adjustable pressure-limiting valve
{{short description|Flow control valve used in anaesthesiology}}
File:Adjustable pressure-limiting valve anaesthetic machine.jpg
An adjustable pressure-limiting valve (commonly abbreviated to APL valve, and also referred to as an expiratory valve, relief valve or spill valve) is a type of flow control valve used in anaesthesiology as part of a breathing system. It allows excess fresh gas flow and exhaled gases to leave the system while preventing ambient air from entering.{{cite book|author1=Baha Al-Shaikh|author2=Simon Stacey|title=Essentials of Anaesthetic Equipment|url=https://books.google.com/books?id=9jNV55CcAXcC|date=2013|publisher=Elsevier Health Sciences|isbn=978-0-7020-4954-5|pages=55–73|chapter=Breathing systems}}
Mechanism
Such valves were first described by the American dentist Jay Heidbrink, who used a thin disc that was held in place by a spring.{{cite book|author1=Steven M. Yentis|author2=Nicholas P. Hirsch|author3=James K. Ip|title=Anaesthesia and Intensive Care A-Z: An Encyclopaedia of Principles and Practice|chapter=Adjustable pressure-limiting valves|url=https://books.google.com/books?id=M_W-55fYDyAC|date=2013|publisher=Elsevier Health Sciences|isbn=978-0-7020-4420-5|page=12}} The valve is adjustable and spring-loaded, allowing the opening pressure of the valve to be controlled by screwing the valve top which modifies the pressure on the spring. A very light spring is used, so that at its minimum setting the valve can be opened by the patient's breathing alone using low pressures.{{cite book|last1=Davis|first1=Paul D|last2=Kenny|first2=Gavin N C|title=Basic Physics and Measurement in Anaesthesia|url=https://books.google.com/books?id=oi96QgAACAAJ|year=2003|publisher=Butterworth-Heinemann|isbn=978-0-7506-4828-8|page=3|chapter=The Interrelationship of Pressure and Force}} In contemporary APL valves, three orifices or "ports" are present: one for intake of gas, one for return of gas to the patient, and an exhaust port for waste gas which can be connected to a scavenging system.{{cite web| url=https://wellawaresystems.com/ |title= CPAP Machine Guide }} Wednesday, April 22, 2020
References
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Category:History of anesthesia