CHAPTER 2 HUMIDITY AND AEROSOL
PRETEST QUESTIONS
1. Secretions tend to become thicker if the inspired air has which of the following characteristics?
2. A patient receiving 38 mg H2O per liter of gas from a nebulizer has a humidity deficit of which of the following?
3. After connecting a nasal cannula to the humidifier outlet, you kink the tubing and hear a whistling noise coming from the humidifier. Which of the following most likely has caused this?
4. You notice that the patient’s secretions have become thicker and more difficult to suction since replacing the ventilator humidifier with a heat moisture exchanger. The respiratory therapist should recommend which of the following?
5. Which of the following are indications for bland aerosol therapy?
6. You notice that very little mist is being produced by a nebulizer attached to an aerosol mask. Which of the following could be responsible for this?
REVIEW
Note: Questions regarding humidity therapy appear on the CRT exam only.
CRT Exam Content Matrix: IIA3, IIIB8, IIIF2, IIIF2g2
A. Humidity is the quantity of moisture in air or gas that is caused by the addition of water in a gaseous state, or vapor. Also called molecular water or invisible moisture.
The objective of humidity therapy is to make up for water loss that occurs when dry gas is delivered or when the upper airway is bypassed.2. To provide 100% body humidity of the inspired gas for patients with ET tubes or tracheostomy tubes
3. By the time inspired air reaches the oropharynx, it has been warmed to approximately 34° C and is 80% to 90% saturated with H2O.
4. By the time the inspired air reaches the carina, it has been warmed to body temperature (37° C) and is 100% saturated.
5. When the inspired air is fully saturated (100%) at 37° C, it holds 44 mg H2O per liter of gas and exerts a water vapor pressure of 47 mm Hg.
1. Absolute humidity is the amount of water in a given volume of gas; its measurement is expressed in milligrams per liter.
2. Relative humidity is a ratio between the amount of water in a given volume of gas and the maximum amount it is capable of holding at that temperature (capacity). Its measurement is expressed as a percentage and is obtained with a hygrometer.
The capacity of water at body temperature is 44 mg/L.
3. A 48% body humidity indicates that the inspired air is holding only 48% of the water it takes to fully saturate the gas in the airway at body temperature. The body’s humidification system adds the other 52% by the time the air reaches the carina.
1. Inspired air that is not fully saturated at body temperature creates a humidity deficit. This deficit is corrected by the body’s own humidification system.
or when expressed as a percent:
4. This is why it is important to deliver humidified gas at body temperature to a patient with an artificial airway that bypasses the patient’s upper airway.
5. If adequate humidity is not provided, the patient’s airway can dry out, which can lead to thickening of secretions and result in increased airway resistance.
6. Gas being delivered to a patient with an ET tube or tracheostomy tube that contains less than 44 mg H2O per liter of gas or a water vapor pressure of less than 47 mm Hg tends to dry secretions, making them thicker and more difficult to mobilize.
a. The higher the flow rate used, the less time of contact between the gas and water and therefore the lower the humidity output.
b. The lower the water level in the jar, the less time of contact between the gas and water and therefore the lower the humidity output.
a. Gas simply passes over the surface of the water, picking up moisture and delivering it to the patient.
b. Produces low humidification because of the limited time of gas and water contact and the small surface area involved.
b. O2 entering the humidifier travels through a tube under the surface of the water and exits through a diffuser at the lower end of the tube.
a. Gas from the flowmeter or ventilator enters the humidifier and is exposed to the wick (made of cloth, sponge, or paper), which is partially under the surface of the water.
c. Because the water bath or the gas is heated, a body humidity approaching 100% is delivered to the patient. This method is ideal for patients with artificial airways and those who receive mechanical ventilation.
Exam Note
a. Gas travels down the tower and hits the bottom of the jar, which displaces the water upward over a grid, forming a liquid film. The gas travels back up through the grid (from underneath), picking up moisture and delivering it to the patient.
1. Most nonheated humidifiers have a pressure pop-off valve set at 2 psi. After the device is set up, the tubing of the oxygen delivery device (e.g., cannula, mask) should be kinked to obstruct flow. If the pop-off sounds, there are no leaks. If no sound is heard, all connections, as well as the humidifier top, should be tightened.
2. Water levels of all humidifiers should be maintained at the levels marked on the humidifier jar to ensure maximum humidity output.
3. Condensation occurs in the tubing of heated humidifiers. This water should be discarded in a trash container or basin and should never be put back into the humidifier.
4. The temperature of inspired gas should be monitored continuously with an in-line thermometer when heated humidifiers are used. The thermometer should be as close to the patient’s airway as possible.
