Why should pressure trigger be avoided in auto positive end-expiratory pressure (auto PEEP)?

Why Pressure Triggering Should Be Avoided in Auto-PEEP

Flow triggering should be used instead of pressure triggering in patients with auto-PEEP because pressure triggers worsen patient-ventilator asynchrony and increase the work of breathing required to initiate a breath. 1

The Fundamental Problem with Pressure Triggers in Auto-PEEP

Intrinsic PEEP (auto-PEEP) creates an inspiratory threshold load that patients must overcome before any breath can be triggered. 1 This means the patient must generate sufficient negative pressure to:

• First overcome the auto-PEEP pressure (which can be 10-15 cm H₂O in severe COPD) 1
• Then generate additional negative pressure to reach the pressure trigger threshold
• Only after both thresholds are met will the ventilator deliver a breath 2

This double threshold creates excessive inspiratory work and leads to ineffective triggering efforts where the patient attempts to breathe but fails to initiate ventilator support, causing severe patient discomfort and increased work of breathing. 1, 3

Why Flow Triggers Perform Better

Flow triggering systems detect changes in the ventilator's bias flow rather than requiring pressure changes, making them more sensitive to patient effort in the presence of auto-PEEP. 1 The BTS/ICS guidelines explicitly state that "flow rather than pressure triggers reduce the incidence of asynchrony." 1

The physiologic advantage is clear:

• Flow triggers respond to the initiation of inspiratory flow before significant pressure changes occur 4
• This reduces the inspiratory effort required to trigger the ventilator 4
• The pressure differential (work) required is lower with flow triggering compared to pressure triggering at comparable sensitivity levels 4

Clinical Consequences of Using Pressure Triggers with Auto-PEEP

When pressure triggers are used in patients with auto-PEEP, several detrimental outcomes occur:

• Increased trigger delay as patients must overcome both auto-PEEP and the pressure trigger threshold 3, 5
• Ineffective triggering events where up to 45% of respiratory efforts may fail to trigger the ventilator 3, 6
• Patient-ventilator asynchrony that is "common and deleterious" 1
• Poor weaning outcomes: only 16% of patients with trigger asynchrony successfully weaned compared to 57% without asynchrony 3

The Correct Approach: Flow Triggering Plus Applied PEEP

The evidence-based strategy for managing patients with auto-PEEP involves:

1. Use flow triggering instead of pressure triggering to minimize trigger delay and asynchrony 1

2. Apply external PEEP to offset intrinsic PEEP, which reduces the effort required for triggering 1, 7

   • External PEEP helps by "offsetting iPEEP" and reducing perceived effort 1
   • However, setting PEEP greater than intrinsic PEEP can be harmful 1
   • Typically use external PEEP of 5 cm H₂O or less, even when auto-PEEP is 10-15 cm H₂O 1

3. Monitor for auto-triggering when using sensitive flow triggers, as cardiogenic oscillations or circuit leaks can cause false triggering 8

Critical Pitfall to Avoid

Never set external PEEP levels in excess of intrinsic PEEP, as this is explicitly stated to be deleterious. 1 The goal is to partially offset auto-PEEP (typically 80% or less), not to exceed it, which would worsen hyperinflation and potentially cause hemodynamic compromise. 7, 2