Transcript Document

fetal distress
up to date 2014
TERMINOLOGY FOR
FETAL ACID-BASE
DISORDERS
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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
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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 acidbase assessment .
Metabolic alkalosis rarely affects the fetus.
Fetal acid-base
physiology
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The fetus produces both volatile
(carbonic acid) and nonvolatile acids
(noncarbonic or organic acids).
Carbonic acid
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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
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
Buffers
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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
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
FETAL VERSUS ADULT ACIDBASE PHYSIOLOGY
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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.
Normal values
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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 cutoff for significant pathologic acidemia
is a pH less than 7.00, and may even
be a pH of less than 6.90
Antepartum assessment
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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.
Intrapartum assessment
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.
The pH of fetal capillary scalp blood is
usually lower than that of umbilical
venous blood and approaches that of
umbilical arterial blood
pH > 7.25-------- labor is observed
7.20 >PH > 7.25 -------the pH 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
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The only benefits reported for
scalp pH testing are fewer
cesarean deliveries for fetal
distress
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
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 bpm for at
least 15 seconds occurs within 15
seconds after the stimulation and with
prolonged fetal movement
Fetal Pulse Oximetry
There were no neonatal benefits or
adverse effects associated with fetal
pulse oximetry
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
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
Units Network has one in progress.
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
Assessment at birth
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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
Meconium in the
Amnionic Fluid
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Three theories:
response to hypoxia
normal gastrointestinal tract maturation
vagal stimulation
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
Importantly, such acidemia occurs
acutely, and therefore meconium
aspiration is unpredictable and likely
unpreventable.
clear amnionic fluid was also a poor
predictor
many infants with meconium aspiration
syndrome have suffere chronic hypoxia
before birth
Blackwell and associates (2001) found that 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
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.
Diagnosis
Because of the above uncertainties, it
follows that identification of “fetal
distress” based on fetal
heart rate patterns is imprecise and
controversial.
Question
 What
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