??Partial Pressure for dummies

[MAC10]

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In reference to page 50 in Lange (Morgan/Mikhail)

Can someone help me understand why you need to increase the concentration dial when giving desflurane at high altitude. I understand that the Tec 6 does not control for changes in atm pressure. However it seems that higher altitude and lower atm pressure would result in an increased partial pressure of des 😕

Also how does partial pressure of a gas relate to the concentration of a gas.

 

[Justin4563]

it has to do with the vapor pressure of the agent

 

[UTSouthwestern]

From Miller:

"The Tec 6 works at absolute pressures; therefore, altitude makes no difference to the vaporizer's performance. It can accurately deliver the dialed volume percent of desflurane. However, when this gas is brought to ambient atmospheric pressure at high altitudes, the volume percent represents an absolute decrease in the partial pressure of the anesthetic, unlike the contemporary variable-bypass vaporizers, which deliver a constant partial pressure of anesthetic. To compensate for the reduction of partial pressure of vapor at altitude, the Tec 6 rotary valve must be advanced to maintain the required partial pressure of anesthetic. The required dial setting may be calculated using the following formula[92] :

Required dial setting = Normal dial setting (vol %) × 760 mm Hg/Ambient pressure (mm Hg)

For example, at an altitude of 2000 m or 6564 feet, where the ambient pressure is 608 mm Hg, the operator must advance the concentration control dial from 10% to 12.5% to maintain the required anesthetic partial pressure.[92] In hyperbaric settings, the operator must decrease the dial setting to prevent delivery of an overdose."

Essentially what he's saying is that the vaporizer's internal pressure is unaffected by altitude, but once the gas leaves the vaporizer and is exposed to an atmospheric pressure lower than that at sea level, the gas' effective partial pressure is lower than what it would be at sea level due to a volume dilution effect (less desflurane per unit of carrier gas).

 

[MAC10]

From Miller:

"The Tec 6 works at absolute pressures; therefore, altitude makes no difference to the vaporizer's performance. It can accurately deliver the dialed volume percent of desflurane. However, when this gas is brought to ambient atmospheric pressure at high altitudes, the volume percent represents an absolute decrease in the partial pressure of the anesthetic, unlike the contemporary variable-bypass vaporizers, which deliver a constant partial pressure of anesthetic. To compensate for the reduction of partial pressure of vapor at altitude, the Tec 6 rotary valve must be advanced to maintain the required partial pressure of anesthetic. The required dial setting may be calculated using the following formula[92] :

Required dial setting = Normal dial setting (vol %) × 760 mm Hg/Ambient pressure (mm Hg)

For example, at an altitude of 2000 m or 6564 feet, where the ambient pressure is 608 mm Hg, the operator must advance the concentration control dial from 10% to 12.5% to maintain the required anesthetic partial pressure.[92] In hyperbaric settings, the operator must decrease the dial setting to prevent delivery of an overdose."

Essentially what he's saying is that the vaporizer's internal pressure is unaffected by altitude, but once the gas leaves the vaporizer and is exposed to an atmospheric pressure lower than that at sea level, the gas' effective partial pressure is lower than what it would be at sea level due to a volume dilution effect (less desflurane per unit of carrier gas).

Thanks UT