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Pediatric anesthesia: what you didn't learn in kindergarten (Proceedings)
What is a neonate or pediatric patient? Neonate patients are between 0 to 6 weeks of life and a pediatric patient is between 6 to 16 weeks of life.
What is a neonate or pediatric patient? Neonate patients are between 0 to 6 weeks of life and a pediatric patient is between 6 to 16 weeks of life. Neonate and pediatric patients have limited organ reserve, decreased ability to respond to physiological changes and challenges and an increased sensitivity to anesthetic and sedation drugs which results in prolonged drug effects. Drug dosages should be reduced by 1/3 to 1/2 of calculated adult dosages. These patients also have an increased risk of perianesthetic complications. Monitoring and following trends is a must!
Issues Confronting Neonate & Pediatric Patients:
1. Hypoalbuminemia results in a greater active portion of protein bound drugs being available so less anesthetic or sedation drug is bound to protein which can result in a deep and often fatal plane of anesthesia e.g. ketamine, etomidate, NSAIDS etc.
2. Increased permeability of the blood/brain barrier so a greater amount of drug enters the brain.
3. More susceptible to hypovolemia due to a fixed circulating fluid volume. This small fluid volume increases delivery of anesthetic drug to highly perfused tissues such as the brain.
4. Low body fat % - these patients have a smaller "fat" compartment for anesthetic drug redistribution.
5. Immature liver at least for the first 3-4 weeks of life (perhaps longer). Drug metabolism is prolonged so drug duration and effects are extended e.g. diazepam, opioids
6. Immature GFR (glomerular filtration rate): GFR is not mature until 2-3 weeks of age and tubular secretion not until 4-8 weeks of age. Immature GFR results in prolonged drug activity e.g. increased ½ life e.g. gabapentin
7. High metabolic rate (MR) causes increased O2 consumption and hence greater alveolar ventilation This causes rapid mask induction.
Neonate and Pediatric Physiology
Cardiac output (CO) is dependent on heart rate (HR) with stroke volume (SV) and cardiac reserve being limited in these patients. Remember: CO=HR X SV. In addition, the neonate and pediatric patient's sympathetic nervous system (SNS) is not fully developed and stimulation of the SNS only results in a MINIMAL increase in HR and contractility so increases in CO are limited. Neonate and pediatric patients can only increase contractility by 30%, but adults by 300%! Also, SNS immaturity can result in an incomplete and inadequate baroreceptor response to hypotension. Remember: Blood pressure (BP) is dependent on CO so if HR decreases than CO decrease and so does BP. Neonate kidneys are less efficient (than adults) at eliminating fluid overloads and fluid overloading may cause pulmonary edema to develop. Pulmonary reserve is small which increases the possibility of hypoxia during apnea, airway obstruction or induction. The neonate rib cage is pliable so it is less efficient at ventilating and greater energy is expended during breathing. This predisposes this group of patients to hypoxia and ventilatory fatigue. Minute ventilation is high because of increased O2 demand. Hematocrit DECREASES initially in neonates (3-4 weeks of age) and minor hemorrhaging may decrease O2 delivery to tissues. Neonate and pediatric patients are susceptible to hypothermia because of an immature thermoregulatory system, high body mass to surface area ratio and a limited ability to vasoconstrict to conserve heat. Fetal circulation may persist in some patients: PDA, hepatic shunts.
Pre-anesthetic Evaluation/Blood work
A TPR should be performed on all neonate and pediatric patients. This should include a HR (feline/canine) ≈ 200bpm, RR ≈ 15-35 breaths/minute and temperature. Minimal blood work should include a blood glucose, PCV, TP and fractionated protein (albumin), but more is always better. Assess hydration status and correct if necessary. Preoperative blood pressure should be performed as well.
ASA levels were established by the American Society of Anesthesiologists. An ASA level is assigned to a patient after reviewing the patient's medical history. This ASA level designates the patient's anesthetic risk with regard to its medical history. There are 5 ASA levels that range from 1 (healthy, young) – 5 (morbid, will probably not survive next 24 hours with or without surgery). An ASA Level with an "E" after it denotes an emergency surgical procedure. Neonate and pediatric patients are designated as an ASA 2.
Anticholinergics may not be necessary unless HR drops. This class of drugs may increase myocardial work and O2 consumption if given indiscriminately. Brachycephalics have a tendency toward high vagal tone and some procedures increase vagal stimulation as well e.g. ocular, bronchoscopy and endoscopy. Anticholinergics may need to be given in these situations.
Benzodiazepines are more consistent sedatives and anxiolytics in neonate and pediatric patients, but they do not provide analgesia. They are reversible (flumazenil) and produce little or no cardiovascular & respiratory depression. Acepromazine should be avoided with neonate and pediatric patients as it will cause hypotension and is non reversible. Alpha-2s should NOT be used in patients under 4 months of age as this can cause profound reflective bradycardia which may result in hypotension.
