o_1977r8vv9vk1ts2ms0kd8pksa.pdf

is inherent in its formuation and is reflected in its distribution
characteristics [28] (Fig. 4.16) and its total
variance (Fig. 4.17). Despite these limitations, this index
is used extensively in obstetrics, particularly in
the United States. The RI values, on the other hand,
have defined limits with a minimum value of 0 and a
maximum value of 1.0. Unlike the S/D, the RI shows
gaussian distribution and is therefore amenable to
parametric statistical analyses (Fig. 4.18). The limitation
of RI is due to its inability to reflect impedance increases
with the reversal of end-diastolic flow. Theoretically,
the PI provides more hemodynamic information
than the RI and S/D ratio, as it includes data on
the whole cardiac cycle in the form of its denominator,
which is the time-averaged value of the maximum frequency
shift envelope over one cardiac cycle. Furthermore,
it expresses hemodynamic alterations associated
with absent or reversed end-diastolic flow. In practice,
however, computation of the time-averaged value is not
as precise as determination of the peak systolic or enda
Chapter 4 Spectral Doppler Sonography: Waveform Analysis and Hemodynamic Interpretation 45
4
Doppler Shift Frequency
1
3
2
5
6
7
8
9
10
sociates [26] described yet another technique of comprehensive
waveform analysis that involved a four-parameter
curve-fitting analysis of an averaged waveform.
More recently, MarsÏaÂl reported use of a classification
system based on the PI value and end-diastolic
flow characteristics [27]. This classification system
was superior to other measures of the waveform for
predicting fetal distress and operative delivery due to
fetal distress. Most of these techniques have not been
thoroughly evaluated, and currently there is no evidence
that they offer any advantages over the simpler
Doppler indices.
Choice of Indices
Time
Fig. 4.14. Reference points in a typical velocity envelope
waveform obtained from the umbilical arteries. 1 trough,
2±3 ascending slope, 4 peak, 5±6 initial descending slope,
8±9 final descending slope, 10 trough. (Reprinted from [2]
with permission)
Fig. 4.15. Correlation between the resistance index (RI)
and the systolic/diastolic (S/D) ratio
Of the various indices, the systolic/diastolic (S/D) ratio,
RI, and PI have been used most extensively in
obstetric practice. Of these, the S/D ratio and RI,
being based on the same set of Doppler parameters
(peak systolic and end-diastolic frequency shifts), are
related to each other as shown in Eq. (8) and
Fig. 4.15:
RI ˆ 1
S
D
…9†
As the end-diastolic frequency shift declines the S/D
ratio rises exponentially; and when the end-diastolic
flow disappears the value of the index becomes infinity.
Hence the S/D ratio value becomes meaningless beyond
a certain point as the fetoplacental flow impedance
continues to rise. This behavior of the S/D ratio
Fig. 4.16. Distribution of the umbilical arterial systolic/diastolic
ratio in the study population. Note its nongaussian
distribution. (Based on data from [28])