The four main classes of drugs used as premedicants in the pediatric patients are opioids, benzodiazepines, anticholinergics, and tranquilizers. These drugs can be used alone or in combinations.

Opioids

Opioids in general have very little effect on cardiac contractility but may produce a reduction in heart rate. The μ agonists (fentanyl, morphine, methadone, hydromorphone, and oxymorphone) provide the best analgesia but will cause greater cardiovascular and respiratory depression than the partial μ agonists(buprenorphine)and the κ agonist/partial μ agonists(butorphanol). These latter drugs provide only moderate analgesia but may be a more appropriate choice dependent on the procedure and analgesic requirements.

Benzodiazepines

Benzodiazepines usually do not provide reliable sedation in a healthy adult patient, but are often quite effective in the pediatric patient. Midazolam and diazepam cause minimal cardiovascular depression and provide good muscle relaxation. Benzodiazepines do not provide any analgesia, so they are best used in combination with an opioid. Both opioids and benzodiazepines have specific antagonists, naloxone and flumazenil, respectively, which can be administered if undesirable or prolonged effects of these drugs occur.

Anticholinergics

Anticholinergics are indicated in the pediatric patient. Cardiac output in the pediatric patient is dependent on heart rate; thus, care should taken to maintain heart rate. The use of atropine or glycopyrrolate is also indicated in patients with high vagal tone or in procedures that may likely stimulate a vagal reflex.

Acepromazine

Acepromazine, a phenothiazine, is one of the most common drugs used in premedication in the veterinary patient. Acepromazine provides excellent sedation, but its deleterious side effects make it an unwise choice in the pediatric patient. It may cause hypotension and heat loss due to vasodilatation in a patient that is less able to compensate for these effects.

All of these drugs require hepatic metabolism and renal clearance; and with both of these organ systems underdeveloped, prolonged and exaggerated effects may be seen. Care must be taken with drug doses because the pediatric patient is less able to tolerate absolute or relative overdoses.

α₂ adrenergic agonists

Although α₂ adrenergic agonists will provide effective sedation with some analgesia, they can also produce profound bradycardia. This is of particular importance in pediatric patients due to their rate-dependent cardiac output. At higher dose rates respiratory depression may also be an issue due to the high metabolic oxygen demand of the patient. α₂ adrenergic agonists are also extensively metabolized in the liver, so prolonged and more profound effects of these drugs would be expected. Their use is best avoided in the pediatric patient. Table 26.1 lists suggested dosage for pediatric patients.

Induction

Induction of anesthesia is achieved by the administration of injectable drugs or by the use of inhalational agents. Injectable drugs have the benefit of minimizing stress to the patient and a rapid loss of consciousness, which will facilitate rapid control of the airway. Venous access is required for administration, and this may prove challenging in the conscious pediatric patient.

The injectable drugs commonly used as induction agents are propofol, ketamine/diazepam, etomidate, and alfaxalone. All injectable anaesthetics have side effects that need to be considered when selecting the most appropriate drug.

Propofol

Propofol is a short-acting hypnotic agent that can be used for the induction and maintenance of general anesthesia via incremental doses or constant rate infusion. If used in conjunction with opioids and phenothiazines, the use of propofol may cause unwanted cardiovascular effects, including bradycardia and vasodilatation. It can also cause significant respiratory depression and should be titrated to effect. Propofol is highly lipid-soluble and requires hepatic metabolism, though some extra hepatic metabolism does occur (Saint-Maurice 1994).

Table 26.1. Suggested drug doses (mg/kg) for pediatric small animals.

• Extrapolated from adult doses
  
• Combining drug groups provides balanced premedication.
  
• Drug doses should be further reduced for neonatal patients.

*Induction dose rates are for premedicated patients and should be titrated slowly to effect.

Ketamine

Ketamine, a dissociative anesthetic is often used in combination with a benzodiazepine as an induction agent. Ketamine has a rapid onset of action, causes minimal cardiovascular depression, and provides a short duration of analgesia. Ketamine often produces an increase in blood pressure and cardiac output due to stimulation of the sympathetic nervous system (Wright 1982).

Etomidate

Etomidate is a short-acting, nonbarbiturate intravenous anesthetic agent. It causes minimal adverse cardiovascular effects and is less likely to cause a significant drop in blood pressure when compared to other induction agents. Etomidate produces minimal respiratory depression.

Alfaxalone

Alfaxolone in cyclodextran (Alfaxan®) is a new formulation of the older steroid-injectable anesthetic (Saffan). Alfaxalone is a rapid and noncumulative injectable agent that provides reasonable muscle relaxation, has excellent cardiovascular stability, and causes minimal respiratory depression (Best and Pearson 1997).

All of the injectable drugs require hepatic metabolism and renal clearance. When used in the patient with immature renal and hepatic function, prolonged duration of action and recovery may result, with the possible exception of propofol due to its extrahepatic metabolism.

Accurate dosing of injectable induction agents is important. It is beneficial to dilute the concentration of a particular drug with a compatible diluent to assist in the titration process.

All of the injectable agents have the potential to produce respiratory depression, so the technician needs to be able to take rapid control of the airway, supply oxygen, and ventilate when necessary.

Inhalation agents

The inhalation agents commonly used for induction are isoflurane and sevoflurane. Both agents may cause dose-dependent cardiovascular and respiratory depression. Sevoflurane has a lower solubility in blood than isoflurane, and thus results in a more rapid uptake (induction) and elimination (recovery). A small well-fitting face mask with minimal dead space should be used for an inhalational induction (Fig. 26.2). Wrapping the patient in a warm towel can reduce the stress of restraint.

Figure 26.2. Pediatric mask.

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