Comparison of the reproducibility of 2D doppler and 3D STIC in the measurement of fetal cardiac output

doi:10.4236/ojog.2011.14031

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Open Journal of Obstetrics and Gynecology, 2011, 1, 163-167

doi:10.4236/ojog.2011.14031 Published Online December 2011 (http://www.SciRP.org/journal/ojog/ OJOG

Published Online December 2011 in SciRes. http://www.scirp.org/journal/OJOG

Comparison of the reproducibility of 2D doppler and 3D

STIC in the measurement of fetal cardiac output

Rajeswari Parasuraman1, Clive Osmond2, David T. Howe1

1Wessex Maternal and Fetal Medicine Unit, Princess Anne Hospital, Southampton, UK;

2MRC Lifecourse Epidemiology unit, Southampton General Hospital, Southampton, UK.

Email: [email protected]

Received 4 September 2011; revised 25 October 2011; accepted 30 October 2011.

ABSTRACT

Objectives: Two methods have been described to as-

sess fetal cardiac output (CO). It has usually been

calculated by using 2D ultrasound to measure the

diameter of outflow valves and Doppler ultrasound to

measure flow velocity through the valves. Recently

CO has been assessed using 3D spatio-temporal im-

age correlation (STIC) to measure stroke volume. We

aimed to compare the reproducibility of these tech-

niques. Methods: In 27 women with singleton preg-

nancies, examinations were performed in three gesta-

tional age groups: 13 - 15, 19 - 21 and >30 weeks of

gestation. Each mother was scanned once. Using 2D

pulsed wave Doppler the duration of flow and avera-

ge flow velocity in systole were measured through

aortic and pulmonary valves. We averaged values f-

rom three consecutive Doppler complexes. The outlet

valve diameters were measured and the cardiac out-

put was calculated for each valve. The measurements

were repeated to assess reproducibility. In the same

women, we acquired STIC volumes of the fetal heart.

The volume measurements were made using the 3D

Slice method by one observer. Using 2 mm slices the

circumference of the ventricles was traced at the end

of systole and diastole to calculate ventricular volume

before and after contractions to calculate stroke vo-

lume and hence cardiac output. The measurements

were repeated to assess reproducibility. Results: The

root mean square difference of log (CO) of repeat

measurements ranged between 0.12 and 0.21 using

Doppler compared to 0.7 to 1.47 using STIC. The

differences in reproducibility reached statistical sig-

nificance for both sides of the heart at all but one

gestation. Conclusions: We found that Doppler asse-

ssment of fetal cardiac output was more reproducible

than measurement using STIC.

Keywords: Cardiac Output; Fetal; STIC; 2D Doppler;

Ultrasound

1. INTRODUCTION

Fetal cardiac output has been measured by various tech-

niques using both 2D Doppler [1-3] and 3D ultrasono-

graphy [3-11]. Cardiac outflow has been estimated by

using Doppler ultrasound to measure the average flow

velocity across the aortic and pulmonary valves and 2D

ultrasound to measure the valve diameters [3].

After the advent of 3D ultrasonography, Hamill et al

suggested it might have advantages over 2D assessment

of cardiac output. Using a single acquired volume ana-

lysed with STIC (spatio-temporal image correlation) and

VOCAL (virtual organ computer aided analysis) they

assessed ventricular volume at end diastole and end-sys-

tole to calculate stroke volume. They felt that these volu-

me assessments would be more accurate than using 2D

methods such as multiplanar assessment and Simpson’s

rule. They felt it might also avoid the variability that

occurs with 2D measurements due to small changes in

measurement of valvular diameter having a large effect

on calculated cardiac output [6].

Uitttenbogaard et al [10] recently described calculation

of cardiac output using 3D spatio-temporal image corre-

lation (STIC) to provide cross-sectional slices of the

heart based on Simpson’s rule [12]. They provided refe-

rence ranges of cardiac output from 12 weeks until term

in a longitudinal study of 63 fetuses and argued that be-

cause of the potential for errors in conventional 2D Do-

ppler assessment of cardiac output calculations from

STIC volumes were likely to become the method of choi-

ce for assessment of fetal cardiac function. Their study

did not compare their method with Doppler assessment

in the same fetuses so we aimed to compare directly the

reproducibility of these techniques from 13 weeks until

term.

2. MATERIALS AND METHODS

This was a cross-sectional observation study of women

undergoing normal singleton pregnancy taking part in a

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164

larger study of fetal cardiac function [13]. When women

attended the ultrasound department for routine dating or

anomaly scans they were invited to take part in the study

by coming for an additional research scan. In 27 women

with singleton pregnancies, examinations were perfor-

med in three gestational age groups: 13 to 15, 19 to 21

and more than 30 weeks of gestation. The scans were

arranged so that each woman attended for a single asse-

ssment of fetal cardiac function. The study was appro-

ved by the local research ethics committee.

