Anesthesia Equipment and Physics Flashcards
1
Q
1. A 58-year old patient has severe shortness of breath and “wheezing”. On examination, it is found that the patient has inspiratory and expiratory stridor. Further evaluation reveals marked extrinsic compression of the midtrachea by a tumor. The type of airflow at the point of obstruction within the trachea is A. laminar flow B. orifice flow C. undulant flow D. stenotic flow E. none of the above
A
- B Orifice flow occurs when gas flows through a region of severe constriction such as described in this question. Laminar flow occurs when gas flows down parallel-sided tubes at a rate less than critical velocity. When the gas flow exceeds the critical velocity, it becomes turbulent. Hall 1
2
Q
- Concerning the patient with extrinsic compression of the midtrachea by a tumor and severe SOB and wheezing, administration of 70% helium in O2 instead of 100% O2 will decrease the resistance to airflow through the stenotic region within the trachea because
A. helium decreases the viscosity of the gas mixture
B. helium decreases the friction coefficient of the gas mixture
C. helium decreases the density of the gas mixture
D. helium increases the reynolds number of the gas mixture
E. none of the above
A
- C During orifice flow, the resistance to gas flow is directly proportional to the density of the gas mixture. Substituting helium for nitrogen will decrease the density of the gas mixture, thereby decreasing the resistance to gas flow (as much as threefold) through the region of constriction. Hall 2
3
Q
3. A 56-year-old patient is brought to the OR for elective replacement of a stenotic aortic valve. An awake 20-gauge arterial catheter is placed into the right radial artery and is then connected to a transducer located at the same level as the patient’s left ventricle. The entire system is zeroed at the transducer. Several seconds later, the patient raises both arms into the air such that his right wrist is 20 cm above his heart. As he is doing this, the blood pressure (BP) on the monitor reads 120/80. What would this patient’s true BP be at this time? A. 140/100 mm Hg B. 135/95 mm Hg C. 120/80 mm Hg D. 105/65 mm Hg E. 100/60 mm Hg
A
- C Modern electronic blood pressure monitors are designed to interface with electromechanical transducer systems. These systems do not require extensive technical skill on the part of the anesthesia provider for accurate usage. A static zeroing of the system is built into most modern electronic monitors. Thus, after the zeroing procedure is accomplished, the system is ready for operation. The system should be zeroed with the reference point of the transducer at the approximate level of the aortic root, eliminating the effect of the fluid column on the system of arterial BP readings. Hall 3
4
Q
4. An admixture of room air in the waste gas disposal system during an appendectomy in a paralyzed, mechanically ventilated patient under general volatile anesthesia can best be explained by which mechanism of entry? A. venous air embolism B. positive pressure relief valve C. negative pressure relief valve D. soda lime canister E. ventilator bellows
A
- C Waste gas disposal systems, also called scavenging systems, are designed to decrease pollution of the OR by anesthetic gases. These scavenging systems can be passive (waste gases flow from the anesthesia machine to a ventilated system on their own) or active (anesthesia machine connected to a vacuum system then to the ventilation system). The amount of air from a venous gas embolism would not be enough to be detected in the disposal system. Positive pressure relief valves open if there is an obstruction between the anesthesia machine and the disposal system, which would then leak the gas into the OR. A leak in the soda lime canisters would also vent to the OR. Since most ventilator bellows are powered by oxygen, a leak in the bellows would not add air to the evacuation system. The negative pressure relief valve is used in active system and will entrap room air if the pressure in the system is less than -0.5 cm H2O. Hall 4
5
Q
5. The relationship between intra-alveolar pressure, surface tension, and the radius of an alveolus is described by A. Graham’s law B. Beer’s law C. Newton’s law D. Laplace’s law E. Bernoulli’s law
A
- D The relationship between intra-alveolar pressure, surface tension, and the radius of alveoli is described by Laplace’s law for a sphere, which states that the surface tension of the sphere is directly proportional to the radius of the sphere and pressure within the sphere. With regard to pulmonary alveoli, the mathematical expression of Laplace’s law is as follows: T = ½ PR where T is the surface tension, P is the intra-alveolar pressure, and R is the radius of the alveolus. In pulmonary alveoli, surface tension is produced by a liquid film lining the alveoli. This occurs because the attractive forces between the liquid film and gas. Thus, the surface area of the liquid tends to become as small as possible, which could collapse the alveoli. Hall 5
6
Q
6. A size “E” compressed-gas cylinder completely filled with N2O contains how many liters? A. 1160 L B. 1470 L C. 1590 L D. 1640 L E. 1750 L
A
- C The World Health Organization requires that compressed-gas cylinders containing N2O for medical use be painted blue. Size “E” compressed-gas cylinders completely filled with N2O contain approximately 1590 L of gas. Hall 6
7
Q
- Which of the following methods can be used to detect all leaks in the low-pressure circuit of any contemporary anesthesia machine?
