Variable bypass vaporizer
A variable bypass vaporizer (Concentration calibrated/ dial controlled vaporizer) is a type of vaporizer that can alter the volume of fresh gas, flowing through the bypass chamber. This is possible with the help of an adjustable dial.
The total fresh gas is supplied from the rotameter. This gas then enters the vaporizer and then splits into two. The major portion (around 80%) of gas goes into the bypass chamber. The remaining portion goes into the vaporizing chamber. The carrier gas which enters into the vaporizing chamber (VC) picks up the vapor of anesthetic agents. Then the gas from the vaporizing chamber goes back to the bypass chamber. Since a low concentration of anesthetic is necessary for the patient, the bypass gas dilutes the saturated carrier gas.
Vaporizer Output Concentration (VOC) is a combination of
- bypass gas which directly comes from the bypass chamber,
- carrier gas saturated with the inhalation anesthetics and
- carrier gas which has failed to carry vapor along with it.
Splitting Ratio is the ratio of the volume of bypass gas to the fresh carrier gas. This splitting ratio is dependent upon the saturated vapor pressure (SVP) of the inhalation anesthetics.
You can see in the chart that the splitting ratio for a 1% dose of sevoflurane is 25:1. This means that for 26 ml of total gas supply, 25 ml of gas goes to the bypass chamber and 1 ml goes to the vaporizing chamber. Also, less is the saturated vapor pressure of the anesthetics; less will be the splitting ratio.
The splitting ratio can be manually controlled with the help of a dial setting. When the dial is rotated, the opening of the bypass chamber at the valve alters. This obstructs the volume of gas flow through the bypass chamber. Obstructed gas goes through the vaporizing chamber. Also, if the value of TGF is constant then bypass and carrier gas are inversely related to each other.
Factors affecting the output of variable bypass vaporizer
- Total gas flow (TGF) rate:- Increase or decrease of the total gas flow can lower the output of VOC. However, clinically, this factor is not that important.
- Intermittent back pressure:- It can either be due to positive pressure ventilation or due to oxygen flush.
- Carrier gas composition:- Nitrous oxide is soluble in liquid anesthetics. So, an increase in the volume of nitrous oxide will decrease the value of VOC