patients do not fully exhale before taking their next breath. In this situation, the patient is no longer receiving the lung-protective TV that has been set despite being on a volume control mode (answer A is incorrect). If the ventilator setting were changed to pressure control, his TVs would vary with respiratory effort and therefore he would not be guaranteed to have more lung-protective TVs (answer E is incorrect). 

The patient has evidence of intrinsic PEEP with an end-expiratory pressure that is greater than the set PEEP. In order to trigger breaths on the ventilator, or breath spontaneously, a patient needs to decrease their airway opening pressure and (depending on a ventilator trigger mode) create inspiratory flow. When a patient has intrinsic PEEP, they must overcome this PEEP to decrease their airway opening pressure or initiate inspiratory flow; this is the inspiratory threshold load and it increases the work of breathing both in spontaneously breathing and patients who are mechanically breathing and triggering breaths (answer D is correct). Changing the ventilator mode to pressure control will not necessarily decrease intrinsic PEEP (answer C is incorrect). The measurement of endexpiratory airway pressure often underestimates intrinsic PEEP because lung units that are slow emptying may not equilibrate with the airway opening and therefore the sickest lung units with the highest intrinsic PEEP are not fully represented by bedside measurements (answer B is incorrect). 

Reference:

  1. Nilsestuen JO, Hargett KD. Using ventilator graphics to identify patientventilator asynchrony. Respir Care. 2005;50(2):202-234.