COMPLIANCE AND RESISTANCE IN MECHANICAL VENTILATION

COMPLIANCE AND RESISTANCE IN MECHANICAL VENTILATION

Introduction to Respiratory Mechanics

• Respiratory Mechanics: The study of lung and chest wall behavior during breathing.

• Key Concepts in Mechanical Ventilation:

  • Compliance

  • Resistance

Compliance

• Definition of Compliance:

  • Measure of lung distensibility.

  • Defined as change in volume per unit change in pressure.

  • Formula: C = \frac{\Delta V}{\Delta P}

  • $C$ = Compliance

  • \Delta V = Change in volume

  • \Delta P = Change in pressure

  • Units: mL/cmH₂O

  • Importance: Crucial for assessing lung function and guiding ventilator management.

Types of Compliance

• Static Compliance:

  • Measured during no-flow conditions.

  • Reflects elastic properties of lungs and chest wall.

• Dynamic Compliance:

  • Measured during active breathing.

  • Influenced by both elastic and resistive forces.

• Specific Compliance:

  • Compliance per unit of lung volume.

  • Useful for comparing patients of different sizes.

Factors Affecting Compliance

• Lung Tissue Elasticity:

  • How easily lung tissue stretches.

• Chest Wall Elasticity:

  • The flexibility of the thoracic cage.

• Surfactant Function:

  • Substance that reduces surface tension in the alveoli.

• Lung Volume:

  • Compliance can change at different lung volumes.

• Patient Positioning:

  • The position of the patient can influence lung mechanics.

• Disease States:

  • Conditions such as ARDS (Acute Respiratory Distress Syndrome) and pulmonary fibrosis decrease compliance.

• Mechanical Ventilation Settings:

  • Settings can impact lung mechanics and patient outcomes.

Measuring Compliance

• Static Compliance Measurement:

  • Conditions: Air is stagnant.

  • Calculation:
    C = \frac{VT}{(P{plat} - PEEP)}

  • Where:

    • $V_T$: Set tidal volume collected.

    • $P_{plat}$: Plateau pressure measurement.

  • Normal Adult Static Compliance:

  • Ranges from 60-100 mL/cmH₂O.

• Dynamic Compliance Measurement:

  • Conditions: Air is in motion.

  • Calculation:
    C = \frac{VT}{(P{IP} - PEEP)}

  • Where:

    • $P_{IP}$: Peak inspiratory pressure.

Clinical Implications of Altered Compliance

• Low Compliance:

  • Indicates stiff lungs, potentially due to:

  • ARDS.

  • Pulmonary edema.

  • Kyphosis.

  • Decreased surfactant.

  • Results in increased work of breathing and risk of barotrauma.

• High Compliance:

  • Overly distensible lungs, possibly due to:

  • Emphysema.

  • Risk of dynamic hyperinflation.

  • Effects of position change and flail chest.

Scenarios

• Case Study: A patient manifests difficulty breathing post-motor vehicle accident, indicating likely issues with lung compliance.

• Problem-Solving: Evaluate interventions to improve compliance based on clinical presentation.

Resistance

• Definition of Resistance:

  • Opposition to airflow in the respiratory system.

  • Defined as change in pressure per unit flow rate.

  • Formula: R = \frac{\Delta P}{Flow}

  • $R$ = Resistance

  • \Delta P = Change in pressure

  • $Flow$ = Air flow rate

  • Units: cmH₂O/L/sec

  • Importance: Critical for assessing airway function and optimizing ventilator settings.

Types of Resistance

• Airway Resistance:

  • Major component of total respiratory resistance encountered in conducting airways.

• Tissue Resistance:

  • Related to lung and chest wall tissue deformation.

• Endotracheal Tube Resistance:

  • Additional resistance present in mechanically ventilated patients.

Factors Affecting Resistance

• Airway Diameter:

  • Narrower airways increase resistance.

• Length of Airways:

  • Longer airways can contribute to increased resistance.

• Gas Density and Viscosity:

  • Heavier or thicker gases create more resistance.

• Laminar vs. Turbulent Flow:

  • Turbulent flow usually results in higher resistance.