Two-dimensional fetal echocardiography was perfor-

med using a Toshiba Aplio ultrasound machine. Exami-

nation of blood flow was made by Doppler assessment

of the duration and average flow velocity in systole th-

rough the aortic and pulmonary valves. In order to make

the measurements, the Doppler gate was reduced to its

minimum size (1 mm) and placed centrally just beyond

the outlet valve. The valves ‘clicks’ are evident in the

Doppler flow and help to defin e the opening and closing

of the valves. The ultrasound probe was placed so that

the beam was as near to parallel as possible with the

direction of blood flow and the offset was always less

than 30˚.. The machine settings were used to correct for

the angle of insonation if the beam was not truly parallel.

There is a single peak of flow through the aortic and

pulmonary valves, during cardiac systole and we mea-

sured the average velo city of systolic flow and th e length

of the ejection time (tsyst) as shown in Figure 1. For each

measurement we used the aver age values from three con-

secutive Doppler com plexes.

To measure the outlet valve diameter, the ultrasound

beam was placed perpendicular to the valve to define the

vessel borders clearly and the diameter measured three

times between the valve leaflets, with the average value

used for the calculations. The cross-sectional area of the

outlet valve was calculated from the average measured

diameter.

The stroke volume was calculated as the time avera-

ged velocity × output time × cross-sectional area of the

outlet valve, and the cardiac output as the stroke volume

× heart rate.

In the same women, we acquired STIC volumes of the

fetal heart using a Voluson 730 (GE Medical Systems)

ultrasound machine. The mean number of STIC volumes

for each fetus was 5 with a range from 3 to 7. The STIC

acquisition time was 7.5 seconds. A short acquisition ti-

me was used to limit degradation of the volumes due to

fetal movement during capture. The volumes were stored

and the clearest two analysed by one observer using the

3D Slice method described by Uittenbogaard with the

GE post processing software [10]. We used multiple 2

mm slices (Figure 2) and calculated the volume by a

sum of the areas traced each multiplied by the thickness

(2 mm). The circumference of the ventricles was traced

Figure 1. Doppler waveform of an outlet valve showing

measurement of the length of the e jection time t(syst).

Figure 2. Images of 2 mm slices obtained from STIC volume.

R. Parasuraman et al. / Open Journal of Obstetrics and Gynecology 1 (2011) 163-167 165

at the end of systole (just before opening of the atrio-

ventricular valves) and the end of diastole (just after

closure of the atrio-ventricular valves) to calculate ven-

tricular volume before and after contractions to calculate

stroke volume and hence cardiac output. We followed

Uittenbogaard’s technique by tracing ventricular con-

tours on the echogenic side of the endocardial border

and considered papillary muscles to be part of the ven-

tricular cavity. The measurements were repeated on a se-

parately acquired STIC volume to assess reproducibility.

3. RESULTS

We included 10 women in the 13 week - 15 week gesta-

tional period, 8 women between 19 week - 21 weeks and

9 women after 30 weeks of gestation. All fetuses had

biometric measurements within the normal range for ge-

station at the time of the scan. For each gestation and te-

chnique.

We calculated the root means square of the differences

in repeat measurements and these are shown in Tabl e 1 .

Using 2D echocardiography, the root mean square dif-

ference (rms) of log (cardiac output) across the gesta-

tional age group 13 to 15 weeks was 0.18 on the left and

0.17 on the right side. The rms across the 19 to 21 weeks

group was 0.16 on the lef t and 0.21 on the right whereas

the respective values in the >30 weeks gestation group

were 0.12 and 0.14 using 2D echocardiography. Doppler

measurements were achieved in 26 of the 27 fetuses e-

xamined.

The rms for the STIC method at 13 to 15 weeks was

0.70 for the left ventricle and 1.07 for the right. on the

left and right sides in the 13 to 15 weeks gestation group,

At 19 to 21 weeks it was 1.47 and 1.39 and at >30 weeks

it was 0.62 and 0.9 for the left and right ventricles re-

spectively. Volume measurements were not possible with

STIC in 2 fetuses (8%), one in the 13 to 15 week gesta-

tional group and the other in the 19 to 21 week group as

the clarity of the views in the captured volumes were

suboptimal.

We found that Doppler assessment of fetal cardiac

output was more reproducible than measurement using

STIC as the rms values are much higher in the latter

method. We demonstrated that this reached statistical si-

gnificance (p < 0.05) in all but one of the gestation

groups on both left and right sides of the heart (Table 1).

