Introduction

Gonadectomy in very young dogs and cats has become a routine surgery performed by veterinarians. Reported benefits of early gonadectomy include population control, decreased surgery times, and reduced incidence of mammary neoplasia and pyometra.¹ In addition to gonadectomy, puppies and kittens may need surgery for correction of congenital defects. For the purposes of this review, neonatal patients are defined as those in the first 6 weeks of life and pediatric patients are those from 6 to 12 weeks.²

Age itself was not considered an increased risk factor for anesthetic-related death in neonatal or pediatric dogs and cats.^3,4 However, dogs weighing <5 kg were nearly 8 times more likely to die and cats weighing <2 kg were nearly 16 times more likely to die during anesthesia.^3,4 Although age was not associated with increased anesthetic-related death, small body weight is a very common condition associated with neonatal or pediatric anesthesia. Additionally, neonatal and pediatric patients present physiological particularities that influence the responses to anesthetic and analgesic drugs. The anesthetist must be aware of these peculiarities in order to predict and minimize anesthetic-related complications in such patients.

Neonatal/Pediatric Physiological Particularities

Very young patients are physiologically immature and neonates are likely to present a greater degree of immaturity than pediatric patients. During the first 12 weeks of age, the contractile tissue of the myocardium is less than in adults, which makes the neonate/pediatric patient able to increase stroke volume by only 1/10 the capacity of an adult. Furthermore, immaturity of the sympathetic nervous system results in poor vasomotor control. As a result, cardiac output and blood pressure are dependent on high heart rates.^2,5 Normal heart rates for neonatal dogs and cats are above 200 beats/min and mean arterial pressure is approximately 50 mm Hg.²

The metabolic rate, oxygen consumption and minute volume are higher, and the intercostal muscles are weaker in very young patients. Respiratory chemoreceptors are less responsive to blood increases in CO₂ and decreases in O₂. Finally, small patients may have narrow airways that are more easily obstructed. Altogether, these particularities make the neonate/pediatric patient more predisposed to ventilatory fatigue and hypoxemia.^2,5

Immaturity of the hepatic and renal systems is associated with decreased drug metabolism and elimination, which may result in prolonged effect of anesthetics.^2,5 Although glucose levels are maintained adequately, neonate/pediatric patients have lower glycogen stores and a reduced ability to induce gluconeogenesis, which may lead to hypoglycemia if fasted for long periods.⁵

Small body size animals have a low mass-to-surface area ratio. In addition, thermoregulation by vasoconstriction and shivering is impaired, rendering neonates and pediatric patients prone to develop hypothermia.^2,5

Anesthetic Management

Young patients eating solid food may be fasted for 3 to 4 hours. Neonates not receiving solid food should not be separated from the mother for more than 2 hours before anesthesia. A thorough physical examination should be performed including assessment of signs of congenital abnormalities. A minimum of hematocrit, total protein, albumin and blood glucose should be performed. Reference intervals for young patients differ from adults and results should be interpreted accordingly. Other laboratory analyses and diagnostic procedures may be performed according to the history and findings of the physical exam.

Neonate and pediatric patients may need lower drug doses compared to adults. Therefore, the initial doses should be lower than for adults and then be titrated "to effect." Reversible drugs and those that are not dependent on liver metabolism or kidney excretion are preferable. This will allow faster anesthetic recovery, earlier return to food intake and regain of thermoregulation and glucose control.

Premedication in very young patients may consist of administration of an opioid analgesic alone (morphine, methadone, fentanyl, pethidine, hydromorphone, buprenorphine, butorphanol). The opioid adverse effect of greatest concern in neonate and pediatric patients is bradycardia, because cardiac output and blood pressure are dependent on high heart rates.

To avoid decreases in heart rate, an anticholinergic (atropine or glycopyrrolate) may be administered in combination with the opioid. Mild to moderate sedation is expected following opioid administration. In more agitated animals, a benzodiazepine (midazolam or diazepam) may be combined with the opioid to improve sedation without causing additional adverse effects. Acepromazine should be avoided in neonate/pediatric patients because it induces prolonged vasodilation, reduction in blood pressure, and impaired thermoregulation, which are not reversible. Alpha-2 adrenoceptor agonists should also be avoided because of the pronounced decrease in heart rate and cardiac output.