A. oxygen flush test
B. oxygen gas outlet occlusion est
C. traditional positive-pressure leak test
D. negative-pressure leak test
E. no test can verify the integrity of all contemporary anesthesia machines
A
- D Many anesthesia machines have a check valve downstream from the rotameters and vaporizers but upstream from the oxygen flush valve. When the oxygen flush valve button is depressed and the Y piece (which would be connected to the ETT or the anesthesia mask) is occluded, the circuit will be filled and the needle on the airway pressure gauge will indicate positive pressure. The positive pressure reading will not fall, however, even in the presence of a leak in the low-pressure circuit of the anesthesia machine. If a check valve is present on the common gas outlet, the positive-pressure leak test can be dangerous and misleading. In 1993, the US FDA established the FDA Universal Negative Pressure Leak Test. With the machine master switch, the flow control valves and the vaporizers turned off, a suction bulb is attached to the common gas outlet and compressed until it is fully collapsed. If a leak is present the suction bulb will inflate. It was so named because it can be used to check all anesthesia machines regardless of whether they contain a check valve in the fresh gas outlet. Hall 7
8
Q
8. Which of the following valves prevents transfilling between compressed-gas cylinders? A. fail-safe valve B. pop-off valve C. pressure-sensor shutoff valve D. adjustable pressure-limiting valve E. check valve
A
- E Check valves permit only unidirectional flow of gases. These valves prevent retrograde flow of gases from the anesthesia machine or the transfer of gas from a compressed-gas cylinder at high pressure into a container at a low pressure. Thus, these unidirectional valves will allow an empty compressed-gas cylinder to be exchanged for a full one during operation of the anesthesia machine with minimal loss of gas. The adjustable pressure-limiting valve is a synonym for a pop-off valve. A fail-safe valve is a synonym for a pressure-sensor shutoff valve. The purpose of a fail-safe valve is to discontinue the flow of N2O if the O2 pressure within the anesthesia falls below 25 psi. Hall 8
9
Q
9. The expression that for a fixed mass of gas at constant temperature, the product of pressure and volume is constant is known as A. Graham’s law B. Bernoulli’s law C. Boyle’s law D. Dalton’s law E. Charles’ law
A
- C Boyle’s law states that for a fixed mass of gas at constant temperature, the product of pressure and volume is constant. This constant can be used to estimate the volume of gas remaining in a compressed-gas cylinder by measuring the pressure within the cylinder. Hall 9
10
Q
10. The pressure gauge on a size “E” compressed-gas cylinder containing O2 reads 1600 psi. How long could O2 be delivered from this cylinder at a rate of 2L/min? A. 90 minutes B. 140 minutes C. 250 minutes D. 320 minutes E. cannot be determined
A
- C US manufacturers require that all compressed-gas cylinder containing O2 for medical use be painted green. A compressed-gas cylinder completely filled with O2 has a pressure of approximately 2000 psi and contains approximately 625 L of gas. According to Boyle’s law the volume of gas remaining in a closed container can be estimated by measuring the pressure within the container. Therefore, when the pressure gauge on a compressed-gas cylinder containing O2 shows a pressure 1600 psi, the cylinder contains 500 L of O2. At a gas flow of 2L/min, O2 could be delivered from the cylinder for approximately 250 minutes. Hall 10
11
Q
