Anaesthesia Workstation MCQs

Liquid oxygen is stored in a vacuum-insulated evaporator (VIE) at a temperature of -150°C to -170°C and at a pressure of 5-10 atmospheres. In simple terms this works like a giant thermos flask. At 15°C, one volume of liquid oxygen occupies 842 times its volume as gas. Oxygen constantly evaporates from the top of the VIE. As this oxygen is very cold, it needs to be passed through a super heater. When the oxygen flows at a faster rate, the temperature of the liquid falls due to the latent heat of evaporation. To overcome this problem additional heat is provided by a pressure-raising vaporiser. Regular maintenance, monitoring of usage of oxygen and topping up the tank at regular intervals is necessary. Reference: Al-Shaikh B, Stacey S, Eds. Liquid oxygen. In: Essentials of anaesthetic equipment, 3rd ed. London, UK: Churchill Livingstone, Elsevier, 2007; Chapter 1: 9-10.

The more volatile a liquid, the faster it vaporizes and the greater the loss of latent heat of vaporization Liquid cools à rate of vaporization falls Reference : Anaesthesia Review a Comprehensive Texbook 5th Edition Chapter 44 Page No .423

The ideal vaporizer should be Lightweight, robust and durable Easy to transport Corrosion and solvent resistant Leak proof Economical and safe to use Require minimal servicing Accurate over a wide range of: Freah Gas Flow Liquid agent levels Ambient pressures Ambient temperatures Unaffected by: Heat loss due to vaporization Pressure changes downstream of the vaporizer Tilting or tipping Compatible with vaporizer interlock systems Reference :Anaesthesia Review a Comprehensive Texbook 5th Edition Chapter 44 Page No .423 to 428

The cylinders used in medical practice are either solid drawn or made from seamless steel tube, and never welded at the ends. After manufacturing, the cylinders are kept in a furnace at a temperature of 860-890 degree C and then carefully removed and cooled at atmospheric pressure . As the cylinders must withstand a high gas pressure and rough handling during transport, they are subjected to mechanical testing before marketing for use. The tensile test, flattening test, impact test and bend test are performed on one cylinder from a batch of every 100 finished cylinders. Reference: Anaesthesia Review: A Comprehensive Textbook by Atul, 5th Edition, Page Number 396-409.

Explanation: Pressure Conversions: 1 kPa = 10 cm H2O is a valid approximation, which is useful in clinical settings for measuring pressures in respiratory physiology and anaesthesia. To understand this, atmospheric pressure is often measured in various units, including mmHg, mbar, and psi. Atmospheric Pressure Reference: Atmospheric pressure is defined as the pressure exerted by the weight of air in the atmosphere, commonly measured using a barometer. 760 mmHg (Torr) is equivalent to 1 bar = 1000 mbar = 101.3 kPa . Conversions for Context: 1 kPa ≈ 7.5 mmHg (Option C) 1 bar = 14.5 psi , thus 1 kPa ≈ 0.145 psi (Option B) 1 kPa is equivalent to 10 mbar , not 100 mbar (Option D) Key Takeaways: 1 kPa is approximately 10 cm H2O, a key conversion for understanding pressures in clinical practice. Familiarity with pressure units is essential in anaesthesia and critical care settings.

The anesthesia machine, now more properly called the anesthesia workstation, has two main pressure circuits. The higher-pressure circuits consist of the gas supply from the pipelines and tanks, all piping, pressure gauges, pressure reduction regulators, check valves (which prevent backward gas flow), the oxygen pressure sensor shutoff valve (also called the oxygen failure cutoff or fail-safe valve), the oxygen supply failure alarm, and the oxygen flush valve-or, simplistically, everything up to the gas flow control valves and the machine common gas outlet. The low-pressure circuit starts with and includes the gas flow control valves, flowmeters, vaporizers, and vaporizer check valve and goes to the machine common gas outlet. See also figure for explanation to Question 12 Reference: Barash: Clinical Anesthesia, ed 7, pp 641-650; Miller: Basics of Anesthesia, ed 6, pp 198-204.

The fail-safe valve automatically closes nitrous oxide (and other gases) to prevent delivery of hypoxic gas mixture to the patient. The fail-safe valve is designed to be activated when oxygen pressure falls below 25 psi. Reference: Lippincott's ANESTHESIA REVIEW: 1,001 QUESTIONS AND ANSWERS Paul Sikka, MD, PhD

The oxygen-flush valve provides gas flow at pipeline pressures of about 45 to 55 psi at 35 to 75 L/min. The high flow of oxygen is provided directly to the common gas outlet, bypassing the flowmeters and vaporizers. One should be careful when using the oxygen-flush valve, as high gas flows at high pressures can cause lung barotrauma in the patient. Reference: Lippincott's ANESTHESIA REVIEW: 1,001 QUESTIONS AND ANSWERS Paul Sikka, MD, PhD