Biologic Limits of Viability

Objectives

After completing this article, readers should be able to:

1. Define the gestational range for extremely preterm birth.

2. Explain the difference between possibility and probability in reference to survival following extremely preterm birth.

3. Describe the key lung development required for adequacy of gas exchange.

4. Describe the renal functional limitations for water, sodium, and creatinine homeostasis in the first postnatal week of the extremely preterm infant.

5. List at least one structural and/or functional limitation of at least five organ systems that increases mortality risks for the extremely preterm infant.

Introduction

With the advent of neonatal intensive care in the 1960s, preterm infants born at progressively lower gestational ages began to survive. The progression of the decrement did not follow a recognizable relationship; rather, it was erratic as the knowledge base of neonatal physiology and pathology expanded and new technical capabilities for life support were developed.

The most dramatic increase in survival of extremely preterm birth (<26 completed weeks’ gestation) came with the introduction of surfactant replacement therapy in the late 1980s. The increased use of maternal antenatal corticosteroid therapy for induction of lung maturation with threatened preterm delivery following the National Institute of Child Health and Human Development Consensus Conference recommendations in 1994 also contributed to improved survival. Since that time, the survival of infants of gestational ages as low as 22 weeks’ has been reported. The purpose of this article is to review the current understanding of the biologic factors limiting survival potential, particularly as they inform clinical decision-making.

The Concept of Viability

Viability or survival potential, when used in reference to a fetus, frequently is used to indicate the possibility for a fetus to be live born and capable of living to a specified endpoint of either a designated time, reaching a certain age or a landmark event, admission to a neonatal intensive care unit, …