Difference between SIMV Vs APRV for My MD Anaesthesiology exam notes
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SIMV vs APRV: Comparison for MD Anaesthesiology Exam Notes
1. SIMV - Synchronized Intermittent Mandatory Ventilation
Definition
A ventilator mode that delivers a preset number of mandatory breaths (volume- or pressure-targeted) synchronized to the patient's own inspiratory effort. Between mandatory breaths, the patient can breathe spontaneously without ventilator assistance (unless Pressure Support is added).
Breath Sequence Classification
- PC-IMV or VC-IMV (Intermittent Mandatory Ventilation)
- The mandatory breath is "synchronized" = timed to coincide with the beginning of a spontaneous effort to avoid breath stacking
How It Works
- Clinician sets: Rate (RR), Tidal Volume (VT) or Pressure, FiO2, PEEP
- Ventilator delivers the set number of mandatory breaths per minute
- If patient's RR > set rate: additional breaths above the set rate are unsupported spontaneous breaths (patient does all the work)
- If patient is paralyzed: SIMV and AC look identical - both deliver at the set rate
- SIMV + PSV: Pressure support is typically added to assist spontaneous breaths and reduce work of breathing imposed by the ET tube and circuit resistance
Key Settings
| Parameter | Typical Value |
|---|---|
| Set RR | 10-14 breaths/min (full support) → reduced during weaning |
| VT | 6-8 mL/kg IBW |
| PEEP | 5 cm H2O (adjust per ABG/SpO2) |
| FiO2 | Titrate to SpO2 ≥94% |
| PSV (add-on) | 5-10 cm H2O for spontaneous breaths |
Work of Breathing (Key Exam Point)
- Mandatory breaths: ventilator does the work
- Spontaneous breaths between mandatories: patient does ALL the work, including overcoming ET tube resistance
- This unequal work distribution is SIMV's major disadvantage
- PSV must be added to reduce this burden
Weaning with SIMV (Morgan & Mikhail)
- Progressively decrease RR by 1-2 breaths/min
- Check ABG after min. 15-30 min at each setting
- Criteria to continue decreasing: PaCO2 <45-50 mmHg, RR <30 breaths/min
- When IMV rate reaches 2-4 breaths/min with acceptable oxygenation: discontinue ventilation
- If PSV is co-used: reduce to 5-8 cm H2O before extubation
Disadvantages (Exam High-Yield)
- Delays liberation from mechanical ventilation vs T-piece trials or PSV-alone weaning (Murray & Nadel, Roberts & Hedges)
- High work of breathing during spontaneous breaths
- Patient-ventilator dyssynchrony if rate set too high (masks effort) or too low (insufficient backup)
- If rate is set too low (e.g., 4 breaths/min) and patient is weak - backup may be insufficient
2. APRV - Airway Pressure Release Ventilation
Definition
A time-cycled, pressure-targeted mode that maintains a prolonged high CPAP level (P-high) during which the patient breathes spontaneously, with brief, intermittent pressure releases (P-low) to eliminate CO2. Also known as bilevel ventilation, biphasic airway pressure, or (when paralyzed) inverse ratio ventilation.
Breath Sequence Classification
- PC-IMV variant / CPAP-based mode
- Modification of pressure-targeted SIMV that allows unrestricted spontaneous breathing at ALL phases of the cycle (including T-low)
How It Works
The four key parameters:
| Parameter | Description | Typical Value |
|---|---|---|
| P-high | High CPAP level - maintains alveolar recruitment; oxygenation phase | 20-35 cm H2O |
| T-high | Time at P-high (patient breathes spontaneously here) | 4-6 seconds |
| P-low | Low CPAP/release pressure - CO2 clearance phase | 0-5 cm H2O |
| T-low | Duration of release/exhalation phase | 0.2-0.8 s (restrictive); 0.8-1.5 s (obstructive) |
- Releases occur 10-12 times/min (starting point)
- T-low is set to allow only 50-75% of exhalation to complete (flow termination criterion), creating auto-PEEP
- Patient can breathe in AND out at any point during T-high
The "Floating Valve" Concept
A demand valve responds to patient effort regardless of where the patient is in the cycle - this is what makes APRV fundamentally different from cyclic ventilation. There is no traditional set inspiratory or expiratory time per breath.
Oxygenation vs Ventilation in APRV (Exam High-Yield)
| Goal | Mechanism | Parameter to Adjust |
|---|---|---|
| Oxygenation | Long T-high maintains high mean airway pressure, recruits alveoli | ↑ P-high, ↑ T-high, ↑ FiO2 |
| Ventilation (CO2 clearance) | Pressure drop from P-high to P-low + elastic recoil drives large expiratory flow | ↑ Number of releases (↓ T-high), adjust T-low |
APRV Waveform
The pressure-time trace shows prolonged P-high phase with brief drops to P-low. The flow-time trace shows spontaneous breathing efforts (small positive/negative deflections) during T-high, and a large negative deflection (high expiratory flow) during the release. Small exhalations during T-high (50-200 mL) should not be counted as full breaths.
