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  • Involves intermittent administration of positive pressure:
    • Expiratory phase is passive:
      • Flow out of patient is determined by airway resistance and lung elastance (1/compliance)
  • In general, ventilators in anesthesia machines are simpler than ventilators seen in an ICU setting; however, the distinction between the two is increasingly blurred


  • Controlled mechanical ventilation:
    • Designed for anesthetized, and often paralyzed, patients
    • Volume controlled and pressure controlled:
      • Volume controlled (Figure 49-1):
        • A tidal volume and a respiratory rate are programmed
        • Tidal volumes are constant, but airway pressures vary with airway resistance and lung and chest wall compliances
      • Pressure controlled (Figure 49-2):
        • Inspiratory pressure, inspiratory time, and respiratory rate are programmed
        • The ventilator delivers a constant pressure during inspiration
        • The inspired pressure with each breath is constant, but tidal volumes may change with any changes in airway resistance and lung and chest wall compliances (i.e., abdomen insufflation, bronchospasm, ETT kinking, secretions in ETT)
        • Need to closely monitor actually delivered Vt
        • May be useful in conditions under which high peak pressures are generated under volume-controlled ventilation, for example, ALI/ARDS, morbidly obese patients, laparoscopic surgery in patients in steep Trendelenburg position
        • No evidence that “protective ventilation” used for ARDS patients improves outcome in patients with healthy lungs
  • Partially controlled mechanical ventilation:
    • Designed for spontaneously breathing patients. Not all OR ventilators have these modes
    • Modes:
      • Synchronized intermittent mandatory ventilation (SIMV; Figure 49-3):
        • A tidal volume and a respiratory rate are programmed
        • The patient may spontaneously breathe at a rate higher than that which is programmed
        • The ventilator will allow this, but will not assist the patient’s spontaneous breaths
      • Pressure support ventilation (PSV; Figure 49-4):
        • The patient breathes spontaneously
        • With each breath, the ventilator adds a programmed amount of pressure to assist the patient
        • Typically 5 cm H2O will eliminate the extra work of breathing due to the resistance of the circuit and the ETT. Beyond that, the machine assists the patient’s breathing
      • SIMV + PSV combines the two previous modes:
        • Mechanical breaths are delivered
        • Spontaneous breaths are assisted

Figure 49-1. Volume-Targeted Square Wave Flow Controlled-Mode Ventilation
Graphic Jump Location

Note that inspiratory flow is constant while pressure increases until the start of expiration. Also, there is no negative deflection before each breath, as the breaths are not initiated by the patient. Vt 700 mL, RR 15, I:E 1:2, PEEP 5 cm H2O. Reproduced with permission from Hess DR, MacIntyre NR, Mishoe SC, et al, eds. Respiratory Care: Principles and Practice. Philadelphia: WB Saunders; 2002:786–791. © Elsevier.

Figure 49-2. Pressure-Targeted Controlled-Mode Ventilation (PCV)
Graphic Jump Location

Note that inspiratory flow increases to achieve the target pressure, and then decreases as the lungs fill. Pinspired 25 cm H2O, RR 15, I:E 1:1, PEEP 5 cm H2O. Reproduced with permission from Hess DR, MacIntyre ...

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