OXYGEN AND MEDICAL GAS THERAPY

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CHAPTER 1 OXYGEN AND MEDICAL GAS THERAPY

PRETEST QUESTIONS

Answer the pretest questions before studying the chapter. This will help you determine your strong and weak areas in the material covered.

1. A patient is receiving oxygen from an “E” cylinder at 4 L/min through a nasal cannula. The cylinder pressure is 1900 psig. How long will the cylinder run until it is empty?

A. 47 min
B. 1.7 h
C. 2.2 h
D. 3.6 h

2. After setting up a partial rebreathing mask on a patient at a flow of 8 L/min, the reservoir bag collapses before the patient finishes inspiring. The respiratory care practitioner should do which of the following?

A. Change to a 6-L/min nasal cannula.
B. Decrease the flow.
C. Change to a non-rebreathing mask.
D. Increase the flow.

3. A patient with carbon monoxide (CO) poisoning can best be treated with which of the following therapies?

A. Nasal cannula at 6 L/min
B. Hyperbaric O2
C. Continuous positive airway pressure (CPAP)
D. Non-rebreathing mask

4. The following blood gas levels have been obtained from a patient using a 60% aerosol mask.

pH 7.47
PaCO2 31 mm Hg
PaO2 58 mm Hg

What should the respiratory care practitioner recommend at this time?

A. Place the patient on CPAP.
B. Increase the oxygen to 70%.
C. Intubate and place the patient on mechanical ventilation.
D. Change to a non-rebreathing mask.

5. Given the following data, what is the patient’s total arterial O2 content?

pH 7.41
PaCO2 37 mm Hg
PaO2 88 mm Hg
HCO3− 26 mEq/L
SaO2 95%
Hb 14 g/dL
A. 12 vol%
B. 14 vol%
C. 16 vol%
D. 18 vol%

6. A patient is using a 30% Venturi mask at an O2 flow of 5 L/min. The total flow delivered by this device is which of the following?

A. 36 L/min
B. 45 L/min
C. 54 L/min
D. 60 L/min

See answers and rationales at the back of the text.

REVIEW

I. STORAGE AND CONTROL OF MEDICAL GASES

CRT Exam Content Matrix: IIA9a

RRT Exam Content Matrix: IIA4d, IIC6

A. Storage of Medical Gases and Cylinder Characteristics
1. Cylinders are constructed of chrome molybdenum steel.
2. Gas cylinders are stored at high pressures; a full O2 cylinder contains 2200 psig pressure.
3. Cylinders are constructed in various sizes. The most common sizes for O2 storage are the “H” cylinder and the “E” cylinder.
a. The “H” cylinder holds 244 cu ft (6900 L) of O2.
b. The “E” cylinder, used for transport, holds 22 cu ft (622 L) of O2.

image There are 28.3 L in 1 cu ft.

4. Valves on the cylinder allow attachment of regulators that release the gas at various flow rates. Cylinder and regulator safety systems dictate that the valves be constructed to allow the connection of only one type of gas regulator. For example, an O2 regulator cannot be attached to a helium cylinder.
a. Large cylinders use the American Standard Safety System. Each type of gas cylinder valve has a different number of threads per inch and a different thread size, and the cylinder may require either a right- or left-hand turning motion for attachment to the regulator.
b. Small cylinders use the Pin Index Safety System. Each cylinder valve has two holes drilled in unique positions that line up with corresponding pins on the appropriate regulator. There are six different hole placement positions.
5. Safety relief devices on cylinder valves allow escape of excess gas if the pressure in the cylinder increases. There are two types of safety relief devices:
a. Frangible disk—breaks at 3000 psig
b. Fusible plug—melts at 208° to 220° F (caused by high ambient temperature or high pressure, which increase the temperature)
6. The Compressed Gas Association (CGA) developed a color code system for cylinders to distinguish the various gases.

