Anesthesia Equipment & Physics Flashcards
Ch.1 Hall's Comprehensive Review (32 cards)
The driving force of the ventilator (Datex-Ohmeda 7000, 7810, 7100, and 7900) on the anesthesia work- station is accomplished with A. Compressed oxygen B. Compressed air C. Electricity alone D. Electricity and compressed oxygen
A
The control mechanism of standard anesthesia ventilators, such as the Ohmeda 7000, uses compressed oxygen (100%) to compress the ventilator bellows and electric power for the timing circuits. Some ventilators (e.g., North American Dräger AV-E and AV-2+) use a Venturi device, which mixes oxygen and air. Still other ventilators use sophisticated digital controls that allow advanced ventilation modes. These ventilators use an electric stepper motor attached to a piston (Miller: Miller’s Anesthesia, ed 8, p 757; Ehrenwerth: Anesthesia Equipment: Principles and Applications, ed 2, pp 160–161; Miller: Basics of Anesthesia, ed 6, pp 208–209).
Select the correct statement regarding color Doppler imaging.
A. It is a form of M-mode echocardiography
B. The technology is based on continuous wave
Doppler
C. By convention, motion toward the Doppler is red
and motion away from the Doppler is blue
D. Two ultrasound crystals are used: one for trans-
mission of the ultrasound signal and one for reception of the returning wave
C
Continuous wave Doppler—Continuous wave Doppler uses two dedicated ultrasound crystals, one for continuous transmission and a second for continuous reception of ultrasound signals. This permits measurement of very high frequency Doppler shifts or velocities. The “cost” is that this technique receives a continuous signal along the entire length of the ultrasound beam. It is used for measuring very high velocities (e.g., as seen in aortic stenosis). Also, continuous wave Doppler cannot spatially locate the source of high velocity (e.g., differentiate a mitral regurgitation velocity from aortic stenosis; both are systolic velocities).
Pulsed Doppler—In contrast to continuous wave Doppler, which records the signal along the entire length of the ultrasound beam, pulsed wave Doppler permits sampling of blood flow velocities from a specific region. This modality is particularly useful for assessing the relatively low velocity flows associated with transmitral or transtricuspid blood flow, pulmonary venous flow, and left atrial appendage flow or for confirming the location of eccentric jets of aortic insufficiency or mitral regurgitation. To permit this, a pulse of ultrasound is transmitted, and then the receiver “listens” during a subsequent interval defined by the distance from the transmitter and the sample site. This transducer mode of transmit- wait-receive is repeated at an interval termed the pulse-repetition frequency (PRF). The PRF is therefore depth dependent, being greater for near regions and lower for distant or deeper regions. The distance from the transmitter to the region of interest is called the sample volume, and the width and length of the sample volume are varied by adjusting the length of the transducer “receive” interval. In contrast to continuous wave Doppler, which is sometimes performed without two-dimensional guidance, pulsed Doppler is always performed with two-dimensional guidance to determine the sample volume position.
Because pulsed wave Doppler echo repeatedly samples the returning signal, there is a maximum limit to the frequency shift or velocity that can be measured unambiguously. Correct identification of the frequency of an ultrasound waveform requires sampling at least twice per wavelength. Thus, the maximum detectable frequency shift, or Nyquist limit, is one half the PRF. If the velocity of interest exceeds the Nyquist limit, “wraparound” of the signal occurs, first into the reverse channel and then back to the forward channel; this is known as aliasing (Miller: Basics of Anesthesia, ed 6, pp 325–327).
When the pressure gauge on a size “E” compressed- gas cylinder containing N2O begins to fall from its previous constant pressure of 750 psi, approxi- mately how many liters of gas will remain in the cylinder? A. 200 L B. 400 L C. 600 L D. Cannot be calculated
B
The pressure gauge on a size “E” compressed-gas cylinder containing liquid N2O shows 750 psi when it is full and will continue to register 750 psi until approximately three fourths of the N2O has left the cylinder (i.e., liquid N2O has all been vaporized). 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 (Miller: Basics of Anesthesia, ed 6, p 201; Butterworth: Morgan & Mikhail’s Clinical Anesthesiology, ed 5, pp 12–13).
