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

#Arrangoiz #Surgeon #Teacher

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