mech vent lec

• Mechanical ventilation


  Used for patients with acute exacerbation of COPD, acute severe asthma, neuromuscular disease, acute hypoxemic respiratory failure, heart failure and cardiogenic shock, acute brain injury, and flail chest
• Ventilatory failure


  Occurs when the patient's minute ventilation cannot keep up with CO2 production
• Oxygenation failure


  Occurs when the cardiopulmonary system cannot provide adequate oxygen needed for metabolism
• Airway resistance


  The degree of airflow obstruction in the airways
• According to the simplified form of Poiseuille's Law, the driving pressure (DP) to maintain the same airflow (V#) must increase by a factor of 16-fold when the radius (r) of the airway is reduced by only half of its original size
• Clinical conditions that increase airway resistance

  • COPD (emphysema, chronic bronchitis, chronic asthma, bronchiectasis)
  • Mechanical obstruction (postintubation obstruction, foreign body aspiration, endotracheal tube, condensation in ventilator circuit)
  • Infection (laryngotracheobronchitis, epiglottitis, bronchiolitis)
• Normal airway resistance in healthy adults

  Between 0.5 and 2.5 cm H2O/L/sec
• Airway resistance varies directly with the length and inversely with the diameter of the airway or ET tube
• The major contributor to increased airway resistance is the internal diameter of the ET tube
• Secretions inside the ET tube greatly increase airway resistance
• The ventilator circuit may also impose mechanical resistance to airflow and contribute to total airway resistance, particularly when there is a significant amount of water in the ventilator circuit due to condensation
• Airway resistance


  Calculated by Pressure Change/Flow
• Increase in airway resistance

  Increases the work of breathing
• If pressure change (work of breathing) is held constant and airway resistance increases

  Flow decreases and minute ventilation decreases
• Patients with chronic airway obstruction may develop highly compliant lung parenchyma and use a breathing pattern that is deeper but slower
• Patients with restrictive lung disease (low compliance) breathe more shallowly but faster, since airflow resistance is not the primary disturbance in these patients
• Lung compliance


  Volume change (lung expansion) per unit pressure change (work of breathing)
• Static compliance


  Reflects the elastic resistance of the lung and chest wall
• Dynamic compliance


  Reflects the airway resistance (nonelastic resistance) and the elastic properties of the lung and chest wall
• Examples of conditions that lead to decreased lung compliance

  • Acute respiratory distress syndrome (ARDS)
  • Restrictive lung defects
  • Low lung volumes
  • Low minute ventilation
• Emphysema is an example of high compliance where the gas exchange process is impaired due to chronic air trapping, destruction of lung tissues, and enlargement of terminal and respiratory bronchioles
• Static compliance


  Calculated by dividing the volume by the pressure (plateau pressure) measured when the flow is momentarily stopped. Reflects the elastic resistance of the lung and chest wall.
• Dynamic compliance


  Calculated by dividing the volume by the pressure (peak inspiratory pressure) measured when airflow is present. Reflects the airway resistance (nonelastic resistance) and the elastic properties of the lung and chest wall (elastic resistance).
• Conditions causing changes in plateau pressure and static compliance

  Invoke similar changes in peak inspiratory pressure and dynamic compliance
• When airflow resistance is increased (e.g. bronchospasm)


  Peak inspiratory pressure is increased while plateau pressure stays unchanged
• Pressure-Volume (P-V) loop


  Essentially a "compliance loop" that provides information on the characteristics of a patient's compliance
• Shift of P-V slope toward pressure axis

  Indicates a decrease in compliance
• Shift of P-V slope toward volume axis

  Indicates an increase in compliance
• Shift of P-V slope and loop toward pressure axis

  Shows a higher pressure is required to deliver the same volume (decrease in compliance)
• For critically ill patients, dynamic compliance is between 30-40 mL/cm H2O and static compliance is between 40-60 mL/cm H2O
• Compliance and volume change are directly related. In low compliance situations, minute ventilation decreases with decreased tidal volumes and increased frequencies.
• Deadspace ventilation


  Wasted ventilation or a condition in which ventilation is in excess of perfusion
• Anatomic deadspace


  The volume occupying the conducting airways that does not take part in gas exchange (estimated to be 1 mL/lb ideal body weight)
• Decrease in tidal volume causes a relatively higher anatomic deadspace to tidal volume percent
• Alveolar deadspace


  The normal lung volume that has become unable to take part in gas exchange because of reduction or lack of pulmonary perfusion
• Physiologic deadspace


  Sum of anatomic and alveolar deadspace. Under normal conditions, it is about the same as anatomic deadspace.
• Physiologic deadspace to tidal volume ratio (VD/VT)

  Calculated as (PaCO2 - PE-CO2)/PaCO2. A value less than 60% is considered acceptable upon weaning from mechanical ventilation.
• Ventilatory failure


  The inability of the pulmonary system to maintain proper removal of carbon dioxide, resulting in hypercapnia and respiratory acidosis
• Mechanisms leading to ventilatory failure

  • Hypoventilation
  • Persistent ventilation/perfusion (V/Q) mismatch
  • Persistent intrapulmonary shunting
  • Persistent diffusion defect
  • Persistent reduction of inspired oxygen tension (PIO2)
• Hypoventilation


  Caused by depression of the central nervous system, neuromuscular disorders, airway obstruction, and other conditions. Characterized by reduced alveolar ventilation (VA) and increased arterial carbon dioxide tension (PaCO2).