Classic APRV trace: patient breathes spontaneously at P-high (~20 cm H2O), with periodic drops to P-low (~5 cm H2O). Spontaneous breaths visible at both P-high and P-low levels.
Advantages of APRV
- Maintains spontaneous breathing → better V/Q matching, diaphragm recruitment, hemodynamic preservation
- Less sedation/NMB required compared to IRV or CMV (no chemical paralysis needed)
- Higher mean airway pressure without proportionally higher peak pressure → better recruitment
- Reduces barotrauma risk compared to PC-IRV
- Less circulatory depression than other high-PEEP modes
- Open-lung strategy - ideal for ARDS with poor compliance
- Avoids need for paralysis (unlike IRV) - patient comfort maintained
Indications
- ARDS (early, diffuse) - especially when traditional LPV is failing
- ALI with refractory hypoxemia
- Post-op respiratory failure
- Alternative to PC-IRV when paralysis is undesirable
3. SIMV vs APRV: Direct Comparison Table
| Feature | SIMV | APRV |
|---|---|---|
| Mode type | Volume or pressure-targeted IMV | Pressure-targeted, time-cycled |
| Primary pressure level | Variable (set by VT or Pinsp) | High CPAP (P-high): 20-35 cm H2O |
| Time distribution | Equal I:E ratio (e.g., 1:2) | Majority time at P-high (4-6s high : 0.2-1.5s low) |
| I:E ratio | Normal (1:2 or 1:3) | Effectively INVERSE (I:E >>1, like IRV) |
| Spontaneous breathing | Between mandatory breaths only (unsupported unless PSV added) | During ALL phases - at P-high AND P-low |
| CO2 clearance | Via mandatory + spontaneous breaths | Via pressure release (T-low) + elastic recoil |
| Oxygenation mechanism | FiO2 + PEEP | High mean Paw via prolonged P-high |
| Sedation requirement | Moderate | Low (minimal sedation, no paralysis needed) |
| NMB required? | No | No (key advantage over IRV) |
| Work of breathing | High during unsupported spontaneous breaths | Distributed, supported throughout cycle |
| Weaning | Step-down RR by 1-2/min | Lengthen T-high, reduce P-high gradually |
| Liberation from MV | Delays liberation (evidence-based) | Potentially fewer ventilator days (single-center data) |
| Primary use | General MV support, weaning | ARDS, refractory hypoxemia, open-lung strategy |
| Mean airway pressure | Lower | Higher (lung recruitment) |
| Peak airway pressure | Can be high (VC mode) | Limited by P-high setting |
| Barotrauma risk | Higher in VC mode | Lower than IRV, comparable to lung-protective CMV |
| Hemodynamic effects | Less depression than APRV (lower mean Paw) | More circulatory depression (higher mean Paw) - monitor RV |
| Settings | RR, VT (or Pinsp), PEEP, FiO2, (+PSV) | P-high, P-low, T-high, T-low, FiO2 |
| Breath sequence | PC-IMV or VC-IMV | PC-IMV (CPAP-based variant) |
4. Key Exam Mnemonics & High-Yield Points
APRV = "CPAP with a release valve"
- P-high = Platform for oxygenation
- T-high = Time to recruit (long)
- P-low = Pressure drops for CO2 clearance
- T-low = Tiny window (short - to preserve auto-PEEP)
SIMV Key Facts
- "S" = Synchronized - breath timed to patient's effort (prevents stacking)
- High RR SIMV ≈ full support; Low RR SIMV ≈ near spontaneous breathing
- Always add PSV to unsupported breaths
- SIMV delays weaning compared to T-piece or PSV trials (Esteban et al.)
APRV Key Facts
- Inverse ratio without paralysis = APRV's signature
- Mean Paw > PEEP (auto-PEEP from short T-low)
- Terminate T-low when expiratory flow reaches 50-75% of peak (prevents derecruitment)
- APRV = PC-IRV when patient is paralyzed (functionally identical)
5. Clinical Decision: When to Choose Which?
| Scenario | Preferred Mode |
|---|---|
| Standard post-op ventilation | SIMV+PSV or AC |
| Weaning from MV | PSV > T-piece > SIMV (SIMV slowest) |
| ARDS with refractory hypoxemia | APRV (open-lung) |
| Patient requiring heavy sedation or NMB | Avoid APRV (defeats its purpose) |
| Obstructive lung disease (COPD/asthma) | Avoid APRV (air trapping risk, T-low adjustment needed) |
| Head injury (need tight CO2 control) | Avoid APRV (CO2 control less precise) |
| Spontaneously breathing ARDS patient | APRV preferred |
Sources:
- Morgan and Mikhail's Clinical Anesthesiology, 7e, Chapters 58 (Mechanical Ventilation, Weaning with SIMV, APRV)
- Roberts and Hedges' Clinical Procedures in Emergency Medicine, Chapter 8 (SIMV, APRV)
- Murray & Nadel's Textbook of Respiratory Medicine, Chapter 135 (Modes of Ventilation, APRV)
- Current Surgical Therapy, 14e (SIMV, APRV)
- Sabiston Textbook of Surgery (SIMV, APRV/Bilevel)
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