Ventilation modes

Background

  • Ventilation modes define the pattern of breath delivery by a mechanical ventilator
  • Understanding modes is critical for optimizing oxygenation and ventilation while minimizing ventilator-associated lung injury
  • Mode selection is determined by the clinical scenario, degree of respiratory failure, and patient effort
  • All modes control some combination of volume, pressure, and flow during the respiratory cycle
  • The variable that is set (controlled) determines the mode category; the other variable becomes dependent

Key Terminology

  • Tidal volume (TV): volume of gas delivered per breath
  • Peak inspiratory pressure (PIP): maximum airway pressure during inspiration
  • Plateau pressure (Pplat): pressure measured during an inspiratory hold; reflects alveolar pressure
  • Driving pressure: Pplat minus PEEP; values < 14 cmH2O associated with improved survival[2]
  • PEEP: positive end-expiratory pressure; prevents alveolar collapse and improves oxygenation
  • FiO2: fraction of inspired oxygen
  • I:E ratio: ratio of inspiratory to expiratory time
  • Trigger: mechanism by which patient initiates a breath (flow or pressure)
  • Cycle: mechanism by which inspiration ends and expiration begins

Volume Control vs. Pressure Control

  • Volume-controlled modes: TV is set; airway pressure is variable
    • Guarantees minute ventilation
    • PIP varies with changes in compliance and resistance
  • Pressure-controlled modes: inspiratory pressure is set; TV is variable
    • Limits peak airway pressures
    • TV varies with compliance and resistance changes
    • Decelerating flow pattern may improve gas distribution

Modes

Volume Assist Control (AC)

  • The recommended default mode for ED ventilation[3]
  • Prevents patient fatigue by offering full respiratory support with every breath
  • The safety and ease of this mode typically outweighs the theoretical benefits of other modes[3]
  • Preset rate and TV; every breath (mandatory and triggered) delivers the full set tidal volume
  • Patient able to trigger additional breaths above set rate (each delivers full assisted tidal volume)
  • Beneficial for patients requiring a high minute ventilation (reduces O2 consumption and CO2 production of the respiratory muscles)
  • May worsen obstructive airway disease by air trapping or breath stacking
  • Set: RR, FiO2, PEEP, TV, I:E ratio (or inspiratory flow rate)
  • ED Pearl: Start with lung protective settings (TV 6-8 mL/kg ideal body weight, RR 16-18, PEEP 5, FiO2 100% and wean)[4]

Pressure Assist Control (Pressure Control Ventilation, PCV)

  • Set inspiratory pressure, PEEP, RR, and inspiratory time
  • Every breath (mandatory and triggered) delivers gas at the set pressure
  • TV varies based on lung compliance and airway resistance
  • Decelerating flow pattern may improve gas distribution and comfort
  • Useful when high peak pressures are problematic (e.g., bronchopleural fistula, subcutaneous emphysema)
  • Limitation: TV not guaranteed; must closely monitor for hypoventilation if compliance changes
  • Set: inspiratory pressure, PEEP, RR, I-time, FiO2

Synchronous Intermittent Mandatory Ventilation (SIMV)

  • Preset mandatory breaths delivered in coordination with the patient's respiratory effort
  • Spontaneous breathing allowed between mandatory breaths
    • Spontaneous breaths receive only whatever pressure support is additionally set (not full TV)
  • Synchronization attempts to limit barotrauma by not delivering a breath when already maximally inhaled (vs. older IMV)
  • Because of need for patient effort, not recommended for tired or septic patients
  • Historically believed to ease weaning but has not demonstrated superiority over AC for weaning[5]
  • For the paralyzed patient, there is no difference in minute ventilation or airway pressures between AC and SIMV
  • Set: RR, FiO2, PEEP, TV, I:E ratio, PS (for spontaneous breaths)

Pressure Support Ventilation (PSV)

  • Patient-triggered, pressure-limited, flow-cycled mode
  • Level of pressure set (not TV) to augment each spontaneous effort
  • Limits barotrauma and decreases the work of breathing in the spontaneously breathing patient
  • Breath terminates when inspiratory flow drops below a threshold (typically 25% of peak flow)
  • Most ventilators allow a back-up respiratory rate (apnea backup) in case of apnea
  • Mode of choice in patients whose respiratory failure is not severe and who have adequate respiratory drive
    • Improved patient comfort, reduced cardiovascular effects, reduced risk of barotrauma, improved gas distribution[6]
  • Commonly used during weaning and spontaneous breathing trials
  • Limitation: TV not guaranteed; should not be used in patients with unreliable respiratory drive
  • Set: PS level, FiO2, PEEP (RR is patient-determined)

Pressure Regulated Volume Control (PRVC)

  • Dual-control mode that combines features of volume and pressure control
  • Provides pressure-limited breaths with a decelerating flow pattern
  • Ventilator adjusts inspiratory pressure breath-to-breath to achieve a target tidal volume based on measured compliance
  • Benefits: minimum PIP, guaranteed tidal volume, patient can trigger additional breaths, breath-by-breath adaptation
  • Not recommended for asthma or COPD (auto-PEEP may cause the ventilator to inappropriately lower pressure support)
  • Set: FiO2, RR, TV (target), upper pressure limit, I:E ratio, PEEP