When the pressure gauge on a size “E” compressed- gas cylinder containing N2O begins to fall from its previous constant pressure of 750 psi, approxi- mately how many liters of gas will remain in the cylinder? A. 200 L B. 400 L C. 600 L D. Cannot be calculated
D
Desflurane is unique among the current commonly used volatile anesthetics because of its high vapor pressure of 664 mm Hg. Because of the high vapor pressure, the vaporizer is pressurized to 1500 mm Hg and electrically heated to 23° C to give more predicable concentrations: 664/1500 = about 44%. If desflurane were used at 1 atmosphere, the concentration would be about 88% (Barash: Clinical Anesthe- sia, ed 7, pp 666–668; Miller: Basics of Anesthesia, ed 6, pp 202–203; Butterworth: Morgan & Mikhail’s Clinical Anesthesiology, ed 5, pp 60–64).
If the internal diameter of an intravenous catheter were doubled, flow through the catheter would be
A. Decreased by a factor of 2
B. Decreased by a factor of 4
C. Increased by a factor of 8 D. Increased by a factor of 16
D
Factors that influence the rate of laminar flow of a substance through a tube are described by the Hagen- Poiseuille law of friction. The mathematic expression of the Hagen-Poiseuille law of friction is as follows:
V = [πr^4 (∆ P)] / [8 Lμ]
where V ̇ is the flow of the substance, r is the radius of the tube, ΔP is the pressure gradient down the tube, L is the length of the tube, and μ is the viscosity of the substance. Note that the rate of laminar flow is proportional to the radius of the tube to the fourth power. If the diameter of an intravenous catheter is doubled, flow would increase by a factor of two raised to the fourth power (i.e., a factor of 16) (Ehrenwerth: Anesthesia Equipment: Principles and Applications, ed 2, pp 377–378).
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
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 liquid N2O contain approximately 1590 L of gas. See table from Explanation 10 (Miller: Basics of Anesthesia, ed 6, p 201; Butterworth: Morgan & Mikhail’s Clinical Anesthesiology, ed 5, p 12).
Which of the following methods can be used to detect all leaks in the low-pressure circuit of all contemporary anesthesia machines?
A. Negative-pressure leak test
B. Common gas outlet occlusion test
C. Traditional positive-pressure leak test D. None of the above
D
Anesthesia machines should be checked each day before their use. For most machines, three parts are checked before use: calibration for the oxygen analyzer, the low-pressure circuit leak test, and the circle system. Many consider the low-pressure circuit the area most vulnerable for problems because it is more subject to leaks. Leaks in this part of the machine have been associated with intraoperative awareness (e.g., loose vaporizer filling caps) and hypoxia. To test the low-pressure part of the machine, several tests have been used. For the positive-pressure test, positive pressure is applied to the circuit by depressing the oxygen flush button and occluding the Y-piece of the circle system (which is connected to the endotracheal tube or the anesthesia mask during anesthetic administration) and looking for positive pressure detected by the airway pressure gauge. A leak in the low-pressure part of the machine or the circle system will be demonstrated by a decrease in airway pressure. With many newer machines, a check valve is positioned downstream from the flowmeters (rotameters) and vaporizers but upstream from the oxygen flush valve, which would not permit the positive pressure from the circle system to flow back to the low-pressure circuit. In these machines with the check valve, the positive-pressure reading will fall only with a leak in the circle part, but a leak in the low-pressure circuit of the anesthesia machine will not be detected. In 1993, use of the U.S. Food and Drug Administration universal negative-pressure leak test was encouraged, whereby the machine master switch and the flow valves are turned off, and a suction bulb is collapsed and attached to the common or fresh gas outlet of the machine. If the bulb stays fully collapsed for at least 10 seconds, a leak did not exist (this needs to be repeated for each vaporizer, each one opened at a time). Of course, when the test is completed, the fresh gas hose is reconnected to the circle system. Because machines continue to be developed and to differ from one another, you should be familiar with each manufacturer’s machine preoperative checklist. For example, the negative-pressure leak test is recommended for Ohmeda Unitrol, Ohmeda 30/70, Ohmeda Modulus I, Ohmeda Modulus II and II plus, Ohmeda Excel series, Ohmeda CD, and Datex-Ohmeda Aestiva. The Dräger Narkomed 2A, 2B, 2C, 3, 4, and GS require a positive-pressure leak test. The Fabius GS, Narkomed 6000, and Datex-Ohmeda S5/ADU have self-tests (Butterworth: Morgan & Mikhail’s Clinical Anesthesiology, ed 5, pp 83–85; Miller: Miller’s Anesthesia, ed 8, pp 752–755).
