INTRODUCTION

Sedation of critically ill veterinary patients is often required to permit minor surgical procedures and diagnostic techniques. Advantages over general anesthesia include flexibility and ease of drug administration, while avoiding the need for intubation and inhalant anesthetics. Significant cardiovascular and respiratory depression may result, however, leading to life-threatening patient compromise. To minimize the influence of preexisting conditions and the extent of crises that may occur, evaluation and adequate preparation of the patient are essential before administering any drug.

PATIENT EVALUATION AND MANAGEMENT

On presentation, immediate attention should be paid to the ABCs (airway, breathing, circulation; see Chapter 2, Patient Triage). These should be deemed adequate before proceeding. Evaluation of neurologic status, including mental status and evidence of spinal cord or head trauma, should be included. A complete history should be obtained if possible, including presenting complaint, known medical conditions, any current medications, and previous anesthesia history.

Oxygen supplementation and ventilatory support are provided as necessary. Indications for securing an airway early include poor ventilation or oxygenation, deteriorating mental status, lack of a gag reflex, or upper airway obstruction. Stabilization of fluid balance and cardiovascular function are also essential before drug administration, although assessing the adequacy of intravascular volume can be difficult. Inadequate intravascular volume is aggravated by the peripheral vasodilation caused by many pharmacologic agents and the generalized decrease in sympathetic tone and attenuation of compensatory mechanisms that occur with sedation. Adequate intravascular access is extremely important.

Cardiac arrhythmias, especially premature ventricular contractions (PVCs), are seen commonly in critically ill patients, including those with gastric dilation-volvulus, hemoabdomen, and/or thoracic trauma. In addition, they can occur secondary to electrolyte abnormalities, hypoxemia, or hypercarbia. The type and significance of arrhythmias should be determined and the underlying cause treated, if possible. Indications for antiarrhythmic therapy before drug administration include frequent multiform PVCs or paroxysmal ventricular tachycardia that adversely affects blood pressure (see Chapter 47, Ventricular Tachyarrhythmias).

When time and the animal’s condition permit, electrolyte abnormalities should be normalized before drug administration. Severe hyperkalemia (potassium >6 mmol/L) is seen frequently in patients with renal compromise, urinary tract obstruction or rupture, massive tissue trauma, or severe dehydration with metabolic acidosis (see Chapter 55, Potassium Disorders). Drug administration in a patient with hyperkalemia is associated with a high incidence of arrhythmias and cardiac arrest, and therefore appropriate treatment should be instituted before proceeding. Hypocalcemia may be seen transiently after citrated blood product administration, although this generally self-resolves.

CHOICE OF AGENT

The choice of sedative agent will depend on the patient’s current physical status, reason for presentation, pertinent history, and the procedure to be performed. Special attention should be paid to both cardiovascular and respiratory effects of these agents, but contraindications to drug use should also be considered. Intravenous administration allows for titration of the drugs and is generally preferred. Intramuscular sedation may be helpful, especially in the fractious patient or one with severe respiratory distress that becomes easily stressed with restraint. A complete list of commonly used sedative drugs can be found in Table 162-1.