Gas Color of Cylinder
Oxygen Green; white (internationally)
Helium Brown
Carbon dioxide Gray
Nitrous oxide Light blue
Cyclopropane Orange
Ethylene Red
Air Yellow
CO2/O2 Gray and green
He/O2 Brown and green

7. Cylinder markings

image

FIGURE 1-1 A, Cylinder markings.

B, Cylinder markings.

8. Cylinder testing
a. Cylinders are visually tested by dropping a lightbulb inside to look for corrosion.
b. Cylinders are hydrostatically tested every 5 or 10 yr, depending on the cylinder marking. A “star” next to the latest test date means the next test must be done 10 yr from that date. Hydrostatic testing determines the amount or number of
(1) Wall stress
(2) Cylinder expansion
(3) Leaks
9. Liquid gas systems
a. 1 cu ft of liquid O2 expands to 860 cu ft as it changes to a gas. Liquid O2 is much more economical than gas.
b. The liquid O2 is stored in Thermos containers at a pressure not to exceed 250 psig and a temperature below −297° F (the boiling point of O2).
c. Liquid O2 is most commonly produced by the process of fractional distillation.
B. Control of Medical Gases
1. Regulators are devices attached to the cylinder valve to regulate flow and reduce cylinder pressure to working pressure (i.e., 50 psig).
2. Types of reducing valves
a. Single-stage, which reduces the cylinder pressure directly to 50 psig and has one safety relief device
b. Double-stage, which reduces the cylinder pressure to approximately 150 psig and then to 50 psig and has two safety relief devices
c. Triple-stage, which reduces the cylinder pressure to approximately 300 psig, then to 150 psig, and finally to 50 psig and has three safety relief devices
3. Regulators may be preset or adjustable.
a. Mechanics of a preset regulator:

image

FIGURE 1-2 Gas under high pressure from the cylinder enters the inlet (A) of the reducing valve where cylinder pressure is read on the Bourdon gauge (B). Gas then passes through the inlet valve (H) into the pressure chamber (C) and pushes down on the diaphragm (E), spring (F), and valve stem (G) assembly, closing the valve and cutting off gas flow into the pressure chamber. At the moment the valve closes, the gas pressure is equal to the spring pressure in the ambient chamber (D). Then, as gas exits the pressure chamber through the outlet (I), pressure drops in the pressure chamber and the diaphragm assembly moves upward, which allows gas to enter the chamber again. In fact, the diaphragm assembly moves continually up and down, opening and closing the valve repeatedly, as gas passes through the chamber flowmeter (J) and needle valve (K). Excessive pressure in the pressure chamber is vented through a pressure relief valve (L). The spring tension on this regulator is preset at 50 psig.

b. Adjustable regulator:

image

FIGURE 1-3 This is an example of a Bourdon gauge regulator. It works just as a preset regulator does, except the spring tension is adjustable by the user. The second gauge (J) indicates the pressure exerted by the spring. If a flowmeter or high-pressure hose leading to a ventilator is attached to the outlet of the regulator, the spring should be adjusted to 50 psig. The gauge may also be calibrated in L/min to read flow. This is called a Bourdon gauge flowmeter.

4. Technical problems associated with reducing valves and regulators
a. Dust or debris entering the regulator from the cylinder valve may rupture the diaphragm. Always “crack” the cylinder before attaching a regulator. This is accomplished by turning the cylinder on and back off quickly to blow out the debris from the cylinder outlet.
b. Constant pressure “trapped” in the pressure chamber after the cylinder is turned off may rupture the diaphragm. Always vent pressure in the regulator by turning the flowmeter back on after the cylinder is turned off.
c. A hole in the diaphragm will result in a continuous leak into the ambient chamber and out the vent hole (see diagram of regulator), causing failure of the regulator.
d. A weak spring can result in diaphragm vibration and inadequate flows that are caused by premature closing of the inlet valve.
e. When attaching a regulator to a small cylinder, make sure the plastic washer is in place or gas will audibly leak around the cylinder valve outlet and regulator inlet.
5. Calculating how long cylinder contents will last:

image

Cylinder factors: “H” cylinder = 3.14 L/psig
“E” cylinder = 0.28 L/psig

EXAMPLE:

