Clinical meaning
The second stage of labor begins when the cervix is fully dilated (10 cm) and ends with the delivery of the infant. During this stage, the laboring patient actively bears down with contractions to push the fetus through the birth canal. Understanding fetal oxygenation during pushing requires knowledge of the uteroplacental oxygen transfer system and the physiologic stresses placed on the fetus during each contraction and pushing effort. Fetal oxygenation depends entirely on maternal blood flow through the uterine arteries to the intervillous spaces of the placenta. Oxygen-rich maternal blood enters the intervillous space, where oxygen diffuses across the placental membrane (syncytiotrophoblast layer) into fetal capillaries within the chorionic villi. Fetal hemoglobin (HbF) has a higher affinity for oxygen than adult hemoglobin (HbA) due to its reduced binding of 2,3-diphosphoglycerate (2,3-DPG), which shifts the oxygen-hemoglobin dissociation curve to the left. This allows the fetus to extract oxygen from maternal blood even at relatively low oxygen partial pressures. During each uterine contraction, the myometrial muscle fibers compress the spiral arteries and reduce or temporarily interrupt blood flow to the intervillous space. A normal contraction lasting 60-90 seconds followed by adequate resting tone (30-60 seconds between contractions) allows sufficient time for the intervillous space to refill with oxygenated maternal blood between contractions. During the second stage, when the patient performs a Valsalva maneuver (closed-glottis, sustained pushing for 10 or more seconds), several additional hemodynamic changes occur that further stress fetal oxygenation. The Valsalva maneuver increases intrathoracic pressure, which decreases venous return to the heart, reduces cardiac output, and can decrease uteroplacental perfusion. Simultaneously, the sustained bearing-down effort compresses the umbilical cord between the fetal presenting part and the bony pelvis, particularly during fetal descent. Head compression during passage through the birth canal stimulates a vagal response, which may cause early decelerations on the fetal heart rate tracing (a benign, symmetric decrease in heart rate that mirrors the contraction pattern). Variable decelerations (abrupt, sharp decreases in heart rate) may occur from intermittent umbilical cord compression. Late decelerations (gradual decreases that begin after the peak of the contraction and do not recover until after the contraction ends) are the most concerning pattern because they indicate uteroplacental insufficiency -- the placenta cannot deliver adequate oxygen during the stress of contractions. Tachysystole, defined as more than 5 contractions in 10 minutes averaged over 30 minutes, is particularly dangerous during the pushing phase because it reduces the recovery time between contractions, preventing adequate replenishment of the intervillous oxygen reserve. Fetal oxygen reserves are limited, and prolonged or excessive pushing without adequate recovery time can lead to progressive fetal acidemia (accumulation of lactic acid from anaerobic metabolism in fetal tissues). The practical nurse must understand two primary pushing techniques. Directed pushing (closed-glottis or Valsalva pushing) involves coaching the patient to take a deep breath, hold it, bear down for 10 seconds, and repeat three times per contraction. While effective for fetal descent, this technique can reduce fetal oxygenation with each prolonged push. Physiologic pushing (open-glottis or spontaneous pushing) allows the patient to follow her body's natural urge to push, using shorter pushing efforts (3-5 seconds) with breathing between efforts. Research suggests that physiologic pushing is associated with better fetal oxygenation, fewer abnormal fetal heart rate patterns, and less maternal exhaustion compared to directed Valsalva pushing. Delayed pushing (laboring down) involves waiting 1-2 hours after full dilation before beginning active pushing, allowing the fetus to descend passively with contractions. This technique may reduce pushing duration, decrease maternal fatigue, and improve fetal oxygenation, particularly in patients with epidural analgesia where the urge to push may be diminished.