I usually avoid mask inductions in patients with pulmonary disease because inhalation anesthetics depend on the respiratory
tract as the administration route. This is particularly important in patients with upper airway obstruction. In these patients,
the goal of rapid induction and intubation is usually accomplished with intravenous induction agents (e.g. thiopental, propofol). Patients experiencing upper airway obstruction or hypoxemia should receive oxygen through a mask or
nasal cannula for three to five minutes before induction. Struggling can be avoided by sedation with an opioid; however, excessive
sedation may compromise a patient's respiratory drive and should be avoided in dyspneic patients. Patients with cardiac disease
may benefit from preoxygenation as well to minimize deficits in oxygen delivery.
In general, inducing anesthesia in patients with cardiac disease usually relies on high doses of opioids (e.g. fentanyl, hydromorphone) that have minimal effects on cardiovascular performance (i.e. blood flow to the tissues). I usually intravenously administer an opioid and a benzodiazepine followed by a low dose of thiopental,
propofol, ketamine and diazepam, or etomidate, if necessary. As stated previously, low concentrations of an inhalant may also
be used to complete induction. The goal is a slow transition toward a surgical plane of anesthesia that allows the cardiovascular
and autonomic nervous systems more time to respond. This also allows the anesthetist more opportunity to prevent overdose.
Q: What are the differences between the cardiovascular effects of isoflurane and sevoflurane?
I am not aware of a single cardiopulmonary characteristic I would use to persuade someone to switch from one of these agents
to the other. Sevoflurane has a low blood:gas solubility coefficient, thus changes in anesthetic depth occur more rapidly
than with isoflurane. This includes changes during anesthesia as well as inductions and recoveries. However, I think many
other factors affect recovery, and I don't use mask inductions frequently enough to desire a more rapid induction. I do like
the ability to change anesthetic depth more quickly, but as many practitioners will remember, that characteristic made our
change from halothane to isoflurane frustrating because it was difficult to achieve a steady plane of anesthesia.
Q: For patients with organ system dysfunction, I've heard that the best protocol is to induce anesthesia with propofol and
maintain with isoflurane. Is there really anything better?
While some fantastic anesthetic agents have become available over the past 20 years, none of them is perfect. It is not the
anesthetic agent itself that makes it safe, but how it is used. Knowing the advantages and disadvantages of any anesthetic
agent or combination of agents will enhance its value.
Propofol with isoflurane is a valuable option for induction and maintenance of anesthesia. Both can be used to produce rapid
changes in anesthetic depth and are associated with short recovery times.
However, remember that neither propofol nor isoflurane provides analgesia. Both propofol and isoflurane are dose-dependent
respiratory depressants, and the anesthetist should be prepared to support ventilation. Likewise, both anesthetic agents cause
dose-dependent peripheral vasodilation,6-8 and patients predisposed to hypotension will be at risk. Propofol is short-acting, and keeping a patient anesthetized with
isoflurane alone can require high vaporizer settings that are more likely to result in these cardiopulmonary consequences.
Using opioid premedication for analgesia with this combination will allow the use of less propofol and lower vaporizer settings.
This will be particularly important in patients with organ dysfunction. In addition, high-dose or multiple administrations
of propofol in cats has caused Heinz body formation.9,10
Typically, there are no advantages to using propofol rather than thiopental when inducing anesthesia before general anesthesia
with inhalant anesthetics. Cardiopulmonary effects are similar, and when large doses of thiopental are avoided by using premedication,
no differences in recovery times are noted. Along the same lines, isoflurane typically has no advantages over sevoflurane.
Q: What are your recommendations for sedating and anesthetizing dogs that are heartworm-positive?
I would avoid anesthetizing patients with heartworm disease. Anesthesia may be necessary to remove heartworms in patients
with caval syndrome, but this situation is not common. If sedation is required for diagnostic evaluation of heartworm-positive
dogs, an opioid with or without a benzodiazepine can be used. All elective procedures should be postponed until after treatment.