How should I adjust the expiratory time and trigger settings for an adult female on SIMV who has inappropriate expiratory time settings causing poor synchrony and reduced tolerance?

Adjusting Expiratory Time and Trigger Settings in SIMV for Poor Synchrony

Shorten the expiratory time setting immediately—on some ventilators, setting a long expiratory time creates a long "lock out" period that prevents patient-triggered breaths, directly causing the poor synchrony and reduced tolerance you're observing. 1

Understanding the Problem

The issue stems from how SIMV (Synchronized Intermittent Mandatory Ventilation) handles the relationship between expiratory time and patient triggering:

• In assist/control modes like SIMV, the ventilator uses a "lock out" period to prevent breath stacking 1
• As respiratory rate increases, the lock out period must shorten proportionally 1
• Critical pitfall: On some ventilators, setting a long expiratory time automatically sets a long lock out period, which blocks patient-triggered breaths and causes intolerance 1

Specific Adjustment Algorithm

Step 1: Adjust Expiratory Time Based on Respiratory Rate

Target I:E ratio of at least 1:2, with longer expiration for obstructive disease:

• At respiratory rate 15 breaths/min: Use inspiratory time 1.2-1.6 seconds (30-40% of cycle time) 2
• For concurrent obstructive airways disease: Use shorter inspiratory time (~30% IPAP time) to allow adequate exhalation, especially at higher respiratory rates 2
• For restrictive disease: Use longer inspiratory time (~40% IPAP time) 2

Step 2: Optimize Trigger Sensitivity

Address intrinsic PEEP (PEEPi) which is the primary cause of trigger failure:

• Add EPAP (expiratory positive airway pressure) 3-5 cm H₂O to offset intrinsic PEEP 1
• This helps triggering by reducing the inspiratory effort needed before airflow begins 1
• Although PEEPi may reach 10-15 cm H₂O in severe COPD, EPAP levels >5 cm H₂O are rarely tolerated 1

Step 3: Consider Alternative Mode if Synchrony Remains Poor

If adjustments fail, switch to timed/ACV mode rather than continuing to struggle with patient-triggered breaths:

• Set mandatory breaths to match the patient's natural ventilatory pattern 1
• This is particularly important when the patient has advanced respiratory failure or insufficient respiratory effort to reliably trigger breaths 1

Key Mechanisms of Asynchrony

Patient-ventilator asynchrony results from:

• Undetected inspiratory effort
• Delay in response to start of inspiration
• Delay in detection of end of breath
• Excessive leakage 1

These causes may be difficult to resolve individually—hence the recommendation to use timed mode as an alternative 1

Common Pitfalls to Avoid

1. Don't assume all ventilators behave the same—the relationship between expiratory time and lock out period varies by manufacturer 1

2. Don't ignore the role of EPAP—it serves multiple functions: eliminates exhaled CO₂, aids triggering, and counteracts upper airway collapse 1

3. Don't persist with spontaneous triggering in advanced respiratory failure—patients may cease making spontaneous effort when "captured" or have hypoxic drive dependence 1

4. Don't overlook excessive leakage—this impairs both inspiratory and expiratory trigger function 1