Compliance and Resistance

• Resistance:
  • Is the impedance of flow:
    • In the tubing and airways and therefore:
      • Can only occur when there is:
        • Airflow
  • According to Ohm’s Law:
    • Resistance (R) = Δ pressure /Δ volume
      • R = (Peak inspiratory pressure – Plateau pressure) / Tidal volume
        • R = (PIP- Pplat) / (TV)
• Assuming a constant tidal volume:
  • The resistance equation can be simplified to:
    • R ≈ (PIP- Pplat)
• Normal airway resistance:
  • Should be ≤ 5 cmH20
• Resistance is a factor in ventilating all patients but can become particularly important:
  • When ventilating patients with COPD or asthma:
    • The resistance in a system:
      • Increases with decreasing diameter
        • While common examples include:
          • A very small endotracheal tube (ETT) or bronchospasm leading to narrowing of the airways:
            • Recall that a “decrease in the diameter” can also occur at just one point:
              • Such as with kinking or biting of the ETT, or a mucous plug in a large airway
• Compliance refers to:
  • The distensibility of the system and is the inverse of elastance:
    • In other words:
      • It a measure of the lung’s ability to stretch and expand:
        • The more elastic a system, or higher the “recoil,”:
          • The lower the compliance:
            • A common analogy to understand the concepts of elastance is to analyze the recoil of springs:
              • Imagine a very tightly wound and stiff spring
              • This spring is difficult to stretch and wants to stay in the coiled position
              • This spring would have high elastance and low compliance
            • Envision a second, loosely coiled spring:
              • Very little force is required to stretch out this spring, and therefore, it has low elastance but high compliance
• Although compliance commonly is used to describe the lung parenchyma:
  • Remember that compliance actually involves all components of the system:
    • In other words:
      • A patient with pulmonary edema may have low compliance:
        • Due to an issue with the lung parenchyma
      • But another patient may have similarly low compliance due to severe chest wall stiffness after a third-degree burn
      • Clinically, knowing the exact cause of decreased compliance in a given patient can be challenging:
        • Physicians should not, therefore, always assume that it is always related to “stiff lungs.”
• In the figure below, the top “lungs” are healthy:
  • The lungs on the left have a resistance problem or impairment in airflow
  • The lungs on the right have a compliance problem or impairment in stretch and recoil
  • In this picture:
    • Both figures could have elevated peak inspiratory pressures (PIP):
      • Due to the excess pressure generated in the system:
        • However:
          • Only the right-hand figure would have an elevate plateau pressure (Pplat):
            • Since this process occurs when there is an absence of airflow

• Compliance (C) = ∆ volume / ∆ pressure
  • C = Tidal volume / Plateau pressure – Peak inspiratory pressure
    • C = (TV) / (Pplat – PEEP)
• Therefore, when troubleshooting high-pressures on the ventilators:
  • Two values are needed:
    • The peak inspiratory pressure (PIP):
      • Is the maximum pressure in the system and includes both:
        • Resistance and compliance:
          • An inspiratory pause stops all airflow:
            • Thereby removing resistance, and only leaving compliance, as illustrated in this diagram below:
              • The plateau pressure, or Pplat, is, therefore:
                • A measure of compliance

• These values can be displayed on the ventilator screen:
  • On most ventilators:
    • The PIP is always seen
    • While the Pplat is seen by pushing the “inspiratory hold” or “inspiratory pause” button on the ventilator
  • An elevated PIP and normal Pplat is:
    • Indicative of increased airway resistance
  • An elevated PIP and elevated Pplat is:
    • Indicative of abnormal compliance

• Determining whether the patient has a:
  • Resistance problem or a compliance problem:
    • Can assist in the differential diagnosis of respiratory failure:
      • High Resistance:
        • High PIP, Low / Normal Pplat:
          • Kinked / obstructed ETT
          • Mucus plugging
          • Brochospasm
          • Endotracheal tube to narrow:
            • Small
          • Coughing
          • Bronchospasm:
            • Obstructive lung disease
  • Low Compliance:
    • High PIP, High Pplat:
      • Atelectasis
      • Pulmonary edema
      • ARDS
      • Hemothorax /pneumothorax
      • Pneumonia
      • Pulmonary fibrosis:
        • Restrictive lung disease
      • Air-trapping with accumulated auto-PEEP
      • Obesity
      • Abdominal compartment syndrome
      • Circumferential burns of the chest
      • Scoliosis
      • Supine position
• Air trapping:
  • Also referred to as breath-stacking:
    • Can lead to the development of auto-PEEP, or intrinsic PEEP (iPEEP):
      • These pressures should be differentiated from the set PEEP, or extrinsic PEEP (ePEEP):
        • ePEEP refers to the additional end-expiratory positive pressure set during mechanical ventilation:
          • To prevent alveolar collapse and derecruitment
    • In contrast:
      • Auto-PEEP, or iPEEP:
        • Is a pathophysiological process:
          • That can occur when the ventilator initiates the next breath prior to complete exhalation:
            • While this is most common in patients with prolonged expiratory phases, such as asthma or COPD:
              • It can also occur in patients:
                • Who have a fast respiratory rate or
                • Those who are being ventilated with large tidal volumes
            • The amount of auto-PEEP can be measured by:
              • Pressing the “expiratory hold” or “expiratory pause” button on the ventilator:
                • When this button is pressed, the ventilator will display the total PEEP:
                  • The auto-PEEP is the difference between the total PEEP and the set PEEP:
                    • Auto-PEEP (iPEEP) = Total PEEP – ePEEP
• The Figure represents the effects of air trapping:

• Air trapping, or autoPEEP:
  • Can lead to significant adverse cardiopulmonary effects
  • The increased intrathoracic pressure from autoPEEP can:
    • Decrease venous return and lead to hemodynamic instability, even cardiac arrest in severe cases
  • The increased pressures may also result in:
    • A pneumothorax or pneumomediastium
  • Additionally, air trapping can lead to:
    • Ineffective ventilation due to:
      • Collapse of the capillaries responsible for gas exchange:
        • With worsening hypercarbia and hypoxemia
    • While this may seem like a paradox:
      • As one may assume that increasing the minute ventilation, or moving more air:
        • Will improve ventilation, there is a limit to the beneficial effects:
          • Once the lungs are overdistended, gas exchange is ineffective
          • In these circumstances, allowing the patient sufficient time to exhale can decrease CO2 retention

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