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• Humidification


  The process of adding moisture to dry gases to prevent damage to the respiratory tract
• Filtration

  The process of removing particles, bacteria, and viruses from the gas flow
• Inhaling dry gases can cause damage to the cells lining the respiratory tract, impairing ciliary function
• Within 10 min of ventilation with dry gases, cilia function will be disrupted, increasing the patient's susceptibility to respiratory tract infection
• A decrease in body temperature (due to the loss of the latent heat of vaporization) occurs as the respiratory tract humidifies the dry gases
• Absolute humidity

  The amount of water vapour present in a given volume of air, measured in mg/L
• Air fully saturated with water vapour has an absolute humidity of about 44 mg/L at 37°C
• Relative humidity


  The ratio of the partial pressure of water vapour in the air to the equilibrium vapour pressure of water at the same temperature
• During nasal breathing at rest, inspired gases become heated to 36°C with a relative humidity of about 80–90% by the time they reach the carina
• Mouth breathing reduces the relative humidity to 60–70%
• The humidifying property of soda lime can achieve an absolute humidity of 29 mg/L when used with the circle breathing system
• Isothermic boundary point

  The point where 37°C and 100% humidity have been achieved, normally a few centimetres distal to the carina
• Insertion of a tracheal or tracheostomy tube bypasses the upper airway and moves the isothermic boundary distally
• Heat and moisture exchanger (HME) humidifiers


  Compact, inexpensive, passive and effective humidifiers that retain a portion of the patient's expired moisture and heat, and return it to the respiratory tract during inspiration
• Characteristics of the ideal humidifier

  • Capable of providing adequate levels of humidification
  • Has low resistance to flow and low dead space
  • Provides microbiological protection to the patient
  • Maintains body temperature
  • Safe and convenient to use
  • Economical
• Mechanism of action of HME humidifiers

  1. Warm humidified exhaled gases pass through the humidifier, causing water vapour to condense on the cooler HME medium

  2. The condensed water is evaporated and returned to the patient with the next inspiration of dry and cold gases, humidifying them

  3. The greater the temperature difference between each side of the HME, the greater the potential for heat and moisture to be transferred

  4. The HME humidifier requires about 5-20 min before it reaches its optimal ability to humidify dry gases

  5. Some designs can filter out bacteria, viruses and particles from the gas flow
• Heat and moisture exchanging filters (HMEF)


  HME designs with a pore size of about 0.2 µm that can filter out bacteria, viruses and particles from the gas flow in either direction
• Apparatus dead space


  The volume of the HME, which ranges from 7.8 mL (paediatric practice) to 100 mL
• Factors affecting the performance of HME

  • Water vapour content and temperature of the inspired and exhaled gases
  • Inspiratory and expiratory flow rates affecting the time the gas is in contact with the HME medium
  • The volume and efficiency of the HME medium
• Hot water bath humidifier

  A humidifier used to deliver relative humidities higher than the heat moisture exchange humidifier, usually in intensive care units
• Mechanism of action of hot water bath humidifier


  1. The water is heated to between 45°C and 60°C

  2. Dry cold gas enters the container where some passes close to the water surface, gaining maximum saturation, and some gas passes far from the water surface, gaining minimal saturation and heat

  3. The container has a large surface area for vaporization to ensure the gas is fully saturated at the temperature of the water bath

  4. The tubing has poor thermal insulation properties causing a decrease in the temperature of inspired gases, partly compensated by the release of the heat of condensation

  5. The temperature of gases at the patient's end is measured by a thermistor which controls the temperature of water in the container via a feedback mechanism
• Nebulizers

  Devices that produce a mist of microdroplets of water suspended in a gaseous medium, used to deliver medications to peripheral airways and radioactive isotopes in diagnostic lung ventilation imaging
• Mechanism of action of a gas-driven (jet) nebulizer

  1. A high-pressure gas flows through a Venturi constriction, creating a negative pressure that draws water up through a capillary tube and breaks it into a fine spray

  2. The majority of the droplets are in the range of 2-4 µm, depositing on the pharynx and upper airway with a small amount reaching the bronchial level
• Nebulizers


  • Produce microdroplets of water of different sizes, 1–20 µm
  • The quantity of water droplets is not limited by the temperature of the carrier gas
  • They can be gas driven, spinning disc or ultrasonic
• Gas-driven (jet) nebulizer


