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Neonatal and Infant Physiology Relevant to Anesthesia
Introduction
Understanding the unique physiology of neonates (birth to 28 days) and infants (1 month to 1 year) is essential for safe anesthetic management. Organ systems undergo rapid maturation during this period, and the pharmacokinetic and pharmacodynamic profiles of anesthetic agents differ markedly from those in older children and adults.
Cardiovascular Physiology
The neonatal myocardium has fewer contractile elements and greater dependence on extracellular calcium for contraction. Cardiac output is heart rate dependent because stroke volume is relatively fixed due to a noncompliant ventricle. Bradycardia is therefore the most common cause of decreased cardiac output in neonates.
The transitional circulation presents unique challenges. Functional closure of the ductus arteriosus occurs within 24 to 72 hours of birth, but anatomic closure takes 2 to 3 weeks. Persistent pulmonary hypertension of the newborn (PPHN) can result in right-to-left shunting through the foramen ovale and ductus arteriosus. Sympathetic innervation is immature and vagal tone predominates, predisposing neonates to bradycardia with laryngoscopy and hypoxia.
Key Hemodynamic Parameters
The normal neonatal heart rate is 120 to 160 bpm, and normal systolic blood pressure is approximately 60 to 80 mmHg in the term neonate. A rough guideline for minimum acceptable systolic blood pressure is the gestational age in weeks.
Respiratory Physiology
Oxygen consumption in neonates is approximately 6 to 8 mL/kg/min, roughly twice the adult rate, leading to rapid desaturation during apnea. Functional residual capacity is low relative to closing capacity, promoting atelectasis and ventilation-perfusion mismatch. Alveolar ventilation is high relative to FRC, resulting in rapid inhalational induction and emergence.
The rib cage is horizontal and cartilaginous, and the intercostal muscles contribute less to ventilation. The diaphragm is the primary muscle of respiration and is more susceptible to fatigue because it contains fewer type I (slow-twitch) fibers. Neonates are obligate nasal breathers until approximately 3 to 5 months of age. Control of breathing is immature, and neonates, especially preterm infants, are prone to periodic breathing and apnea.
Airway Anatomy
Neonates have a large head and occiput with a relatively large tongue. The larynx is cephalad, positioned at C3-C4 compared to C5-C6 in adults. The epiglottis is long, narrow, and omega-shaped. Traditionally, the subglottis at the level of the cricoid cartilage was considered the narrowest portion of the pediatric airway, but recent evidence suggests the glottic opening may actually be the narrowest point. Cuffed endotracheal tubes are now preferred even in neonates, using an appropriate size of 3.0 to 3.5 mm ID for term neonates.
Thermoregulation
Neonates have a high surface area-to-body weight ratio, thin skin, and minimal subcutaneous fat. Nonshivering thermogenesis via brown fat is the primary heat-generating mechanism. Cold stress increases oxygen consumption, can worsen metabolic acidosis, and may trigger pulmonary vasoconstriction.
Strategies to prevent hypothermia include warming the operating room to 26 to 28 degrees Celsius, using forced-air warming devices, heated mattresses, and warmed IV fluids, minimizing exposed body surface area, and using humidified and heated inspired gases.
Renal and Fluid Physiology
Glomerular filtration rate is low at birth (approximately 20 mL/min/1.73 m2) and reaches adult values by 1 to 2 years of age. Neonates have limited ability to concentrate urine, with a maximum of approximately 600 to 700 mOsm/kg, and limited ability to handle sodium and water loads. Total body water is higher in neonates at approximately 75 to 80% of body weight compared to 60% in adults. The maintenance fluid rate for neonates is approximately 4 mL/kg/hr based on the Holliday-Segar formula for the first 10 kg. Isotonic crystalloid (lactated Ringer's or Plasmalyte) should be used for replacement because hypotonic fluids carry a risk of hyponatremia.
Hepatic and Metabolic Function
Hepatic enzyme systems, including Phase I and Phase II pathways, are immature at birth. Reduced conjugation capacity (such as glucuronidation) affects the metabolism of morphine, acetaminophen, and bilirubin. Lower plasma protein levels, including albumin and alpha-1 acid glycoprotein, increase the free fraction of protein-bound drugs.
Glucose stores in the form of glycogen are limited, and neonates are at risk for hypoglycemia, especially preterm or small-for-gestational-age infants. Blood glucose should be monitored intraoperatively and maintained above 45 to 50 mg/dL.
