topic 2

• 4 Basic Functions of Mechanical Ventilation

  • Input Power
  • Power Transmission and Conversion
  • Control System
  • Output (Pressure, Volume, and Flow Waveforms)
• Input Power


  Where the mechanical ventilator gets its power from
• Power sources for mechanical ventilator

  • Electrical energy
  • Compressed gas
• Electrical energy


  Typical ventilators used in hospitals, plugged into direct outlet, need backup batteries
• Compressed gas


  Specialized ventilators in ICU that use compressed gas as power source, no need for electrical power supply
• Pneumatically powered ventilator


  Uses compressed gas as power source
• Modes of pneumatic transport ventilator

  • Spontaneous
  • CPAP
  • Invasive
• Power Transmission and Conversion

  The system that converts the input power into the actual force needed to deliver gas under pressure
• Drive Mechanism


  • Direct application of compressed gas via pressure-reducing valve
  • Indirect application via electrical motor or compressor
• Compressor


  Accumulates air then compresses it to be used by the ventilator to deliver breath, ventilator won't function without the compressor
• Turbine


  Some machines use a turbine to generate high flow, no need for a compressor
• Output control mechanism


  Regulates the flow of gas to the patient, can shape the output waveform
• Output control valves


  • Pneumatic diaphragm
  • Electromagnetic poppet/plunger valve
  • Proportional valve
• Inhalation valve


  Controls the flow rate and how the flow is delivered to the patient
• Equation of motion


  Equation showing how variables (elastance, volume, resistance, flow) affect ventilatory pressures generated by mechanical ventilator and respiratory muscles

  
• Elastance


  Ability of the lungs to recoil
• Resistance


  Degree of obstruction in the airway
• Compliance


  Ability of the lungs to expand
• Increased elastance


  Requires higher pressures to overcome
• Pneumonia


  Increases elastance, decreases compliance
• Increased airway resistance


  Increases pressures
• Normal airway resistance range: 0.5-2.5 cmH2O/L/sec (Chang's), 0.6-2.4 cmH2O/L/sec (Pilbeam's)
• Higher flow


  Requires higher pressure
• Control Circuit


  System of components that measures and directs the output of the ventilator to replace or assist the patient's breathing efforts
• Components of control circuit

  • Mechanical
  • Pneumatic
  • Electrical
  • Electronic
  • Fluidic
• Control Variables


  Primary variables the ventilator manipulates to cause inspiration: Pressure, Volume, Flow
• Pressure Control Ventilation (PCV)

  Pressure is constant, volume is variable

  
• Volume Control Ventilation (VCV)


  Volume is constant, pressure is variable

  
• Pneumonia worsening in VCV

  Requires higher pressures to deliver set tidal volume
• Pneumonia worsening in PCV

  Tidal volume decreases as lungs become more stiff
• Increased airway resistance in VCV

  Pressures generated by ventilator increase
• Increased airway resistance in PCV

  Tidal volume decreases
• Increased compliance in COPD

  Pressures required to deliver tidal volume decrease
• Effect of ↓ elastance

  ↓ Pvent and ↓ Pmuscles
• In COPD cases with ↑ CL, when on volume-controlled ventilation, volume is constant, but pressure is not normal or lower because of the ↑ CL in the lungs

  
• In COPD patients on volume-controlled ventilation, the mechanical ventilator doesn't need higher inspiratory pressure to deliver the preset tidal volume
• Need to understand the influence diagram for volume-controlled ventilation

  
• Need to memorize and understand the relationships between the parameters in the influence diagram
• Minute ventilation = tidal volume x respiratory rate

  
• If tidal volume ↓

  Minute volume ↓