Ventilator Wave Form Analysis

• In waveform analysis:
  • The scalars of interest are:
    • Volume vs time
    • Pressure vs time
    • Flow vs time
• In volume assist control:
  • It is common to have a constant flow pattern:
    • Which will show up as straight in the flow scalar
  • Adding an inspiratory pause (when flow = 0):
    • Will indicate a plateau pressure in the pressure scalar
  • In volume assist control:
    • Effort will cause changes to the pressure waveform
• In pressure assist control:
  • Pressure is held constant over time
  • The rise time:
    • Is the time it takes to reach the pressure provided during inspiration
  • To find the plateau pressure:
    • Use an inspiratory pause
  • In pressure assist control:
    • Effort by the patient will change the flow waveform
• To understand patient behavior:
  • It is important to understand asynchrony, which includes:
    • Neurological timing and ventilator timing:
      • Is out of sync
  • Flow starvation:
    • The ventilator does not meet the demand of the patient
  • Patient is unable to meet the trigger criteria
• In volume assist control:
  • You will see patient effort:
    • In the form of:
      • Negative scooping:
        • In the pressure waveform:
          • If flow demand is not entirely met for the patient
    • Patients with COVID-19:
      • Often have excessive efforts when they interact with the ventilator
        • As a result, it is important to monitor the occlusion pressure, or p0.1:
          • To ensure it is not too high and to watch for significant scooping in the pressure waveform

• The figure above shows patient effort in volume assist control with:
  • A normal amount of scooping:
    • With a p0.1 of 2.35 cmH2O
• The figure below indicates excessive effort:
  • Which many COVID-19 patients display when they begin to interact with the ventilator:
    • With p0.1 of 4.7

• In pressure assist control:
  • Patient effort should change the:
    • Flow waveform
  • In this, it is important to check if the flow reaches 0 as pressure reaches 0:
    • During inspiration or
    • If there is dampening of the peak expiratory flow during exhalation:
      • To ensure the patient is not experiencing asynchrony with breath timing:
        • This can be adjusted by shortening or lengthening the inspiratory time
        • Similarly, p0.1 can be monitored to evaluate whether a patient is experiencing excessive effort:
          • In which increasing support should be done:
            • In an attempt to reduce respiratory drive:
              • Provided tidal volume is not excessive
        • PEEP can also be adjusted to attempt to improve the patient’s drive
  • In pressure support, it may be necessary to alter the % of peak inspiratory flow:
    • That causes cycling off to eliminate asynchrony
  • If the patient is demonstrating excessive effort:
    • Increasing the pressure support:
      • Will help decrease the patient’s drive:
        • However, if increasing pressure support does not decrease the p0.1:
          • Reducing the drive must be prioritized
    • PEEP can also be adjusted to attempt to improve the patient’s drive
• Delayed cycling is another phenomena:
  • Where the patient wants to exhale:
    • But the machine has not cycled off yet
  • In pressure assist control:
    • If there is an increase in pressure at the end of inspiration with a subsequent rapid deceleration in the expiratory flow:
      • Then the patient is most likely experiencing delayed cycling
      • As a result, the inspiratory time should be shortened:
        • However, the time should be not shortened so much that it causes premature cycling:
          • In which the patient’s effort lasts longer than the ventilator’s cycle off criteria
          • This will manifest as a dampening in the peak expiratory flow
        • Premature cycling can also occur in pressure support:
          • When the cycling-off percentage is too high:
            • This flow scalar will look similar to that for premature cycling in pressure assist control

• In the figures above, delayed cycling (top) and premature cycling (bottom) in pressure assist control:
  • Are noticeable by analyzing the flow scalar:
    • The inspiratory time:
      • Should be shortened for delayed cycling
      • Lengthened in premature cycling
• Ineffective efforts are another form of asynchrony that generally occurs:
  • When patients are overassisted (too much pressure support) or have too high of airway resistance:
    • Delayed cycling is a common cause of ineffective efforts
• Reverse triggering:
  • Is another form of asynchrony:
    • where the ventilator triggers a breath:
      • That then triggers an effort from the patient:
        • This can also lead to breath stacking
  • The first thing to check when noting reverse triggering:
    • Is to see if it is bad timing or reflex
      • To check this:
        • First reduce the respiratory rate
          • After reducing the respiratory rate:
            • If the patient is triggering the breaths, it was simply bad timing
        • However, if the reverse triggering still continues, a few steps should be taken:
          • Turn off sedation if possible
          • Increase tidal volume to a maximum of 8 mL / kg of IBW
          • Keep plateau pressure:
            • Less than or equal to 27 cmH2O
            • Less than 30cmH2O for COVID-19 patients:
              • Typical ARDS guidelines
          • If there is a known injurious pattern (breath stacking) and sedation cannot be stopped:
            • Consider NMB agents to protect the lung and minimize the possibility of barotrauma

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