Humidity Therapy PDF

HUMIDITY THERAPY EGANS CHAPTER 39  Outcomes 1. Define humidity therapy 2. Describe the physiological mechanisms for achieving BTPS of inhaled gases. 3. Differentiate between absolute humidity and relative humidity. 4. Calculate relative humidity, humidity deficit and %body humi...

HUMIDITY THERAPY EGANS CHAPTER 39  Outcomes 1. Define humidity therapy 2. Describe the physiological mechanisms for achieving BTPS of inhaled gases. 3. Differentiate between absolute humidity and relative humidity. 4. Calculate relative humidity, humidity deficit and %body humidity. 5. Recognize the indications for humidity including heated humidity. 6. Define evaporation and describe the factors which influence the rate of evaporation. 7. For Bubble Humidifier, Heated Passover and HME describe Operation Indications for use Efficiency and limitations  HUMIDITY THERAPY Addition of water vapor and possibly heat to inspired gas beyond what is added by the surrounding environment and the body’s own humidification mechanisms  HUMIDITY THERAPY BTPS = Body Temperature and Pressure Saturated BTPS = a temp. of 37˚c and 43.8 (44) mg/L of water vapor the nose and upper airway to heat and humidify inspired gas to BTPS BTPS is achieved approximately 5 cm below the carina  HUMIDITY THERAPY Within the nose: The inspired air passes across the turbinate bones Turbinate bones create ridges which increase surface area for contact between moist mucosa and air Within the Upper Airway:  Heat and moisture is added from the secretions of the mucous membranes ❑These two anatomical mechanisms allow BTPS to be achieved  During exhalation both areas collect some of the humidity from the exhaled gases  When BTPS cannot be achieved just below carina, As the inspired air travels further into the smaller airways more moisture and heat will be added to the air. The loss of moisture and heat will continue until BTPS is Rationale achieved. for Loss of moisture and heat from the small airways has Humidity consequences which include: ◼Damage to airway epithelium Therapy ◼Impairs movement of cilia ◼Airway secretions become very thick and difficult to clear Retain secretions which incr. risk of infection and adds to airway obstruction  ABSOLUTE HUMIDITY the actual amount or content of water vapor within a gas. It is expressed as mg/L or mmHg. Humidity RELATIVE HUMIDITY Definitions the ratio of the absolute humidity to the saturated capacity of the gas at a particular temperature. %RH = content (absolute humidity) x 100 saturated capacity  % BODY HUMIDITY ratio of the water vapor content of an inspired gas to the saturated capacity of the inspired gas at 37˚c. Definitions saturated capacity at 37˚c is 44mg/L Continued %BH = content (absolute humidity) x 100 44mg/L  Humidity Deficit The amount of water vapor the body must add to an inspired gas to achieve BTPS Humidity Deficit =44mg/L –absolute humidity of inspired gas The larger the humidity deficit, the greater the chance that BTPS will not be achieved at the level just below carina.  Humidity Deficit Additional Complication of a significant humidity deficit include: Hypothermia Water vapor adds heat to the inspired gas. A significant deficit can contribute to hypothermia. Bronchospasm Cold dry air may irritate the airway leading to bronchospasm  Indications For Humidity Therapy 1. Humidify dry medical gases (liter flows >4 L/min) inhaled through the upper airway. 2. Upper airway bypassed due to artificial airway. 3. Strategy for managing hypothermia or bronchospasm caused by exposure to cold air.  Evaporation The process where water molecules escape into a gas when the temperature of the water is below it’s boiling point. Humidifiers operate on the principle of evaporation. When gas is passed across the surface of water, molecules will evaporate into the gas Within the gas the water molecules exert a pressure, and this is referred to as Water Vapor Pressure  Factors Influencing the Rate of Evaporation During Humidification TEMPERATURE PRESSURE SURFACE AREA TIME  Temperature and Evaporation Rate of Evaporation varies directly with Temperature As the temperature of the gas is increased, the gas molecules move farther apart and therefore the CAPACITY for water vapour molecules INCREASES. As the temperature of the water is increased, the movement of the water molecules increases which increases the rate of evaporation.  