Question 21.1

Last updated Fri, 11/06/2020 - 00:22
 Topic: Respiratory Medicine and Mechanical Ventilation
Pass rate
?
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The following image is a snapshot of ventilator graphics for an 80kg patient in the ICU intubated and mechanically ventilated.

ventilator waveform

a) Describe the abnormalities displayed in this image.

b) What changes (if any) would you make to the ventilator settings?

c) Give three possible causes for the appearance of the pattern demonstrated by the pressure time curve.

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College Answer

a)

Resistance to inspiratory flow - peak inspiratory pressure approx. 40 cmH2O and plateau pressure less than 20 – high peak to plateau pressure

Prolonged expiration with low peak flow indicative of expiratory resistance

Expiratory flow not returned to baseline at end of expiration indicating auto-PEEP / gas trapping / dynamic hyperinflation

b)

  • Reduce rate
  • Reduce T insp
  • Reduce VT
  • (Check intrinsic PEEP)

c)

  • Kinked tubing / blocked filter
  • Kinked / blocked ETT
  • Asthma

Discussion

a)

The ventilator graphic I have created to replace the one which the college has omitted is designed to yield the answer to somebody who understands ventilator pressure waveforms.

The peak pressure is very high, and during the inspiratory pause one glimpses the plateau pressure, which is comparatively low- this suggests that airway resistance is to blame. The flow waveform confirms this - the flow curve never reaches zero in expiration, suggesting that the passive expiratory gas flow is occurring agaisnt some sort of resistance. This is evidence of autoPEEP. In short, specific features of bronchospasm seen here are:

  • High peak airway pressure, but a normal plateau pressure
  • Slow return of the flow-time curve to baseline
  • The flow-time curve does not reach baseline (indicating that emptying is incomplete)

b)

The college makes sensible suggestions. One would need to decrease the respiratory rate in order for the flow to reach zero, to prevent gas trapping. One may also consider decreasing the tidal volume, while understanding the the combination of these two manoeuvrs may actually create even greater hypercapnea. Reducing the inspiratory time may result in increased peak airway pressure, but will help increase the expiratory time, thereby increasing the total CO2 clearance.

c)

Differentials for this revolve around the airway resistance, and to a lesser extent the compliance of the whole circuit. This could be approached systematically:

  • Machine problems:
    • kinked ventilator tubing
    • "rain-out" in the ventilator tubing
    • old waterlogged HME
    • kinked or obstructed ETT
    • old expiratory filter in the ventilator
  • Patient problems
    • biting and chewing on the tube
    • increased upper airway resistance due to some sort of sputum plug
    • bronchospasm (most likely)
    • increased chest wall rigidity, eg. due to massive fentanyl bolus, or hypothermia.

References

No specific references exist, but there is a good broadoverview article which touches upon the troubleshooting process and has some waveforms :

Santanilla, Jairo I., Brian Daniel, and Mei-Ean Yeow. "Mechanical ventilation."Emergency medicine clinics of North America 26.3 (2008): 849-862.

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