© 2012 by the American Institute of Ultrasound in Medicine
More
than a decade ago, Mari et al1,2
achieved a major breakthrough in the treatment of Rh-sensitized fetuses with
their pioneering work that showed a correlation between the Doppler middle
cerebral artery peak systolic velocity (PSV) and fetal hemoglobin levels. This
technique has virtually eliminated the need for invasive procedures such as
amniocentesis and cordocentesis that have been used for diagnosis of fetal
anemia with their inherent complications. Since then, the middle cerebral
artery PSV has been the standard of care for treatment of anemic fetuses.
Doppler studies have also been used in neonates with different cerebral
conditions (eg, intraventricular hemorrhage, brain lesions, and hydrocephalus).3–6
However, this approach has not been attempted in neonates suspected to have
blood volume disorders such as anemia and polycythemia. If a correlation
between neonatal hemoglobin levels and the middle cerebral artery PSV is found,
it may be similarly applied for rapid, noninvasive bedside diagnosis of acute
life-threatening conditions in neonates until the standard blood tests can be
performed.
The
aims of this study were therefore to determine whether a correlation exists
between the neonatal middle cerebral artery PSV and hemoglobin levels and to
assess the possibility of implementing this indicator for rapid, noninvasive
diagnosis of blood volume disorders in neonates.
Materials and Methods
Study Population
This prospective
study included 151 healthy neonates, weight appropriate for gestational age,
born at our medical center during a 6-month period. All neonates were delivered
at 37 weeks’ gestation or later with Apgar scores of 7 or higher at 5 minutes.
We excluded all neonates with malformations, intrauterine growth restriction,
perinatal asphyxia, and infections. The local Research Ethics Institutional
Review Committee approved the study, and informed consent was obtained from the
mother of each neonate enrolled in the study. Medical examinations by a senior
pediatrician confirmed that all neonates enrolled in the study were healthy
without dysmorphic features. The neonates were born by spontaneous vaginal
delivery or cesarean delivery. All neonates were prospectively studied on the
second day of life (between 24 and 36 hours after delivery). Anemia was defined
as a hemoglobin level of 13.5 g/dL or less or a hematocrit value 45% or less,
and polycythemia was defined as a hemoglobin level greater than 22 g/dL or a
hematocrit value greater than 65%.7
Doppler Studies
Doppler
examinations of the middle cerebral artery were performed with a Voluson 730
ultrasound system (GE Healthcare, Solingen ,
Germany ) and a
convex transducer (4–8 MHz) in a quiet room. The neonates were sleeping in a
crib without gross body or limb movements and were breathing quietly. The
examinations were performed by using an axial plane on the temporal bone anterior
to the external auditory canal and superior to the zygomatic process,
identifying the middle cerebral artery. Measurements were obtained just distal
to the middle cerebral artery origin from the internal carotid artery. The
angle of insonation was close to 0°, thus obviating the need for angle
correction (Figure 1).
The sample gate was 3 to 4 mm. The total examination time was 1 to 3 minutes.
Five Doppler waves were recorded, and the highest PSV waveform was used for
analysis.
Statistics
An
analysis of variance was performed to evaluate the different variables in the 3
groups studied (normal, anemic, and polycythemic neonates). Multiple comparison
analyses were performed as well to determine whether the variable means were
statistically different from each other. A regression analysis was conducted to
test correlations between hemoglobin levels and middle cerebral artery PSVs in
the whole groups. P < .05 was considered significant.
Results
The
study population included 122 normocythemic, 24 anemic, and 5 polycythemic
neonates. The mean gestational age ± SD of the neonates at delivery was 39 ±
1.5 weeks, with a median Apgar score of 10 at 5 minutes and a mean birth weight
of 3290 ± 446 g.
Table 1 presents
the hemoglobin, hematocrit, and PSV values of the 3 groups. There were
significant differences in the hemoglobin, hematocrit, and PSV values between
the normocythemic neonates and the anemic and polycythemic neonates (P
< .001). Of the 24 anemic neonates, 20 (83%) had a middle cerebral artery
PSV that was higher than the 95% confidence interval (CI) for normocythemic
neonates, and all 5 polycythemic neonates had a PSV that was lower than the 95%
CI for normocythemic neonates.
In Figure 2, the means
and 95% CIs of the middle cerebral artery PSV values in the 3 groups (anemic,
normocythemic, and polycythemic) are shown. Although there are overlapping
values, the means of the 3 groups are significantly different and can be easily
distinguished from each other (P < .01). Figure 3 depicts
the middle cerebral artery PSV according to different hemoglobin levels (with
the means and 95 percent CIs). A clear decrease in the PSV is evident with
increasing hemoglobin levels (P < .01).
In Figure 4, the
middle cerebral artery PSV of the 3 groups combined is depicted with a
third-order polynomial fit regression line. Although there are overlapping
values, the trend is clear (especially at the extremes of the hemoglobin
levels) that the lower the hemoglobin concentration, the higher the PSV and
vice versa. A significant correlation between the PSV and hemoglobin levels was
found (P < .01). It is interesting to note that a plateau exists at
hemoglobin levels considered to be within the normal range (±2 SDs) for
neonates (at ≈13–22 g/dL), but below or above these limits, there are acute
changes in the PSV.
In Figure 5, we show a
vector plot of several anemic neonates who underwent partial exchange
transfusion. The hemoglobin level and middle cerebral artery PSV were obtained
at the bedside before the blood transfusion and 1 hour after the transfusion.
