Key words
fetus - obstetrics - pregnancy - uterus - ultrasound-color doppler
Abbreviations
PI: pulsatility index
RI: resistance index
TAmax: time-averaged peak velocity
BMI:
body mass index
R: correlation coefficient
Introduction
The volume of the utero-placental blood flow is an important parameter in assessing
normal placental function and fetal development [1]. Using ultrasound color Doppler, it is possible to assess the flow resistance in
the placenta [2]. The pulsatility index (PI) and resistance index (RI) are commonly used to quantify
the ratio of blood flow velocities in the uterine artery and the placenta [3]. High resistance based on high PI values and notches in the uterine arteries are
used as predictors of adverse maternal and fetal outcomes [1]
[4]
[5]
[6]
[7]. It is assumed that a precise measurement of the total blood flow (Q) would be the
best way to assess placental function and well-being of the fetus. Blood flow can
be calculated from the diameter of the vessel and time-averaged maximum velocity (TAmax)
or the mean velocity (TAmean) using the formula 0.5*TAmax (cm/s) * cross section area
of the vessel (CSA) (cm2) * 60. CSA is calculated as π * (diameter/2)2 assuming that the vessels are circular [8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]. It has recently been suggested that color Doppler with the PixelFlux technique
can be used to calculate flow by examining the area and color of each pixel in a vessel
section [16]
[17]. It is a post-processing method in which each pixel is assigned a spatial angle
correction. The spatial angle is calculated by a mathematical algorithm of the PixelFlux
technique. This algorithm uses the shape of the elliptical vessel and its orientation
in the frontal plane to extract the angle of the vessel with the frontal as well with
the sagittal plane. After calculation of the spatial angle, each color pixel is corrected
and the flow volume and the area corresponding to each pixel are calculated. All pixels
during a heart cycle are added, and the total flow is estimated. The PixelFlux software
is a commercial software application based on the recording made by conventional Doppler
and analysed off-line from a two-dimensional video-clip [18].
The aims of the study were to investigate correlations between fetal weight gain and
birth weight with blood flow estimates in the uterine arteries calculated with the
PixelFlux technique and with measurements from TAmax. We also aimed to examine agreement
between estimates using the two methods.
Materials and Methods
We conducted a prospective observational pilot study of women with single pregnancies
who, due to high-risk pregnancies, were referred to the fetal medical center at St.
Olavs Hospital, Trondheim, Norway, from March 2016 to June 2016. They were referred
to have an examination of the pulsatility index (PI) with Doppler ultrasound in the
uterine arteries in gestational week 24–25. An extended examination including fetal
weight estimation and measurements of blood flow in the uterine arteries was performed.
The outcome measures were birth weight, weight gain/day from the ultrasound examination
to delivery and preterm deliveries. All women gave written consent and the local ethics
committee approved the study (Rek Midt 2015/2304).
Transabdominal ultrasound measurements were performed using Voluson E8 ultrasound
equipment (GE Medical Systems, Zipf, Austria) with a 3.5–7.5-MHz three-dimensional
curved multi-frequency transabdominal transducer. All measurements were carried out
by specially trained midwives. Blood flow volume (ml/minute) in the uterine arteries
was first calculated from TAmax ([Fig. 1]). The angle of the ultrasound waves to the vessels was corrected, and kept as close
as possible to zero. The diameter of the vessel was measured in the systole on the
color Doppler image using the method published in other studies [11]
[15]
[19]. The blood flow in the two uterine arteries was summarized.
Fig. 1 Flow calculated from TAmax (time-averaged maximum velocity) using the formula 0.5*TAmax
(cm/s) * cross section area of the vessel (CSA) (cm2) * 60. CSA was calculated as
π * (diameter/2)2.
Thereafter, the blood flow was calculated using the PixelFlux method ([Fig. 2]). The flow volumes of all pixels inside a vessel’s section were added to calculate
the flow volume of the vessel during a heart cycle. [Fig. 3] and Video. 1 illustrate variation during a heart cycle. All the calculations and measurements
were done automatically by the PixelFlux program [18].
Video. 1 Variation in velocity (color) in left uterine artery during a
heart cycle. The color bar is shown to the left.
Fig. 2 PixelFlux is a Doppler ultrasound method. Every pixel represents a spatial angle
corrected velocity, which is coded by a color. All velocities as well as the area
of all pixels are summarized and the total flow is calculated during a full heart
cycle.
Fig. 3 Variation in velocity (color) in left uterine artery during a heart cycle (systole
to left and diastole to right).
Fetal weight in the second trimester was estimated using the mean of three measurements
of the biparietal diameter (BPD) and abdominal circumference (AC). We calculated fetal
weight using the algorithm implemented in eSnurra [20], as recommended by the Norwegian Directory of Health. The birth weight was obtained
immediately after birth. The mean fetal weight gain/day was calculated as the difference
between the birth weight and estimated fetal weight in the second trimester, and thereafter
compared to the remaining days in pregnancy from the ultrasound examination to delivery.