Which of the following valves prevents transfilling be- tween compressed-gas cylinders? A. Fail-safe valve B. Check valve C. Pressure-sensor shutoff valve D. Adjustable pressure-limiting valve
B
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 con- tainer at a lower 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 (or pro- portionally reduce it) if the O2 pressure within the anesthesia machine falls below 30 psi (Miller: Miller’s Anesthesia, ed 8, p 756).
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. Charles’ law C. Boyle’s law D. Dalton’s law
C
Boyle’s law states that for a fixed mass of gas at a constant temperature, the product of pressure and volume is constant. This concept can be used to estimate the volume of gas remaining in a compressed- gas cylinder by measuring the pressure within the cylinder (Ehrenwerth: Anesthesia Equipment: Principles and Applications, ed 2, p 4).
The pressure gauge on a size “E” compressed-gas cylin- der containing O2 reads 1600 psi. How long could O2 be delivered from this cylinder at a rate of 2 L/min?
A. 90 minutes
B. 140 minutes C. 250 minutes D. 320 minutes
C
U.S. manufacturers require that all compressed-gas cylinders 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 of 1600 psi, the cylinder contains 500 L of O2. At a gas flow of 2 L/min, O2 could be delivered from the cylinder for approximately 250 minutes (Ehrenwerth: Anesthesia Equipment: Principles and Applications, ed 2, p 4; Butterworth: Morgan & Mikhail’s Clinical Anesthesiology, ed 5, pp 10–12).
A 25-year-old healthy patient is anesthetized for a femo- ral hernia repair. Anesthesia is maintained with isoflu- rane and N2O 50% in O2, and the patient’s lungs are mechanically ventilated. Suddenly, the “low-arterial saturation” warning signal on the pulse oximeter gives an alarm. After the patient is disconnected from the anes- thesia machine, he undergoes ventilation with an Ambu bag with 100% O2 without difficulty, 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. Flow of O2 through the O2 rotameter
B. No flow of O2 through the O2 rotameter
C. A leak in the O2 rotameter below the bobbin D. A leak in the O2 rotameter above the bobbin
B
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 (Ehrenwerth: Anesthesia Equipment: Principles and Applications, ed 2, pp 43–45).
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. 30 psi C. 50 psi D. 100 psi
B
Fail-safe valve is a synonym for pressure-sensor shutoff valve. The purpose of the fail-safe valve is to prevent the delivery of hypoxic gas mixtures from the anesthesia machine to the patient resulting from failure of the O2 supply. Most modern anesthesia machines, however, would not allow a hypoxic mix- ture, because the knob controlling the N2O is linked to the O2 knob. When the O2 pressure within the anesthesia machine decreases below 30 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 the delivery of hypoxic gas mixtures or pure N2O when the O2 rotameter is off, because the O2 pressure within the cir- cuits of the anesthesia machine is maintained by an open O2 compressed-gas cylinder or a central supply source. Under these circumstances, an O2 analyzer will be needed to detect the delivery of a hypoxic gas mixture (Ehrenwerth: Anesthesia Equipment: Principles and Applications, ed 2, pp 37–40; Miller: Basics of Anesthesia, ed 6, pp 199–200).
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 com- pare with the true blood pressure (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
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). The BP expressed by the arterial line will therefore be accurate, provided the stopcock remains at the wrist and the transducer is not moved once zeroed. Raising the arm (e.g., 15 cm) decreases the BP at the wrist but increases the pressure on the transducer by the same amount (i.e., the vertical tubing length is now 15 cm H2O higher than before) (Ehrenwerth: Anesthesia Equipment: Principles and Applications, ed 2, pp 276–278; Miller: Miller’s Anesthesia, ed 8, pp 1354–1355).
