Article reprinted with permission of DVM360 – June 23,
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ANAESTHESIOLOGY
Just Ask the Expert
Light Anaesthesia Anxiety
By Andrew Claude, DVM, DACVAA
Are your surgical patients all sufficiently anesthetised?
There are lessons for all veterinary practices in
deciphering the cause of inadequate anesthetic depth
in routine procedures. Here is a step-wise guide on
what to consider as you perform your root-cause
analysis. Based on the context of the question above it seems
you have placed most of the concerns for the patient
issues on the scavenging system. Granted, there are
key points about the scavenging system to consider,
but there are also important details concerning the
patient that are of equal importance.
Q. Recently, we have noticed that our anesthetized
patients have heart rates that don't correspond
with their respiration rates in terms of their plane of
anaesthesia. Their heart rates seem to reflect stage 3,
plane 2 anaesthesia, but their respiration rates reflect
stage 3, plane 1 anaesthesia (e.g. a 100-lb dog with a
heart rate of 100 to 110 beats/minute but respiration
is 60 breaths/minute or more). These patients are
easily awakened by stimuli such as turning them
over or stimulating a sore tooth when performing a
dental cleaning. We find we are constantly having to
turn up the isoflurane. We originally thought we had
a problem with our medical gas scavenging system.
However, we recently performed a test to determine
if the scavenging system has the correct amount of
suction, and the results were normal. We are back to
square one. I would greatly appreciate any thoughts
you have on this. 1. What are the typical anaesthetic protocols being
used for these patients?
A. This is a very interesting question with two major
categories to consider—the circumstances regarding
the patient and those of the scavenger system.
Commonly, premedication protocols use the
combination of a tranquilizer and an opioid
(neuroleptanalgesia). Multiple publications state that
analgesics used before and during surgery decrease
the dose of both the induction drugs and the
maintenance inhalants (mean alveolar concentration,
or MAC). Local and regional anesthetic techniques
are also effective in decreasing the MAC of inhalant
anesthetics. Without preoperative and intraoperative
analgesia, anesthetised patients will be subject to
sudden light planes of anaesthesia necessitating
multiple top-up administration of induction drugs and
increased doses of inhalant anaesthesia.
In my opinion, in dogs and cats, if you are routinely
running your isoflurane vaporizer greater than 1.75%
(sevoflurane > 2.5%) during surgical procedures (e.g.
ovariohysterectomy, orthopaedics), your analgesia
protocols should be reconsidered. Contrary to
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