5. Warm moist areas, such as heated humidifiers, are a breeding ground for microorganisms (especially Pseudomonas species). The humidifier should be replaced every 24 hours.
Exam Note
1. This device is placed in line between the patient’s ET tube and the ventilator adapter of the ventilator circuit.
2. As the patient exhales, gas at body humidity and body temperature enters the heat moisture exchanger (HME), which heats the hygroscopic filter (made of felt, plastic foam, or cellulose sponge) and condenses water into it. During the next inspiration, gas passes through the HME and is warmed and humidified.
Exam Note4. The HME may be used for temporary humidification during patient transport or for long-term humidification for ventilator-dependent patients. If ventilatory pressure begins to increase, check for water or secretions in the HME, and if there are obstructions, change to a new HME. If secretions are causing obstruction, a conventional heated humidifier should be used.
CRT Exam Content Matrix: IIA4, IIA12, IIA21, IIIC3, IIID5a, IIIF2c1-4
RRT Exam Content Matrix: IIIC3, IIID5a
A. Aerosol is defined as a suspension of water in particulate form (or a mist) in gas. Nebulizers produce aerosolized gas. Nebulizers are often referred to as aerosol generators.
a. Gravitational sedimentation: The larger a particle is, the more effect gravity has on it and the sooner it will deposit.
b. Brownian movement: Affects particles of 0.1 µm or smaller in size, which will deposit too soon (possibly in the aerosol tubing).
c. Inertial impaction: Larger particles have greater inertia, which keeps them moving in a straight line. Because they cannot make directional changes in the airway, they deposit sooner.
d. Hygroscopic properties: Aerosol particles are hygroscopic (retain moisture). As they travel down the airway, they may increase in size as they retain moisture, which may alter the time at which they deposit.
e. Ventilatory pattern: To obtain optimal particle penetration, the patient should be instructed to take slow, moderately deep breaths with a 2- to 3-second breath hold at end inspiration.
a. Uses viscous shear forces (see later note) to draw water up the capillary tube into the gas stream then to a baffle that breaks the water into fine aerosol particles.
b. Uses either a Venturi or Pitot tube for the entrainment of air to obtain various oxygen percentages.
(1) Venturi tube: The objective is to allow air entrainment, increase flow, and restore postrestriction pressure toward prerestriction pressure.
(2) Pitot tube: The objective is to maintain high forward velocity of the gas after air entrainment occurs, which results in a high forward pressure that is better able to counteract back pressure into the device and to maintain more stable oxygen percentages of the gas. (Postrestriction lateral pressure is not restored to prerestriction levels.)
Current theories in the literature suggest that the negative pressure created to entrain air is the result of viscous shear forces that occur as gas passes through a restriction. As the gas travels through a narrowed orifice, the flow becomes turbulent and the swirling of the gas causes a decrease in pressure, which allows air to be entrained in the area of lower pressure.
a. Water is pumped up into a reservoir above a glass sphere and drops out of the reservoir onto the sphere. The sphere has a small hole with high-velocity gas coming through it that decreases the pressure pulling the water over the sphere. Water is hit by high-velocity gas, producing an aerosol, and the particles hit a baffle, further reducing particle size.
b. About 97% of the aerosol particles produced fall within the 1- to 10-µg size range, and 50% measure less than 5 µg.
a. Used as a handheld nebulizer or as a nebulizer on a ventilator or intermittent positive pressure breathing (IPPB) circuit to deliver medications.
a. This device delivers medication in aerosol form by squeezing the vial (in which the medication is stored) upward into the delivery port. This activates a small valve that allows the pressurized gas to nebulize the medication and deliver it to the patient.
b. Metered-dose inhalers (MDIs) have become a very popular method for delivering aerosolized drugs to the respiratory tract.
c. The particle size produced varies from 2 to 40 µg. Only about 10% of the dose actually reaches the lower respiratory tract.
d. The patient must be instructed thoroughly and correctly on the proper use of the MDI to ensure optimal aerosol particle penetration.
(1) Do not place your lips around the delivery port, but keep your mouth opened wide so that the teeth and lips do not obstruct the flow of aerosol.
(2) Hold the MDI about 1 inch from the mouth, with the delivery-port opening directed inside the mouth.
(3) Inhale as slowly and as deeply as possible. The MDI should be activated just after you start inhaling.
(4) Depending on the prescription, two or three aerosol doses (puffs) may be taken, waiting 30 s to 1 min between puffs.
f. MDIs are also used for ventilator-dependent patients by placement of the device directly into the inspiratory limb of the circuit and activation by the respiratory care practitioner. Ideally, the aerosol should be delivered during a sigh breath with an inspiratory hold.
g. Spacers and holding chambers, which are extensions placed on the outlet of the MDI, have proved to be effective in minimizing aerosol loss and increasing the evaporation of the propellant. This increases the stability of the aerosol and results in deeper penetration of the particles.