A few minutes of preoxygenation with 100% oxygen prior to induction of anesthesia is advisable. An injectable agent should be employed because the use of an inhalation anesthetic alone for induction and maintenance of anesthesia was associated with increased odds of anesthetic-related death in dogs.⁴ Propofol has a short duration of action and its elimination is little influenced in patients with impaired liver or kidney function. Therefore, propofol may be considered as first option for anesthetic induction in young patients. However, propofol is known to decrease blood pressure and cardiac output.⁶ For this reason, in patients with hemodynamic instability, such as those with congenital heart disease (e.g., patent ductus arteriosus), etomidate may be a safer induction agent. Ketamine combined with a benzodiazepine may also be considered although it can result in prolonged recovery due to mandatory hepatic metabolism and renal clearance.²

Maintenance of anesthesia should be performed with an inhalation anesthetic, such as isoflurane or sevoflurane, and nonrebreathing systems are usually preferable for neonate and pediatric dogs and cats. Intubation can be difficult in very small-sized patients and care should be taken not to cause trauma to the larynx. Both isoflurane and sevoflurane induce dose-dependent cardiovascular depression that can be attenuated by employing lower concentrations for maintenance of anesthesia.⁷ This can be achieved by co-administering analgesic drugs/techniques. Remifentanil constant rate infusion is a very attractive option because of its very fast clearance from plasma regardless of the duration of infusion.⁸ Local anesthetic techniques are also recommended but care should be taken because toxic doses may be easily achieved in small patients.

Recommended intraoperative monitoring include ECG, pulse oximetry, blood pressure, capnography, blood glucose levels, and body temperature. Assisted or controlled ventilation may be required to avoid ventilatory fatigue. Hypoglycemia may be prevented with dextrose- or glucose-containing fluids. The use of heat sources, such as thermal pads, starting in the preoperative period, may help minimize hypothermia and prolonged recovery from anesthesia. Postoperative pain may be treated with opioid analgesics as indicated. Nonsteroidal anti-inflammatory drugs should be used judiciously in very young patients due to immaturity of the cardiovascular, hepatic and renal systems which could result in more pronounced adverse effects.

Final Considerations

Anesthesia of neonate and pediatric patients has several challenges. Small body size constitutes a difficulty for venous access and monitoring of vital signs because most probes and sensors of monitoring devices are not designed for such small patients. Furthermore, immaturity of organ systems may result in unpredictable responses to anesthetics and anesthetic adjuncts. In general, the smaller and younger the patient, the greater is the challenge of the anesthetic procedure. Non-emergency procedures in neonates should be delayed, if possible, and performed once the patient has reached the pediatric age. The anesthetic plan should take into consideration the procedure to be performed, existence of immaturity of organ systems, the results of the physical exam and other diagnostic procedures.

References

1.  Root Kustritz MV. Pros, cons, and techniques of pediatric neutering. Vet Clin North Am Small Anim Pract. 2014;44(2):221–33.

2.  Grubb TL, Perez Jimenez TE, Pettifer GR. Neonatal and pediatric patients. In: Grimm KA, Lamont LA, Tranquilli WJ, Greene SA, Robertson SA, editors. Veterinary Anesthesia and Analgesia. Ames, IA: Wiley Blackwell; 2015:983–987.

3.  Brodbelt DC, Pfeiffer DU, Young LE, Wood JL. Risk factors for anaesthetic-related death in cats: results from the confidential enquiry into perioperative small animal fatalities (CEPSAF). Br J Anaesth. 2007;99(5):617–23.

4.  Brodbelt DC, Pfeiffer DU, Young LE, Wood JL. Results of the confidential enquiry into perioperative small animal fatalities regarding risk factors for anesthetic-related death in dogs. J Am Vet Med Assoc. 2008;233(7):1096–104.

5.  Cunha AF. Neonatal, pediatric, and geriatric concerns. In: Snyder LBC, Johnson RA, editors. Canine and Feline Anesthesia and Co-existing Disease. Ames, IA: Wiley Blackwell; 2015:310–319.

6.  Berry SH. Injectable anesthetics. In: Grimm KA, Lamont LA, Tranquilli WJ, Greene SA, Robertson SA, editors. Veterinary Anesthesia and Analgesia. Ames: Wiley Blackwell; 2015:277–296.

7.  Mutoh T, Nishimura R, Kim HY, Matsunaga S, Sasaki N. Cardiopulmonary effects of sevoflurane, compared with halothane, enflurane, and isoflurane, in dogs. Am J Vet Res. 1997;58(8):885–90.

8.  Glass PS. Pharmacology of remifentanil. Eur J Anaesthesiol Suppl. 1995;10:73–4.