The HME is covered in greater detail elsewhere in a small chapter dedicated to its rugged beauty. Here, one will find only a brief point form summary, designed with the intention of answering Question 27.1  from the first paper of 2012. The question shows a picture of the HME, and asks the candidate to "list the features of the device depicted". Only 38% of the candidates passed that question. Earlier, in Question 15 from the first paper of 2005, the college also wanted  their candidates to compare the HME with a wet circuit. In Question 30 from the first paper of 2018, they wanted too know when this sort of humidifier is inappropriate.

In brief, the HME is a device for the passive humidification of inspired gas in short-term invasively ventilated patients. It is not as effective as a water-bath humidifier, insofar as it achieves less gas humidification; however in the short term (in the first 24-28 hours) it is equivalent to a "wet" circuit, and much cheaper

Indications for the use of a HME:

  • Humidification of inspired gas in short-term invasively ventilated patients, where equipment cost is a factor.

Contraindications:

  • Conditions which demand minimsation of apparatus dead space
  • Large volume of secretions or froth 
  • Large minute volume ( over 10L/min) 
  • Large air leak, eg. bronchopleural fistula
  • Long term ventilation
  • Frequent nebulised medications

Safety features

  • Transparent outer casing, to better inspect the filter material
  • Standardised connectors to ventilation equipment
  • Antibacterial filter material in some models
  • Small volume, to minimise apparatus dead space
  • Single use item

Complications

  • The inspissation of secretions
  • Difficulty aspirating thickened secretions
  • The incrustation of endotracheal tubes with thick secretions, with increasing peak airway pressures
  • Possibility of undetected circuit disconnection, or airway obstruction
  • Increased airway resistance with an aging filter
  • Colonisation of filter with respiratory pathogens (but this is an unproven allegation)

References

Drager have a nice brochure, spruiking the virtues of their HME systems.

However, the depicted item is the Pall Ultipor 25 Breathing System Filter.

Its all I had. Thank you to the theatre staff for kindly letting me break this piece of equipment.

The international standard for adequate humidifcation can be found here, at the ISO:

Respiratory tract humidifiers for medical use -- Particular requirements for respiratory humidification systems

Wilkes, A. R. "Heat and moisture exchangers and breathing system filters: their use in anaesthesia and intensive care. Part 1–history, principles and efficiency."Anaesthesia 66.1 (2011): 31-39.

Wilkes, A. R. "Heat and moisture exchangers and breathing system filters: their use in anaesthesia and intensive care. Part 2–practical use, including problems, and their use with paediatric patients." Anaesthesia 66.1 (2011): 40-51.

Cole, P. H. I. L. I. P. "Some aspects of temperature, moisture and heat relationships in the upper respiratory tract." J Laryngol Otol 67.8 (1953): 449-456.

Williams, Robin, et al. "Relationship between the humidity and temperature of inspired gas and the function of the airway mucosa.Critical care medicine24.11 (1996): 1920-1929.

COHEN, IAN L., et al. "Endotracheal tube occlusion associated with the use of heat and moisture exchangers in the intensive care unit." Critical care medicine16.3 (1988): 277-279.

Chiaranda, M., et al. "Use of heat and moisture exchanging (HME) filters in mechanically ventilated ICU patients: influence on airway flow-resistance."Intensive care medicine 19.8 (1993): 462-466.