FIGURE 2-10 Types of spacers and holding chambers.
A, From Monaghan Medical Corporation. B, From Gardenhire DS: Rau’s respiratory care pharmacology, ed 7, St Louis, 2008, Mosby.
a. The disk rotates rapidly, drawing water up from the reservoir and throwing it through a slotted baffle, which reduces the size of the particles.
b. These devices are popular for home use, but clinically they do not produce adequate aerosol output.
a. A piezoelectric transducer located in the bottom of the couplant chamber of the unit is electrically charged and produces high-frequency vibrations. These vibrations are focused on the bottom of the medication cup that sits in the couplant chamber. The vibrations break the medications in the cup into small particles, which are delivered to the patient.
b. The frequency (which determines the particle size) of the electric energy supplied to the transducer is approximately 1.35 Mc (1 Mc = 1 million Hz). Particle size cannot be adjusted by the user.
c. The couplant chamber contains tap water to help absorb mechanical heat and to act as a transfer medium for the sound waves to the medication cup.
d. The amplitude control determines the volume of the aerosol output. Volume may be as high as 6 mL/min, which is twice the output of pneumatic nebulizers, making the ultrasonic nebulizer the best choice for patients with thick, retained sputum or when sputum induction is necessary for laboratory cultures.
e. A built-in blower delivers 20 to 30 L/min of air to the medication cup to aid in aerosol delivery and help evacuate heat.
g. The temperature of the delivered aerosol is between 3° C and 10° C above room temperature during normal operation.
(7) Changes in drug dosage, caused by the drug reconcentrating because of the solvent in the medication cup, leading to an increasingly stronger dose as the treatment continues
1. Of all respiratory equipment, heated nebulizers are the greatest source of delivery of contaminated moisture to the patient; Pseudomonas species are the most common contaminate. These nebulizers should be replaced every 12 to 24 h.
2. Make sure that jets and capillary tubes are clear of debris or buildup of minerals by cleaning after each use. If the nebulizer is not producing adequate mist, a clogged capillary tube or jet may be the cause.
3. Some older, nondisposable nebulizers have pressure pop-off valves that should be checked after the nebulizer is set up (valves are usually set at 2 psi). The pop-off valve is effective only when the nebulizer is set on 100% because an open entrainment port constitutes a leak in the nebulizer.
4. Keep water drained out of the aerosol tubing because this increases the percentage of oxygen delivered to the patient. Water in the tubing obstructs gas flow, resulting in back pressure into the nebulizer. As pressure increases in the nebulizer, less air is entrained in the entrainment port; therefore delivered FiO2 increases, total flow to the patient decreases, and less aerosol is delivered.
5. Keep the water level at the appropriate markings on the nebulizer to ensure optimal aerosol output.
6. Soap should not be used to clean the couplant chamber or medication cup of the ultrasonic nebulizer because residue interferes with the ultrasonic activity. If this occurs, a small amount of isopropyl alcohol in the couplant chamber or medication cup helps.
7. Make sure the filter at the end of the capillary tubes of pneumatic nebulizers are decontaminated properly. A dirty or clogged filter does not allow the fluid to be drawn up the tube adequately, which reduces aerosol output.
POSTCHAPTER STUDY QUESTIONS
1. List five hazards of aerosol therapy.
2. If the inspired air can hold 32 mg of H2O per liter and is holding 8 mg/L, what is the relative humidity?
3. How much of a humidity deficit exists when a patient inspires air holding 30 mg of H2O per liter?
4. Give an example of a humidifier that can deliver gas at 100% body humidity.
5. List four hazards of ultrasonic nebulizer therapy.
6. A clogged filter or capillary tube on a jet nebulizer has what effect on the operation of the device?
7. List five clinical indications for aerosol therapy.
8. How should a patient be instructed to breathe while a bronchodilating agent is being administered via a small-volume nebulizer?
Branson R, Hess D, Chatburn R. Respiratory care equipment, ed 2. Philadelphia: Lippincott; 1999.
Cairo J, Pilbeam S. Mosby’s respiratory care equipment, ed 8. St Louis: Mosby; 2010.
Gardenhire DS. Rau’s respiratory care pharmacology, ed 7. St Louis: Mosby; 2008.
Hess D, Respiratory care principles and practice, ed 1, Philadelphia, Saunders, 2002.
Wilkins RL, Stoller JK, Kacmarek R. Egan’s fundamentals of respiratory care, ed 9. St Louis: Mosby; 2009.
