Ventilator Mechanics

Lung Compliance

Static Compliance

  • Reflects the distensibility of the lungs and chest wall (but not airway
    resistance, since it is measured at point of no airflow)
  • Static Compliance = VT/Plateau Pressure-Total PEEP
  • Use the exhaled volume shown on the ventilator for the VT in this equation, not the preset VT (since ventilator tubing will expand during positive pressure: expands about 3 cc for every 1 cm H2O
    increase in the inflation pressure)
  • Compliance is most accurate when performed during passive ventilation (since patient’s respiratory muscle efforts will decrease the chest wall compliance)
    (normal = 50-80 mL/cm H2O)

Dynamic Compliance

  • Reflects the distensibility of the lungs and chest wall and airway resistance (since it is measured at point of maximal inspiratory airflow)
  • Dynamic Compliance = VT/PIP-Total PEEP
  • Use the exhaled volume shown on the ventilator for the VT in this equation, not the preset VT (since ventilator tubing will expand during positive pressure: expands about 3 cc for every 1 cm H2O
    increase in the inflation pressure)
  • Compliance is most accurate when performed during passive ventilation (since patient’s respiratory muscle efforts will decrease the chest wall compliance)
    (normal = 50-80 mL/cm H2O)

Evaluation of Respiratory Decompensation Using PIP and Plateau Pressures

Decreased Peak Inspiratory Pressure (PIP)

  • Air Leak Around ETT Cuff
    • Decreased exhaled VT
    • Intra-breath drop in airway pressure may occur in cases where cuff has herniated above the vocal cords, as air begins to leak past the cuff
  • Air Leak from Ventilator Circuit
    • Decreased exhaled VT
  • Patient’s Inspiratory Efforts are Exceeding the Peak Flow Rate Set on the Ventilator
    • Normal or increased exhaled VT
  • Inadvertent Placement of NG into Lung
    • Decreased exhaled VT
    • Airway pressure may be decreased continuously (if on continuous suction) or intermittently (if on intermittent suction)

Normal Peak Inspiratory Pressure (PIP)

  • Acute PE:
  • Non-Pulmonary Etiology of Decompensation: pain, etc.

Increased Peak Inspiratory Pressure (PIP)

Increased PIP-Plateau (>5)

  • Kinked Endotracheal Tube
  • Bronchoconstriction (see Obstructive Lung Disease, [[Obstructive Lung Disease]])
  • Mucous Plugging: of large airway

Normal PIP-Plateau (<5 cm H2O)

  • Abdominal Wall
  • Agitation
  • Alveolar Filling
  • Chest Wall Restriction
    • Kyphoscoliosis (see Kyphoscoliosis, [[Kyphoscoliosis]])
    • Fentanyl (see Fentanyl, [[Fentanyl]]): may produce increased chest wall/abdominal wall rigidity in some cases (especially with high doses used during cardiothoracic surgery)
  • Pleural Filling
    • Pneumothorax (see Pneumothorax, [[Pneumothorax]])
    • Large Pleural Effusion (see xxxx, [[xxxx]])
    • Hemothorax (see xxxx, [[xxxx]])

“Lost Volume” on Ventilator

Definition

  • Lost Volume: disparity between inspiratory tidal and expiratory tidal volume (i.e. expiratory tidal volume is lower than inspiratory tidal volume)

Etiology

  • Bronchopleural Fistula (see Bronchopleural Fistula, [[Bronchopleural Fistula]]): air leak from chest tube
  • Endotracheal Tube (ETT) Cuff Leak
  • Inadvertent Nasogastric (NG) Tube Placement in Lung (see Nasogastric Tube, [[Nasogastric Tube]]): with suction applied
  • Leak in Ventilator Circuit
  • Technical Problem with Ventilator

Patient-Ventilator Dyssynchrony

Factors Contributing to Patient-Ventilator Dyssynchrony

Ventilator Factors

  • Extraneous Flow: from a nebulizer or added oxygen
  • Flow Delivery System of the Ventilator
  • Flow Pattern Selected
  • Flow Triggering vs. Pressure Triggering of Ventilator: there is not a significant difference in patient effort required to trigger on either type of modern ventilators
  • Inappropriate I:E Ratio: inadequate expiration time may contribute to the development of auto-PEEP
  • Pressure-Targeting Difficulties: pressure target for ventilator to reach is detected at the proximal end of the ETT, but since the ETT is a high resistance tube, the pressure at distal end of ETT will be different
    • Solutions to this problem include using “tracheal target” for pressure (with the sensor located at distal end of the ETT) or adjusting slope of pressure rise for breath
    • Automatic Tube Compensation (ATC): this ventilator feature is purported to overcome the resistance of the ETT
  • Rise Time Capability: rise time is the rate of valve opening
  • Sensitivity Setting

Patient Factors

  • Abdominal Pathology
  • Auto-PEEP: presence of auto-PEEP requires the patient to generate much more negative pressure to trigger the ventilator to overcome the already positive airway pressure
  • Circuit Leak
    • Endotracheal Tube Cuff Leak
  • Endotracheal Tube/Tracheostomy Size
  • Inspiratory Effort/Respiratory Drive/Neural Timing
  • Pain
    • Splinting
  • Respiratory System Pathology
  • Secretions
  • Sedation

Adverse Effects of Patient-Ventilator Dyssynchrony

  • Delayed Ventilator Weaning/Increased Duration of Mechanical Ventilation
  • Dynamic Hyperinflation
  • Increased Cost
  • Increased Length of ICU Stay
  • Increased Sedation Requirement
  • Increased Work of Breathing
  • Muscle Damage
  • Patient Fighting of Ventilator
  • Ventilation/Perfusion Problems

References

  • Using ventilator graphics to identify patient-ventilator asynchrony. Respir Care. 2005;50:202-234; discussion 232-234 [MEDLINE]
  • Applied respiratory physiology: use of ventilator waveforms and mechanics in the management of critically ill patients. Respir Care. 2005;50(2):287–293 [MEDLINE]