Table 162-1 Commonly Used Sedative Drug Dosages

Drug	Dosage (mg/kg)	Route
Anticholinergic Agents
Atropine	0.02	IM, SC
	0.01	IV
Glycopyrrolate	0.01	IM
	0.005	IV
Sedatives and Tranquilizers
Acepromazine	0.005 to 0.1	IM, IV, SC
Diazepam	0.2 to 0.5	IM, IV
Midazolam	0.1 to 0.5	IM, IV
Flumazenil (reversal agent)	0.01 to 0.02	IV
Medetomidine	0.001 to 0.03	IM
Atipamezole (reversal agent)	0.1 to 0.25	IV, IM
Opioid Agonists and Partial Agonists
Buprenorphine	0.005 to 0.02	IM, IV
Butorphanol	0.1 to 0.5	IV
	0.2 to 0.5	IM
Fentanyl	0.025 to 0.01	IV
Hydromorphone	0.05 to 0.2 (½ dose in cats)	IM, IV
Morphine	0.2 to 2 (½ dose in cats)	IM, IV
Methadone	0.1 to 0.5 (½ dose in cats) 0.2 to 2.0 (½ dose in cats)	IV SC, IM
Nalbuphine	0.1 to 2.0	IM, IV
Oxymorphone	0.02 to 0.2 (½ dose in cats)	IM, IV
Naloxone	0.01 to 0.04	IV, IM (reversal agent)
Other Agents
Ketamine	2 to 8 (cat only)	IM
	2 to 5	IV
Telazol	2 to 4	IM (may have prolonged recovery)
Propofol	1 to 6	IV (titrate slowly to effect)

IM, Intramuscular; IV, intravenous; SC, subcutaneous.

Opioids

Most sedative techniques in the critically ill patient involve a sedative or tranquilizer in combination with an opioid analgesic (see Chapter 184, Narcotic Agonists and Antagonists). This neuroleptanalgesic combination generally produces a greater degree of sedation and analgesia with less cardiovascular depression than that achieved by comparable doses of either drug alone.

Most of the clinically used pure opioid agonists (morphine, methadone, oxymorphone, hydromorphone, fentanyl) bind primarily to the μ-receptor in the central nervous system, although they interact with the other receptors (κ, δ), especially at higher dosages.¹ In healthy animals, opioids cause behavioral changes ranging from sedation to excitement; however, in critically ill patients, opioids usually cause sedation. Cardiovascular function, including left ventricular function, cardiac output, and systemic blood pressure, is well maintained. Vagally mediated bradycardia may be seen and can be treated with anticholinergic agents (atropine, glycopyrrolate), if necessary. Oxymorphone, hydromorphone, and fentanyl are particularly useful intravenous agents, especially in combination with benzodiazepine tranquilizers (midazolam, diazepam), because they provide the most cardiovascular stability.^2,3 Histamine release with subsequent vasodilation and hypotension may occur after meperidine or morphine administration.⁴

Although opioids are relatively sparing of the cardiovascular system, they may act as respiratory depressants, causing a decreased ventilatory response to hypercarbia. Respiratory depression may be exacerbated by concomitant administration of other sedatives. Therefore opioids should be used judiciously and at decreased dosages if respiratory depression and hypoventilation are contraindicated, as in cases with upper airway obstruction or increased intracranial pressure. Materials for intubation and positive-pressure ventilation should be readily available before drug administration in these patients. Do not mistake panting, a common side effect of opioids, with effective ventilation; look carefully at the depth of each breath, the rate of respiration, and the arterial carbon dioxide level, if indicated.

Butorphanol, a κ-agonist and μ-antagonist, or buprenorphine, a partial μ-agonist, may cause less respiratory depression and be preferable in some critically ill cases. Other clinically significant side effects such as vomiting and decreased gastrointestinal motility may also be less pronounced with these drugs, although they tend to provide less analgesia. If undesirable side effects should occur, the opioid can be reversed using naloxone (0.01 to 0.02 mg/kg IV, IM, or SC). Buprenorphine may be difficult to reverse and require up to 10 times the normal naloxone dosage. Opioid reversal with naloxone will also remove the analgesic effects, and this should be anticipated by the clinician. Alternatively, small doses of butorphanol (0.05 mg/kg IV) or nalbuphine (0.05 mg/kg IV) may be titrated to reverse some of the sedative effect of a pure opioid agonist, while retaining some of the analgesia by enhancing the κ effects.

Before administering an opioid (or any other drug), the veterinarian should carefully observe the animal and consider the underlying disease process. Commonly used opioid drugs and their pertinent information can be found in Table 162-2.

Table 162-2 Suggested Dosages for Opioids in Small Animals