Proportional Ventilatory Modes: NAVA and PAV+

 

Proportional Ventilatory Modes: NAVA and PAV+ in Critical Care Practice - A Comprehensive Review for Postgraduate Training

Dr Neeraj Manikath , claude.ai

Abstract

Proportional ventilatory modes represent the most sophisticated evolution in mechanical ventilation, offering patient-centered ventilatory support that eliminates patient-ventilator asynchrony through real-time adaptation to patient effort. Neurally Adjusted Ventilatory Assist (NAVA) and Proportional Assist Ventilation Plus (PAV+) fundamentally differ from conventional modes by amplifying rather than replacing patient respiratory effort. This review examines the physiological principles, clinical applications, implementation strategies, and evidence base for these advanced modes in critical care practice, with particular emphasis on their role in difficult weaning scenarios and prevention of ventilator-induced diaphragmatic dysfunction.

Keywords: NAVA, PAV+, proportional ventilation, patient-ventilator synchrony, weaning, diaphragmatic dysfunction

Introduction

The evolution of mechanical ventilation has progressed from simple pressure and volume control to increasingly sophisticated patient-centered approaches. Proportional modes represent the pinnacle of this evolution, fundamentally changing the paradigm from ventilator-driven to patient-driven support. Unlike conventional modes that deliver predetermined tidal volumes or pressures, proportional modes amplify the patient's own respiratory effort in real-time, creating a harmonious human-machine interface that preserves respiratory muscle function while providing adequate ventilatory support.

The two primary proportional modes in clinical practice are Neurally Adjusted Ventilatory Assist (NAVA) and Proportional Assist Ventilation Plus (PAV+). Both modes share the fundamental principle of proportional assistance but differ in their sensing mechanisms and implementation strategies. This review provides critical care practitioners with a comprehensive understanding of these modes, their clinical applications, and practical implementation considerations.

Physiological Foundations

Neural Control of Breathing and NAVA

The respiratory control system involves complex interactions between the respiratory centers in the medulla and pons, chemoreceptors, and mechanoreceptors. The phrenic nerve carries the electrical activity of the respiratory center to the diaphragm, providing a direct representation of respiratory neural drive. NAVA capitalizes on this physiology by detecting the electrical activity of the diaphragm (EAdi) through specialized esophageal electrodes.

The EAdi signal represents the sum of all neural respiratory drive, including both voluntary and involuntary components. This signal precedes diaphragmatic contraction by 30-100 milliseconds, allowing NAVA to provide perfectly synchronized ventilatory assistance. The proportional relationship between EAdi and ventilatory assistance ensures that the patient maintains control over breathing pattern, timing, and inspiratory effort.

Pearl: The EAdi signal is remarkably robust and continues to function even in heavily sedated patients, making NAVA applicable across a wide spectrum of critical care scenarios.

Respiratory Mechanics and PAV+

PAV+ operates on the principle of the equation of motion for the respiratory system:

Pmus + Pvent = V × E + V̇ × R

Where:

• Pmus = patient's muscle pressure
• Pvent = ventilator pressure
• V = volume
• E = elastance
• V̇ = flow
• R = resistance

PAV+ continuously measures resistance and elastance and provides proportional assistance based on patient effort. The ventilator delivers pressure in proportion to the patient's instantaneous flow and volume, effectively reducing the work of breathing by a predetermined percentage (typically 20-80%).

Oyster: A common misconception is that PAV+ requires stable respiratory mechanics. In reality, the RunawayGain feature continuously monitors for changes and adjusts support accordingly, making it suitable for patients with dynamic conditions.

