Interpreting the Shape of the Pressure-Volume Loop

Context and Relevance

• Relevant to Sections F3(iv) and F3(vii) of the 2023 CICM Primary Syllabus
• Discusses pressure-volume and flow-volume relationships in the respiratory system.
• Historically important for understanding ARDS and optimizing PEEP.

Pressure-Volume Loop in Spontaneous Breathing

• Inspiration: Creates negative pressure, trends to zero as lungs fill.
• Expiration: Elastic recoil creates positive pressure, decreases towards zero as volume is exhaled.
• PEEP Application: Shifts loop right as baseline pressure increases.

Anatomy of the Pressure-Volume Loop

• Ventilator Mode: Volume-control, mandatory mode with constant flow.
• Optimal Compliance Curve: Represents ideal alveolar inflation.

Inflection Points in the Loop

• Lower Inflection Point (Pflex): Indicates critical opening pressure of alveoli.
  • Represents alveolar recruitment; may suggest PEEP settings.
• Upper Inflection Point: Occurs on expiratory limb, indicating elastic recoil.
  • Dereliction throughout the curve; suggests balance between recruitment and hyperinflation.

Challenges in Interpretation

• Heterogeneity in ARDS: Complicates identification of inflection points due to variable lung conditions.
• Alveolar Overdistension: Beak shape in curve indicates pressure not contributing to volume increase.

Idealized Pressure-Volume Loop Scenarios

• Volume-Controlled Ventilation: Predictable pressure rise with constant flow.
• Pressure-Controlled Ventilation: Maintains constant pressure with changing flow; lacks overdistension beak.

Realistic Pressure-Volume Loop

• SIMV-PRVC Mode: Features decelerating flow and constant inspiratory pressure.
  • Lower inflection point difficult to identify due to flow variations.
  • Focus on zero-flow points for lung compliance estimation.

References and Further Reading

• Harris (2005) for detailed discussion.
• Falke et al (1971) for early discussions on PEEP and compliance.
• Lu et al (2001) for alveolar pressure curves.