Pressure and Evaporation Rate of Evaporation Varies Inversely with Pressure. as the pressure of the gas increases it opposes the evaporation of the water molecules into the gas. How Does This Apply To Clinical Situations ?? As atmospheric pressures change, the rate of evaporation can change. With CPAP/BIPAP therapy, as the prescribed pressures increase the rate of evaporation will decrease. * Overall this has minimal significance for typical medical gas therapies.  Surface Area and Evaporation Evaporation Varies Directly with Surface Area As the surface area for contact between the water molecules and the gas increases, the rate of evaporation will increase.  Time and Evaporation Evaporation Varies Directly with Time The longer the gas is in contact with the water molecules the greater the rate of evaporation.  Incorporating Time, Surface Area and Temperature to Humidification Devices Temperature when the upper airway is bypassed by an artificial airway the humidification device must be heated so BTPS can be achieved. Surface Area and Time All the humidifiers have been designed to maximize the surface area and time for contact between the water and gas.  TYPES OF HUMIDIFIERS USED FOR MEDICAL GAS DELIVERY BUBBLE HUMIDIFIERS PASSOVER HUMIDIFIER HEAT AND MOISTURE EXCHANGER (HME) BUBBLE HUMIDIFIERS  Bubble Humidifiers Provide humidified gas delivered via Nasal Cannula and Medium Concentration Mask. Operation 1. Gas from the flowmeter is directed down a small tube into the reservoir of water. 2. As gas leaves the tube it passes through a diffuser head which breaks the gas flow into several tiny bubbles. 3. Gas bubbles rise through the reservoir of water until they reach the surface. 4. As the GAS bubble moves through the water, the water molecules evaporate into the bubble. Key Design Element is Diffuser Head Diffuser breaks the gas into several smaller gas bubbles Increases the surface area for contact between the water and the gas  Only capable of 25% Body Humidity ( 15-20 mg/L ) Factors Limiting the Efficiency 1.No Heat The water w/in the reservoir is not heated Efficiency of 2.Increasing Flows Impact Contact Time Bubble ↑ flow =↓contact time between gas and water Humidifier Therefore, as input flows increase the rate of evaporation will decrease and the % body humidity decreases. Increasing the input flows also cools the water which will further decrease the rate of evaporation.  Resistance Bubble humidifiers contain a pressure relief valve Downstream resistance will create backpressure which can cause the pressure relief valve to be activated, makes a chirping bird sound. Additional Limitations of Downstream Resistance Can Be Created By; Bubble Jet orifice of an air entrainment mask Humidifiers Jet orifice of a small volume nebulizer Use of a Bubble Humidifier Bottle is limited to gas delivery with a Nasal Cannula and Simple Mask  Passover Humidifier Gas is directed across the surface of the water in a chamber. Evaporation occurs as the gas flows across the surface of the water. Heating element heats the water and gas as it enters the chamber. Gas can reach BTPS.  Servo Controlled Heated Passover Humidifiers Temperature probes at the output of the humidifier and patient wye of the ventilator circuit monitor gas temperatures. If temperature decreases a signal is created which increases the temperature at the chamber. The servo control ensures BTPS is maintained despite conditions which cause changes to rate of evaporation. Heated Wire Circuits Heated wires run through the ventilator circuit which maintain BTPS conditions after the gas exits the humidifier. Heated Passover Humidifier and Circuit Heat and Moisture Exchanger (HME)  HME 1. Material within the HME collects heat and moisture from patient’s exhaled air. 2. Through evaporation the moisture and heat is added to the inspired air.  Indication For HME Use Used to add heat and humidity to gas inspired through an artificial airway. ◦ Added to the ventilator circuit ◦ Added to a bag and valve assembly ◦ Added to the circuit during anesthesia  Advantages Lightweight Adds minimal resistance Adds minimal deadspace (10-98 cc’s) for adults Decrease risk of hospital acquired infections Provides up to 75% body humidity (30 mg/L) Advantages and Contraindications Contraindications Cannot be used for infants due to significant addition of deadspace. Cannot be used for patients with +++ secretions. Recommended that HME be changed at least every five days.