- A 25-year-old healthy patient is anesthetized for a femoral hernia repair. Anesthesia is maintained with isoflurane and N2O 50% in O2 and the patient’s lungs are mechanically ventilated. Suddenly, the “low-arterial saturation” warning signal on the pulse oximeter alarms. After the patient is disconnected from the anesthesia machine, he is ventilated with an Ambu bag with 100% O2 without difficult and the arterial saturation quickly improves. During inspection of your anesthesia equipment, you notice that the bobbin in the O2 rotameter is not rotating. This most likely indicates
A. the flow of N2O through the O2 rotameter
B. no flow of O2 through the O2 rotameter
C. a flow of O2 through the O2 rotameter that is markedly lower than indicated
D. a leak in the O2 rotameter above the bobbin
E. a leak in the O2 rotameter below the bobbin
A
- B All the choices listed in this question can potentially result in inadequate flow of O2 to the patient; however, given the description of the problem, no flow of O2 through the O2 rotameter is the correct choice. In a normally functioning rotameter, gas flows between the rim of the bobbin and the wall of the Thorpe tube, causing the bobbin to rotate. If the bobbin is rotating you can be certain that gas is flowing through the rotameter and that the bobbin is not stuck. Hall 11
12
Q
12. The O2 pressure-sensor shutoff valve requires what O2 pressure to remain open and allow N2O to flow into the N2O rotameter? A. 10 psi B. 25 psi C. 50 psi D. 100 psi E. 600 psi
A
- B Fail-safe valve is a synonym for pressure-sensor shutoff valve. The purpose of the fail-safe valve is to prevent delivery of hypoxic gas mixtures from the anesthesia machine to the patient due to failure of the O2 supply. When the O2 pressure within the anesthesia machine decreases below 25 psi, this valve discontinues the flow of N2O or proportionally decreases the flow of all gases. It is important to realize that this valve will not prevent delivery of hypoxic gas mixtures or pure N2O when the O2 rotameter is off, but the O2 pressure within the circuits of the anesthesia machine is maintained by an open O2 compressed-gas cylinder or central supply source. Under these circumstances, an O2 analyzer would be needed to detect delivery of a hypoxic gas mixture. Hall 12
13
Q
13. A 78-year-old patient is anesthetized for resection of a liver tumor. After induction and tracheal intubation, a 20-gauge arterial line is placed and connected to a transducer that is located 20 cm below the level of the heart. The system is zeroed at the stopcock located at the wrist while the patient’s arm is stretched out on an arm board. How will the arterial line pressure compare with the true BP? A. it will be 20 mm Hg higher B. it will be 15 mm Hg higher C. it will be the same D. it will be 15 mm Hg lower E. it will be 20 mm Hg lower
A
- C It is important to zero the electromechanical transducer system with the reference point at the approximate level of the heart. This will eliminate the effect of the fluid column of the transducer system on the arterial BP reading of the system. In this question, the system was zeroed at the stopcock, which was located at the patient’s wrist (approximate level of the ventricle). Blood pressure expressed by the arterial line will, therefore, be accurate, provided the distance between the patient’s wrist and the stopcock remains 20 cm. Hall 13
14
Q
14. The second-stage O2 pressure regulator delivers a constant O2 pressure to the rotameters of A. 4 psi B. 8 psi C. 16 psi D. 32 psi E. 64 psi
A
- C O2 and N2O enter the anesthesia machine from a central supply source or compressed-gas cylinders at pressure as high as 2200 psi (oxygen) and 720 psi (N2O). First stage pressure regulators reduce these pressures to approximately 45 psi. Before entering the rotameters, second-stage O2 pressure regulators further reduce the pressure to approximately 14 to 16 psi. Hall 14
15
Q
15. The highest trace concentration of N2O allowed in the OR atmosphere by the National Institute for Occupational Safety and Health (NIOSH) is A. 1 part per million (ppm) B. 5 ppm C. 25 ppm D. 50 ppm E. 100 ppm
A
