Anatomy and Physiology

Airway. Understanding the anatomic differences between infant and adult upper airways is key to managing infants' airways safely (Table 42-1: Anatomic Features of the Upper Airway in Infants Compared with Adults) (Lerman J. Pediatric anesthesia. In: Barash PG, Cullen BF, Stoelting RK, Cahalan MK, Ortega R, Stock MC, eds. Clinical Anesthesia. Philadelphia: Lippincott Williams & Wilkins; 2013:1216–1256).
1. As a result of these anatomic features, the head in the child is naturally in the “sniffing” or flexed position.
2. The large tongue:mouth ratio presents difficulty if the mouth is closed during mask ventilation, particularly with the narrowed nares.
3. The most common airway problem in infants and young children is upper airway obstruction caused by laryngomalacia.
4. A number of airway anomalies may present problems during anesthesia (Table 42-2: Airway Anomalies that may Present Problems During Anesthesia).
5. The cricoid ring represents the only solid cartilaginous structure in the upper airway because it is covered with pseudostratified columnar epithelium.
  1. This epithelium is subject to swelling if irritated, swelling into the lumen and reducing the radius of the airway.
  2. A 50% reduction in the radius of the cricoid ring increases the work of breathing, which cannot be sustained resulting in respiratory failure.
6. The short trachea in the infant and child facilitates accidental endobronchial intubation (persistent <85% saturation).
Cardiovascular
1. In the early years, the heart has limited ability to increase stroke volume, rendering cardiac output more dependent on heart rate than in the adult. (Hypotension in the child with a normal or increased heart rate is caused by hypovolemia and should be treated with volume expansion rather than vasopressors [except in children with congenital heart disease].)
2. Systemic vascular tone is low in children up to 8 years of age, as evidenced by the lack of a change in blood pressure when caudal or epidural blocks are administered.
3. Both heart rate and blood pressure increase with increasing age in childhood (Table 42-3: Normal Range of Resting Heart Rates and Blood Pressure in Children).
Central Nervous System
1. Oxygen consumption in the brains of children (5.5 mL/100 g/min) is 50% greater than that in adults (3.5 mL/100 g/min). In children 6 months to 3 years of age, the cerebral blood flow is 50% to 70% greater than in adults (50 mL/min/100 g vs. 70–110 mL/min/100 g).
2. Apoptosis. Most general anesthetics (with the exception of xenon, dexmedetomidine, and opioids) and sedatives, that also act on N-methyl-d-aspartic acid (NMDA) and GABA_A receptors, cause apoptosis in newborn rodents and nonhuman primates after relatively prolonged exposure (>12 hours) and when administered in combination.
  1. Brief anesthesia with ketamine (≤3 hours) or isoflurane (<2 hours) fails to induce neurocognitive dysfunction.
  2. Whether the rodent and nonhuman primate evidence of anesthetic-induced neurotoxicity is applicable to humans remains hotly debated. (Because most anesthetics in young children last <3 hours, these data may have limited validity in humans.)
  3. Also, the doses of intravenous (IV) anesthetics and sedatives in rodents and primates are up to 10-fold greater than in humans.
  4. Studies in humans who received anesthesia at a young age suggested that cognitive disability in those who received anesthesia before the age of 3 years may be more prevalent than in those who did not. However, most of those studies were seriously flawed in terms of their design, and a large cohort of identical twins who were discordant for general anesthesia before 3 years of age in one study, when tested for intellectual aptitude 10 years later, were similar.

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