Newer Modes of Ventilation

1. Pressure-Regulated Volume Control (PRVC)

• After each breath, the ventilator checks whether the delivered VT was adequate; if too low or too high, it adjusts the next breath's pressure accordingly.
• The result is a decelerating, variable flow pattern that is generally more comfortable for the patient than a fixed-flow volume-controlled breath.
• If respiratory mechanics improve (compliance rises), the ventilator reduces the applied pressure; if mechanics worsen, pressure increases to maintain the target volume.
• Volume Support (VS) is a closely related mode but applies to spontaneous (pressure-support style) breaths where the pressure target is auto-adjusted to achieve a set VT.

Murray & Nadel's Textbook of Respiratory Medicine, p. 3175; Washington Manual of Medical Therapeutics, p. 288

2. Airway Pressure Release Ventilation (APRV)

• Spontaneous breathing is permitted throughout the entire cycle (at any phase), which may enhance alveolar recruitment and improve cardiac filling compared to controlled modes.
• The short Tlow prevents complete lung derecruitment on release.
• Patients may be permitted to become hypercapnic to pH 7.20 ("permissive hypercapnia").
• A meta-analysis of nearly 19 years of data in acute hypoxemic respiratory failure suggested a mortality benefit compared to conventional modes (Sabiston); however, most randomized studies comparing APRV to true lung-protective strategies have not shown consistent differences in outcomes.
• A single-center RCT in ARDS (n=138) found more ventilator-free days (19 vs. 2, P<0.001) and shorter ICU stay with APRV vs. volume assist-control, though methodological limitations exist.

Murray & Nadel's, p. 3175; Fishman's Pulmonary Diseases, p. 2505; Sabiston Textbook of Surgery; Washington Manual, p. 288

3. High-Frequency Oscillatory Ventilation (HFOV)

• Taylor dispersion
• Coaxial flow (fresh gas flows centrally while CO2 exits peripherally)
• Augmented molecular diffusion
• Asymmetric velocity profiles

• The mean airway pressure is set at a level that maintains oxygenation (alveolar recruitment).
• The small oscillatory pressure swings on top of the mean pressure handle CO2 clearance.
• Theoretically the "ultimate" low-tidal-volume ventilator - small alveolar pressure swings minimize cyclic overdistention and derecruitment.

• OSCAR trial: No mortality benefit vs. usual care.
• OSCILLATE trial: Stopped early - HFOV arm showed higher in-hospital mortality vs. low VT/high-PEEP control group.
• Currently used only as a salvage mode in refractory hypoxemia or as a bridge to ECMO; even in this role, controlled trial evidence is lacking.
• May have a role in the pediatric population.

Fishman's Pulmonary Diseases, p. 2505; Murray & Nadel's, p. 3175; Washington Manual, p. 288

4. Adaptive Support Ventilation (ASV)

• The clinician sets only the desired minute ventilation and the patient's height (used to estimate anatomic dead space).
• The ventilator continuously measures resistance and compliance, calculates the expiratory time constant (R x C), and adjusts the frequency-VT pattern to minimize the work of ventilation (integral of pressure over volume).
• The breathing pattern is also modulated to avoid air trapping (using the expiratory time constant).
• When the patient triggers breaths, ASV behaves like volume-support mode.
• As respiratory mechanics change (e.g., during weaning), the pattern is automatically adjusted.

Murray & Nadel's Textbook of Respiratory Medicine, p. 3175-3176

5. Proportional Assist Ventilation (PAV / PAV+)

• Uses transient end-inspiratory occlusions to measure respiratory system elastance (1/compliance) and resistance.
• Estimates the pressure required to inflate the respiratory system using the equation of motion:

  Pventilator = Gain × (Elastance × Volume + Resistance × Flow)

• The ventilator delivers a fixed proportion (the "gain," set by clinician, e.g., 50-80%) of this total inflation pressure.
• Result: Greater patient effort → greater delivered pressure → respiratory muscles are unloaded in proportion.

• In pressure-targeted modes, delivered ventilation is essentially independent of effort.
• In volume-targeted modes, delivered ventilation actually decreases when effort increases (since increased flow from patient reduces ventilator flow delivery).
• PAV+ corrects this mismatch, making it more physiologic.

Murray & Nadel's Textbook of Respiratory Medicine, p. 3176

6. Neurally Adjusted Ventilatory Assist (NAVA)

• A specially designed nasogastric tube with EMG electrodes is placed esophageally to detect the electrical activity of the diaphragm (Edi) via the diaphragmatic crura.
• The ventilator delivers pressure in direct proportion to Edi magnitude: Pdelivered = NAVA level × Edi.

• Triggering occurs at neural onset (before airway pressure or flow signals change) - eliminates trigger delay.
• Cycling-off is also neural - ends when Edi falls, eliminating over-assistance or under-assistance.
• Improves patient-ventilator synchrony, especially in patients with intrinsic PEEP (auto-PEEP), high respiratory drive, or weak efforts.
• Reduces asynchrony events (double triggering, missed triggers, premature cycling).
• Applicable even in non-invasive ventilation (NIV-NAVA).
• Used in neonates, pediatric patients, and adults.

Murray & Nadel's Textbook of Respiratory Medicine, p. 3176; Miller's Anesthesia, 10e; Fishman's Pulmonary Diseases

7. Inverse Ratio Ventilation (IRV)

• Raises mean airway pressure and recruits slowly-filling alveoli, improving oxygenation in ARDS.
• The short expiratory time can create auto-PEEP (intrinsic PEEP), which may be the actual mechanism of improved oxygenation.
• Hazard: In obstructive lung disease, auto-PEEP can cause severe hyperinflation and hemodynamic compromise.
• Patients are permitted hypercapnia to pH 7.20.
• APRV is considered an extreme form of IRV.

Washington Manual of Medical Therapeutics, p. 288

8. Bilevel Positive Airway Pressure (BiPAP / BIPAP as invasive mode)

• Similar to APRV but with a longer Tlow (more balanced I:E ratio).
• Minimizes pressures seen by alveoli.
• Most commonly used in patients with severely reduced lung compliance (ARDS).

Summary Table

Sources: Murray & Nadel's Textbook of Respiratory Medicine; Fishman's Pulmonary Diseases and Disorders; Sabiston Textbook of Surgery; Washington Manual of Medical Therapeutics; Miller's Anesthesia, 10e

• All 7 modes covered in detail (PRVC, APRV, HFOV, ASV, PAV+, NAVA, IRV)
• APRV settings table
• Clinical evidence summaries for each mode
• A full summary comparison table at the end
• Clinical notes on ARDSNet strategy and driving pressure targets
• Source references throughout