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Functions of anesthesia reservoir bag in a breathing system

The safe administration of general anesthesia requires an understanding of the technological advances in highly sophisticated anesthetic equipment. When using modern anesthetic workstations, it is important to understand the complex interaction between the delivery systems – which conduct anesthetic gases from an anesthetic machine to the patient – and the reservoir bag.

The anesthesia reservoir bag is a collapsible gas container, which is a fundamental part of most anesthesia breathing frameworks. The reservoir bag inflates slightly with every exhalation and deflates slightly with each inhalation, permitting the operator to determine the respiratory rate easily. [1] During inhalation sedation, the reservoir bag works as a reservoir from which additional gas may be drawn as the patient’s respiratory demands exceed gas flow delivered from the machine. The reservoir bag also serves as a monitoring device for respiration during sedation.

The anesthesia reservoir bag permits manual ventilation and acts as a visual or tactile indicator of spontaneous breathing. The most important function of a reservoir bag during a patient’s spontaneous breathing is the collection of respiratory gases. During spontaneous breathing, the patient inhales fresh respiratory gases along with gases from the reservoir bag, which are devoid of CO2 as they pass through the CO2 absorber and the inspiratory valve. During expiration, the reservoir bag fills up with exhaled air, and there is an inflow of fresh gases as the inspiratory valve closes. When the reservoir bag is filled up to its nominal volume, the excess gases are released through the adjustable pressure limiting valve. [2]

Another function of the reservoir bag is its use as a means of providing O2 during manual or controlled ventilation. In this ventilation mode, the adjustable pressure limiting valve is only partly closed to provide the pressure required for efficient ventilation. During manually controlled or assisted ventilation, the reservoir bag is pressed to create pressure in the circle breathing system, which results in the flow of respiratory gases from the reservoir bag to the patient. The excess gas is discharged through the adjustable pressure limiting valve. [3]

The modern anesthesia workstation serves as a mechanized substitute for the manual squeezing of the reservoir bag. The inclusion of the reservoir bag in the circuit during mechanical ventilation is an additional safety feature. [4] During mechanical ventilation, the reservoir bag has a special selector switch. The reservoir bag in an anesthesia machine allows manual ventilation as well as “visual” monitoring of spontaneous breathing. It is important to monitor it, particularly in the case that negative pressure or fresh gas low alarms are triggered. [2]  

Modern anesthesia workstations have an extra safety automatic mechanism where the reservoir bag has a function that protects the patient’s lung from barotrauma. The reservoir bag is the most flexible part of the breathing system. During gas filling, it is the reservoir bag that first shows evidence of filling. When the pressure limiting outflow valve is closed, the reservoir bag is filled beyond the nominal volume. Nevertheless, it must maintain a safety pressure within limits below 60 cm H2O [1]. For instance, The Dräger Primus anesthesia workstation activates an alarm of the highest priority (pressure high) if, on continuous inflation of the reservoir bag, the pressure set as the upper limit for alarm by the user is exceeded. If the pressure is below the upper level of alarm, this triggers an alarm of continuous pressure that also represents the highest priority (red color), thus requiring immediate action. As the last safety mechanism, the device itself reduces the pressure by automatically relieving the breathing system. [5]

 [1]Križmarić M. Functions of anesthesia reservoir bag in a breathing system  Zdrav Vestn. 2017; 86:226–35.

[2] Barash P, Cullen BF, et al. Clinical Anesthesia Lippincott Williams & Wilkins, 2013, 1880. ISBN1469830248, 781469830247

[3] Malamed S, Sedation (Sixth Edition), Mosby, 2018, Pages 164-166, ISBN 9780323400534,

 [4] Milner Q. Anaesthetic Breathing Systems  Updates in anesthesia 1997, Pages, 145-150.

[5] Vinay B, Gopalakrishna KN, Sriganesh K. Low-Flow Anesthesia, Low Fresh Gas Flow, and Oxygen

Flush: An Interesting Interplay. AANA journal. 2015;83(3):201.