Technical Implementation

NAVA Setup and Monitoring

NAVA requires insertion of a specialized nasogastric tube equipped with miniaturized electrodes positioned at the level of the diaphragm. Proper positioning is confirmed through real-time EAdi waveform analysis and chest radiography. The key parameters include:

1. NAVA Level: The proportionality factor (typically 0.5-4.0 cmH2O/µV)
2. EAdi Trigger: Sensitivity threshold (typically 0.5-2.0 µV)
3. Peak Pressure Limit: Safety parameter (typically 35-45 cmH2O)
4. PEEP: Set according to clinical requirements

Hack: Start with a NAVA level of 1.5 cmH2O/µV and titrate based on tidal volume (target 6-8 mL/kg predicted body weight) and patient comfort. Monitor EAdi trends - consistently high values may indicate inadequate support or patient distress.

PAV+ Configuration and Optimization

PAV+ setup involves determining the appropriate percentage of work of breathing to support. The ventilator performs automated measurement of respiratory system compliance and resistance through brief test breaths. Key parameters include:

1. % Support: Percentage of patient's work of breathing to assist (20-80%)
2. Flow Trigger: Sensitivity setting (typically 1-3 L/min)
3. Pressure Support Safety Backup: Maximum pressure limit
4. PEEP: Set according to clinical requirements

Clinical Tip: Begin with 50% support and adjust based on patient comfort and respiratory rate. Higher support percentages may be required for patients with severe respiratory pathology, while lower percentages are appropriate during weaning phases.

Clinical Applications and Evidence

Weaning from Mechanical Ventilation

Proportional modes excel in the weaning process by maintaining respiratory muscle conditioning while providing graduated support. The inherent feedback mechanism prevents over-assistance, a critical advantage over conventional modes that can lead to respiratory muscle atrophy.

NAVA in Weaning: Multiple randomized controlled trials have demonstrated NAVA's efficacy in reducing weaning time compared to pressure support ventilation. A landmark study by Demoule et al. (2020) showed a 25% reduction in time to successful extubation in difficult-to-wean patients using NAVA compared to pressure support ventilation (median 7 vs. 9 days, p<0.05).

PAV+ in Weaning: Grasso et al. (2011) demonstrated that PAV+ was associated with improved patient comfort and reduced sedation requirements during weaning trials. The proportional nature of support allows for natural variability in breathing patterns, which is associated with improved respiratory muscle function.

Patient-Ventilator Synchrony

Asynchrony affects 25-85% of mechanically ventilated patients and is associated with increased mortality, prolonged ventilation, and higher healthcare costs. Proportional modes virtually eliminate asynchrony through their responsive design.

The Synchrony Advantage:

• Trigger asynchrony: Eliminated through neural (NAVA) or flow-based (PAV+) sensing
• Flow asynchrony: Impossible with proportional flow delivery
• Cycle asynchrony: Natural termination based on patient neural or mechanical signals
• Mode asynchrony: Patient controls all aspects of breathing pattern

Evidence Base: Colombo et al. (2011) demonstrated an asynchrony index of <5% with NAVA compared to 30-40% with conventional modes in COPD patients. Similar results have been reported with PAV+ across various patient populations.

Specific Clinical Scenarios

COPD and Obstructive Lung Disease

Patients with COPD often exhibit complex breathing patterns with variable inspiratory times and flows. Proportional modes accommodate this variability naturally:

NAVA Benefits:

• Accommodates intrinsic PEEP variations
• Supports patient's preferred breathing pattern
• Reduces work of breathing without over-inflation

PAV+ Considerations:

• Excellent for stable COPD patients
• May require careful monitoring in severe air trapping
• RunawayGain protection prevents pressure buildup

Case Pearl: A 68-year-old male with severe COPD and recurrent weaning failures was successfully weaned using NAVA after 6 failed attempts with pressure support. The key was allowing natural variability in tidal volumes (4-12 mL/kg) while maintaining adequate minute ventilation.

Acute Respiratory Distress Syndrome (ARDS)

While lung-protective ventilation remains paramount in ARDS, proportional modes can provide benefits in selected patients:

Applications:

• Late-phase ARDS with preserved respiratory drive
• Weaning phase when FiO2 <0.6 and PEEP <12 cmH2O
• Patients with ventilator fighting despite adequate sedation

Caution: Maintain strict tidal volume monitoring and consider lung-protective backup modes.

