Case 37: A -Over Vaporizer with a Non-rebreathing Circuit
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You are the only anesthesiologist who is part of a medical team to visit an outlying village in Colombia, South America. Unfortunately, all your anesthetic equipment has not arrived, and today is the day for operations on 12 orthopedic patients. You are presented with an Ohmeda Cyprane PAC (portable anesthesia complete) isoflurane draw-over circuit. This is the only available anesthesia delivery system (Fig. 1). The anesthesia nurse, who is a Colombian and works in the hospital, says she had been recommended this equipment by an overseas anesthesiologist some years ago. She has used it with great success both for spontaneously breathing patients and those she had to paralyze. In the latter case, she used an Ambu bag attached to the draw-over circuit. She tells you that she was recommended to use the Penlon Oxford ventilator (POV) when she needs to ventilate the patient instead of the Ambu bag. However, she has not used the above ventilator yet as she not familiar with it. She never saw it in use and is very keen for you to show her how to use the Penlon ventilator. You have never seen this equipment before but ask for any information she may have on this equipment. She hands you a paper  describing the use of the draw-over vaporizer and the Penlon Oxford ventilator (the latter is to be placed at F in Fig. 1). You read the text and study the diagram below. The vaporizer D is fitted with a 900 ml corrugated anesthetic tubing (B) with a dust filter at the inlet (A). Oxygen can be given through inlet C. Air is drawn in and through the vaporizer by the patient during inspiration. B acts as a reservoir of oxygen between inspirations if oxygen is used. However, in a healthy patient, you may elect not to use oxygen. The outlet E of the PACU vaporizer incorporates a one-way valve that is connected via a second anesthetic tube to a T-connector F. This T-connector is the ventilation port to which an Ambu bag or a ventilator can be attached. From the T-connector right exit, another anesthetic tube leads to a one-way Ruben’s valve (G) and the patient’s mask (H) or to an endotracheal tube. An exhaust tube (I) carries expired gases from the patient to a scavenging system or into the air. A sidestream adaptor (J) is used for end-tidal CO2 and/or gas analyzer. Unidirectional flow during ventilation is maintained in the PAC draw-over circuit by the one-way valves at E and G, such that negative pressure at F draws only carrier gas through the vaporizer and into the circuit. Positive pressure applied at F directs the flow only to the patient, as reverse flow is blocked at the E. Back flow of exhaust gas into the circuit is also blocked during all phases of respiration by the Ruben’s valve. You also discover that the Penlon Oxford ventilator is fed by a compressed gas source and thus provides only a positive pressure cycle.
KeywordsDraw-over vaporizer Non-rebreathing circuit Ohmeda Cyprane PAC Penlon Oxford ventilator Controlled ventilation Ambu bag Vapor concentration Ruben’s valve Compressed gas source