- C NIOSH sets guidelines and issues recommendations concerning the control of waste anesthetic gases. NIOSH mandates that the highest trace concentration of N2O contamination of the OR atmosphere should be less than 25 ppm. In dental facilities where N2O is used without volatile anesthetics, NIOSH permits up to 50 ppm. Hall 15
16
Q
16. A sevoflurane vaporizer will deliver an accurate concentration of an unknown volatile anesthetic if the latter shares which property with sevoflurane? A. molecular weight B. viscosity C. vapor pressure D. blood/gas partition coefficient E. oil/gas partition coefficient
A
- C Agent specific vaporizers, such as the Sevotec (sevoflurane) vaporizer, are designed for each volatile anesthetic. However, volatile anesthetics with identical saturated vapor pressures could be used interchangeably with accurate delivery of the volatile anesthetic. Vapor pressures D HI SE. Desflurane (669) > Halothane (243) ~ Isoflurane (240) > Sevoflurane (160) ~ Enflurane (172). Hall 16
17
Q
17. The portion of the ventilator (Ohmeda 7000, 7810, and 7900) on the anesthesia machine that compresses the bellows is driven by A. compressed oxygen B. compressed air C. electricity alone D. electricity and compressed oxygen E. electricity and compressed air
A
- A The control mechanism of standard anesthesia ventilators, such as the Ohmeda 7000, uses compressed oxygen (100%) to compress the ventilator bellows and electrical power for the timing circuits. Hall 17
18
Q
18. Which of the following rotameter flow indicators is read in the middle of the dial? A. bobbin B. “H” float C. ball float D. skirted float E. nonrotating float
A
- C Five types of rotameter indicators are commonly used to indicate the flow of gases delivered from the anesthesia machine. As with all anesthesia equipment, proper understanding of their function is necessary for safe and proper use. All rotameter indicators should read at the upper rim except ball floats, which should be read in the middle. Hall 18
19
Q
19. When the pressure gauge on a size “E” compressed-gas cylinder containing N2O begins to fall from its previous constant pressure of 750 psi, approximately how many liters of gas will remain in the cylinder? A. 200 L B. 400 L C. 600 L D. 800 L E. cannot be calculated
A
- B The pressure gauge on a size “E” compressed-gas cylinder containing N2O shows 750 psi when it is full and will continue to register 750 psi until approximately three-fourths of the gas has left the cylinder. A full cylinder of N2O contains 1590 L. Therefore, when 400 L of gas remain in the cylinder, the pressure within the cylinder will begin to fall. Hall 19. (why does N2O not follow Boyle’s law? Boyle’s law only applies for constant n and constant T. N2O undergoes association reaction 2N2O N2O4. so n is not constant)
20
Q
20. A 3-year-old child with severe congenital facial anomalies is anesthetized for extensive facial reconstruction. After inhalation induction with sevoflurane and oral tracheal intubation, a 22-gauge arterial line is placed in the right radial artery. The arterial cannula is then connected to a transducer that is located 10 cm below the patient’s heart. After zeroing the arterial line at the transducer, how will the given pressure compare with the true arterial pressure? A. it will be 10 mm Hg higher B. it will be 7.5 mm Hg higher C. it will be the same D. it will be 7.5 mm Hg lower E. it will be 10 mm Hg lower
A
- B In this question the reference point is the transducer, which is located 10 cm below the level of the patient’s heart. Thus, there is an approximate 10 cm H2O fluid column from the level of the patient’s heart to the transducer. This will cause the pressure reading from the transducer system to read approximately 7.5 mm Hg higher than a true arterial pressure of the patient. A 20 cm column of H2O will exert a pressure equal to 14.7 mm Hg. Hall 20
