Chapter 39 Flashcards
Humidity
Humidity is water in its gaseous form and it’s quantity depends on the temperature of the gas. It’s expressed as AH Absolute Humidity
Absolute Humidity
The amount of water in a given volume of gas, is usually depressed in milligrams per liter
Relative Humidity
The amount of water vapor in a volume of gas, it is expressed as a percentage of the amount of water vapor required to fully saturate that gas at the same temperature and pressure.
Control of heat and moisture exchange
HME is a primary function of the upper respiratory tract that’s mainly the nose.
The nose heats humidifies gas on inspiration and it cools and reclaims water in the gas that is exhaled. The nasal mucosal lining is kept moist by secretions from the mucous glands, goblet cells, transduction of fluid through cell walls and condensation of exhaled humidity.
Control of heat and moisture exchange continued
During inspiration through the nose the path of gas through the turbinates increase surface area contact between the inspired air and the mucosa. As the inhaled ambient air enters the nose, it warms to 28.5 degree Celsius and picks up water vapor from the moist mucosal lining, cooling the mucosal surface to 30.2 degree Celsius at the end of inspiration.
During exhalation the expired gas (34.1 degrees C ) transfers heat back to the cooler tracheal and nasal mucosa which is 32.2 degree C at the end of expiration.
When a person inhales through the mouth at normal room temp. Pharyngeal temperatures are approximately 3 degrees C less than when a person breaths through the nose with 20% less RH.
Isothermic saturation boundary
As inspired has moves into the lungs it achieves body temperature and pressure saturated conditions, body temperature 37 degree C , barometric pressure, saturated with water vapor 100% RH at 37 degree C. Normally 5cm below the carina and is called isothermic saturation boundary.
Above the ISB temperature and humidity decrease during inspiration and increase during exhalation. Below the ISB temperature and RH remain constant.
The ISB shifts when a person breaths through the mouth, when they breath cold dry air, when the upper airway is bypassed, or when the minute ventilation is higher than normal. These shifts can compromise the body’s HME and then humidity therapy would be needed.
Indications for humidification and the warnings of inspired gas
The primary goal of humidification is to maintain normal physiological conditions in the lower airways. Proper levels of heat and humidity help to ensure normal function of the mucociliary transport system.
Administration of dry medical gas at flows greater than 4 L/min to the upper airway causes immediate heat and water loss. If prolonged causes structural damage to the epithelium.
Breathing dry gas through an endotracheal tube (ETT) can cause damage to tracheal epithelium within minutes.
As long as the inspired humidity is at least 60% of the body temperature and pressure saturated ( BTPS) conditions, no injury occurs in normal lungs.
Prolonged breathing of improper conditioned gases through the tracheal airway can result in hypothermia ( reduced body temperature)
Indications for humidification and the warning of inspired gases. Continued.
Delivery of inspired gas at 30 degree C or even 34 degrees C with 100% RH may not be sufficient to prevent epithelial damage occurring during a 6 hour exposure.
A reduction of 20 mg/ L below BTPS ( 44mg/L) is less than 60% RH at BTPS
The amount of heat and humidity a patient needs depends on the site of gas delivery.
Warming and humidification of the inspired gas can help alleviate bronchospasm in patients who develop airway narrowing after exercise after exercise or if they breath in cold air.
The delivery of cool humidified gas is used to treat upper airway inflammation resulting from cojo, epiglottis, and postextubation edema.
Equipments for humidification
A humidifier is a device that adds molecular water to gas. This process occurs by evaporation of water from a surface to a gas that is not 100% saturated with water vapor.
Physical principles governing humidifier function
These four variables effect the performance of humidifiers.
1. Temperature
2. Surface area
3. Time of contact,
4. Thermal mass
Temperature the first variable affecting humidifier performance
Temp : is an important factor effecting humidifier performance. The higher the temperature of gas the more water vapor it can hold. As gas expansion and evaporation cool water in ingested humidifiers to 10 degrees c below ambient temperature, the humidifiers become less efficient.
Simply heating the humidifier to 40 degrees C increases the output to 51 mg/L which is sufficient to meet BYPs conditions.
Surface Area the second variable that effects humidifiers
Surface Area: the greater the are of contact between water and gas the more port unity there is for evaporation to occur. Pass- over humidifiers pass gas over a large surface area of water. More efficient ways to increase the ratio of water to gas surface area include bubble diffusion, aerosol, and wick technologies.
Bubble diffusion directs a stream of gas under water where it’s broken up into small bubbles. The smaller the bubble the greater the ratio of water to air surface area.
Wicks use porous water absorbent materials to draw water (like a sponge) into it’s fine honeycombed structures by means of capillary action. The surfaces of the wick increase the area of contact between the water and gas which aids evaporation.
Contact time, The third variable that effects humidifiers
Contact Time: the longer has remains in contact with the water the greater the opportunity for evaporation to occur.
With bubble humidifiers contact time depends on the depth of the water column, the deeper the column the greater the time of contact as the bubbles rise to the surface
Thermal mass, the fourth variable that effects humidifiers
Thermal Mass: the greater the amount of water in a humidifier the greater the thermal mass. Increased thermal mass equals an increase capacity to hold and transfer heat to therapeutic gases. Larger reservoir humidifiers can provide more consistent heat and humidification with a broader range of gas flow.
Types of Humidifiers
There’s two categories active and passive.
Active which consists of bubble humidifier, pass-over humidifiers, nebulizers of bland aerosol, vaporizers
Passive consist of heat and moisture exchangers (HME)
