Neurosurgery notes/Other specialities/ITU/Respiratory/Mechanical ventilation

Mechanical ventilation

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Respiratory
  • Types
    • Assist-control (volume-cycled or pressure-targeted)
      • Volume cycled
        • Settings: tidal volume (TV), flow rate, flow waveform, fraction of inspired oxygen (FiO₂), positive end-expiratory pressure (PEEP), and frequency
        • Airway pressures and auto-PEEP are dependent on respiratory system
      • Pressure-targeted
        • Settings: pressure target, inspiratory time, inspiratory rise rate, FiO₂, PEEP, and frequency
        • Tidal volume and auto-PEEP are dependent upon respiratory system
    • Pressure-support ventilation
      • Usually with set continuous positive airway pressure (CPAP)
      • Spontaneous mode where support can be adjusted
      • Cannot set frequency or tidal volume
        • These are dependent upon patient effort, strength, and compliance of respiratory system
      • Tolerated well because flow, depth, and length are all patient controlled
    • Synchronized intermittent mechanical ventilation (SIMV)
      • Combination of spontaneous breathing and set number of ventilator breaths that are fully supported but coincide with spontaneous efforts
      Category
      Continuous mechanical ventilation (CMV) or “volume assist”
      Pressure controlled ventilation (PCV) or “pressure assist”
      Pressure support ventilation (PSV)
      Synchronized intermittent mechanical ventilation (SIMV)
      Breath trigger
      Time (e.g. 10/min) or patient spontaneous breath sensor (patient draws flow from circuit or creates negative pressure)
      Time or patient spontaneous breath
      Patient spontaneous breath
      Time or patient spontaneous breath
      Breath delivery
      Fixed flow rate
      Fixed pressure
      Fixed pressure
      Fixed flow rate or pressure
      Breath termination (cycling)
      Delivered preset tidal volume
      Completed preset inspiratory time
      Inspiratory flow decreased to preset percentage of peak flow
      Delivered preset tidal volume
      Comments
      Flow target and frequency that at least equals the preset rate
      Pressure target and frequency that at least equals the preset rates
      Tidal volume, inspiratory time and frequency are determined by the patient
      Patient can breathe spontaneously with or without PSV between machine breaths
      Advantages
      Not comfortable but delivery of minute ventilation guaranteed
      Comfortable, minute ventilation not guaranteed
      Comfortable, inadequate minute ventilation if insufficient respiratory drive
      Not comfortable but delivery of minute ventilation guaranteed
    • PEEP (positive end expiratory pressure) or CPAC (continuous positive airway pressure)
      • At the end of expiration to prevent lung atelectasis and improve oxygenation
      • The benefits are redistribution of
        • Lung water, (the redistribution of extravascular water leads to improved oxygenation, lung compliance, and ventilation-perfusion matching)
        • Increasing FRC (shunting is decreased and thus oxygenation improved)
        • Decreasing work of breathing. Patients who benefit are suitable are cardiovascularly stable, do not have raised ICP, and lungs that can be expanded by PEEP, and bilateral lung lesions.
      • PEEP can cause hypotension due to excessive positive pressure.
        • At lower level of PEEP (3-10 cmH₂O) prevents the alveolar collapse, at higher levels reopen or recruit collapsed alveolar unit (alveolar recruitment)
    • FiO₂: 1.0% or 100% oxygen during unstable hemodynamic, CPR or initially put on ventilator 0.4-0.5 to prevent hypoxemia (PaO₂ 100-50 mmHg) 0.3-0.4 to keep PaO₂ 60-80 mmHg or SpO₂ 90-92% for patient with high risk for oxygen toxicity e.g. preterm
    • Tidal volume
      • Normally 6-8 cc/kg of ideal body weight e.g. patient who weighs 70 kg, the tidal volume 7 x 70 = 490 cc and look for PIP which is not too high (not more than 50 cmH₂O)
    • Inspiratory: Expiratory ratio
      • 1:2 in normal patients,
      • 1:3 in COPD or asthmatic patient who need longer expired time
      • Reverse I:E ratio for severe ARDS patients
    • Respiratory rate 8-25/min to keep normocarbia or permissive hypercarbia.
      • Inspired time 1-2 s depends to respiratory rate. If higher respiratory rate, lower inspired time to keep constant I:E ratio
    • Peak flow rate inversely correlate with inspired time and also affects I:E VAC or SIMV, the inspiratory flow rate is usually set at 40-90
    • Dr Hawthrone
      • Things you can control in ventilation
        • Tidal volume=400ml/breath
          • 6ml/kg=70*6=420mls/breath
        • RR=12-16
        • FiO₂
        • PEEP: 5 cmH₂O. Increase to keep recruitment but increase too much can affect venous return and potential increase ICP