Setting positive end-expiratory pressure in the severely obstructive patient

Abstract

Purpose of review: The response to positive end-expiratory pressure (PEEP) in patients with chronic obstructive pulmonary disease (COPD) requiring mechanical ventilation depends on the underlying pathophysiology. This review focuses on the pathophysiology of COPD, especially intrinsic PEEP (PEEPi) and its consequences, and the benefits of applying external PEEP during assisted ventilation when PEEPi is present.

Recent findings: The presence of expiratory airflow limitation and increased airway resistance promotes the development of dynamic hyperinflation in patients with COPD during acute respiratory failure. Dynamic hyperinflation and the associated development of PEEPi increases work of breathing and contributes to ineffective triggering of the ventilator. In the presence of airflow limitation, application of external PEEP during patient-triggered ventilation has been shown to reduce inspiratory effort, facilitate ventilatory triggering and enhance patient-ventilator interaction. To minimize the risk of hyperinflation, it is advisable to limit the level of external PEEP during assisted ventilation after optimization of ventilator settings to about 70% of the level of PEEPi (measured during passive ventilation).

Summary: In patients with COPD and dynamic hyperinflation receiving assisted mechanical ventilation, the application of low levels of external PEEP can minimize work of breathing, facilitate ventilator triggering and improve patient-ventilator interaction.

Figure 1.

Airway pressure (Paw), esophageal pressure (Pes), and gastric pressure (Pga) in a patient with COPD being ventilated with assist-control ventilation. The airway was occluded during expiration in order to quantify intrinsic PEEP (PEEPi). The patient made an expiratory effort during the occlusion (arrow), and thus the recorded pressure overestimated the true PEEPi. Reproduced from Jubran (Figure 48–14 of ref 24).

Figure 2:

Flow, airway pressure (Paw), and esophageal pressure (Pes) in Patient 1 (left) and Patient 2 (right) with COPD receiving pressure support ventilation set at 20 cmH₂O (no external PEEP is applied). Dynamic intrinsic PEEP, estimated as the difference in esophageal pressure between the onset of inspiratory effort (vertical blue line) and the onset of inspiratory flow (vertical red line), was 0.5 cmH2O in the patient on the left and 10.6 cm H₂O in the patient on the right. Reproduced from Tobin (Figure 4 of ref 10).

Figure 3:

Flow, airway pressure (Paw) and esophageal pressure (Pes) in a patient with COPD who is receiving assist-control ventilation at the following settings: tidal volume 600 ml, inspiratory flow 60 L/min, trigger sensitivity −2 cm H₂O and positive end-expiratory pressure 0 cm H₂O. The patient’s intrinsic respiratory rate is 28 breaths per minute, whereas the number of breaths delivered by the ventilator is 16 breaths per minute. That is, 43% of the patient’s inspiratory efforts fail to trigger ventilator assistance. Reproduced from Tobin (Figure 53–12 of ref 32).

Figure 4:

Airway pressure (Paw) versus time during controlled mechanical ventilation as external positive end-expiratory pressure (PEEP) is increased from 0 to 12 cm H₂O in a patient with expiratory flow limitation (upper panel) and in a patient without expiratory flow limitation (lower panel). In both patients, static intrinsic PEEP (PEEPi) (arrow) is 10 cm H₂O. In the presence of expiratory flow limitation upper panel), peak airway pressure increases only when external PEEP is 12 cm H₂O (dotted red line). In the absence of expiratory flow limitation (lower panel), application of external PEEP causes a proportional increase in peak airway pressure. The qualitative responses to external PEEP, seen with peak airway pressure, are expected to occur also with plateau pressure. Reproduced from Laghi (Figure 31–10 of ref 53).