The second-stage O2 pressure regulator delivers a con- stant O2 pressure to the rotameters of
A.4psi
B.8psi
C. 16 psi D. 32 psi
C
O2 and N2O enter the anesthesia machine from a central supply source or compressed-gas cylinders at pressures as high as 2200 psi (O2) and 750 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 (Miller: Miller’s Anesthesia, ed 8, p 761).
The highest trace concentration of N2O allowed in the operating room (OR) atmosphere by the National In- stitute for Occupational Safety and Health (NIOSH) is A. 1 part per million (ppm)
B.5ppm C. 25 ppm D. 50 ppm
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 (Butterworth: Morgan & Mikhail’s Clinical Anesthesiology, ed 5, p 81).
A sevoflurane vaporizer will deliver an accurate con- centration of an unknown volatile anesthetic if the lat- ter shares which property with sevoflurane?
A. Molecular weight
B. Oil/gas partition coefficient C. Vapor pressure
D. Blood/gas partition coefficient
C
Agent-specific vaporizers, such as the Sevotec (sevoflurane) vaporizer, are designed for each volatile anesthetic. However, volatile anesthetics with identical saturated vapor pressures can be used interchangeably, with accurate delivery of the volatile anesthetic. Although halothane is no longer used in the United States, that vaporizer, for example, may still be used in developing countries for administration of isoflurane (Butterworth: Morgan & Mikhail’s Clinical Anesthesiology, ed 5, pp 61–63; Ehrenwerth: Anesthesia Equipment: Principles and Applications, ed 2, pp 72–73).
A 58-year-old patient has severe shortness of breath and “wheezing.” On examination, the patient is found to have 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. Turbulent flow C. Undulant flow D. Stenotic flow
B
Turbulent flow occurs when gas flows through a region of severe constriction such as that 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 (Butterworth: Morgan & Mikhail’s Clinical Anesthesiology, ed 5, pp 488–489).
Concerning the patient in Question 17, administra- tion 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
C
During turbulent flow, the resistance to gas flow is directly proportional to the density of the gas mixture. Substituting helium for oxygen will decrease the density of the gas mixture, thereby decreasing the resistance to gas flow (as much as threefold) through the region of constriction (Butterworth: Morgan & Mikhail’s Clinical Anesthesiology, ed 5, pp 498–499, 1286–1287; Ehrenwerth: Anesthesia Equipment: Principles and Applications, ed 2, pp 230–234).
A 56-year-old patient is brought to the OR for elec- tive replacement of a stenotic aortic valve. An awake 20-gauge arterial catheter is placed into the right ra- dial artery and is then connected to a transducer lo- cated 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 until his right wrist is 20 cm above his heart. As he is doing this the BP on the monitor reads 120/80 mm Hg. 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
C
Modern electronic BP 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 use. 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 of the system on arterial BP readings (Ehrenwerth: Anesthesia Equipment: Principles and Applications, ed 2, pp 276–278).
An admixture of room air in the waste gas disposal system during an appendectomy in a paralyzed, me- chanically ventilated patient under general volatile an- esthesia can best be explained by which mechanism of entry?
A. Positive-pressure relief valve B. Negative-pressure relief valve C. Soda lime canister
D. Ventilator bellows
B
Waste gas disposal systems, also called scavenging systems, are designed to decrease pollution in the OR by anesthetic gases. These scavenging systems can be passive (waste gases flow from the anesthesia machine to a ventilation system on their own) or active (anesthesia machine is connected to a vacuum system, then to the ventilation 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. Given that most ventilator bellows are powered by oxygen, a leak in the bellows will not add air to the evacuation system. The negative-pressure relief valve is used in active systems and will entrap room air if the pressure in the system is less than −0.5 cm H2O (Miller: Miller’s Anesthesia, ed 8, p 802; Miller: Basics of Anesthesia, ed 6, pp 212; Ehrenwerth: Anesthesia Equipment: Principles and Applications, ed 2, pp 101–103).
The relationship between intra-alveolar pressure, sur- face tension, and the radius of an alveolus is described by A. Graham’s law B. Beer’s law C. Bernoulli’s law D. Laplace’s law
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 mathematic expression of Laplace’s law is as follows: T = (1/2)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 molecules of the liquid film are much greater than the attractive forces between the liquid film and gas. Thus, the surface area of the liquid tends to become as small as pos- sible, which could collapse the alveoli (Butterworth: Morgan & Mikhail’s Clinical Anesthesiology, ed 5, pp 493–494; Miller: Miller’s Anesthesia, ed 8, p 475).