Hematologic Considerations
Fetal hemoglobin (HbF) constitutes 60 to 80% of total hemoglobin at birth. HbF has a higher oxygen affinity, reflected in a left-shifted oxyhemoglobin dissociation curve with a P50 of approximately 19 mmHg compared to 27 mmHg for adult hemoglobin A. A physiologic nadir of hemoglobin occurs at 8 to 12 weeks of life, with hemoglobin typically falling to 9 to 11 g/dL in term infants.
Blood volume is approximately 80 mL/kg in term neonates and 90 to 100 mL/kg in preterm neonates. Allowable blood loss calculations must account for the small total blood volume.
Neonatal vs. Adult Physiology Summary
| Parameter | Neonate | Adult | Anesthetic Implication |
|---|---|---|---|
| Heart rate | 120–160 bpm | 60–100 bpm | CO is HR-dependent; bradycardia = low CO |
| SBP | 60–80 mmHg | 100–140 mmHg | Min acceptable SBP ~ gestational age in weeks |
| O2 consumption | 6–8 mL/kg/min | 3–4 mL/kg/min | Rapid desaturation during apnea |
| FRC | Low (relative to closing capacity) | Normal | Prone to atelectasis and V/Q mismatch |
| Total body water | 75–80% | 60% | Larger Vd for water-soluble drugs |
| Blood volume | 80 mL/kg (term); 90–100 mL/kg (preterm) | 65–70 mL/kg | Small absolute volumes; MABL must be calculated |
| Hemoglobin type | 60–80% HbF (P50 = 19 mmHg) | HbA (P50 = 27 mmHg) | Left-shifted curve; higher PaO2 needed for same O2 delivery |
| GFR | ~20 mL/min/1.73 m2 | ~120 mL/min/1.73 m2 | Prolonged drug clearance for renally eliminated agents |
| Hepatic enzymes | Immature Phase I/II | Mature | Prolonged drug metabolism; higher free drug fraction |
| Thermoregulation | High SA:volume; minimal subQ fat | Normal | Rapid heat loss; brown fat thermogenesis |
Pharmacologic Implications
MAC for inhalational agents is lower in neonates younger than one month compared to infants, with peak MAC occurring at 1 to 6 months of age. The increased volume of distribution for water-soluble drugs requires higher weight-based doses of certain agents, such as succinylcholine at 2 to 3 mg/kg IV. An immature blood-brain barrier increases sensitivity to opioids and sedatives, and reduced hepatic and renal clearance prolongs the duration of many drugs. Sevoflurane is the inhalational agent of choice for induction. Regarding muscle relaxants, neonates are more sensitive to nondepolarizing agents on a per-receptor basis but may require similar or higher mg/kg doses due to a larger volume of distribution.
Postoperative Considerations
Former preterm infants younger than 60 weeks postconceptual age are at risk for postoperative apnea and should be monitored with continuous pulse oximetry for at least 12 to 24 hours. Pain assessment requires validated neonatal scales such as CRIES, NIPS, or N-PASS. Regional anesthesia techniques, including caudal and spinal blocks, may reduce opioid requirements and apnea risk.
Clinical Pearls
The most critical response to neonatal bradycardia is to ensure adequate oxygenation and ventilation before reaching for atropine. Rapid desaturation during apnea is the rule in neonates, making preoxygenation and efficient intubation technique paramount. Hypothermia is not a benign side effect in neonates; it increases oxygen consumption, prolongs drug metabolism, and can precipitate acidosis and coagulopathy. Maximum allowable blood loss should always be calculated before the procedure begins using the formula MABL = EBV x (starting Hct - minimum Hct) / starting Hct. The left-shifted curve of fetal hemoglobin means that PaO2 must be higher to achieve the same oxygen delivery, and pulse oximetry readings may appear reassuring while tissue oxygen delivery is marginal.
References
- Costarino AT, Davis PJ. Neonatal anesthesia. In: Smith's Anesthesia for Infants and Children. 10th ed. Elsevier; 2022.
- Bissonnette B, et al. Pediatric Anesthesia: Basic Principles, State of the Art, Future. Shelton, CT: PMPH-USA; 2011.
- Davidson AJ, Morton NS, Arnup SJ, et al. Apnea after awake regional and general anesthesia in infants: the General Anesthesia compared to Spinal Anesthesia Study. Anesthesiology. 2015;123(1):38-54.
- Sury MR, et al. The state of UK neonatal anaesthesia: a survey. Br J Anaesth. 2014;113(1):56-64.