Humidification Flashcards
Humidification
* Definition
* Relevance to anaesthesia
* Complications of failure to provide adequate humidification
Humidification of gases = warming and adding moisture to inspired gases
* Provides optimum environment for gas exchange
* Reduces risk of infection
Usually carried out by the upper airway
However, under anaesthesia:
* Patient is exposed to dry medical gases. Compressed gases from cylinders or central supply points are all supplied at room temp and zero humidity
* ETT, tracheostomy, extra-glottic airway devices -> bypass the upper airway, preventing the physiological heating and humidification of inspired gases.
Within 10 minutes of ventilation with dry gases cilia function will be disrupted. Potential complications of failing to provide adequate humidification:
- Increased workload of patient
- Exposure to infection
- Thickened secretions, risk of tracheal tube blockage, more difficult ot clear via coughing
Note non-invasive ventilation circuits must also be humidified
Absolute humidity
What is the AH for air at 37 degrees?
= total mass of water as vapour in a given volume of gas at a given temperature (mg/L)
There is a maximum value for AH, above which condensation occurs. For air at 37 degrees, AH is 44mg/L
Relative humidity
= the actual mass of water in a given volume of gas defined as a percentage of the maximum amount achievable at a given temperature.
Saturated vapour pressure
* Definition
* How is this affected by temperature
= Pressure exerted by the maximum amount of water in gaseous state (or vapour), in equilibrium with the liquid state.
Increasing temperature -> more water goes into gaseous state
Decreasing temperature -> more water goes into liquid state
What % saturation is required for most humidifiers at 37 degrees
Most humidifier standards require >85 % saturation at 37 oC which corresponds to full saturation at 34 oC.
Heat and moisture exchangers: structure, mechanism of action, humidification achieved
Passive exchangers of heat and moisture
* Contain a mesh made of hygroscopic or hydrophobic material with large surface area. This is positioned inside a plastic case
* Exhaled gases (at body temperature and fully saturated) pass over the mesh -> increase temperature of mesh medium and cause water to condense on their surface
* Dry cool inspired gas is inhaled over the mesh, warmed and humidified
* Provide up to 70% humidification
Additional features
* Bacterial and viral filter can be added to the humidifying element to reduce the risk of cross infection
* Side port for gas sampling and monitoring is incorporated into most HMEs
Heat and moisture exchanges: key considerations for use
- Efficiency decreases over time
- Should not be relied upon as the sole means of humidification in the ICU
- Obstruction with mucus or due to the expansion of saturated heat exchanging material may occur -> increased resistance to breathing
- No need for external power source, allowing use during patient transfers
- Provide up to 70% humidification which is adequate for short term use
Heat and moisture exchangers: position in the breathing circuit
Which position is used in adult and paediatric patients?
Must be placed in the breathing system close to the patient.
Position 1: Correct position in adults. When placed at the Y connection, both expiratory and inspiratory gases pass over HME.
Position 2: If placed on inspiratory limb, exhaled gases do not pass over the filter so will not warm or humidify. However not uncommon to see an extra HME/filter in this position ot stop soda lime dust entering the breathing system
Position 3: If placed on expiratory limb, inspiratory gases do not pass over HME. However not uncommon to see an additional HME/filter here in the breathing system to protect the machine from any infectious agent in the exhaled gases. If an HME is already in the correct position it is unnecessary, but common to see a HME/filter in this position in ICU as a hot water bath humidifier is used with no filter.
Position 4: Ideal position for heat and moisture exchange and reduces dead space, but risks blockage by patient secretions. Commonly used in paediatrics (due to reduced dead space)
Cold water bath humidifiers: structure, maximum humidity achievable, disadvantages
- Resevoir of water at room temperature through which inspired gases are passed on the way to the patient
- Maximum humidity achievable 40%
- Require supplemental oxygen as a driving gas
Disadvantages
* Not very efficient as the temperature of the water resevoir decreases due to loss of latent heat of vaporisation
* Can act as a potential resevoir for microorganisms
Hot water bath humidifiers: features
- Powered by electricity, a disposable resevoir of water is heated to 45-60 degrees
- Dry gasses enters the hot water resevoir, and leave the chamber at 100% humidity -> pass down the tubing where they cool down. Water may condense on the inner surface of the tubing - heated elements can be placed on the inspiratory and expiratory limbs to maintain the temperature and prevent condensation.
- Temperature of the resevoir is thermostatically controlled and usually set to >body temperature to ensure full saturation. Temperature sensors in the resevoir and in the breathing system close to the patient
- Large surface area in the resevoir allows full saturation of inspired gases
Parts of a non invasive humidified CPAP circuit
- Patient temperature sensors maintain temperature and protect against airway burns
- PEEP valve, provides desired end-expiratory pressure
- Sterile water for topping up heating chamber
- HME used as filter to protect ventilator from contamination
- Heating chamber to heat water to required temp
- Control panel to set temperature and alarms
Hot water baths: potential disadvantages (4)
- Mixture of electricity and water -> risk of electric shock
- Potential for resevoir to become colonized with bacteria
- Risk of scalding the airway if temperature sensors become disconnected or faulty
- Cannot be used during patient transfer
Drug delivery to airway: What particle sizes are delivered to different parts of the airway?
- Upper airway: 40 micrometers
- Bronchi and bronchioles: 8-15 micrometers
- Peripheral conducting airway: 3-5 micrometeres
- Alveoli: 0.8-3 micrometres
Note: bacteria are 0.2-10 micrometres, viruses are 0.017-0.3 micrometres, nebulised liquid water is 1-40 micrometres
Studies have shown that only 1-3% of nebulised drug reaches the bronchioles where it will have a clinical effect, the rest is lost in the breathing system and deposited in the upper airway.
NB 1 micrometre (SI) = 1 micron (non SI) = 1x10-6 of a metre
Drug delivery to airway: which medications may be delivered
- Bronchodilators: beta agonists, anticholinergics, corticosteroids
- Antibiotics, antivirals
- Saline for sputum induction
- Mucolytics
- Inhaled gases e.g. helium, nitric oxide, anaesthetic agents
- Local anaesthetics (e.g. awake fibreoptic intubation), including cocaine
- Drugs in cardiac arrest if IV access cannot be obtained e.g. epinephrine, atropine
- Analgesics e.g. nasal diamorphine
NOT: sodium bicarbonate, which is irritant to the airway
Nebulisers: function, effect on other equipment
- Can be used to deliver drugs to the airway by breaking up liquid into smaller particles which are then carried by the fresh gas flow
- Can be used as humidifiers: but note humidify by adding water particles (not water vapour), therefore it is possible to over-saturate the airway and deliver more water than could be achieved by 100% humidity
Effects on other equipment
* Require driving gas which will affect FiO2 and gas flows
* If ventilated, necessary to interrupt breathing system to introduce nebuliser-> may affect PEEP