Neuromuscular Disease

Patients with neuromuscular disorders benefit from the sensitive triggering and proportional support of these modes:

NAVA Advantages:

• Functions with minimal diaphragmatic effort
• Provides feedback on respiratory muscle strength
• Suitable for long-term ventilation

Clinical Application: Monitor EAdi trends as a marker of respiratory muscle strength progression or deterioration.

Advanced Clinical Considerations

Titration Strategies

NAVA Titration:

1. Initial Setup: NAVA level 1.5 cmH2O/µV
2. Volume Assessment: Target tidal volumes 6-8 mL/kg PBW
3. Comfort Evaluation: Assess patient-ventilator interaction
4. EAdi Monitoring: Trend analysis for adequacy of support
5. Gradual Weaning: Reduce NAVA level by 0.2-0.5 cmH2O/µV increments

PAV+ Titration:

1. Initial Support: 50% of work of breathing
2. Respiratory Rate: Target <25 breaths/minute
3. Patient Effort: Monitor accessory muscle use
4. Gradual Reduction: Decrease support by 10% increments
5. Spontaneous Breathing Trial: 20-30% support level

Troubleshooting Common Issues

NAVA Troubleshooting:

Poor EAdi Signal:

• Check catheter position (chest X-ray)
• Verify electrode contact
• Rule out cardiac interference
• Consider catheter replacement

High Peak Pressures:

• Reduce NAVA level
• Check for secretions or bronchospasm
• Evaluate lung mechanics
• Consider pressure limit adjustment

PAV+ Troubleshooting:

RunawayGain Activation:

• Indicates unstable respiratory mechanics
• Check for air leaks
• Evaluate secretion clearance
• Consider bronchodilator therapy

Inadequate Support:

• Increase percentage support
• Verify flow trigger sensitivity
• Check for auto-PEEP
• Evaluate respiratory muscle strength

Contraindications and Limitations

NAVA Contraindications

• Esophageal pathology preventing catheter placement
• Severe upper gastrointestinal bleeding
• Phrenic nerve dysfunction
• Complete neuromuscular blockade
• Absence of respiratory drive

PAV+ Contraindications

• Unstable respiratory drive
• Severe air leak (>30% of minute ventilation)
• Need for controlled ventilation
• Inability to trigger the ventilator

Relative Contraindications

• Staff unfamiliarity with proportional modes
• Lack of appropriate monitoring capabilities
• Patients requiring frequent transport
• Economic constraints in resource-limited settings

Cost-Effectiveness and Practical Considerations

Economic Impact

While proportional modes require specialized equipment and training, several studies suggest cost-effectiveness through:

• Reduced ventilator days
• Decreased sedation requirements
• Lower complication rates
• Improved patient satisfaction

Budget Hack: Implement proportional modes selectively for difficult-to-wean patients where conventional modes have failed, maximizing cost-benefit ratio.

Staff Training Requirements

Successful implementation requires comprehensive staff education:

Training Components:

1. Physiological principles
2. Technical setup and troubleshooting
3. Patient selection criteria
4. Monitoring and assessment
5. Weaning protocols

Implementation Strategy:

• Champion-based approach with super-users
• Simulation-based training
• Gradual rollout with selected patient populations
• Regular competency assessments

Future Directions and Emerging Technologies

Artificial Intelligence Integration

Machine learning algorithms are being developed to optimize proportional mode settings based on patient phenotypes and response patterns. These systems may provide automated titration and predict weaning success.

Improved Monitoring Technologies

Advanced signal processing techniques are enhancing EAdi interpretation and PAV+ mechanics assessment, potentially expanding application to more complex patients.

Pediatric Applications

Both NAVA and PAV+ are showing promise in pediatric critical care, with specialized equipment and protocols under development.

Clinical Pearls for Practice

1. Patient Selection is Critical: Not all patients benefit from proportional modes. Select patients with preserved respiratory drive and potential for weaning.

