Understanding the operation and maintenance of the anesthesia deliver system (ADS) is essential for operator and patient safety.
One should spend time understanding the below information regarding compressed gas & ADS safety as well as the function and
its various breathing systems.
There are two types of gases you may see used in veterinary anesthesia. Oxygen (green or white colored tank) and Nitrous Oxide
(blue tank). The diameter index and safety system is a coding system used with compressed gas cylinders. The cylinders are
attached to the machine by a length of high-pressure hose connected to a diameter index safety system (DISS) fitting on the
machine. An 'E' cylinder has a similar safety system call the pin index safety system (PISS). E cylinders are held in a yoke.
These safety connections prevent a person from accidentally connecting the wrong gas to the regulator or hose. For example,
attaching a nitrous oxide (N2O) cylinder where an oxygen cylinder needs to go.
Compressed Gas Storage & Handling
- Never leave a cylinder sitting upright. Store E cylinders in a rack and secure H cylinders to a wall
- Separate full from empty cylinders. Always keep compressed gases away from heat and electrical wiring.
- It is ideal to have a minimum of 20 feet distance between cylinders and combustibles. Open cylinder valves slowly and completely.
Room air contains 21% oxygen. Patients under general anesthesia will have much higher inspired oxygen. Most veterinary anesthesia
machines will deliver oxygen only at 100%. This will enter the breathing system and be delivered to the patient with or without
inhalation anesthetic gases. An oxygen concentration of 21% is adequate for an awake patient. Anesthetized patients encounter
respiratory depression from anesthesia; therefore without supplemental oxygen hypoxemia is real possibility.
Nitrous Oxide (N2O)
This is a gas that may be used in addition to oxygen. It's MAC in small animals range from 180- 200%. Using N2O as a second
gas in veterinary patients will provide additional analgesia as well as less cardiovascular compromise. It's storage and delivery
is similar to oxygen. N2O exists as a liquid of room temperature. The N2O pressure gauge reads only pressure of gas within
the cylinder. As gas leaves the tank, more liquid evaporates and enters gas state. The result is that the cylinder pressure
gauge will not change until all of the liquid has evaporated. You know when to change the tank because a full E cylinder will
weigh about 8kg but 6kg when empty. Once liquid nitrous oxide is exhausted, the gauge will begin to change; leaving you with
about 5-10 minutes remaining.
The flowmeter allows the anesthetist to set the gas flow rate(s) that are delivered to patient. It contains a ball or bobbin
that rises within a glass tube to a height proportional to the flow of gas through tube. The gas flow rate is read at widest
diameter of ball or bobbin. When using the N2O, the N2O:O2 ratio should never exceed 2:1 as the patient may develop hypoxemia. Avoid excessive torque when closing the flowmeter knob.
This may break or strip the pin valve. The gas passing through the flowmeter is reduced from 50 psi (pounds per square inch)