Calculate how long a full “H” cylinder will last running at 8 L/min.

image

EXAMPLE:

An “E” cylinder of O2 contains 1800 psig. If the respiratory care practitioner runs the cylinder at 4 L/min through a nasal cannula, how long will it take for the cylinder to reach a level of 200 psig?

image

6. Calculating the duration of flow for a liquid O2 system

image One liter of liquid oxygen weighs 2.5 lb (1.1 kg).


image

image

EXAMPLE:

A liquid oxygen tank contains 5 lb of liquid oxygen. The patient is receiving oxygen at 3 L/min through a nasal cannula. How long will the liquid oxygen last?

image

image

Math shortcut: Because 860 and 2.5 are constants in the equation, cancel them out by dividing 860 by 2.5. The answer is 344. Now simply multiply 344 by the pounds of liquid oxygen. The answer will be the same as when you use the longer equation.

7. Flowmeters
a. Uncompensated flowmeter:

image

FIGURE 1-4

(1) The needle valve is located proximal to (before) the float; therefore atmospheric pressure is in the Thorpe tube. Any back pressure in the tube affects the rise of the float.
(2) When a restriction, such as a humidifier or a nebulizer, is attached to the outlet, back pressure into the tube forces the float down and compresses the gas molecules closer together so that more molecules go around the float than what the float indicates. Therefore the flowmeter reading is lower than what the patient is actually receiving.
(3) Uncompensated flowmeters should not be used clinically.
b. Compensated flowmeter

image

FIGURE 1-5

(1) The needle valve is located distal to (after) the float; therefore 50 psig is in the tube. Only back pressure that exceeds 50 psig will affect the rise of the float.
(2) The flowmeter reads accurately with an attachment, such as a humidifier or nebulizer on the outlet, or with any obstruction downstream. If the oxygen tubing is completely obstructed with no gas flowing to the patient, the flowmeter will reflect that with a flow reading of near zero.
(3) There are three ways to determine whether a flowmeter is compensated for pressure:
(a) It is labeled as such on the flowmeter.
(b) The needle valve is located after the float.
(c) The float jumps when the flowmeter, while it is turned off, is plugged into a wall outlet.
(4) Flowmeter outlets use the Diameter Index Safety System, as do all gas-administering equipment that operate at less than 200 psig, so that attachment to the wrong gas source is avoided.
(5) If the flowmeter is turned off completely but gas is still bubbling through the humidifier or is heard coming from the flowmeter, the valve seat is faulty and the flowmeter should be replaced.
c. Bourdon gauge flowmeter

image

FIGURE 1-6

(1) The Bourdon gauge flowmeter is a pressure gauge that has been calibrated in liters per minute. It is uncompensated for back pressure.
(2) When a humidifier or nebulizer is attached to the outlet of the Bourdon gauge, back pressure is generated into the gauge (which measures pressure) and the gauge reading is higher than what the patient is actually receiving.
(3) Gas enters the hollow, flexible question mark–shaped tube, which tends to straighten as pressure fills it. A gear mechanism is attached to the tube, and as the tube straightens, it rotates a needle indicator that shows the pressure (flow).
(4) The advantage of the Bourdon gauge is that it, unlike Thorpe tube flowmeters, is not position dependent. It reads just as accurately in a horizontal position as it does in a vertical position.
(5) Because this gauge actually measures pressure, an obstruction to flow through the tubing attached to a Bourdon gauge flowmeter resulting in back pressure will cause the gauge reading to increase slightly. In other words, the gauge will indicate flow to the patient while the patient is receiving little or no oxygen.
8. Air compressors are used to provide medical air through either portable compressors or large medical air piping systems. Two types of air compressors are generally used:
a. Piston air compressor:

image

FIGURE 1-7

Air is drawn into the compressor, where it travels to a reservoir tank. From this tank, the air passes through a dryer to remove moisture and on to a pressure-reducing valve, which reduces the pressure to 50 psig to power a compressed-air wall outlet. As the piston drops, gas is drawn in through a one-way intake valve. On the upstroke, the intake valve closes and gas exits through a one-way outflow valve. Piston air compressors are seen most commonly on large medical air piping systems.

b. Diaphragm air compressor:

image

FIGURE 1-8

A diaphragm is used instead of a piston. On the downstroke, the flexible diaphragm bends downward, drawing air through a one-way intake valve. On the upstroke, air is forced out the one-way outflow valve. Diaphragm air compressors are commonly used on O2 concentrators and portable air compressors.

