Question 1

During the transport of an intubated, ventilated patient, the end—tidal carbon dioxide (ETCO2) trace on the transport monitor indicates that CO2 is no longer detectable. List the possible causes and outline your response.

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College answer

  • Patient related possibilities 
    • Airway issue – blockage or dislodgement of ETT o Patient disconnected from ventilator/ventilator tubing 
    • Patient not ventilating – for example pneumothorax or high peak pressures resulting in transport ventilator ‘cutting out’ flow
    • CO2 not being produced by the patient – cardiac arrest, massive pulmonary embolism
  • Equipment related possibilities 
    • Capnography calibrating
    • Disconnection of ventilator/ventilator tubing (no mark if already mentioned above) o Transport monitor/cable failure
    • Capnography related problem – for example disconnection of ETCO2 tubing or H2O within the tubing/capnography
  • Response 
    • Immediate patient assessment – check pulse, check ventilating
    • May need to modify transport to enable thorough assessment and treatment if needed – for example to gain access to the patient.
    • If patient issue:
      • CPR if arrest, or resuscitation as indicated 
      • Change to bag mask ventilation
      • Check ETT position/blockage – reintubation if necessary
      • Treat if intrinsic lung pathology – thoracostomy for pneumothorax.
    • If confident patient is stable check equipment for disconnection or leak

Decision on continuation of transport to be made if equipment faulty; whether to continue or abort transfer will depend on factors including patient stability, reason for and urgency of transfer and estimated time remaining.

Examiners Comments:

 Overall done reasonably well. Few candidates considered transport related issues or gave a good systematic approach.


So, the capnography trace; it is no longer detectable. If one iholds onto the belief that college questions undergo careful review and are very precisely worded, then this specific choice of phrase gains a particular significance. The trace did not peter out and go flat, it disappeared entirely. The possible causes and response to this therefore fall into "blame the patient" and "blame the equipment" categories. The college classification is sufficiently brief and comprehensive, so that minimal mprovement on the college answer can be offered. One is merely able to rearrange the wording somewhat, to make it easier for the time-poor candidate to score more points.

In short:

  • Patient causes:
    • Airway
      • Upper obstruction (eg. patient suddenly bit down on the tube)
      • Lower obstruction (eg. anaphylaxis, bronchospasm)
    • Breathing
      • No expired gas
        • Apnoea; you forgot to switch the patient to a mandatory mode
        • No ventilation; no flow is being delivered because of some sort of catastrophic respiratory problem, eg. pneumothorax or severe bronchospasm
      • No CO2 in the expired gas (oesophageal intubation, though the scenario we are given makes this unlikely)
    • Circulation
      • No CObeing circulated to the alveolus (massive PE)
      • No CObeing produced (cardiac arrest)
  • Equipment causes:
    • Ventilator
      • Powered down (battery loss)
      • Empty (gas supply failure)
    • Circuit
      • Disconnected
      • Kinked
    • Endotracheal tube
      • Perforation (the end tidal gas is escaping via the hole before it gets to the capnograph)
      • Cuff leak 
    • EtCO2 monitor
      • Disconnected infra-red monitoring module
      • Disconnected or waterlogged entrainment port of an inline monitor
    • Monitoring display
      • The plug has fallen out
      • Miscalibrated monitor (waveform exists but is out of the display scalar range)

A possible systematic response to this might consist of:

  • Check the patient:
    • Other monitoring on the screen: does it support the diagnosis of cardiac arrest or massive PE?
    • Airway: is the ETT obstructed or leaking?
    • Breathing:
      • is there evidence of pneumothorax or bronchospasm?
      • is there air entry, or is the patient not ventilating for some reason?
    • Circulation: is there still pulsatile blood flow?
  • Check the equipment
    • Pass a suction catheter down the ETT to make sure it is not blocked
    • Check the tubing to make sure the capnometry module is intact and atached
    • Check the monitor atachments to make sure all the plugs are in place
    • Check the monitor to make sure it is properly scaled
    • Recalibrate the monitor
  • Check your transport progress
    • Is this transport so vital that it can't wait for equipment that works?

In terms of published evidence, there is little literature out there to guide one's written response to this question, except for papers which support the intra-transport CO2 monitoring. For example, Silvestri et al (2005) determined that it is helpful on the basis of the fact that among the patients who were monitored in this way, none ended up having undetected oesophageal intubation. However, unlike other such situations (eg. the tracheostomy patient who just won't ventilate) this scenario has not yeat attracted the attention of anybody who might create a flowchart management algorithm.