Pressure Support and SIMV Modes
Key Points
- Pressure support ventilation (PSV) assists only patient-triggered breaths with a set inspiratory support pressure.
- SIMV guarantees a minimum number of breaths while permitting spontaneous breathing between mandatory cycles.
- PSV requires reliable spontaneous effort; deterioration may require return to controlled ventilation.
- SIMV can increase work of breathing and may prolong ventilator dependency compared with spontaneous-breathing-trial-driven liberation approaches.
Pathophysiology
Spontaneous breathing modes reduce full-machine control and rely more on patient respiratory drive. In PSV, every initiated breath receives a constant support pressure that reduces inspiratory effort while the patient determines rate and volume pattern. This can improve comfort and synchrony when spontaneous effort is preserved.
SIMV combines mandatory timed support with spontaneous intervals. During each ventilator cycle window, the ventilator synchronizes mandatory support with patient effort when present, or provides controlled support if no effort is detected. Although this preserves spontaneous participation, mismatch and high patient drive can increase work of breathing and reduce efficiency.
Classification
- PSV (pressure support ventilation): Set PS, PEEP, and FiO2; patient triggers all breaths.
- SIMV (synchronized intermittent mandatory ventilation): Set RR, Vt or inspiratory pressure, PEEP, FiO2, and optional pressure support.
- Observed variables: In PSV and SIMV, clinicians trend non-set outputs such as Vt, PIP, I-time, and I:E depending on mode implementation.
Nursing Assessment
NCLEX Focus
Priority questions often test which parameters are directly set versus observed in PSV versus SIMV and when mode failure should prompt escalation.
- Assess whether patient has adequate spontaneous drive and work-of-breathing tolerance for spontaneous-breathing modes.
- In PSV, monitor RR, Vt, and breathing pattern because the patient controls timing and volume.
- In SIMV, assess synchrony between mandatory cycle windows and spontaneous respiratory effort.
- Watch for signs of mode intolerance, rising work of breathing, and hemodynamic strain.
Nursing Interventions
- Verify ordered mode and ensure all directly set values are correct for selected strategy.
- In PSV, titrate support with team to keep work of breathing within acceptable limits.
- In SIMV, use pressure support on spontaneous breaths when ordered to reduce excessive effort.
- Reassess trend data after any adjustment and communicate early signs of fatigue or failure.
- Prioritize timely transition to evidence-supported liberation pathways when SIMV weaning is ineffective.
Hidden Work-of-Breathing Burden
Apparent spontaneous breathing can mask unsustainable respiratory effort and delay needed escalation.
Pharmacology
| Drug Class | Examples | Key Nursing Considerations |
|---|---|---|
| sedative-hypnotics | Synchrony and tolerance context | Excess sedation can suppress drive; insufficient sedation can worsen dyssynchrony and distress. |
| bronchodilators | Airflow-support context | Optimize airway resistance when elevated work of breathing limits spontaneous mode success. |
Clinical Judgment Application
Clinical Scenario
A patient on SIMV develops tachypnea and visible accessory-muscle use despite stable mandatory settings.
Recognize Cues: Spontaneous effort is present but appears inefficient and high burden. Analyze Cues: Spontaneous breaths may be inadequately supported or poorly synchronized. Prioritize Hypotheses: Immediate priority is reducing work of breathing and preventing fatigue. Generate Solutions: Reassess support pressure, evaluate synchrony, and consider alternative liberation strategy. Take Action: Notify team and implement ordered mode/support optimization. Evaluate Outcomes: Respiratory effort decreases and gas-exchange goals remain stable.
Related Concepts
- invasive-mechanical-ventilation-modes - Baseline mode framework before spontaneous-mode transitions.
- ventilator-parameter-adjustment-principles - Adjustment logic for oxygenation/ventilation while on spontaneous modes.
- nippv-initial-setup-and-reassessment - Noninvasive pathways may be considered during liberation planning.
- extubation-readiness-and-procedure - Final liberation step after sustained spontaneous-mode success.
- pressure-control-ventilation-monitoring - Trend interpretation skills transfer to SIMV pressure-targeted configurations.
Self-Check
- Which variables are directly set in PSV versus SIMV?
- Why can SIMV increase work of breathing in some patients?
- What cues suggest PSV or SIMV failure and need for escalation?