A 49-year-old female is intubated and ventilated in your ICU following a motor vehicle accident. You are called to the bedside when the ventilator low pressure alarm is triggered. List the potential causes and outline your approach to this problem.

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College answer

Potential causes:

Most likely air leak:

ETT cuff rupture or incompetent pilot balloon valve, ETT dislodgement,

Disconnection/ defect in ventilator circuit,

Leak into chest drain,

Bronchopleural fistula

Pressure alarm or Tidal Volume set too low

Ventilator failure                                                                                                               (3 marks)

Approach

  1. Examine the patient. – will indicate urgency of problem: e.g.:
    1. SpO2
    2. ET CO2
    3. Chest movement
    4. Breath sounds
    5. Bubbling in chest drains if present
  2. Carefully check ETT
    1. Insertion depth? any change from previous to indicate dislodgement
    2. Air pressure in cuff- re-inflate if low
    3. Audible air leak
    4. If suspect dislodgement have direct look with laryngoscope and reintubate if necessary

(2 marks)

  1. Check ventilator settings and alarms? any inadvertent change to these (1 mark)
  2. Check total length of ventilator circuit, specially connections and any access ports

(1 mark)

Depending on status of patient may have to take off ventilator and hand-bag patient, +/- exchange ETT if thought to be culprit. (1 mark)

Exchange circuit if necessary. Internal ventilator problem unlikely but may need to change out ventilator.   (1 mark)

Examiners Comments:

Those candidates who clearly identified that a low-pressure alarm is usually associated with an air leak (anywhere in the system/patient) and had a logical approach to managing the problem were able to score good marks. Those who gave a random selection of 'any problem that a patient may develop and may trigger any alarm' did not score so well.

Discussion

Low pressure alarm, eh? Why is this happening?

  • The low pressure alarm on most ventilators is sounded when the circuit pressure drops below a certain pressure value 
  • The alarm is usually set (manually) to about 2cm H2O below the PEEP value.
  • The sensor for this is usually in the afferent (pre-patient) limb of the circuit, and it is involved in a feedback loop with the solenoid flow-limitng valve which contols flow into the circuit
  • The ventilator is usually able to compensate for a depressurised circuit by increasing the flow rate.

So, the low pressure alarm has sounded. This can mean several things:

  • There is no problem:
    • The pressure sensor is malfunctioning
    • The pressure sensor is working fine, but somebody has either set the alarm threshold too high, or has decreased the PEEP without also adjusting the alarm.
  • There is a real probem:
    • The problem is with the circuit
    • The problem is with the patient
    • The ventilator has failed in some fundamental way and is not producing flow (eg. in a turbine-driven ventilator, the turbine has failed)
    • The flow is being "stolen" by the suction set

It's impossible to determine which one it is without doing a bit of troubleshooting. If there is a real problem, it could be:

  • Patient problem:
    • Leak could be around the ETT
      • The tube has migrated out past the cords, and now and there is a massive leak
      • The cuff has deflated, and now there is a cuff leak
      • The ETT itself is damaged, and now there is a leak out of the defect (this is rare, as it takes quite a lot of effort to damage those things).
    • Leak could be via the patient:
      • Large volume air leak via the ICC
  • Circuit problem:
    • Disconnection of the circuit from the ventilator
    • Disconnection of the patient from the circuit
  • Failure of the ventilator

So, the best way to approach this:

1) Ensure the patient is safe:

  • Disconnect the patient from the ventilator and the inline suction set, and bag them manually, noting the pressure generated thereby
  • Note whether there is an audible leak; if yes, then the endotracheal tube is to blame
  • Assure that the patient's oxygenation is adequate
  • Assure that the end-tidal COis still generating a waveform (i.e. the tube is not displaced into the oesophagus)

3) Systematically troubleshoot the circuit

  • Start with the patient
    • Listen to the chest: is there air entry?
    • ETT:
      • Make sure it is at the same depth as was previously documented
      • Check the cuff pressure with the manometer
      • Check for cuff leak
      • Perform laryngoscopy if there is any doubt
    • Check the chest drain: is there now a vigorous air leak?
      May need a chest Xray
    • Check the ventilator: while bagging the patient manually, see if the problem is reproduced with the same circuit and a "test lung".
    • Check the closed suction set: did the trigger valve become disabled by some accident (i.e. is the suction constantly "stealing" airway pressure?)

References

References

Jairo I. Santanilla "The Crashing Ventilated Patient"; Chapter 3 in Emergency Department Resuscitation of the Critically Ill, American College of Emergency Physicians, 2011.

Raphael, David T. "The Low-Pressure Alarm Condition: Safety Considerations and the Anesthesiologist’s Response." Anesthesia Patient Safety Foundation Newsletter 13.4 (1999).