how breath is delivered

• Transrespiratory pressure (PTR)


  Pressure gradient between the airway opening and the body's surface
• Positive pressure ventilation


  Pressure delivered at the upper airway is positive and the pressure at the body surface is atmospheric (ambient pressure, which is given a value of 0 cm H2O)
• PTR represents the pressure gradient that must be generated to achieve a given flow
• Various combinations of Pmus (muscle pressure) and Pvent (ventilator pressure) can be used during assisted ventilation to achieve the total force required
• Alveolar pressure (Palv)


  Produced by the interaction between lung and thoracic compliance and the pressure within the thorax
• Transairway pressure (PTA)


  Produced by resistance to the flow of gases through the conductive airways (resistance = PTA/flow)
• Delivery of an inspiratory volume is perhaps the single most important function a ventilator accomplishes
• Control variable


  The primary variable the ventilator adjusts to achieve inspiration
• Variables the ventilator can control

  • Pressure
  • Volume
  • Flow
  • Time
• The ventilator can only control one variable at a time
• Pressure-controlled ventilation


  The clinician sets a pressure for delivery to the patient
• Volume-controlled ventilation


  The clinician sets a volume for delivery to the patient
• Pressure-controlled breathing


  • The pressure waveform is unaffected by changes in lung characteristics, but the volume and flow waveforms will vary
• Volume-controlled breathing


  • The volume and flow waveforms remain unchanged, but the pressure waveform varies with changes in lung characteristics
• Flow-controlled breathing


  • The flow and volume waveforms remain unchanged, but the pressure waveform changes with alterations in the patient's lung characteristics
• Time-controlled breathing


  • Both the pressure and the volume waveforms are affected by changes in lung characteristics
• Time-controlled ventilation is used less often than pressure- and volume-controlled ventilation
• Microprocessor-controlled ventilators have the capability of displaying waveforms as scalar graphs and loops on a screen
• Shapes of pressure, volume, and flow waveforms

  • Rectangular (also called square or constant)
  • Exponential (may be increasing [rising] or decreasing [decaying])
  • Sinusoidal (also called sine wave)
  • Ramp (available as ascending or descending [decelerating] ramp)
• Phase variables


  • Trigger variable (begins inspiration)
  • Limit variable (limits pressure, volume, flow, or time)
  • Cycle variable (ends inspiration and begins expiration)
  • Expiratory variable (determines the characteristics of the expiratory portion of each breath)
• Waveform


  Pressure caused by the elastic recoil (compliance) of the lung
• Flow waveform


  Has the same shape as the transairway pressure waveform (peak pressure minus plateau pressure)
• The shaded areas represent pressures caused by resistance, and the open areas represent pressure caused by elastic recoil
• Types of ventilation


  • Volume-controlled ventilation
  • Pressure-controlled ventilation
• Volume-controlled ventilation


  • Pressure, Volume, Flow
• Pressure-controlled ventilation


  • Pressure, Volume, Flow
• Ptotal


  Pelastic + Presistive
• Pelastic


  Volume / Compliance
• Presistive


  Resistance x Flow
• Phase variables


  • Trigger variable
  • Limit variable
  • Cycle variable
  • Baseline variable
• Time triggering


  Ventilator delivers a mandatory breath by beginning inspiration after a set time has elapsed
• In the past, time-triggered ventilation did not allow a patient to initiate a breath
• Ventilators are no longer used in this manner, conscious patients can take a breath when they need it
• Patient triggering


  Ventilator measures the patient's effort to breathe and triggers a breath when it detects changes in pressure, flow, or volume
• Pressure triggering


  Inspiration begins if a negative airway opening pressure is detected
• Flow triggering


  Inspiration begins when the ventilator detects a drop in flow through the patient circuit during exhalation
• Volume triggering


  Inspiration begins when the ventilator detects a small drop in volume in the patient circuit during exhalation
• Manual triggering


  Operator can initiate a ventilator breath by pressing a button or touch pad
• Patient triggering can be effective when a patient begins to breathe spontaneously, but the patient may experience an apneic episode
• A respiratory rate is set to guarantee a minimum number of breaths per minute