Topic Progress:

Alarms

Alarm limits protect the patient from harm that can arise from change the patient’s respiratory mechanics (eg changes in resistance and compliance), or acute problems in the function of the ventilator and circuit. They should be individualized for each patient and regularly reviewed.

Many alarms are standard (gas supply, power failure, apnoea alarms, etc) but some require careful consideration based on the mode of ventilation…

  • Because volume is set, pressure is the variable that alters between breaths
  • As such, the alarm limit for peak pressures (Phigh) should be monitored closely and adjusted as appropriate

  • Here, given pressure is constant, tidal volume is the variable that alters between breaths
  • Thus, the Alarm limits for tidal volume and minute ventilation should be monitored closely and adjusted as appropriate.

Lets review some of the common alarms below:

Minute Volumes

High Minute Volume (MV high)

Usually set 5L above minute ventilation and triggered when the minute volume exceeds the upper alarm limit.

In most instances, a high minute ventilation would be driven by the patient – eg from pain or agitation, or due to excessive tidal volumes from inappropriate ventilator settings,

After excluding these patient and settings factors, how would troubleshoot the following

Low Minute Volume (MV low)

Usually set 2 to 3 L/min below mandatory and/or spontaneous minute ventilation rate, minimum setting 3 to 4 L/min.

Again, in the first instance this alarm should prompt a review of the patient. After excluding patient factors, how would you troubleshoot the following

Airway Pressures

Airway Pressure Low

  • Usually set at 10 cmH2O

Airway Pressure High

  • Usually set at 35 cmH2O

 

Apnoea  

  • Apnoea alarm will be triggered if a breath is not registered within the period set for the apnoea time.
  • Apnoea time is usually set at 20 seconds

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