  1. A capillary tube with the bottom end immersed in a water container

  2. The top end of the capillary tube is close to a Venturi constriction

  3. A high-pressure gas flows through the Venturi, creating a negative pressure

  4. Water is drawn up through the capillary tube and broken into a fine spray

  5. Even smaller droplets can be achieved as the spray hits an anvil or a baffle
• Gas-driven (jet) nebulizer


  • The majority of the droplets are in the range of 2–4 µm. These droplets tend to deposit on the pharynx and upper airway with a small amount reaching the bronchial level
  • The device is capable of producing larger droplets of up to 20 µm in size
  • Droplets with diameters of 5 µm or more fall back into the container leaving droplets of 4 µm or less to float out with the fresh gas flow
  • The device is compact, making it easy to place close to the patient
• Spinning disc nebulizer

  • A motor-driven spinning disc throwing out microdroplets of water by centrifugal force
  • The water impinges onto the disc after being drawn from a reservoir via a tube over which the disc is mounted
• Ultrasonic nebulizer


  • A transducer head vibrates at an ultrasonic frequency (e.g. 3 MHz)
  • The transducer can be immersed into water or water can be dropped on to it, producing droplets less than 1–2 µm in size
  • Droplets of 1 µm or less are deposited in alveoli and lower airways
  • This is a highly efficient method of humidifying and also delivering drugs to the airway
  • There is a risk of overhydration especially in children
• Bacterial and viral filters

  • Minimize the risk of cross-transmission of bacteria and/or viruses between patients using the same anaesthetic breathing systems
  • The British Standard defines them as 'devices intended to reduce transmission of particulates, including micro-organisms, in breathing systems'
  • The filter should be positioned as close to the patient as possible, e.g. on the disposable catheter mount, to protect the rest of the breathing system, ventilator and anaesthetic machine
  • A new filter should be used for each patient
  • A humidification element can be added producing a heat and moisture exchanging filter (HMEF)
• Filtration mechanisms

  1. Direct interception: large particles (=1 µm), such as dust and large bacteria, are physically prevented from passing through the pores of the filter because of their large size

  2. Inertial impaction: smaller particles (0.5–1 µm) collide with the filter medium because of their inertia

  3. Diffusional interception: very small particles (<0.5 µm), such as viruses, are captured because they undergo considerable Brownian motion

  4. Electrostatic attraction: charged particles are attracted to oppositely charged fibres by coulombic attraction

  5. Gravitational settling: affects large particles (>5 µm)
• Electrostatic filters

  • The element used is subjected to an electric field producing a felt-like material with high polarity
  • A flat layer of filter material can be used as the resistance to gas flow is lower per unit area
  • They rely on the electrical charge to attract oppositely charged particles from the gas flow
  • The electrical charge increases the efficiency of the filter when the element is dry but can deteriorate rapidly when it is wet
  • The resistance to flow increases when the element is wet
  • The electrical charge on the filter fibres decays with time so it has a limited life
  • A hygroscopic layer can be added to the filter in order to provide humidification
• Pleated hydrophobic filters

  • The very small pore size filter membrane provides adequate filtration over longer periods of time
  • To achieve minimal pressure drop across the device with such a small pore size, so allowing high gas flows while retaining low resistance, a large surface area is required
  • The forces between individual liquid water molecules are stronger than those between the water molecules and the hydrophobic membrane, leading to the collection of water on the surface of the membrane with no absorption
  • Although hydrophobic filters provide some humidification, a hygroscopic element can be added to improve humidification
• Currently there is no evidence showing any type of filter is clinically superior to another
• Size of micro-organisms

  • Hepatitis virus: 0.02 µm
  • Adenovirus: 0.07 µm
  • HIV: 0.08 µm
  • Mycobacterium tuberculosis: 0.3 µm
  • Staphylococcus aureus: 1.0 µm
  • Cytomegalovirus: 0.1 µm
• Characteristics of the ideal filter

  • Efficient: the filter should be effective against both air- and liquid-borne micro-organisms. A filtration action of 99.99–99.999% should be achieved
  • Minimal dead space, particularly for paediatric practice
  • Minimum resistance, especially when wet
  • Not affected by anaesthetic agents and does not affect the anaesthetic agents
  • Effective when either wet or dry. It should completely prevent the passage of contaminated body liquids (blood, saliva and other liquids) which may be present or generated in the breathing system
  • User friendly, lightweight, not bulky and non-traumatic to the patient
  • Disposable
  • Provides some humidification if no other methods being used. Adequate humidification can usually be achieved by the addition of a hygroscopic element to the device
  • Transparent
  • Cost effective
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