1. FLOWMETERS - INFLUENCE OF ALTITUDE AND IMPACT OF GAS PROPERTIES ON ACCURACY Ofer M. Wellisch MD

8. Boring Math In the AM

9. Thus, 1 MAC of anesthesia is: In order to deliver a given partial pressure at high altitudes, one must provide higher % concentration

10. How does this affect the required vaporizer setting? We know that the total gas outflow is the vaporizer inflow + vapor outflow + diluent flow. Bringing these together gives us: Or simply…

11. The calibrated vaporizer figures out what fraction of fresh gas flow is sent to the vaporizer to divert through the vaporizing chamber, The remainder of the gas diverted around the vaporizing chamber and functioning as the “diluent flow.” If we define this fraction diverted as: … and substitute it into the equation for the percent anesthetic concentration, we find that: NOW LETS SEE AN EXAMPLE!!!!!! The above equation rearranged to yield the percent anesthetic concentration

12. So, let’s say that we are at sea level (ATM=760), and want to give isoflurane (PP=240), at a concentration of 1.1% (1 MAC). In that case: However, we now find ourselves using that same vaporizer at 8000 feet (ATM=565), where the 2.4% diverted fraction yields an anesthetic concentration of 1.7% (of 565): We know that to deliver 8.4 mm Hg (1 MAC) of isoflurane at 8000 feet, we need an anesthetic concentration of 1.5%. However, our vaporizer, set for 1.1%, is actually producing 1.7%, or a partial pressure of 9.9 mm Hg. Thus, the vaporizer slightly overcompensates for the reduced atmospheric pressure.

13. Note that this does NOT apply to the desflurane vaporizer. The partial pressure of desflurane (saturated) is 669, which is very close to atmospheric pressure (760). Since the output of the vaporizing chamber is Clearly the output of the desflurane vaporizer would be highly unstable, with small changes in atmospheric pressure causing huge changes in vaporizer output. Thus, the desflurane vaporizer is pressurized to about 1500 mm Hg (2 ATM). This provides for a stable output, regardless of the ambient atmospheric pressure. However, since the output of the desflurane vaporizer does not compensate for atmospheric pressure, the vaporizer will have to be turned to higher settings with increasing altitude.