The plot shows the trend of changes in the PSV, and the lines span from the
starting to ending hemoglobin levels. Once more, it is clear that an increasing
hemoglobin level caused an immediate decrease in the PSV. These fetuses with
initial hemoglobin levels of 7.8 to 11.9 g/dL had PSVs of 51 to 144 cm/s, which
rapidly decreased to approximately 32 to 80 cm/s when the hemoglobin levels
increased to the normal range (>13 g/dL).
Discussion
This
study shows that there is a significant correlation between hemoglobin levels
and the middle cerebral artery PSV in neonates. Although overlapping
measurements in the normal range of hemoglobin levels exist, the more severe
degrees of anemia and polycythemia can be readily diagnosed by examining the
middle cerebral artery PSV. Similarly to the well-established technique used in
fetuses, this procedure can also be suitable in neonates for prompt diagnosis
of life-threatening blood volume disorders. Obviously, we do not imply that
this method can replace the traditional direct blood examination. However, it
may be used as an ancillary, rapid means of noninvasively estimating the degree
of anemia or polycythemia in neonates suspected to have blood volume disorders,
thus allowing prompt action. There are several neonatal conditions in which
middle cerebral artery PSV can be rapidly used for diagnosis of anemia and
polycythemia. These include anticipated deliveries of twins affected by
twin-twin transfusion syndrome, neonates affected by Rh and Kell
isoimmunization, thalassemia, parvovirus B19 infection, and hydrops. In
addition, acute intrapartum events such as intracranial hemorrhage, a large
cephalhematoma, and other hemorrhages associated with traumatic instrumental
delivery with loss of a substantial amount of blood or a sudden decrease in
blood volume can be immediately recognized at the bedside when clinical
suspicion dictates. Even critically ill neonates for whom venous access is
difficult (eg, hydropic neonates) can have a prompt diagnosis until blood tests
can be safely performed. The appealing aspect of this technique is that it can
be easily studied and mastered even by novice users in a very short period.
The
underlying pathophysiologic mechanism of increased cardiac output and decreased
blood viscosity in anemic fetuses is valid also for neonates, as shown in this
study. Anemia causes an increase in the cardiac stroke volume, heart rate, and
peripheral resistance and decreased blood viscosity, leading to an increase in
cerebral blood flow to maintain adequate oxygen transport to the brain.8,9
Neonates, similarly to fetuses, obey the same physical laws of flow velocities
in blood vessels.
Polycythemia,
on the other hand, occurs in 2% to 5% of term neonates,10
usually as a compensatory mechanism in intrauterine hypoxia or uncontrolled
diabetic pregnancies with macrosomic neonates or as a result of delayed cord
clamping.11
This condition may lead to hyperviscosity of the blood with altered rheologic
properties and flow disturbances, which can result in impaired perfusion to
multiple organs. This situation can cause neurologic, cardiorespiratory,
gastrointestinal, and renal abnormalities.12–15
Although these symptoms are usually transient, prompt diagnosis and treatment
may be life saving with reversal of the potential damage.16,17
We
found that for the established normal range of hemoglobin levels, the middle
cerebral artery PSV has a plateau, whereas in anemia and polycythemia, the PSV
changes rapidly (increasing and decreasing, respectively). The correlation
between hemoglobin levels and the PSV becomes more pronounced as the severity
of anemia or polycythemia increases (Figure 2).
This factor may be due to the rheologic properties of the blood in neonates.
Flow remains almost constant for a wide range of hemoglobin levels but rapidly
changes as the hemoglobin levels decrease or increase beyond certain limits. It
is interesting to note that the middle cerebral artery PSVs of the term
neonates in this study (Table 1)
were very similar to those reported by Mari et al2
in term fetuses, and anemic fetuses had PSVs similar to those of anemic
neonates.
As to the
question of whether this technique can be implemented in clinical practice, we
have shown several neonates who underwent partial exchange transfusion because
of anemia and were studied with the Doppler middle cerebral artery PSV before
and after the transfusion (Figure 4).
It is evident that normalizing the hemoglobin level rapidly corrects the PSV.
We think that in polycythemic neonates, the contrary occurs as well.
This study
had some limitations. We studied only term neonates 24 to 36 hours after
delivery, examining our hypothesis that the Doppler middle cerebral artery PSV
can be helpful in managing neonatal emergencies occurring in the first days
after delivery (eg, intracerebral bleeding due to traumatic delivery). Because
it has been reported that the PSV progressively changes during the first month
of life,18
the flow velocities may be different later, and caution should be used in
interpreting hemoglobin levels as a function of the middle cerebral artery PSV
in older neonates.
Although
it is appealing to also use this technique in premature neonates to diagnose
acute anemia caused by massive hemorrhage, a caveat should be addressed in this
group as well. The situation may be different in premature neonates in whom the
proportion of hemoglobin F is different, and there may be different rheologic
properties of the blood due to a different elasticity or size of the red blood
cells. This issue should be further studied in the future. An additional factor
that was not controlled for but may potentially alter the middle cerebral
artery PSV is the presence or absence of a patent ductus arteriosus. However,
the impact of the ductus on blood flow to the brain has been reported to be
minimal19;
therefore, we think that this factor may have only a marginal effect on middle
cerebral artery PSV measurements in anemic and polycythemic neonates.
In
conclusion, Doppler measurement of the middle cerebral artery PSV appears to be
helpful for estimating the hemoglobin concentration in neonates and can be used
as a screening tool for diagnosing neonatal anemia and polycythemia. This
technique may allow a rapid, noninvasive determination of the neonatal
hemoglobin level, dictating the urgency of treatment.