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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