So how about the cardiovascular
system? Does general anesthesia have an impact
upon cardiac function? How about the anesthetic vapours?
Some anesthetic vapours can reduce
the contractility of the heart and reduce cardiac
output. These tended to be the older vapours
such as Halothane, but modern vapours such as
Isoflurane, Desflurane, and Sevoflurane have little
or no effect upon cardiac contractility.
They do unfortunately, or fortunately
depending on the situation, have an impact
upon peripheral resistance, which is
the resistance against which
the heart has to work when it contracts.
And low peripheral resistance may result
in low blood pressure, but it may also allow the heart
to contract more fully. So there's a balance all the time,
trying to maintain it just right. If you reduce blood flow
too much, you can reduce it to the brain,
the kidney, and the heart itself. Obviously,
not a good thing. Opiates and opioids
have little of direct effect upon the heart, other than
they slow heart rate, which in most cases is
a very positive situation. They also
have a moderate effect on lowering blood
pressure. The changes are moderate,
and they have very few side effects.
Induction drugs, unfortunately,
are much more problematic. The most popular
induction drugs are Thiopentone and Propofol.
Both of these have profound effects on reducing
cardiac contractility. In addition, Propofol
causes marked reduction in peripheral
resistance, thus potentiating hypotension,
and sometimes leading to reduced
blood flow to vital organs,
including the heart.
So how about the induction drugs?
We already mentioned Propofol and Pentothal.
We're now going to talk about Ketamine
and how it affects cardiac function.
Ketamine is often used in patients who appear
to be volume depleted at the time of surgery
or have had trauma, because it increases heart rate
and increases blood pressure moderately.
The problem is that it can lead to increases
in blood pressure that are unacceptable,
and it can lead to tachycardia or increased
heart rate that's unacceptable. On the good
side, it acts as a pain killer. So less opioids needs
to be used. You can give Ketamine without
a whole lot of extra narcotic on board.
The problem with Ketamine, and one
of the reasons why it isn't as widely used as it might otherwise be,
is that people have what are called Emergence
Phenomena when they start to wake up from Ketamine,
which can include hallucinations and quite
strange behaviour. Patients will wake up and be picking
at things in the air, they'll think they see bugs.
They'll be very concerned about noises in the room.
So, at the time Ketamine was used fairly
regularly in the early 80's, the recovery rooms were often
kept dark and very, very quiet. Which meant
you really couldn't see your patient and monitor them adequately.
So that was totally unacceptable and we no longer
accept that as a property within our recovery
rooms. Etomidate, which is a drug
that has been available in the United States for many years,
but in many other countries hasn't been available, because
it wasn't marketed due to cost constraints,
is a very good drug to use
in patients who have had brain trauma.
Because it has very little effect on intracerebral blood pressure,
has very little effect upon the cardiovascular system,
and oxygen, cerebral oxygen delivery tends
to remain stable. But it's not a perfect drug.
It suppresses the release of cortisol
from the adrenal cortex. And this may interfere
with the body's ability to handle the stress
of trauma, surgery, or anesthesia.
And there are some reports that suggest,
that unexpected death following surgery.
Not at the time of surgery, but in the weeks
to maybe a month following surgery, that unexpected
death is higher in patients who have had
Etomidate. So, what does the anesthesiologist
do about these changes?
Low blood pressure can reduce blood flow,
thus oxygen supply to vital organs.
Rapid heart rates can increase the work
of the heart and increase the need
for myocardial oxygen, which is difficult to supply
in the presence of low blood pressure.
A blood pressure that is too high
is also unacceptable, as it can lead to strokes,
an increased cardiac work, which may cause
cardiac ischemia or heart attacks.
So the anesthesiologist can modify these variables
by careful administration of either stimulating
drugs, such as Ephedrine or Phenylephrine,
Epinephrine or Norepinephrine, which increase
blood pressure and heart rate. Of course,
all of these drugs have their pros and cons.
So careful understanding of each is necessary
before one uses them. For instance, Phenylephrine
will increase blood pressure quite nicely,
but only for a very short period of time
and often causes quite a profound drop in heart rate,
whereas the others all increase heart rate
and tend to increase blood pressure.
Constant monitoring of the patient's condition,
during and after anesthesia, has shown to be
the most important aspect of anesthesiology.