fetal distress up to date 2014

1. fetal distressup to date 2014

2. TERMINOLOGY FOR FETAL ACID-BASE DISORDERS • Acidosis — an increase in hydrogen ions in fetal tissue • ●Acidemia — an increase in hydrogen ions in fetal blood. Respiratory acidemia refers to a low pH in the presence of a significantly elevated PCO2 and a normal serum bicarbonate concentration. Metabolic acidemia refers to a low pH with a normal PCO2 and low bicarbonate concentration. A mixed acidemia exists when bicarbonate concentration is low and PCO2 is elevated [6]. • ●Hypoxemia — a decrease in oxygen content in fetal blood • ●Hypoxia — a decrease in oxygenation of fetal tissue. • ●Asphyxia — hypoxia with metabolic acidosis. Newborns with hypoxia severe enough to result in hypoxic ischemic encephalopathy (HIE) will usually exhibit an umbilical artery pH of less than 7.00 (often less than 6.90) and a base deficit greater than or equal to 12 mmol/L

3. Historically, asphyxia was defined by a low one-minute and five-minute Apgar score. This was not a reliable criteria because only 30 to 40 percent of newborns who are depressed ( have low Apgars) at birth are acidotic at delivery, which suggests that the depression is related to factors other than prolonged hypoxia .Both the American College of Obstetricians and Gynecologists and the American Academy of Pediatrics consider use of the Apgar score in defining asphyxia as a misuse of this scoring system . • Physiologically, asphyxia refers to interference or cessation of the respiratory process leading first to retention of CO2 (hypercarbia) and eventually to a significant reduction in oxygenation (hypoxia) and ultimately to metabolic acidemia. Since this process is difficult, if not impossible, to measure in the fetus, the terms acidosis/acidemia and hypoxia are utilized in lieu of asphyxia. The diagnosis of intrapartum fetal asphyxia requires a blood gas and acid-base assessment . • Metabolic alkalosis rarely affects the fetus.

4. Fetal acid-base physiology • The fetus produces both volatile (carbonic acid) and nonvolatile acids (noncarbonic or organic acids).

5. Carbonic acid • The fetus produces carbonic acid (H2CO3) during oxidative metabolism (aerobic glycolysis). Since H2CO3 is formed primarily from CO2 via hydration in the presence of erythrocyte carbonic anhydrase, the formation of carbonic acid is equivalent to CO2 generation .The rate of CO2 production, in turn, is equivalent to fetal oxygen consumption • For the most part, the fetus can handle the amount of carbonic acid produced daily from aerobic metabolism since carbonic acid dissociates to water and CO2, which readily diffuses across the placenta. Diffusion of CO2 across the placenta is facilitated by a lower PCO2 in the mother during pregnancy, secondary to hyperventilation

6. Organic acids • Noncarbonic or organic acids result from fetal anaerobic metabolism, which occurs when placental transfer of oxygen is restricted. Unlike carbonic acid, the organic acids are cleared very slowly across the placenta and therefore accumulate in the fetus. Metabolic acidemia develops when the primary buffer, bicarbonate (HCO3), as well as other buffers decrease to a critical level. The most important organic acids are lactic acid and ketoacids

7. Buffers • The two major buffers are bicarbonate and hemoglobin • Other buffers that play a lesser role include inorganic phosphates, erythrocyte bicarbonate, and albumin • The placenta also plays a significant role in helping to maintain a bicarbonate pool and buffering the fetus against changes in maternal pH or blood gas status. As an example, a study using a perfused human placental model reported acidification of the maternal side of the circulation did not significantly alter fetal acid-base status and there was efflux of bicarbonate from the placenta into the maternal circulation

8. Factors affecting fetal acid-base physiology • Maternal perfusion of the placenta : Preeclampsia chronic hypertension hypotension/hypovolemia cyanotic heart disease obstetric complications placental abruption cord prolapse maternal acid-base balance renal tubular acidosis diabetic ketoacidosis

9. FETAL VERSUS ADULT ACID-BASE PHYSIOLOGY • several notable differences: • fetus depends primarily upon the placenta to act as lungs and, to a lesser degree, kidneys to help compensate for acidemia • Uteroplacental hypoperfusion is the major cause of both respiratory and metabolic acidemia • if uteroplacental perfusion is corrected during the respiratory phase, respiratory acidosis resolves as CO2 is rapidly cleared across the placenta. However, if the pathologic process is protracted, then in addition to excretion of CO2 being impaired, organic acids are produced and cleared very slowly across the placenta so that metabolic acidosis develops . Thus, fetal physiology is characterized by inability to compensate for acidemia by compensatory respiratory or renal responses in the same way and to the same degree as in the adult.