II. OXYGEN THERAPY

CRT Exam Content Matrix: IIA1a-c, IIA9a-b, IIA12a-b, IIID2d, IIID6, IIIE10, IIIF2d1-2, IIIG2c

RRT Exam Content Matrix: IIA4b

A. Indications for O2 Therapy
1. Hypoxemia
2. Labored breathing or dyspnea
3. Increased myocardial work
B. Signs and Symptoms of Hypoxemia
1. Tachycardia
2. Dyspnea
3. Cyanosis (unless anemia is present)
4. Impairment of special senses
5. Headache
6. Mental disturbance
7. Slight hyperventilation (tachypnea)
C. Complications of O2 Therapy
1. Respiratory depression: The patient with chronic obstructive pulmonary disease (COPD) who is breathing with the “hypoxic drive” mechanism is most affected. Maintain PaO2 between 50 and 65 mm Hg for these patients.

image Exam Note

Although the validity of the hypoxic drive theory is controversial, it is considered a possible complication on the exam.

2. Atelectasis: High O2 concentrations in the lung can wash out nitrogen in the lung and reduce the production of surfactant, which may lead to atelectasis. Maintain FiO2 below 0.60.
3. Oxygen toxicity: High O2 concentrations result in increased O2 free radicals and therefore lung tissue toxicity. This may lead to acute respiratory distress syndrome (ARDS). Maintain FiO2 below 0.60.
4. Reduced mucociliary activity: Maintain FiO2 below 0.60. The beating of the cilia in the mucociliary blanket is not as active when high FiO2 levels are used.
5. Neonatal retinopathy (retinopathy of prematurity): This is caused by high PaO2 levels in infants and results in blindness. It is more common in premature infants. Maintain PaO2 below 80 mm Hg. Normal level of PaO2 in infants is 50 to 70 mm Hg.
D. Normal PaO2 Values by Age

Age (yr) Normal PaO2
≤60 80 mm Hg
70 70 mm Hg
80 60 mm Hg
E. Four Types of Hypoxia
1. Hypoxemic hypoxia
a. Caused by lack of O2 in the blood as a result of
(1) Inadequate O2 in the inspired air: administering O2 is beneficial.
(2) Alveolar hypoventilation: administering O2 alone may not be beneficial.
(3) Diffusion defects (i.e., pulmonary edema, atelectasis, and pulmonary fibrosis): administering O2 alone may not be beneficial.
(4) A ventilation/perfusion mismatch: administering O2 may be beneficial.
(5) An anatomic right to left shunt: administering O2 is not beneficial.
b. If a normal PaO2 level cannot be maintained with a 60% O2 mask, a large shunt is probable and should not be treated with higher O2 concentrations. CPAP should be administered provided that the PaCO2 is at normal or below normal levels. If the PaCO2 level is elevated in a patient with hypoxemia, mechanical ventilation should be initiated.

image Exam Note

When given a patient using 60% oxygen or higher who is ventilating adequately (normal or low PaCO2) but has hypoxemia, place the patient on CPAP. The exception to this rule is if the question states that the patient has hypotension, a low cardiac output, or an elevated intracranial pressure level. Positive pressure should not be administered because the increased intrathoracic pressure may worsen these conditions. Thus in these situations, increase the FiO2.

2. Anemic hypoxia
a. The blood’s capacity to carry O2 is reduced as a result of
(1) A decreased hemoglobin (Hb) level.
(a) Normal Hb level is 12 to 16 g/dL of blood.

image 

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