Let’s Just Mask Him Down

ZDS kitten

The Quizlet flashcard question asks, “What are the uses of mask induction?”  Quizlet  is an online study tool that uses flashcards to help improve your retention.  They call it a simple tool for learning anything.  The mask induction flashcard is part of a section for veterinary technicians on induction and patient prep for anesthesia.  The answer on the reverse side of the flashcard states: “When IV induction is difficult, mildly uncooperative cats and small dogs, neonates, small mammals, birds, debilitated patients subject to the negative effects of premedications or induction drugs.”

Mask inductions fit nicely into the “less is more” mindset we often adopt when it comes to anesthesia.  It seems safer because the fewer drugs we use the less that can go wrong, right?  In a recent article, John Jacobson DVM, DACVAA, lists these perceived advantages of mask inductions.

  • Inhalation anesthetics can be easily eliminated from the body with ventilation; they are not nearly as dependent on redistribution and metabolism for recovery as most injectable agents are.
  • The change in anesthetic depth is typically gradual compared with boluses of intravenous induction agents, giving the patient time to compensate for cardiovascular changes.
  • Intravenous access is not required.

All of this builds a pretty good story for mask inductions being fairly benign.  However, even Dr. Jacobson goes on to say that he avoids them.

“Significant cardiovascular and respiratory depression occurs under halothane and isoflurane anesthesia,” explains Dr. Nancy Brock, also a veterinary anesthesiologist. She states that although induction of anesthesia by mask is perceived by many as a safer method, gas anesthetic agents are not innocuous or inherently any safer or easier to control than injectable agents.

This cautionary information about mask induction is clearly outlined in the Veterinary Anesthesia and Analgesia Support Group (VASG) section on induction of anesthesia.  VASG is the well-respected online anesthesia resource founded by Dr. Bob Stein, a site that currently hosts over 20,000 visitors per month.  VASG states that mask inductions are not recommended for most patient groups.  Here are the reasons they list as why:

  • Increased patient stress, which could increase arrhythmic risk.
  • Unnecessary staff exposure to anesthetic agents.
  • Time required for complete induction of anesthesia is longer than compared to IV agents.
  • Prolonged period of unsecured airway with an increased risk of airway compromise or obstruction.
  • High concentrations of inhalant agents are required to achieve mask induction. Higher doses produce more cardiovascular and respiratory depression than seen with comparable doses of IV induction agents.
  • Contraindicated in brachycephalic patients.

In a special report on veterinary medical care guidelines for spay-neuter programs, the Association of Shelter Veterinarians also states that mask inductions should not be performed routinely and should be avoided whenever possible.  In addition to the reasons listed by VASG, they include the high cost of delivering high flows of oxygen and high concentrations of anesthetic gas directly into the scavenge system.  The report goes so far as to say, “If mask supplementation becomes frequent or regular, other options should be considered for patient and staff safety.”

I did a fair number of mask inductions early in my career.  In one practice, it was the standard protocol for induction of most cats and small dogs.  I had occasion to use a gas analyzer during that time.  A gas analyzer measures the actual concentration of anesthetic gas that’s delivered from a gas machine (as opposed to the concentration indicated by the vaporizer dial), and also the concentration of anesthetic gas exhaled from the patient.  That taught me that for a while after induction, the concentration of anesthetic gas exhaled by the patient was higher than the concentration of gas coming from the vaporizer.  For example, I had the vaporizer set at 1.5% for the surgery, but the patient was actually exhaling twice that concentration.  So my patient was over anesthetized until it could blow off the excess gas required for the induction.  That was an enlightening realization for me.  I had been inducing with anesthetic gas, the most hypotensive drug I use for anesthesia, in high concentrations, in an uncontrolled manner, to a patient who was not intubated and had no monitoring equipment on, and I thought it was the ‘safest’ plan.

Mask inductions have their place in our anesthesia toolbox.  But, if you routinely mask induce or use a chamber to induce patients, it’s time to reevaluate the safety, the cost, and the efficacy of that protocol. As the AAHA Anesthesia Guidelines for Dogs and Cats  warns, mask inductions should be reserved for situations where other alternatives are not suitable.  VASG is an excellent resource to begin marking a trail through the forest of anesthesia protocol possibilities. I know it can be difficult to change an anesthetic protocol.  We all become comfortable with a few ways to do things, and are reluctant to risk trying something different.  But VASG founder Dr. Stein suggests a way to gain confidence with a variety of advanced anesthesia techniques by periodically utilizing them on low risk patents.

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Ken Crump AAS, AHT is a writer and animal anesthetist, and writes Making Anesthesia Easier for DarvallVet, a division of Advanced Anesthesia Specialists.  He makes dozens of Continuing Education presentations on veterinary anesthesia and oncology across the United States and in Canada.  Ken retired from the Veterinary Teaching Hospital at Colorado State University in 2008
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