Airway Pressure Release Ventilation (APRV)

Main article: Airway pressure release ventilation

  • Pressure-controlled, time-cycled mode with inverse I:E ratio
  • Sustained high constant pressure (Phigh) for alveolar recruitment with brief intermittent releases (Tlow) for ventilation
  • Allows unrestricted spontaneous breathing at both pressure levels
  • "Open lung" ventilation strategy used as rescue therapy in severe ARDS
  • Cannot be used with paralysis (requires spontaneous breathing for full benefit)
  • Main limitation is hypercarbia/respiratory acidosis due to short release times
  • Set:Phigh, Plow, Thigh, Tlow, FiO2

CPAP (Continuous Positive Airway Pressure)

  • Provides constant positive pressure throughout the respiratory cycle to a spontaneously breathing patient
  • Equivalent to PEEP without additional inspiratory support
  • Recruits collapsed alveoli, improves oxygenation, reduces work of breathing
  • Not appropriate for fatiguing patients or those with inadequate respiratory drive
  • Invasive CPAP is used during weaning/spontaneous breathing trials
  • Noninvasive CPAP - see Noninvasive ventilation
  • Set: FiO2, PEEP (CPAP level), back-up RR (if available)

Control Mode (CMV)

  • Ventilator initiates and controls every breath; no patient triggering allowed
  • Fixed rate and TV
  • Primarily used in the OR setting
  • Requires patient to be fully sedated or paralyzed
  • Not used in the ED

High Frequency Oscillatory Ventilation (HFOV)

  • Delivers very small tidal volumes at extremely high respiratory rates (3-15 Hz)
  • Maintains near-constant airway pressure (mean airway pressure) with minimal tidal excursion
  • Historically considered a rescue mode for refractory ARDS
  • OSCILLATE and OSCAR trials showed no mortality benefit and possible harm; largely abandoned[7][8]

ED-Specific Considerations

Mode Selection by Clinical Scenario

Clinical Scenario Recommended Mode Key Settings
Default / undifferentiated Volume AC TV 6-8 mL/kg IBW, RR 16-18, PEEP 5
ARDS Volume AC (lung protective) TV 6 mL/kg IBW, Pplat <30, PEEP per ARDSnet table
Asthma / COPD Volume AC TV 6-8 mL/kg IBW, RR 10, I:E 1:4-1:5, PEEP 0-5
Severe Metabolic acidosis Volume AC TV 6-8 mL/kg IBW, high RR (match pre-intubation minute ventilation)
Refractory ARDS APRV Phigh ≤ 35, Thigh 4.5-6s, Tlow 0.5-0.8s
Weaning trial PSV or CPAP PS 5-8, PEEP 5, FiO2 ≤0.4

Common Pitfalls

  • Forgetting to use ideal body weight for tidal volume calculation (obese patients are at high risk for volutrauma)
  • Not matching pre-intubation minute ventilation in patients with severe Metabolic acidosis (e.g., DKA, severe sepsis)
    • Can cause cardiovascular collapse from worsening acidosis post-intubation[9]
  • Inadequate sedation leading to ventilator dyssynchrony and breath stacking
  • Using SIMV when patient cannot reliably generate adequate spontaneous breaths
  • Not reassessing ventilator settings after initial setup (check plateau pressure within 30 minutes)

See Also

Mechanical Ventilation Pages

External Links

References

  1. ↑ The Acute Respiratory Distress Syndrome Network. Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. N Engl J Med. 2000;342(18):1301-1308.
  2. ↑ Amato MB, Meade MO, Slutsky AS, et al. Driving pressure and survival in the acute respiratory distress syndrome. N Engl J Med. 2015;372(8):747-755.
  3. ↑ 3.0 3.1 Weingart SD. Managing Initial Mechanical Ventilation in the Emergency Department. Ann Emerg Med. 2016;68:614-617.
  4. ↑ Brower RG, et al. Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. N Engl J Med. 2000;342(18):1301-1308.
  5. ↑ Esteban A, Frutos F, Tobin MJ, et al. A comparison of four methods of weaning patients from mechanical ventilation. N Engl J Med. 1995;332(6):345-350.
  6. ↑ MacIntyre NR. Respiratory function during pressure support ventilation. Chest. 1986;89(5):677-683.
  7. ↑ Ferguson ND, Cook DJ, Guyatt GH, et al. High-frequency oscillation in early acute respiratory distress syndrome. N Engl J Med. 2013;368(9):795-805.
  8. ↑ Young D, Lamb SE, Shah S, et al. High-frequency oscillation for acute respiratory distress syndrome. N Engl J Med. 2013;368(9):806-813.
  9. ↑ Weingart SD, Levitan RM. Preoxygenation and prevention of desaturation during emergency airway management. Ann Emerg Med. 2012;59(3):165-175.e1.
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