Neonate and pediatric patients do experience pain!!!! Current evidence suggests that if we do not treat pain in this population of patients it can lead to dynamic changes in their nociceptive pathway, resulting in chronic lifetime pain. Opioids are an excellent choice for this patient population as they are reversible. Opioids can cause bradycardia which if not treated with an anticholinergic will result in hypotension. Full mu agonists give the BEST pain relief e.g. morphine, hydromorphone, methadone, oxymorphone etc. Partial mu agonists give moderate pain relief e.g. buprenorphine Butorphanol is a mu antagonist/kappa agonist and is best utilized for sedation in non painful procedures. Opioid premeds should be given IM as this will result in better analgesia absorption and consistency. Reduce dosages in this patient population by at least 1/3! Naloxone or butorphanol can be used to reverse full mu opioids if necessary.
NSAIDS are a good option in OLDER pediatrics. Carprofen can be used in patients 6 weeks or older. Meloxicam and deracoxib are newer NSAIDS with less history and both are more COX2 specific than carprofen. Acute renal failure in felines has been reported with meloxicam usage and gastric perforations in canines with deracoxib. Firocoxib is the most recent NSAIDS so there is still no long term data however it is the most COX2 specific of all.
Preoxygenate patients 5-7 minutes prior to induction to "preload" tissues with O2. Etomidate and propofol are the best induction choices for neonate and pediatric patients. Use of a benzodiazepine with both of these hypnotics will aid in reducing induction doses and unwanted side effects. Both of these drugs are rapidly eliminated by the body by various routes and are not dependent on one organ system for elimination. Etomidate has minimal cardiovascular and respiratory depression associated with it so a significant drop in BP is usually not seen. Myoclonus, retching and vomiting are some of the unwanted side effects of etomidate, but these can be minimized by using a benzodiazepine prior to induction. Propofol will cause cardiovascular and respiratory depression and patients will become hypotensive initially until the drug begins its elimination process. Both etomidate and propofol should be titrated to effect. A ketamine/benzodiazepine combination can be used and will usually cause mild cardiac and respiratory depression. Initially, however, ketamine will cause sympathomimetic side effects, increased BP and HR, but SNS stimulation may be minimal in neonate and pediatric patients because of immature nervous systems. Ketamine is eliminated via the kidneys in felines and liver in canines so effects maybe prolonged in neonate and pediatric patients in these species. Benzodiazepines are metabolized by the liver so prolonged sedative effects may be seen if dosages are not reduced.
Mask induction is no longer considered a "safe" option. It is the leading cause of death in humans undergoing an anesthetic procedure because of unsecured airways! Also, all gas inhalants are dose dependent cardiac and respiratory depressants as well as potent vasodilator. Gas inhalants also cause a prolonged excitement phase which can be more detrimental physiologically in this patient population. Anesthesia is induced very rapidly with gas inhalants and can lead to death if the anesthetist is not vigilant during induction.
Isoflurane and sevoflurane are the most common gas anesthetics on the market. Either is safe for use in neonate and pediatric patients. However, sevoflurane "may be" a better anesthetic choice for neonate and pediatric patients. Sevoflurane has a lower blood: gas partition coefficient than isoflurane so changes in anesthetic depth occur more rapidly. Remember all inhalants cause hypotension, hypoventilation, hypothermia and impaired cardiac contractility. Titrate carefully! Use of a non rebreathing system is the norm for these patients because these systems have lower resistance to breathing and less apparatus dead space. When initiating IPPV be very careful! Tidal volumes are small in these patients and barotrauma can be caused by over ventilation. Peak inspiratory pressure should not exceed 10-15cm of H20.
Be careful with fluid support. Neonate and pediatric patients are less tolerant of fluid overloading. Total protein and albumin can be decreased to life threatening levels very quickly! Intraoperative fluid rates should not exceed 10mls/kg/hr. Monitor blood glucose regularly and add dextrose if necessary. Hypothermia can be life threatening as shivering increases O2 consumption by 200-300%. Keep them warm!!!!
If possible monitor BP, ETCO2, temperature, ECG, and SPO2. Have an anesthetic sheet and follow trends. Remember that BP is linked to HR. ETCO2 adaptors can increase dead space so use pediatric ETCO2 adaptors whenever possible. Neonate and pediatric patients have large surface area to mass ratios so keep them warm.
Monitor temperature. Shivering can increase O2 consumption by 300% so supply O2 via face mask if necessary post op. Use analgesics such as locals, opioids, NSAIDS etc if appropriate. Carefully calculate and titrate. Assessing pain levels in neonate and pediatric patients may be challenging, but analgesics should not be witheld. Resume normal feedings ASAP especially nursing puppies and kittens.