10. Normal values •  The critical pH level that should be used to define normal acid-base status is somewhat controversial. However, there are data to suggest that the cut-off for significant pathologic acidemia is a pH less than 7.00, and may even be a pH of less than 6.90

11. Antepartum assessment • There is no reliable, noninvasive method of determining fetal acid-base profile prior to delivery. Percutaneous umbilical blood sampling (PUBS) can be used to obtain fetal blood to determine fetal acid-base and blood gas values during the antepartum period. Although this technique has been useful in establishing the acid-base profile of fetuses in utero at various gestational ages , its clinical utility is limited because of a high risk of procedure related fetal loss, particularly among fetuses who are compromised, and the need for serial examinations. Therefore, this technique is generally not recommended for antenatal fetal pH assessment.

12. Intrapartum assessment

13. Fetal Scalp Blood Sampling measurements of the pH in capillary scalp blood may help to identify the fetus in serious distress it also emphasized that neither normal nor abnormal scalp pH results have been shown to be predictive of infant outcome.

15. The pH of fetal capillary scalp blood is usually lower than that of umbilical venous blood and approaches that of umbilical arterial blood

16. pH > 7.25--------labor is observed 7.20 >PH> 7.25 -------thepH measurement is repeated within 30 minutes. pH < 7.20-------another scalp blood sample is collected immediately, and the mother is taken to an operating room and prepared for surgery. Delivery is performed promptly if the low pH is confirmed. Otherwise, labor is allowed to continue, and scalp blood samples are repeated periodically

17. The only benefits reported for scalp pH testing are fewer cesarean deliveries for fetal distress

18. Scalp Stimulation scalp stimulation is an alternative to scalp blood sampling. heart rate acceleration in response to pinching of the scalp with an Allis clamp just before obtaining blood was invariably associated with a normal pH. Conversely, failure to provoke acceleration was not uniformly predictive of fetal acidemia

19. Vibroacoustic Stimulation recommended as a substitute for scalp sampling Response to vibroacoustic stimulation is considered normal if a fetal heart rate acceleration of at least 15 bpmfor at least 15 seconds occurs within 15 seconds after the stimulation and with prolonged fetal movement

20. Fetal Pulse Oximetry There were no neonatal benefits or adverse effects associated with fetal pulse oximetry

22. Fetal Electrocardiography The technique requires internal fetal heart monitoring and special equipment to process the fetal ECG. The rationale behind this technology is based on the observation that the mature fetus exposed to hypoxemia develops an elevated ST segment with a progressive rise in T-wave height that can be expressed as a T:QRS ratio

23. that fetal ST-segment waveform analysis was perhaps useful in preventing fetal acidosis and neonatal encephalopathy when standard fetal heart rate monitoring suggested abnormal patterns. Although no randomized trials have yet been performed in the United States, the Maternal-Fetal Medicine UnitsNetwork has one in progress.

25. Intrapartum Doppler Velocimetry this technique was a poor predictor of adverse perinatal outcomes. They concluded that Doppler velocimetry had little if any role in fetal surveillance during labor

26. Assessment at birth • Umbilical cord blood sampling at birth provides an objective method of assessing the fetal/newborn acid-base profile and provides useful information about intrapartum fetal status and obstetrical management. Guidelines for fetal acid-base determination via umbilical cord blood sampling and interpretation of fetal cord blood gas results are discussed separately

27. Meconium in the Amnionic Fluid • Three theories: response to hypoxia normal gastrointestinal tract maturation vagalstimulation

28. it was concluded that the high incidence of meconium observed in the amnionic fluid during labor often represents fetal passage of gastrointestinal contents in conjunction with normal physiological processes

29. Importantly, such acidemia occurs acutely, and therefore meconium aspiration is unpredictable and likely unpreventable. clear amnionic fluid was also a poor predictor

30. many infants with meconium aspiration syndrome have suffere chronic hypoxia before birth Blackwell and associates (2001) foundthat 60 percent of infants diagnosed with meconium aspiration syndrome had umbilical artery blood pH ≥ 7.20, implying that the syndrome was unrelated to the neonatal condition at delivery. Similarly, markers of chronic hypoxia, such as fetal erythropoietin levels and nucleated red blood cell counts in newborn infants, suggest that chronic hypoxia is involved in many meconium aspiration syndrome cases

31. intrapartum suctioning of the oropharynx and nasopharynx? such infants no longer routinely receive intrapartum suctioning because it does not prevent meconium aspiration syndrome if the infant is depressed, the trachea is intubated, and meconium suctioned from beneath the glottis. If the newborn is vigorous, defined as having strong respiratory efforts, good muscle tone, and a heart rate > 100 bpm, then tracheal suction is not necessary and may injure the vocal cords.

32. Diagnosis Because of the above uncertainties, it follows that identification of “fetal distress” based on fetal heart rate patterns is imprecise and controversial.

33. Question • What is the best way ?????