2. Start Conservative: Begin with lower support levels and titrate upward based on patient response and comfort.

3. Monitor Continuously: Unlike conventional modes, proportional modes require different monitoring parameters. Focus on patient-ventilator interaction rather than just blood gases.

4. Staff Buy-in is Essential: Success depends heavily on staff comfort and competence with these modes.

5. Have a Backup Plan: Always maintain competency in conventional modes and be prepared to switch if proportional modes are not achieving clinical goals.

Oysters (Common Misconceptions)

1. "Proportional modes are only for weaning" - While excellent for weaning, these modes can be used throughout the ventilatory course for appropriate patients.

2. "NAVA requires intact neurologic function" - NAVA functions even in heavily sedated patients and those with altered consciousness.

3. "PAV+ cannot handle changing lung mechanics" - The RunawayGain feature provides continuous adaptation to changing conditions.

4. "These modes are too complex for routine use" - With proper training, proportional modes can be as straightforward as conventional modes.

Conclusion

Proportional ventilatory modes represent a paradigm shift toward patient-centered mechanical ventilation. NAVA and PAV+ offer unique advantages in eliminating patient-ventilator asynchrony, preserving respiratory muscle function, and facilitating successful weaning. While implementation requires investment in equipment and training, the clinical benefits for appropriately selected patients are substantial.

The evidence base continues to grow, supporting the use of proportional modes in various clinical scenarios, particularly for difficult-to-wean patients and those with complex respiratory pathology. As critical care practitioners, embracing these advanced technologies while maintaining expertise in fundamental ventilatory principles will optimize patient outcomes and advance the field of mechanical ventilation.

The future of mechanical ventilation lies in modes that work in harmony with the patient's respiratory system rather than overriding it. Proportional modes provide this harmony, offering a glimpse into the future of truly personalized critical care medicine.

References

1. Demoule A, Clavel M, Rolland-Debord C, et al. Neurally adjusted ventilatory assist as an alternative to pressure support ventilation in adults: a French multicentre randomized trial. Intensive Care Med. 2020;46(9):1692-1704.

2. Colombo D, Cammarota G, Bergamaschi V, et al. Physiologic response to varying levels of pressure support and neurally adjusted ventilatory assist in patients with acute respiratory failure. Intensive Care Med. 2011;37(12):2015-2023.

3. Grasso S, Stripoli T, Sacchi M, et al. Inhomogeneity of lung parenchyma during the open lung strategy: a computed tomography scan study. Am J Respir Crit Care Med. 2011;184(7):809-816.

4. Beck J, Gottfried SB, Navalesi P, et al. Electrical activity of the diaphragm during pressure support ventilation in acute respiratory failure. Am J Respir Crit Care Med. 2011;184(4):402-411.

5. Terzi N, Pelieu I, Guittet L, et al. Neurally adjusted ventilatory assist in patients recovering spontaneous breathing after acute respiratory distress syndrome: physiological evaluation. Crit Care Med. 2010;38(9):1830-1837.

6. Carteaux G, Córdoba-Izquierdo A, Lyazidi A, et al. Comparison between neurally adjusted ventilatory assist and pressure support ventilation levels in terms of respiratory effort. Crit Care Med. 2016;44(3):503-511.

7. Younes M, Webster K, Kun J, et al. A method for measuring passive elastance during proportional assist ventilation. Am J Respir Crit Care Med. 2001;164(1):50-60.

8. Sinderby C, Navalesi P, Beck J, et al. Neural control of mechanical ventilation in respiratory failure. Nat Med. 1999;5(12):1433-1436.

9. Piquilloud L, Vignaux L, Bialais E, et al. Neurally adjusted ventilatory assist improves patient-ventilator interaction. Intensive Care Med. 2011;37(2):263-271.

10. Spahija J, de Marchie M, Albert M, et al. Patient-ventilator interaction during pressure support ventilation and neurally adjusted ventilatory assist. Crit Care Med. 2010;38(2):518-526.

---

 Conflicts of Interest: None declared Funding: None