Scavenging System in anesthesia workstation

What is scavenging in OT? what are five basic parts of a scavenging system? What are the hazards of a scavenging system?

Scavenging is the collection and the subsequent removal of waste anesthetic gases from the operating room 

Scavenging systems generally have five components  (1) the gas-collecting assembly, (2) the transfer means, (3) the scavenging interface, (4) the gas-disposal assembly tubing, and (5) an active or passive gas-disposal assembly An “active system” uses a central evacuation system to eliminate waste gases. The “weight” or pressure of the waste gas itself produces flow through a “passive system.

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Gas-Collecting Assembly

The gas-collecting assembly captures excess anesthetic gas and delivers it to the transfer tubing. Waste anesthetic gases are vented from the anesthesia system either through the APL valve or through the ventilator relief valve which  accumulates in the gas-collecting assembly and is directed to the transfer means. 

Transfer Means

The transfer means carries excess gas from the gas-collecting assembly to the scavenging interface. The tubing must be either 19 or 30 mm, and should be sufficiently rigid to prevent kinking, and as short as possible to minimize the chance of occlusion.and is sometimes  color code with yellow  to distinguish it from 22-mm breathing system tubing.. The two tubes frequently merge into a single hose before they enter the scavenging interface.

Scavenging Interface

The scavenging interface is the most important component of the system because it protects the breathing circuit or ventilator from excessive positive or negative pressure. The interface should limit the pressures immediately downstream from the gas-collecting assembly to between -0.5 and +10 cm water with normal working conditions. Interfaces can be open or closed, depending on the method used to provide positive and negative pressure relief.

Open Interfaces

An open interface contains no valves and is open to the atmosphere, allowing both positive and negative pressure relief. Open interfaces is used  with active disposal systems that use a central evacuation system. Open interfaces require a reservoir because waste gases are intermittently discharged in surges, whereas flow from the evacuation system is continuous.

 An open canister provides reservoir capacity which should be large enough to accommodate a variety of waste gas flow rates. Gas enters the system at the top of the canister and travels through a narrow inner tube to the canister base. Gases are stored in the reservoir between breaths. Positive and negative pressure relief is provided by holes in the top of the canister 

Closed Interfaces

A closed interface communicates with the atmosphere through valves. All closed interfaces must have a positive-pressure relief valve to vent excess system pressure if obstruction occurs downstream from the interface. A negative-pressure relief valve is mandatory to protect the breathing system from subatmospheric pressure if an active disposal system is used. Two types of closed interfaces are commercially available. One has positive pressure relief only; the other has both positive and negative pressure relief.

Gas-Disposal Assembly Conduit

The gas-disposal assembly conduit  conducts waste gas from the scavenging interface to the gas-disposal assembly. It should be collapse-proof and should run overhead, if possible, to minimize the chances of accidental occlusion

Gas-Disposal Assembly

The gas-disposal assembly ultimately eliminates excess waste gas  There are two types of disposal systems: active and passive.

The most common method of gas disposal is the active assembly, which uses a central evacuation system. A vacuum pump serves as the mechanical flow-inducing device that removes the waste gases usually to the outside of the building.

interface with a negative-pressure relief valve is mandatory because the pressure within the system is negative. A reservoir is very desirable, and the larger the reservoir, the lower the suction flow rate needed.

Hazards

Scavenging systems add complexity to the anesthesia system.

 A scavenging system extends the anesthesia circuit all the way from the anesthesia machine to the ultimate disposal site. 

Obstruction of scavenging pathways cause excessive positive pressure in the breathing circuit, resulting in  barotrauma . 

Excessive vacuum  to a scavenging system can cause  undesirable negative pressures within the breathing system. 

Scavenging system catching  fires in engineering equipment rooms that house the vacuum pumps used for waste anesthetic gas evacuation. due to oxygen enrichment 

Trendelenburg position and its anaesthetic considerations

What are the various indication and disadvantages of trendelenburg position and how to prevent them?

Tilting a supine patient head down, the Trendelenburg position (German surgeon, Trendelenburg, who described its use for abdominal surgery)

Indications 

Abdominal and laparoscopic surgery, and to prevent air emboli and facilitate cannulation during central line placement. 

Disadvantages 

The Trendelenburg position has significant cardiovascular and respiratory consequences.

1)  The head-down position increases central venous, intracranial, and intraocular pressures. 

2) Prolonged head-down position  can lead to swelling of the face, conjunctiva, larynx, and tongue with an increased potential for postoperative upper airway obstruction. 

3) The cephalic movement of abdominal viscera against the diaphragm worsens the ventilation–perfusion ratio by gravitational accumulation of blood in the poorly ventilated lung apices and  decreases functional residual capacity and pulmonary compliance. 

4) In spontaneously ventilating patients, the work of breathing increases. 

5) The stomach  lies above the glottis so Endotracheal intubation is often preferred to protect the airway from pulmonary aspiration related to reflux and to reduce atelectasis.

Associated care in trendelenburg position

1) Care is taken to prevent patients  sliding off operating tables with resulting head injuries. With modern surgical tables and procedural techniques, steep head-down tilt is not often warranted. 

2) Retension material or shoulder braces for arm placement is avoided in these position to prevent brachial plexus injury.

3) Ventilation should be assisted or controlled and higher inspiratory pressures are needed to expand the lung.

4) Cranial vascular congestion and increased intracranial pressure can result from head-down tilt and in patients with known or suspected intracranial disease, the position should be used only in those rare instances in which alternate posture cannot be found and  Maintenance of the position should then be as brief as possible.

4)Because of the risk of edema to the trachea and mucosa surrounding the airway during surgeries in the Trendelenburg position for prolonged periods, it may be prudent to verify an air leak around the endotracheal tube or visualize the larynx before extubation.