Currently, the commonly used vaporizers (e.g., GE- Datex-Ohmeda Tec 4, Tec 5, Tec 7; Dräger Vapor 19.n and 2000 series) are described as having all of the following features EXCEPT A. Agent specificity B. Variable bypass C. Bubble through D. Temperature compensated
C
Because volatile anesthetics have different vapor pressures, the vaporizers are agent specific. Vaporizers are described as having variable bypass, which means that some of the total fresh gas flow (usually less than 20%) is diverted into the vaporizing chamber, and the rest bypasses the vaporizer. Tipping the vaporizers (which should not occur) may cause some of the liquid to enter the bypass circuit, leading to a high concentration of anesthetic being delivered to the patient. The gas that enters the vaporizer flows over (does not bubble through) the volatile anesthetic. The older (now obsolete) Copper Kettle and Vern-Trol vaporizers were not agent specific, and oxygen (with a separate flowmeter) was bubbled through the volatile anesthetic; then, the combination of oxygen with volatile gas was diluted with the fresh gas flow (oxygen, air, N2O) and administered to the patient. Because vaporization changes with temperature, modern vaporizers are designed to maintain a constant concentration over clinically used temperatures (20° C-35° C) (Barash: Clinical Anesthesia, ed 7, pp 661–672; Miller: Basics of Anesthesia, ed 6, pp 202–203; Butterworth: Morgan & Mikhail’s Clinical Anesthesiology, ed 5, pp 60–64).
For any given concentration of volatile anesthetic, the splitting ratio is dependent on which of the following characteristics of that volatile anesthetic?
A. Vapor pressure
B. Molecular weight
C. Specific heat
D. Minimum alveolar concentration (MAC) at
1 atmosphere
A
Vaporizers can be categorized into variable-bypass and measured-flow vaporizers. Measured-flow vaporizers (nonconcentration calibrated vaporizers) include the obsolete Copper Kettle and Vernitrol vaporizers. With measured-flow vaporizers, the flow of oxygen is selected on a separate flowmeter to pass into the vaporizing chamber, from which the anesthetic vapor emerges at its saturated vapor pressure. By contrast, in variable-bypass vaporizers, the total gas flow is split between a variable bypass and the vaporizer chamber containing the anesthetic agent. The ratio of these two flows is called the splitting ratio. The splitting ratio depends on the anesthetic agent, the temperature, the chosen vapor concentration set to be delivered to the patient, and the saturated vapor pressure of the anesthetic (Ehrenwerth: Anesthesia Equipment: Principles and Applications, ed 2, pp 68–71).
A mechanical ventilator (e.g., Ohmeda 7000) is set to deliver a tidal volume (VT) of 500 mL at a rate of 10 breaths/min and an inspiratory-to-expiratory (I:E) ra- tio of 1:2. The fresh gas flow into the breathing circuit is 6 L/min. In a patient with normal total pulmonary compliance, the actual VT delivered to the patient would be
A. 500 mL B. 600 mL C. 700 mL D. 800 mL
C
The contribution of the fresh gas flow from the anesthesia machine to the patient’s VT should be considered when setting the VT of a mechanical ventilator. Because the ventilator pressure-relief valve is closed during inspiration, both the gas from the ventilator bellows and the fresh gas flow will be delivered to the patient’s breathing circuit. In this question, the fresh gas flow is 6 L/min, or 100 mL/sec (6000 mL/60 sec). Each breath lasts 6 seconds (60 sec/10 breaths), with inspiration lasting 2 seconds (I:E ratio = 1:2). Under these conditions, the 500 VT delivered to the patient by the mechanical ventilator will be augmented by approximately 200 mL. In some ventilators, such as the Ohmeda 7900, VT is controlled for the fresh gas flow rate in such a manner that the delivered VT is always the same as the dial setting (Butterworth: Morgan & Mikhail’s Clinical Anesthesiology, ed 5, pp 79–81).
