Methods
Consecutive patients referred to GenesisCare Cardiology clinical testing
facilities in Brisbane, Australia were studied prospectively between
April 2014 and November 2015. Standard Bruce protocol treadmill testing12 with digital gated echocardiography before and
after exercise was performed. Patients requiring dobutamine stress
echocardiography, patients with resting left bundle branch block, those
with a paced rhythm or atrial fibrillation, those with reduced ejection
fractions (EF < 50%), those with significant valvular
regurgitation or stenosis and patients with aortic or mitral valve
replacements or mitral valve repairs were excluded. Indications for the
test included chest pain and dyspnea for investigation.
The echocardiographic images were acquired in the parasternal long axis,
short axis, apical four, two, and three chambers. Ejection fraction was
measured by Simpson’s method. The SV was estimated by the formula SV =
cross sectional area (CSA) x velocity time integral (VTI). The CSA of
the aortic annulus was calculated using the formula 0.785 x
d2, where d is the left ventricular outflow tract
(LVOT) diameter measured in the parasternal long axis plane. The VTI was
obtained by measuring the area under the curve of the LVOT pulsed wave
Doppler tracing in the apical five chamber view, using the leading edge
of the velocity spectrum.8-10 (see Figure 1 & 2) The
CO was calculated by multiplying the VTI by the heart rate at the time
of the tracing.8-11 The resting SV was estimated
during the pre-exercise echocardiography scan. The post-exercise value
was captured after the peak regional wall assessment,7between 60 and 120 seconds after completion of exercise (see Figure 1).
The LVOT diameter was not expected to change post
exercise.10 The change in stroke volume (ΔSV) in
millilitres (ml) was defined as the SV estimated post exertion minus the
resting SV.
Exercise was replicated and quantitated using General Electric medical
grade treadmills using Case systems (Milwaukee, USA). Standard Bruce
protocols were used to produce exercise stress in a controlled and
reproducible environment. Imaging was acquired utilising General
Electric Vivid e9 (Horton, USA) and Vivid 7 (Horton, USA), Siemens
SC2000 and SC2000 Prime (Mountain View, USA) and the Phillips ie33 and
Epic (Best, The Netherlands) echocardiography scanners. Metabolic
equivalents (METs) were used to represent exercise capacity, as per
standard protocols.1,2,12
The echocardiogram was performed by cardiac sonographers with
subspecialty training in stress echocardiography. All tests were
supervised and read by cardiologists with subspecialty training in
stress echocardiography, and an exercise physiologist. Results were then
over-read, standardized and recorded by a single stress echocardiography
specialised cardiologist, blinded to the results and the outcomes.
Ischemic stress tests were defined as those with new regional wall
motion abnormalities in two contiguous segments, or cavity dilatation
with a lack of cardiac augmentation (direct comparison of the pre- and
post-exercise echocardiographic images). Non-ischemic stress
echocardiograms were defined as having no evidence of myocardial
ischemia on the stress echocardiogram (no new regional wall motion
abnormalities, no cavity dilatation, and appropriate augmentation of
cardiac contractility post exercise), with an appropriate augmentation
of left ventricular function, as assessed by echocardiography. A wall
motion score index (WMSI)1 was calculated for all
studies by a single stress echocardiography specialised cardiologist,
blinded to the results and the outcomes. A subset of patients underwent
anatomic testing for coronary obstruction (invasive or CT coronary
angiography), as determined by the treating physician, independent of
this study.
In order to test the hypothesis that SV could be a marker of abnormal
cardiac outcomes, a two-phase study design was devised. An initial
cohort of patients was selected and examined (the derivation cohort), to
test the hypothesis and if valid to then determine the SV cut-off value.
To confirm the concept that the LVOT diameter did not change
significantly post exercise, it was measured before and after peak
exertion in this derivation cohort. The validation cohort, a subsequent
larger group of patients, was then studied to examine the predictive
value of an abnormal SV response based on prognostic outcomes.
Patients’ medical records were reviewed for up to five (5) years after
the baseline SE data collection. Predetermined adverse cardiac endpoints
were angina as determined by the patient’s clinical cardiologist, acute
coronary syndrome, cardiac revascularization (percutaneous intervention
or coronary artery bypass grafting, worsening New York Heart Association
(NYHA) class, a reduction in EF of greater than 10%, and cardiovascular
death. A diagnosis of angina, or worsening NYHA class was determined by
the treating cardiologist at the time of follow-up. This assessment was
made independent to the study, and the treating specialist was blinded
to the results. The time interval to a patient’s first adverse cardiac
event (a composite of the above) was analysed using Cox proportional
hazards regression, adjusted for patient’s age, sex, ejection fraction,
exercise capacity and pre-test Framingham risk,13 at
the time of the stress echocardiogram.
The study design and methodology were reviewed and approved by the
Brisbane GenesisCare Cardiology echocardiographic working group ethics
subcommittee. The research protocol was carried out in accordance with
The Code of Ethics of the World Medical Association (Declaration of
Helsinki). All patients provided written consent.
A multivariable model incorporating the resting and peak change in
stroke volume for the prediction of subsequent composite adverse cardiac
events in adult patients referred for stress echocardiography was
developed.14 Data were exported in Microsoft Excel
format for subsequent statistical analysis using MedCalc Statistical
Software (MedCalc Software, Ostend, Belgium). Demographic and baseline
data were tabulated and summarised using descriptive statistics. No
imputation for missing data was performed and the number of analysed
observations was reported for each summary proportion. Unpaired t-tests
were used for cohort analysis of continuous data for separate
populations. Paired t-tests were utilized to compare continuous data for
dependent variables. The Mann-Whitney test for unpaired samples was used
to analyse the median wall motion score index. Data expression was
presented as the mean plus or minus the standard deviation.
The outcome for time-to-event analyses within each patient was the first
observed element of the study composite outcome of pre-determined
endpoints. Results were presented graphically using Kaplan-Meier curves
assessed by log-rank tests, as well as tabulated unadjusted and adjusted
hazard ratios with 95% confidence interval (CI) returned by Cox
proportional hazard regression models. Variables included in adjusted
Cox models were the gender, age in years at the time of baseline
assessment, ejection fraction percentage, exercise capacity in METs and
pre-test Framingham risk score. The selection of the variables was based
on the baseline characteristic differences between groups. The
proportional hazards assumption was evaluated using scaled Schoenfeld
residuals and visual assessment of log-log plots. The cut-point for SV
was determined using receiver operator curves
(ROC).14,15