Gas Distribution Differences Between Spontaneous and Positive Pressure Ventilation
Yes, there are fundamental differences in gas distribution between spontaneous ventilation and positive pressure ventilation (PPV), with spontaneous breathing promoting better ventilation-perfusion matching in dependent lung regions while PPV preferentially distributes gas to non-dependent areas, though these differences can be partially mitigated through specific ventilatory strategies.
Fundamental Physiological Differences
Spontaneous ventilation creates negative intrathoracic pressure that preferentially ventilates dependent (gravity-dependent) lung regions, where perfusion is naturally greatest, resulting in superior ventilation-perfusion (V/Q) matching 1. In contrast, PPV delivers gas under positive pressure, which follows the path of least resistance to non-dependent, more compliant lung regions, creating V/Q mismatch 2.
Key Mechanisms of Gas Distribution
- During spontaneous breathing, diaphragmatic contraction generates negative pleural pressure that preferentially expands dependent lung zones, improving gas flow to well-perfused areas 1
- PPV bypasses this physiological mechanism, instead distributing gas based on regional compliance and resistance rather than perfusion, leading to ventilation of less-perfused non-dependent regions 2
- The interaction of positive pressure with airway resistance and alveolar compliance creates wide variation in ventilation efficacy throughout different lung areas 2
Clinical Evidence in Lung Injury
Spontaneous Breathing Effects
- In oleic acid-induced lung injury, spontaneous breathing during airway pressure-release ventilation (APRV) decreased shunt fraction by 13% and increased perfusion to normal V/Q units by 14% compared to full mechanical support 1
- Spontaneous breathing during APRV improved arterial oxygen tension from 75±3 to 107±8 mmHg and increased cardiac output from 3.9±0.3 to 4.6±0.4 L/min 1
- Dead space ventilation (V/Q >100) decreased by 6% during spontaneous breathing compared to pressure support ventilation 1
The Pendelluft Phenomenon
Spontaneous effort during mechanical ventilation causes "pendelluft"—displacement of gas from non-dependent (more recruited) to dependent (less recruited) lung regions during early inspiration 3. This phenomenon has dual effects:
- Pendelluft improves gas exchange and V/Q matching by redistributing gas to dependent zones 3
- However, at low PEEP levels, pendelluft generates the greatest tidal recruitment and potential for ventilator-induced lung injury 3
- Optimized PEEP (set after recruitment) reduces inspiratory effort from -5.6±1.3 to -2.0±0.7 cmH₂O, while concomitantly reducing pendelluft and tidal recruitment 3
Pressure Support Ventilation Considerations
Breath-to-breath ventilatory support with pressure support ventilation (PSV) does not provide the same V/Q matching benefits as unsupported spontaneous breathing during mechanical ventilation 1. The key distinction:
- PSV provides mechanical support for each spontaneous inspiratory effort, which is insufficient to counteract the V/Q maldistribution of positive pressure lung insufflation during acute lung injury 1
- Spontaneous breathing superimposed on mechanical ventilation (as in APRV) contributes to improved V/Q matching and increased systemic blood flow more effectively than PSV 1
CPAP vs EPAP Effects on Gas Distribution
Continuous positive airway pressure (CPAP) at 10 cmH₂O is more effective than expiratory positive airway pressure (EPAP) for achieving optimal lung volume and gas distribution 4:
- CPAP at 10 cmH₂O produced the greatest arterial oxygenation, functional residual capacity (FRC), and transpulmonary pressure at end-expiration 4
- CPAP allows relaxation of chest wall musculature on expiration, whereas EPAP at 10 cmH₂O increases chest wall muscle tone, potentially limiting lung expansion 4
Inspiratory Flow Pattern Effects
In acute lung injury, decelerating inspiratory flow (pressure-controlled ventilation) versus constant flow (volume-controlled ventilation) does not significantly affect gas distribution when tidal volume and PEEP are equivalent 5:
- No differences were found in recruitment, hemodynamics, or gas exchange between pressure-controlled and volume-controlled ventilation 5
- Pressure-controlled ventilation increased overinflated lung volumes, raising concerns for ventilator-associated lung injury 5
- Recruitment was predominantly associated with PEEP rather than inspiratory flow pattern 5
Clinical Algorithm for Optimizing Gas Distribution
For Patients with Acute Lung Injury:
If spontaneous breathing is present and safe, consider modes that allow unsupported spontaneous breaths (e.g., APRV) rather than breath-to-breath support (PSV) to maximize V/Q matching benefits 1
Apply optimized PEEP after recruitment maneuvers to:
Monitor for excessive spontaneous effort at low PEEP, which improves oxygenation but promotes injurious tidal recruitment 3
If full mechanical ventilation is required, use PEEP strategically as recruitment is predominantly PEEP-dependent rather than flow-pattern dependent 5
For Spontaneously Breathing Patients Requiring Support:
- Use CPAP rather than EPAP when applying 10 cmH₂O positive pressure to maximize FRC and oxygenation 4
- For preterm infants requiring positive pressure ventilation, apply approximately 5 cmH₂O PEEP to improve functional residual capacity and lung compliance 6
Critical Pitfalls to Avoid
- Avoid low PEEP with vigorous spontaneous effort, as this combination generates maximal pendelluft and tidal recruitment despite improved oxygenation 3
- Do not assume PSV provides the same V/Q benefits as true spontaneous breathing—the mechanical support for each breath negates some physiological advantages 1
- Recognize that positive pressure creates hydrostatic forces and raised venous pressures that promote interstitial fluid sequestration and impaired lymphatic drainage 2
- Excessive PEEP (8-12 cmH₂O) may reduce pulmonary blood flow and increase pneumothorax risk, particularly in preterm infants 6