Murray and Nadel Ch 135 Mechanical Ventilation

Positive pressure function

Basics of positive pressure breath

  • triggers, targets, cycles of ventilators
    • trigger: variable chosen to initiate breath
      • patient initiated, assisted or supported breath: either change in flow or pressure
      • machine initiated, controlled breath: set time
    • target: variable chosen to regulate gas delivery
      • set flow or pressure
    • cycle: variable chosen to stop breath
      • set volume
      • set flow
      • set time

Modes of ventilation

Physiologic Effects of PPV

Alveolar Recruitment and Gas Exchange

  • high pressure distends existing lung units more
  • PEEP helps maintain alveolar to be open, improves gas exchange, prevents open alveoli to open and close too much
  • PEEP can be good for one area but overdistends other areas of lung
  • PEEP can reduce cardiac filling
  • recruitment maneuver during initial recruitment can help open up alveoli: 30-40cm PEEP for 30-120 seconds
  • alternatively can also increase inspiratory time

Positive Pressure Ventilation and Cardiac Function

  • positive pressure decreases cardiac filling
  • use volume repletion in high positive pressure to maintain cardiac output

Complications

Ventilator induced lung injury

  • barotrauma: alveolar rupture causing pneumomediastinum, pneumopericardium, subcutaneous emphysema, pneumothorax, air emboli
  • volutrauma: inflammatory reactions can propagate to other organs, leading to multiorgan failure
  • VILI is related to driving pressure (Pplat - PEEP)
  • VILI is also related to frequency and velocity of stress
  • normal lung most vulnerable to VILI
  • VILI can be seen as diffuse alveolar damage

Ventilator induced diaphragm dysfunction

  • diaphragm atrophy can happen chronically on vent, leading to vent dependence

Dyssynchrony

Trigger

  • auto triggering can be caused by cardiogenic oscillations or water build up
  • autoPEEP can cause insufficient triggering. Using PEEP can help reduce trigger load

Flow

Cycle