Statistical analyses
Categorical variables were compared using Chi-square test and continuous variables
using Mann-Whitney U-test and linear regression. In the regression analyses we adjusted
for possible confounders such as parity, maternal age, body mass index (BMI) and systolic
blood pressure. Correlation between methods was analyzed using the intra-class correlation
coefficient. If zero was inside the 95% CI of the mean difference between methods,
no bias was assumed. The analysis of inter-method agreement was performed using limits
of agreement as described by Bland and Altman [21]. P<0.05 was considered statistically significant. Statistical analyses were performed
with SPSS Statistics for Mac, v. 23.0 Armonk, NY: IBM Corp.
Results
In all, 60 women were included in the study; 7 of these were excluded due to a suboptimal
insonation angle, 3 because fetal weight estimation was not performed and 3 because
we were missing information about birth weight, leaving 47 women in the study population.
The characteristics of the study population are presented in [Table 1].
Table 1 Characteristics of study population.
|
Median
|
Range
|
|
Maternal characteristics
|
|
Maternal age (years)
|
32
|
21–45
|
|
BMI
|
26
|
20–48
|
|
Systolic blood pressure (mmHg)
|
114
|
90–141
|
|
Diastolic blood pressure (mmHg)
|
70
|
46–91
|
|
Pregnancy characteristics
|
|
Pregnancy duration (days)
|
280
|
186–293
|
|
Characteristics of the newborn
|
|
Weight gain per day (g)
|
26
|
13–37
|
|
Birth weight (g)
|
3445
|
875–5000
|
|
Apgar score 1 min
|
9
|
3–10
|
|
Apgar score 5 min
|
10
|
4–10
|
|
Apgar score 10 min
|
10
|
8–10
|
|
pH in umbilical artery
|
7.24
|
7.06–7.38
|
The mean flow calculated from PixelFlux was 811 ml/minute (median 777, range 209–1988 ml/minute)
and the mean flow using calculation from TAmax was 787 ml/minute (median 710, range
179–2120 ml/minute). The mean difference was 24 ml/minute (95% CI -45 to 94 ml/minute)
and no significant difference between the two methods was observed because the CI
intervals of the mean difference were crossing zero. The agreement between the mean
flows from the two methods is presented as a scatter plot in [Fig. 4] and as a Bland Altman plot in [Fig. 5]. The intra-class correlation coefficient was 0.83 (95% CI 0.72-0.90) and the limits
of agreement were -441 ml/minute (95% CI -558 to -324 ml/minute) to 489 ml/minute
(95% CI 372 to 606 ml/minute). Details are presented in [Fig. 6].
Fig. 4 Association between flow calculated from TAmax and PixelFlux (Regression equation:
y=155+0.78x).
Fig. 5 Bland-Altman plot for inter-method agreement of total uterine artery flow based on
time-averaged peak velocity and the PixelFlux technique. Mean difference in ml/min
(continues line) with 95% confidence interval (separated dots) and 95% limits of agreement
(tight dots) (i. e., mean difference±1.96 SD) are shown.
Fig. 6 Inter-method agreement of mean flow in uterine arteries calculated from TAmax and
from PixelFlux.
We observed a significant correlation between birth weight and the mean flow calculated
with PixelFlux (r=0.41; p<0.01) as presented in [Fig. 7], and between weight gain/day and the flow calculated with PixelFlux (r=0.33; p=0.02)
as presented in [Fig. 8]. The correlations remained significant after adjusting for maternal age, BMI, parity
and systolic blood pressure (p=0.01 and p=0.02, respectively). The correlations between
flow calculated from TAmax and birth weight or weight gain/day are presented in [Table 2]. We did not observe any significant correlation between PI and flow calculated with
PixelFlux (r=-0.03; p=0.83) as illustrated in [Fig. 9], or between PI and RI with birth weight or weight gain/day ([Table 2]). For every 100-mL/min increase in total blood flow calculated from PixelFlux and
from TAmax, there was an increase in birth weight of 70 grams and 62 g, respectively.
Fig. 7 Association between flow calculated from PixelFlux and birthweight. (Regression equation:
y=2890+0.70x).
Fig. 8 Association between flow calculated from PixelFlux and weight-gain/day (Regression
equation: y=23+0.004x).
Fig. 9 Association between flow calculated from PixelFlux and pulsatile index (Regression
equation: y=0.67 - 0.00001x).
Table 2 Correlations between fetal growth and mean of ultrasound parameters from the two
uterine arteries.
|
Correlation to
|
PixelFlux
|
TAmax
|
PI
|
RI
|
|
r
|
p-value
|
r
|
p-value
|
r
|
p-value
|
r
|
p-value
|
|
Birth weight
|
0.42
|
<0.01
|
0.34
|
0.02
|
−0.25
|
0.09
|
−0.23
|
0.13
|
|
Weight gain/day
|
0.33
|
0.02
|
0.29
|
0.05
|
−0.18
|
0.22
|
−0.15
|
0.33
|
TAmax: time-averaged peak velocity; PI: pulsatile index; r: correlation coefficient.
