1. Classifying newborns by weight leads to overly optimistic conclusions regarding morbidity and mortality, because intrauterine growth-restricted newborns are developmentally more mature than their nonrestricted counterparts.
2. All major organ systems of extremely low gestational age newborns (ELGANs) are immature; thus the patient deserves meticulous preoperative evaluation and intraoperative care to maintain homeostasis during surgery.
3. Low gestational age newborns are substantially more mature than ELGANs, but still warrant many of the same precautions. In particular, while their more mature respiratory systems may allow for a natural airway preoperatively, they are at high risk for postoperative apnea.
Preterm birth is not a trivial problem in the United States. There are >25,000 preterm births annually and many of these small patients will come to the operating room (OR) during their perinatal course (1). As care of the preterm newborn has improved, survival of these tiny patients has increased substantially, but morbidity within the various gestational age and birth weight groups, while lower, is still a significant problem (2).
1. Infant and neonatal mortality have been dropping in the United States since the 1980s.
2. Not unexpectedly, neonatal mortality increases as birth weight and gestational age decrease.
3. Some of the morbidity suffered by these newborns such as necrotizing enterocolitis (NEC) and intraventricular hemorrhage (IVH) brings them to the OR for surgery and anesthesia.
4. In addition, preterm newborns may be born with the same congenital anomalies as term infants that require urgent or emergent surgery such as intestinal atresia, gastroschisis, omphalocele, and esophageal atresia with or without tracheoesophageal fistula or congenital diaphragmatic hernia.
I. Terminology. There has been a recent change in terminology regarding preterm newborns that is worth reviewing (3).
A. Previously, newborns had been categorized primarily on the basis of birth weight, a system that leads to overly optimistic conclusions regarding mortality and morbidity because a given birth weight category will include intrauterine growth-restricted newborns who are more “mature” than newborns at the same weight who were not growth restricted.
B. Morbidity and mortality track much more closely with gestational age than birth weight, and with improved ultrasonographic techniques gestational age estimates are currently much more accurate than in the past.
C. In recent epidemiologic studies, the term extremely low gestational age newborns (ELGAN) is used instead of the term extremely low birth weight (ELBW).
1. The term ELGAN refers to newborns born at 23 to 27 weeks of gestation, whereas the term ELBW refers to newborns with birth weights <1,000 g.
2. For a rough comparison, a 28-week newborn who was not growth restricted will have a birth weight of 1,100 g.
3. Comparing a study that uses the term ELGAN will likely have less optimistic results in comparison to a study that classifies newborns by birth weight because the ELGAN study will exclude growth-restricted newborns and include only “true” 28-week newborns.
4. Gestational age is defined as the time from the mother’s last menstrual period to birth (see Table 36.1).
5. Preterm newborns and infants present different problems depending on their degree of (im)maturity and whether or not intrauterine growth restriction (IUGR) is present.
TABLE 36.1 Terminology for preterm births
Term | Definition |
Gestational age (GA) | First day of LMP to birth |
Chronologic age (CA) | Age since birth |
Postmenstrual age | GA + CA |
Corrected age | CA − weeks preterm |
AGA | Appropriate for gestational age |
LGA | Large for gestational age |
SGA (IUGR) | Small for gestational age/intrauterine growth restricted, weight <2 SD below mean for gestational age |
LMP, last normal menstrual period; SD, standard deviation.
II. Extremely low gestational age newborns
• Often these smallest patients come to the OR for urgent if not emergent procedures. Despite the immediacy of these situations, it is important to thoroughly evaluate ELGAN before the start of an anesthetic.
• Immaturity of all organ systems is present and to focus solely on the respiratory system to the exclusion of the central nervous system (CNS), cardiovascular, or metabolic status among others can expose these patients to increased morbidity during the procedure and anesthetic.
• Mortality and morbidity in this group are significant and vary substantially even among different gestational ages within the ELGAN group (4).
• Infants born at 23 weeks have a survival from as low as 11% to only 30%, whereas those born at 25 weeks have a reported 56% to 76%.
• Morbidity among survivors is approximately 50% in both categories.
• A report from the Netherlands documents the severity of morbidity in ELGAN (23 to 27 weeks) versus low gestational age (27 to 32 weeks) newborns.
• Newborns born after 27 weeks not only had a much higher survival rate (95% vs. 65%) but also a much lower incidence of abnormal neurologic findings at 2 years of age (21% vs. 46%).
A. Review of systems. Every major organ system is altered in ELGAN.
1. Central nervous system
a. Neuronal migration, neuronal proliferation, and glial cell production are completed before the 27th week of gestation.