Between 13 weeks - 15 weeks the probability approa-

ched statistical significance for the right side of the heart

at the 5% level (p = 0.08) and achieved significance for

left heart measurements.

4. DISCUSSION

Rizzo et al. determined the stroke volume of normal fetu-

ses at 20 w eeks - 22 we eks and 28 w eek s - 32 we eks a nd

growth restricted fetuses at 26 weeks - 34 weeks of ges-

tation and reported that there is a good agreement be-

tween 2D Doppler and 4D STIC measurements. They

obtained the outlet valve measurements between the

open aortic and pulmonary valves. They calculated the

time velocity integral by placing the Doppler gate distal

to the outlet valve leaflets with an angle of insonation <

20 and used recordings from six consecutive waveforms.

They reported that the 4D STIC method was less ope-

rator dependent and was more time efficient than the 2D

Doppler but they also noted that fetal movement and

reduced amniotic fluid might affect the quality of the

cardiac volume acquisition [3].

Uittenbogaard et al. [14] reported fetal cardiac volu-

metry in both in-vivo an d in-vitro settings. They used the

volume datasets from fetuses between 16 and 30 weeks

of gestation and a miniature balloon model. Volume cal-

culations were performed using the 3D Slice method and

they reported that measurement errors in the fetus de-

creased with increasing operator experience and that the

reliability was better for stroke volume than for ejection

fraction. They noted that there was variation in measu-

rement of the end-systolic and end-diastolic volumes

which became more noticeable in calculating the ejec-

tion fraction as it included three volume measurements.

Measurement of the volume of the fetal cardiac ven-

tricle has been described by Messing et al. [7] using 4D

STIC combined with the inversion mode after 20 weeks

of gestation. They used this to calculate stroke volume

and ejection fraction and reported that their intra and

Table 1. Root mean square (rms) of repeat m easurements of cardiac output in right and left ventricles assessed by 2D Doppler or 3D STIC.

Right ventricle Left ventricle

Gestation 2D Doppler (rms) 3D STIC (rms)probability (p value)2D Doppler (rms) STIC (rms) probability (p value)

13-15 weeks (n = 10) 0.17 1.07 0.083 0.18 0.7 0.003

19-21 weeks (n = 8) 0.21 1.39 0.001 0.16 1.47 0.001

>30 weeks (n = 9) 0.14 0.9 0.000 0.12 0.62 0.000

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166

interobserver agreement reached 96%. Their method was

not suited to use in early gestation since it was not pos-

sible to use their method in fetal heart volumes under 20

weeks of gestation whereas fetuses >37 weeks of gesta-

tion posed different problems of unfavourable fetal posi-

tion and relative oligohydramnios.

Molina et al. [8] have established reference intervals

for stroke volume and cardiac output using the VOCAL

technique in pregnancies at 12 weeks - 34 weeks of ges-

tation. Their values for cardiac output were lower than

previous reports which used 2D Doppler techniques. They

also reported that the reproducibility of volume measu-

rements is poor in the first trimester and in late gestation

when the myocardium appears thicker. Similar tech-

niques using VOCAL have been described by Simioni et

al. [11] from 20 weeks - 34 weeks of gestation with the

intra- and interobserver agreement reaching 95%.

Ventricular volume, stroke volume, cardiac output and

ejection fraction have been evaluated from 19 to 42

weeks of gestation by Hamill et al. [6] with 4D ultra-

sound using both STIC and VOCAL techniques. They

commented on the limitations of using a computer gene-

rated dataset, the learning curve involved and the sig-

nificant amount of time needed to analyse the data.

When compared directly we found that cardiac output

calculation by 2D Doppler was more reproducible than

the 4D STIC slice method described by Uittenbogaard

[10]. In the latter method, we found the volumes were

difficult to acquire and analyse at early and late gesta-

tions for the same reason s mentio n ed in previou s stud ies.

Fetal movement artefacts affected measurements in early

pregnancy (1 3 weeks - 15 w eeks subgroup) and fetal po -

sition caused shadowing of the heart after 30 weeks.

Uittenbogaard et al. [10] were only able to acquire STIC

volumes in 71% of women attempted and they exclu-

ded fetuses examined after 30 weeks as they only had

three technically acceptable datasets. In addition, the

process was time consuming and accurate tracing of the

ventricular borders was difficult introducing errors into

calculation of stroke volume and hence cardiac output.

Our data do not support their suggestion that their tech-

nique will become the method of choice for assessing

fetal cardiac function.

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