Mode settings

There are three settings common to every conventional mode of ventilation:

  1. FiO2:
    • The amount of oxygen delivered to the patient
  2. PEEP:
    • Pressure maintained in the respiratory system at the end of exhalation:
      • Maintaining PEEP:
        • Keeps an open lung and prevents atelectasis
  3. Trigger sensitivity:
    • Criteria used to see if the patient is making an effort:
      • Flow-triggered
      • Pressure-triggered
  • The three most common modes of ventilation include:
    • Volume assist control
    • Pressure assist control
    • Pressure support
  • Assist control modes (pressure or volume) are:
    • Typically used in the acute phase of mechanical ventilation, or
    • When the patient has no or very minimal drive to breath
  • Pressure support:
    • Is used when patients have an intact respiratory drive
  • Volume Assist Control (AC-VC):
    • Requires a frequency of respirations per minute
    • Patients can trigger additional breaths greater than the devised respirations per minute
    • If the trigger criteria is not being met, the machine will trigger all of the breaths
    • When patients are starting to interact with the ventilator:
      • A spontaneous mode:
        • Such as pressure support should be considered
    • The tidal volume should be 6 to 8 mL/kg of ideal body weight:
      • The weight should be predicted weight, NOT actual weight:
        • As actual weight will overestimate the tidal volume
      • To calculate predicted weight we relied on the work of emDocs, and the group recommends using the equation:
        • 50 + 2.3 x (height [in] – 60) for men
        • 45 + 2.3 x (height [in] – 60) for women
    • Current practice based on several trials suggests:
      • That the patient should be ventilated with “lower” tidal volumes of 6 to 8 mL/kg
    • Flow:
      • Is the speed at which the tidal volume is delivered:
        • 50 to 60 L / min will minimize discomfort when patients start making an effort
    • PEEP should always be set at a minimum of:
      • 5 cmH2O:
        • To reduce atelectasis
    • The inspiratory flow:
      • Is commonly set between 50 and 60 L/min and
      • A minimum I:E ratio of:
        • 1:1.5 to 1:2:
          • Affected by respiratory rate as well
      • Common inspiratory times are:
        • 0.75s to 1 s
      • In certain circumstances:
        • Such as in airway obstruction with asthma:
          • Allowing more time for exhalation is beneficial:
            • In these cases:
              • One can increase the inspiratory flow or
              • Decrease the I:E ratio:
                • To 1:3 or 1:4
    • The inspiratory pause:
      • Helps distinguish between:
        • Resistive pressure and elastic pressure (compliance of the respiratory system)
      • Allows the ventilator to display the:
        • Plateau pressure:
          • Which is helpful for monitoring the patient’s respiratory system mechanics (resistance and compliance)
      • It also prolongs the inspiratory time to the common time of 0.75s to 1 s
  • Pressure Assist Control (AC-PC):
    • Similarly requires a frequency of respirations per minute
    • The inspiratory time:
      • Is the length of time the pressure is maintained
    • The rise time:
      • Is the time the ventilator will take to reach the set pressure:
        • The default setting for rise time is generally acceptable at:
          • 0.1 sec
    • As resistance or elastance of the respiratory system changes:
      • A result will be changes in:
        • Tidal volume and minute ventilation
          • Consequently, it is very important to monitor the tidal volume and keep it in the proper range in AC-PC
    • The I:E ratio:
      • Is the simplest parameter to monitor if there is enough time to exhale:
        • It must be maintained at 1:2 or higher to ensure there is enough time to exhale
          • If the patient starts spontaneously breathing, this will affect I:E and it is worth considering transition to spontaneous breathing
    • Similar to volume assist control:
      • The inspiratory flow is commonly set at:
        • 60 L/min
      • Tidal volume should be:
        • 6 to 8 mL/kg
  • Pressure Support (PS):
    • Is distinguished from AC-PC:
      • Because breaths are cycled off by a % of peak flow:
        • As opposed to time
    • It is important to adjust the default settings for the % of peak flow to initiate the cycling off of each breath:
      • As it depends highly on the resistance and elastance of the lungs

Levels to monitor:

  • Volume Assist Control:
    • Elevated peak pressure and/or plateau pressure:
      • Will result from abnormal resistance and elastance due to:
        • ARDS, COPD, asthma, intra-abdominal hypertension, etc.
  • Pressure Assist Control:
    • Significant changes to tidal volume and minute ventilation:
      • Will result from abnormal resistance and elastance due to:
        • ARDS, COPD, asthma, intra-abdominal hypertension, etc.
  • PEEP:
    • Can be increased to improve oxygenation:
      • However, it should not be so much to overdistend the lungs
      • The risk of potential injury increases:
        • When plateau pressure is greater than:
          • 27 cm H2O
  • pH should be kept between 7.35 and 7.45:
    • To increase pH:
      • Increase minute ventilation
    • To decrease pH:
      • Decrease minute ventilation
    • However:
      • Minute Ventilation should not be increased to the point that PaCO2 < 30 mmHg:
        • Cerebral perfusion may be impacted with levels that low
    • In management of ARDS:
      • Permissive hypercapnia is often considered to minimize injury to the lung and a pH 7.25 is considered the lower acceptable limit

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