(1) Growth of axons and dendrites occurs from 28 weeks of gestation through birth. Programmed cell death, apoptosis, occurs from the 28th week of gestation through the 41st.
(2) There has been much data regarding the effects of various anesthetic agents on apoptosis in various animal models.
b. These ELGANs are susceptible to IVH.
(1) Clinically in the neonatal intensive care unit (NICU), an IVH is heralded by a sudden deterioration without an obvious cause.
(2) The newborn may exhibit hypotension, heart rate instability, color changes, and acidosis. Most IVHs occur within the first week of life.
(3) A newborn may come to the OR having had an IVH, but more importantly the physiologic derangements in the OR may lead to the development of an IVH even for preterm newborns older than 1 week of age.
(4) The severity of IVH is graded I–IV with the more severe grades, III and IV, being associated with significant neurologic disability.
(5) Severe IVH-associated long-term morbidity includes periventricular leukomalacia (PVL), posthemorrhagic hydrocephalus, and neurodevelopmental disabilities.
(6) IVH is a complication of preterm birth.
(7) The germinal matrix, which is prone to hemorrhage in premature newborns, is a site of neuroprogenitor cells that develops early and has essentially resorbed by 35 weeks of gestation (5). In the ELGAN, the germinal matrix is characterized by immature and delicate vasculature (6). Changes in blood flow, blood pressure, and even serum osmolality have all been associated with the development of IVH in preterm newborns.
c. In the OR, swings in blood pressure are difficult to avoid.
(1) Nevertheless, when administering intravenous (IV) fluids to these tiny newborns, it is essential to do so more gently and slowly than with infants or even term newborns.
(2) Passive cerebral perfusion in the preterm infant has been shown in numerous studies, but recent work has shown that cerebral autoregulation begins to develop from as early as gestational week 23 (7).
2. Respiratory system
a. Lung development has been well studied in the fetus and several distinct periods have been identified.
b. During the embryonic period from fertilization through approximately 6 weeks the lung buds off the esophagus, and develops into the mainstem bronchi then segmental divisions of the bronchi.
(1) Vascular development follows the development of the conducting airways.
c. The next stage, called the pseudoglandular stage, extends from approximately the 8th week of fetal life through the 17th week.
(1) During this period, branching of the airways continues through to the level of the future alveolar ducts.
(2) The pulmonary arteries and veins develop along with the airway branching.
d. From week 17 to 25, the canalicular stage, the lung nears the time at which viability is possible.
e. The saccular and alveolar stages extend from the earliest limits of viability, approximately 25 weeks through to term.
f. From 25 weeks to term, there is more than a four-fold increase in both lung volume and the potential surface area for gas exchange (8).
g. There is a wide variation in lung development at any gestational age so that the effectiveness of lung function is difficult to predict.
h. Nearly all ELGAN will have been treated with surfactant, either in the delivery room or shortly after admission to the NICU.
(1) These newborns will also require some sort of respiratory support.
(a) Currently, in an effort to minimize barotrauma and air leak, neonatologists ventilate ELGAN newborns gently, tolerating modest degrees of hypercarbia and SpO2 in the 85% range.
(b) If an air leak does develop, the peak inspiratory pressure (PIP) is adjusted downward and an even higher PaCO2 is tolerated.
(c) With these parameters, PIPs are generally kept in the 14 to 18 cm H2O range, positive end expiratory pressure (PEEP) started at 4 to 5 cm H2O, and the inspiratory time started at 0.3 to 0.4 second. The respiratory rate must be relatively high with such settings in order to achieve satisfactory minute ventilation.
(d) Measured exhaled tidal volume with PIP in the 14 to 18 cm H2O is approximately 4 to 6 mL per kg.
(2) There is some evidence that ventilation of these newborns with lower PIP and higher PaCO2 may lead to a lower incidence of bronchopulmonary dysplasia (BPD) and chronic lung disease (CLD) (9).
(3) There is also evidence that nasal CPAP is a reasonable alternative to intubation (10,11)
CLINICAL PEARL If an ELGAN requires anesthesia, it is unlikely that the ventilator on the anesthesia machine will be adequate to safely provide mechanical ventilation. It is prudent to bring the ventilator used in the NICU to the OR and continue the same setting used there until the situation in the OR is such that alterations are necessary.
3. Cardiovascular system
a. The definition of a normal systemic blood pressure is problematic in the extremely low gestational age newborn (12).
CLINICAL PEARL A safe “rule of thumb” is to treat the mean arterial blood pressure if it is below the gestational age + 5 mm Hg.
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