Diagnostic ultrasound ( PDFDrive )

1576 PART V Pediatric Sonography
TABLE 46.1 Factors That Change
Resistive Index (RI)
Factor
High-pass ilter settings
Transducer scanning pressure
Patent ductus arteriosus
Elevated heart rate
Decreased cardiac output
Effect on RI
Increased
Increased
Increased
Decreased
Decreased
FIG. 46.2 Determination of Peak Systolic Flow Velocity (PSV),
End-Diastolic Flow Velocity (EDV) and Resistive Index (RI) Within
an Arterial Branch of the Anterior Arterial Circulation. The RI can
be derived by placing the Doppler cursor at PSV (green cross), and at
EDV, or minimum velocity (blue cross). The RI is calculated as (PSV − EDV)/
PSV.
the future. hus Doppler exposure should be time limited, and
the signal intensity should be maximized by increasing gain and
not transducer power output settings. he ALARA principle (as
low as reasonably achievable) should be followed to limit energy
exposure to tissues.
Doppler Measurements and Factors That
Change Resistive Index
he peak systolic velocity, end-diastolic low velocity, resistive
index, and time-averaged mean low velocity (TAV) are the
most commonly used semiquantitative spectral Doppler measures
for monitoring intracranial hemodynamics (Fig. 46.2).
One should, however, be aware of the various factors that
may modify the measured or calculated RI values, which include
both technical causes and patient comorbidities outside of the
brain 10-12 (Fig. 46.3, Table 46.1).
With increasing ilter settings, lower velocities are not
depicted, resulting in a falsely elevated RI. Transducer pressure
on the anterior fontanelle may transiently increase intracranial
pressure (ICP), which in turn preferentially reduces low during
diastole and increases RI, especially in neonates with impaired
cerebral autoregulation (e.g., secondary to hypoxic ischemic
injury [HII]). In infants with symptomatic patent ductus
arteriosus, resistance to low in the cerebrovascular bed is higher
than pulmonary vascular resistance. his results in shunting of
blood away from the brain during diastole and an elevated
intracranial RI. During tachycardia, the arterial pressure wave
has less time to dissipate before another systolic ejection occurs.
Intracranial RI is artiicially lower because diastolic velocities
are measured at middiastole, when velocities are higher, rather
than during end diastole. Let ventricular dysfunction (decreased
cardiac output) results in a decreased systolic pressure wave,
lowered systolic velocities, and a reduced RI.
he RI is only a weak predictor of cerebrovascular resistance
under most physiologic conditions. Mean blood low velocity
measures are the most informative indices of cerebral blood
low (CBF). Although accurate placement of the sample volume
and angle of insonation are required, a strong correlation has
been demonstrated between mean blood low velocity and changes
in global CBF under a variety of clinical and experimental
conditions. 13-16
NORMAL HEMODYNAMICS
Normal Arterial Blood Flow Patterns
Arterial hemodynamics in the cerebral circulation are afected
by normal maturational events in the healthy newborn. he RI
in the anterior cerebral artery decreases from a mean of 0.78
(range, 0.5-1.0) in preterm infants to a mean of 0.71 (range,
0.6-0.8) in full-term newborns. 2,3,17-19 his trend is associated
with increasing diastolic low velocities and may be related to
peripheral changes in cerebrovascular resistance or to changes
proximal to the site of recording, such as a closing ductus
arteriosus and a diminishing let-to-right shunt. In full-term
infants, RI may also change over the irst few days of life. 2 In a
study of 476 normal newborns weighing over 2500 g at birth,
anterior cerebral artery RI decreased from a mean of 70.6 ± 7
(range, 51-87) to 68.3 ± 6 (range, 51-83) within the irst 24
hours. 20
Table 46.2 provides the range of published peak systolic and
end diastolic velocities and RI values in several intracranial
arteries. Although the range of normal values is broad, no great
variability should be seen in the individual patient. Changes of
more than 50% from baseline values should be considered
abnormal. here are no consistent diferences in instantaneous
blood low velocity among the major branches of the circle of
Willis or between right-sided and let-sided structures. he
measured blood low velocities may, however, vary with pressure
applied to the anterior fontanel. In healthy preterm and term
neonates a tendency toward higher RI values can be observed
with anterior fontanel compression, and a statistically signiicant
increase in RI values may be observed in children with presumed
impaired autoregulation of brain perfusion caused by hydrocephalus
or brain edema related to HII. 11,21