Pharmacology Revisit: Context Sensitive Half Time

I have been working at an University Medical Center for almost two years. Often times, I take over all day long cases as I am the late person in the OR. Most of those cases are spine cases. As we all know, ERAS protocol has been implemented in most of the big spine cases. They often have multiple drips such as propofol, remifentanyl/fentanyl, lidocaine, ketamine. Hence, it is important to have a good understanding of context sensitive half time of the infusions to ensure a timely wake-up.

First of all, if you take a look at the two graphs above, you can see that propofol and ketamine have similar context sensitive half time. If the surgery is over 6 hours, stopping the infusion 40 minutes prior the emergence will cause 50% reduction of the plasma level. However, if the fentanyl is used as a continuous drip, it would require turning it off a lot sooner or cut down the dosage. As we can see from the graph, 3 hours of infusion of fentanyl needs roughly 2hrs to reduce the plasma level by half. Hence, the first drip that requires to be turned off is fentanyl if it is running over 3 hours above. This applies only to opioid naive patients. This would explain narcotics would not be needed in PACU if the case is a long case. We all know the beauty of running remi in spine cases, there is no change of context sensitive half time due to esterase clearance.

Please keep in mind, all the drips running together may have either additive or synergistic effects. The dosage of the infusion would also affect the context sensitive half time. In summary, to ensure a timely wake-up, the first drip that needs to be turned off is fentanyl. Ketamine would be the next to be turned off followed by propofol. I hope this is helpful in timing a long ERAS wakeup.

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