Late decelerations are one of the precarious decelerations among the three types of fetal heart rate decelerations during labor. They are caused by decreased blood flow to the placenta and can signify an impending fetal acidemia.
Late deceleration is defined as a visually apparent, gradual decrease in the fetal heart rate typically following the uterine contraction. The gradual decrease is defined as, from onset to nadir taking 30 seconds or more. A late deceleration typically follows a uterine contraction meaning, the onset, nadir and the return of the deceleration will follow the onset, peak and the return of a uterine contraction.
Typically, late decelerations are shallow, with slow onset and gradual return to normal baseline. The usual cause of the late deceleration is uteroplacental insufficiency.
The primary etiology of a late declaration is found to be uteroplacental insufficiency. Decreased blood flow to the placenta causes a reduced amount of blood and oxygen to the fetus.
Some of the maternal and fetal conditions which can cause late decelerations are maternal dehydration, anemia, hypoxia, hypotension from epidural analgesia, uterine tachysystole, and placental abruption. Any condition which predisposes to decreased uteroplacental blood flow can cause late decelerations.
Late decelerations are relatively common and correlate with uteroplacental insufficiency.
The central pathophysiology behind late deceleration involves uterine contraction constricting blood vessels in the wall of the uterus which decreases blood flow through the intervillous space of the placenta, reducing diffusion of oxygen into fetal capillaries causing decreased fetal PO2. When fetal PO2 decreases, chemoreceptors initiate an autonomic response in the fetus causing intense vasoconstriction with increased blood pressure. The elevated blood pressure is perceived by the baroreceptors which ultimately stimulate the parasympathetic system to decrease the fetal heart rate, causing late deceleration. The whole process reverses after the contraction is completed, with a relaxation of the uterine muscles allowing for increased blood flow to the placenta and resulting in the fetal heart rate returning to normal.
Late declarations are visually apparent, gradual decreases in the fetal heart rate typically following the uterine contraction. A late deceleration usually follows a uterine contraction meaning that the onset, nadir and the return of the deceleration will follow the onset, peak and the recovery of a uterine contraction.
Late decelerations are the most precarious decelerations among all types. Persistent and recurrent late decelerations need immediate, meticulous assessment to evaluate the cause and to rule out fetal acidemia.
Foremost, the entire fetal heart rate tracing requires evaluation, which includes assessing the uterine activity for tachysystole, presence or absence of variability and accelerations. The fetal heart rate tracing categorizes into I, II or III depending upon the criteria as mentioned above. Presence of moderate variability and accelerations rules out acute fetal acidemia. In the absence of variability and accelerations with persistent late decelerations, immediate attention is necessary as there may be ongoing fetal hypoxia resulting in metabolic acidosis. Attention is also necessary to the full assessment of maternal status; this includes continuous monitoring of maternal vital signs, prevention of dehydration and maintaining adequate intravascular volume, evaluation for ongoing bleeding, awareness of recent medication administration, and assessment of effects from epidural anesthesia. Any maternal hypotension should be addressed immediately.
The principal goal of management of late decelerations is to:
After completing a detailed assessment of the etiology of late declaration, immediate intrauterine resuscitative measures are initiated to prevent fetal acidemia and to decrease fetal morbidity and mortality. The resuscitative measures include:
The scenario of recurrent late decelerations can be precarious as they can become hazardous if not promptly evaluated. Evaluating the entire fetal heart tracing and maternal status is essential. Consideration is necessary to the degree of variability and presence, or absence of accelerations and the underlying cause of the deceleration must be addressed immediately. If the late decelerations are recurrent with minimal or no variability and have not improved with intrauterine resuscitative measures, then expeditious steps must be taken towards delivering the fetus.
If recurrent late decelerations with no variability (Category III) persist or not promptly evaluated and treated, this can lead to increased fetal morbidity and mortality. Complications associated with Category III tracings include fetal acidemia with low APGAR scores, low umbilical cord pH, increased risk of neonatal intensive care unit admission after deliver, neonatal encephalopathy, and cerebral palsy.
The management of labor and delivery is usually done by a multidisciplinary team that includes an obstetrician, labor and delivery nurse, and a midwife. During some deliveries, one may note recurrent late declarations which can become hazardous if not promptly evaluated. Evaluating the entire fetal heart tracing and maternal status is critical. The level of variability and presence or absence of accelerations merits strong consideration. Prompt intrauterine resuscitative measures must commence with concurrent correction of the underlying cause of the late deceleration. If the fetal heart tracing is not improved and late decelerations persist, then expeditious steps must be taken towards delivering the fetus. The outcomes for babies treated promptly are excellent. But if there is a delay, the fetus can suffer a severe anoxic injury to the brain. (Level V)
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