Abstract:
INTRODUCTION: Estimates of left atrial size in patients with
suspected cardiac disease play an important role in predicting
prognosis and events, as well as treatment decisions. Two
methods are commonly used to estimate left atrial size: chest
radiography and cardiac ultrasound. This study aims to
determine the test characteristics by comparing the use of
radiographs to cardiac ultrasound (the gold-standard test).
METHODS: Data from patients older than 18 years admitted
to Steve Biko Academic Hospital during 2000–2003 who
had both chest radiographs and cardiac ultrasound were
included in this cross-sectional, retrospective analysis. Chest
radiographs were classified into three quality classes, and
the sub-carinal angle (SCA) and sub-angle distance (SAD)
were measured twice in all available radiographs by two
observers. Intra- and inter-observer variability (three methods)
as well as the predictive value of the carinal angle and
sub-angle distance measurements were determined using
logistic regression (with left atrial enlargement – determined
by ultrasound as comparator).
RESULTS: Data for 159 patients were available (154 cardiac
ultrasounds and 178 chest radiographs). Intra-observer
variability for chest radiograph measurements was low with
almost perfect concordance (p = 0.000). Inter-observer variability
was higher for supine radiographs. Using logistic
regression, a linear model was identified which was statistically
significant only for erect radiographs. While goodnessof-
fit analysis showed that the model fits the data, performance
characteristics were poor, with high sensitivity and low
specificity, and an area under the ROC curve of 0.62–0.63,
depending on type of radiograph and measurement (SCA
or SAD). Linearity in the logit of the dependent variable
was assessed, and found to be present at the extremes of
SCA measurements for the supine radiograph data and in
the first three quartiles for erect radiograph data. A nonlinear
model determined by fractional polynomial analysis
did not perform significantly better than the original linear
model. Cut-off values for the SCA of 72° and 84° (erect and supine radiographs, respectively) were found to give the best
compromise between sensitivity and specificity. The corresponding
cut-off values for SAD were 24.1 and 26.9 mm.
CONCLUSION: Assessment of either SCA or SAD to determine
left atrial size was equivalent and repeatable, both with the
same observer and between two observers (less so for supine
radiographs). While this measure was precise, it was found
not to be very accurate. Therefore, chest radiographs are not
reliable in predicting left atrial enlargement.
