Archive March 2017 XVIII, No. 3

Anesthesia Alert: One Simple Trick to Preempt Post-op Pain

The magic of a 50 mg dissociative dose of ketamine just before incision.

Barry Friedberg

Barry Friedberg, MD


post-op pain MEASURE THE BRAIN Measure the organ you're medicating with a brain monitor, says Dr. Friedberg, who notes that monitoring changes in heart rate and blood pressure can't ensure you're protecting the patient's brain from incoming pain signals.

Wouldn't it be great if all your patients emerged from their anesthetic pain- and PONV-free? I've developed a reproducible technique that lets you do just that. The key component: a 50 mg ketamine dose 2 to 3 minutes before incision that protects the patient's brain from incoming noxious pain signals soon to be delivered by the surgeon's scalpel. The premise is simple: Patients who've had pain during surgery will have pain after surgery. So to prevent post-op pain, you prevent intraoperative pain. Here's how my preemptive analgesia technique works.

Block NMDA receptors
Regardless of whether patients are unconscious, skin injection or incision is an extremely potent signal to the brain that the "world of danger" has invaded the "protected world of self." The brain can't differentiate between the mugger's knife and the surgeon's scalpel (or trocar). There are other pain receptors, but no signal is more determinant of post-operative pain than incision. It sets off serious internal alarms, and puts the wind-up phenomenon in motion.

I began a propofol-ketamine (PK) clinical trial in 1992, using propofol hypnosis to block ketamine hallucinations or dysphorias. Over the next year, I watched in amazement as my first 50 PK patients quickly emerged from anesthesia with no need for opioids. Until then, I hadn't heard about N-methyl, D-aspartate (NMDA) receptors, much less their critical midbrain location.

Over the next 5 years and 1,200-plus patients, I continued to observe the same effectiveness with the same 50 mg ketamine dose, whether in 100-pound female patients or 250-pound male patients. Ketamine, which prevents transmission of painful sensation to the cortex by blocking midbrain NMDA receptors, is the only IV anesthetic not given on a per-body-weight basis. The effective dissociative dose of ketamine isn't related to body weight.

The adult brain weighs about 3 to 4 pounds and doesn't vary with body weight. The midbrain is a very small part of the adult brain, and the NMDA receptors are a very small part of that very small part. A 50 mg dose of ketamine saturates 98 to 99% of those midbrain NMDA receptors.

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