• Presence of Secretions:

  • Increases airway resistance.

• Bronchospasm:

  • Constriction of airways leads to higher resistance.

• Endotracheal Tube Size and Condition:

  • Affects the resistance in intubated patients.

• Ventilator Circuit Components:

  • Configuration can impact overall resistance.

Poiseuille’s Law

• Definition:

  • A reduction in airway lumen by 50% results in a fourfold increase in airway resistance.

  • Describes how small changes in airway diameter (radius) significantly affect gas delivery to alveoli.

  • Four Factors Affecting Airway Resistance:

  • Viscosity of Gas: Thickness of the gas affects flow.

  • Velocity of Gas: Speed at which gas moves.

  • Length of Airway: Longer length increases resistance.

  • Radius of Airway: Smaller radius increases resistance due to inverse relationship.

• Relationship Clarification:

  • Viscosity, velocity, and length are directly proportional; increases in any increase resistance.

  • Radius is inversely proportional; decrease leads to increased resistance.

Measuring Resistance

• Resistance Calculation:

  • Calculated during mechanical ventilation using:
    R = \frac{(P{IP} - P{plat})}{Inspiratory Flow}

  • Measurements Needed:

  • Peak inspiratory pressure (PIP).

  • Plateau pressure (Pplat).

  • Normal Airway Resistance: Ranges from 0.5-2.5 cmH₂O/L/sec.

  • Increased Resistance: >2.5 cmH₂O/L/sec.

  • Most resistance occurs in upper airways, diminishing toward the alveoli.

Clinical Implications of Altered Resistance

• Increased Resistance Indicates:

  • Obstructive lung diseases (e.g., asthma and COPD).

  • Presence of an artificial airway or tube occlusion.

  • High gas flows increasing work of breathing and risk of dynamic hyperinflation.

• Decreased Resistance Indicates:

  • Effective bronchodilation and airway clearance (e.g., suctioning).

  • Adjusted ventilator settings:

  • Inspiratory flow rate adjustments.

  • I:E ratio adjustments.

  • Consideration for bronchodilator therapy.

Relationship Between Compliance and Resistance

• Time Constant (τ):

  • Defined as the product of compliance and resistance.

  • Formula: \tau = R \times C

  • Represents the time required to fill or empty lung units.

  • Critical in influencing ventilator settings:

  • Inspiratory time adjustments.

  • Expiratory time adjustments.

  • Changes to I:E ratio based on values.

Interaction Dynamics

• Inverse Relationship Between Compliance and Resistance:

  • As compliance decreases, resistance typically increases and vice versa.

• Compliance vs. Elastance:

  • Compliance is the stretch ability of the lungs; elastance is the lungs' recoil ability.

  • Understanding these fluctuations is essential for proper respiratory care practices.

Impact on Mechanical Ventilation Strategies

• Compliance Considerations:

  • Low compliance requires lower tidal volumes and higher PEEP settings.

  • High compliance presents risks for volutrauma; necessitates careful titration of PEEP.

• Resistance Considerations:

  • High resistance mandates longer expiratory times and potentially higher flow rates.

  • Need for monitoring auto-PEEP (inadvertent recruitment of lung units).

• Balancing Act:

  • Critical to optimize ventilation while minimizing the risk of ventilator-induced lung injury.

Monitoring and Troubleshooting

• Assessment of Trends:

  • Regular assessment of compliance and resistance trends is crucial; absolute values are less significant.

  • Sudden Changes May Indicate:

  • Patient-ventilator asynchrony.

  • Airway obstruction.

  • Pneumothorax presence.

  • Signs of pulmonary edema.

Summary and Key Points

• Fundamental Role: Compliance and resistance are essential components of respiratory mechanics.

• Importance of Regular Monitoring: Understanding changes in compliance and resistance helps optimize ventilator settings and improve patient care outcomes.

• Interplay Considerations: Recognize the interplay between compliance, resistance, and time constants during ventilator adjustments.

• Focus on Trends: Always consider trends in compliance and resistance as more informative than static values.

• Goal of Care: The ultimate aim is to provide effective ventilation while minimizing risks associated with mechanical ventilation, particularly ventilator-induced lung injury.