Uptake Question

[Onefellswoop87]

Starting CA-1 this summer (after being out of hospital medicine for a bit as a US Navy GMO), so I'm reading Morgan and Mikhail.

I'm confused about uptake and its effect on induction. Chapter 8 says "the greater the uptake of anesthetic agent, the greater the difference between inspired and alveolar concentration, and the slower the rate of induction."

I get the first part of how uptake is related to the solubility coefficient. But if the agent is radidly soluble in blood and even more so soluble in the brain tissue than blood, why doesn't a high degree of uptake make for fast induction?

Thanks.

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[vector2]

It's a confusing topic for sure, but the key point that you need to repeat to yourself over and over is that onset of inhalation anesthetics is directly proportional to alveolar concentration. More soluble agents get "washed away" by pulmonary blood flow which means it takes longer to reach desired alveolar concentrations (Fa/Fi). Conversely, onset takes less time if pulmonary blood flow is decreased as in states of low cardiac output. Keep in mind that soluble agents may get into blood faster, but onset is also related to partial pressure in the brain...soluble agents will stay in the blood versus dissolving out to act on key brain sites.

Additionally, the rate to reach equilibrium between blood and gas is what's important, not the total amount of agent that is dissolved in blood. Less soluble anesthetics like des and n20 reach equilibrium between the alveolus and blood much faster due to their poor solubility, which means desired alveolar concentration is reached faster, and thus faster onset and offset.

 

[Onefellswoop87]

Thank you. That makes much more sense as partial pressure, not solubility.