Two women delivered preterm; one in week 26 and one in week 27, both due to spontaneous
contractions. These two women did not have preeclampsia or hypertension, and the fetal
weight was appropriate for gestational age. In these women, the flow calculated with
PixelFlux was 307 and 209 ml/minute, respectively, the flow calculated from TAmax
was 388 and 201 ml/min, respectively, and the mean PI was 0.87 and 0.88, respectively.
Discussion
The main finding in the study was a significant correlation between estimated blood
flow in the uterine arteries using the PixelFlux technique with fetal weight gain/day
and with birth weight. TAmax calculations achieved a significant correlation with
birth weight. We observed a good correlation between the PixelFlux and the TAmax technique
to estimate blood flow in the uterine arteries in pregnancy week 24–25.
The pulsatility index is the preferred variable in clinical practice because it is
easy to measure, and independent of the angle of insonation [22]. A high PI in the uterine arteries during pregnancy reflects high resistance in
the placenta, and a high PI in the second trimester has been associated with increased
risk of fetal growth restriction [2], preeclampsia [1]
[3], placental infarction [7] and adverse fetal outcome [6]. In our study, however, we did not observe any association between PI and fetal
growth. PI and blood flow were not associated, and placental flow might thus be more
important for fetal growth than placental resistance.
Calculation of volume blood flow is desirable, but noninvasive techniques are challenging,
especially in small vessels and in pulsating arteries [19]. Using standard calculations from TAmax, it is assumed that the mean cross-sectional
velocity is 0.5 x TAmax. The constant 0.5 is used to calculate flow in tubes with
a parabolic velocity profile [8].
The PixelFlux method calculates flow through at least one entire heart cycle, and
an automatic spatial angle correction in both the sagittal and frontal plane is implemented
in the computer program [18], assuming circular vessel geometry. The main advantage is that simple 2-dimensional
imaging is sufficient to calculate 3-dimensional flow volumes (so-called “PixelFlux
243” technique). An oblique vessel section is made and a short video clip of the vessel
is recorded. Asymmetric flow distribution as well as the changing vessel area and
flow velocities during the heart cycle are detected automatically by the PixelFlux
technique. However, turbulence is still a problem and it is important to adjust pulse
repetition frequency to avoid aliasing.
The total blood flow volumes in the uterine arteries increase during pregnancy [23]. Previous studies have shown varying blood flow volumes in the second trimester
[11]
[24]
[25]. One reason for the variation in blood flow volumes may be that the vessel diameter
has a great influence on calculated flow based on TAmax or TAmean, and it is difficult
to measure the diameter of the uterine arteries using two-dimensional transabdominal
sonography. It is easier to measure the vessel diameter on color Doppler images. However,
the diameter might be slightly overestimated on Doppler images [11]
[19]. The variation might also be related to the insonation angle when the vessel diameter
is measured.
Acharya et al. compared Doppler measurements of blood flow with actual flow using
an invasive method in sheep and found good correlation [19]. They measured the vessel diameter on power Doppler images during systole. Konje
et al. calculated the total uterine artery flow to be around 500 ml/min in pregnancy
week 20 with increasing flow of 39 mL/min per week from week 20 to 24 [11]. McKelvey et al. found the mean total blood flow in the uterine arteries to be more
than 800 ml/min in pregnancy week 23–26 [15]. These findings correlate well with the findings in our study where we found total
uterine artery blood flow to be around 800 ml/min and the mean gestation length at
the time of the ultrasound examinations was 24 weeks and 4 days. In color Doppler
acquisitions, the insonation angle should be as close as possible to zero degrees
and we measured the vessel diameter on these images. This insonation angle is not
optimal for measuring vessel diameter because the lateral resolution in ultrasound
images is lower than the axial resolution. Thus, we might have overestimated the vessel
diameter. However, our aim was not to calculate absolute flow, but to compare methods
and to investigate correlations between flow and fetal growth.
The strengths of the study were that all women were examined in pregnancy week 24–25
and that one examiner used the PixelFlux technique and another examiner calculated
flow using TAmax. The examiner using PixelFlux was blinded and not informed about
clinical indications or about labor outcomes. The limitations of the study are its
small study population and that it was a pilot study without a power calculation.
It was sometimes challenging to achieve an optimal insonation angle of the uterine
arteries and we had to exclude seven cases due to insufficient insonation angles.
We do not have any gold standard for uterine artery blood flow measurement in humans.
Invasive measurements during pregnancy would be unethical, and animal studies are
necessary for validation of Doppler estimates.
In conclusion, we found significant correlations between estimated blood flow in the
uterine arteries using the PixelFlux technique with fetal weight gain/day and with
birth weight. Estimates from the PixelFlux method and from TAmax showed good agreement.
The PixelFlux method might be a promising tool for predicting pregnancy outcome. However,
new and larger studies are necessary.
