Discussion
We evaluated the use of a telemetry-derived ECG in the evaluation of
individuals admitted to the ICU with COVID-19 infection. The population
studied had a high burden of comorbid cardiac disease, and the majority
were treated with hydroxychloroquine and azithromycin, requiring close
cardiac monitoring of QT interval for concern of risk of torsade de
pointes. Our results suggest that telemetry derived ECGs cannot replace
a standard 12-lead ECG, both for interval measurements and morphologic
assessment.
The mean QT or QTc intervals for the entire group as measured on a
12-lead ECG, 7-lead derived ECG, or single-lead tracing were similar,
which is not unexpected given small differences in measurements in a
small population. Though there was a strong linear correlation between
the QT interval as measured on the different tracings, the correlation
was less robust when using the single-lead tracing. The method of using
Pearson correlation only measures the magnitude of a relation between
variables, and not the agreement between two methods. Accordingly, one
would expect a high magnitude of correlation when making measurements of
the same QT intervals. The Bland-Altman method of agreement plots is an
established method for quantifying agreement between two quantitative
measurements. When we evaluated the agreement between measurements of
the QT intervals on a standard 12-lead ECG and telemetry-derived
tracings, a positive bias was found, suggesting that the telemetry
tracings tend to underestimate the “true” QT interval, as defined by
the manually-measured QT on a standard 12-lead ECG. This finding was
also demonstrated when we averaged the individual differences between
the corrected QT intervals on derived leads and the manual 12 lead
measurement, with an underestimation of QT interval by both derived
measurements and a significantly lower value by the single lead method.
We observed differences in measurements often between 25 and 50 msec,
and up to 110 msec when using the single lead tracing.
There was significantly decreased QT dispersion when using the telemetry
derived 7-lead ECG, which is likely explained by fewer leads available
for measurement. This finding may account for the tendency to
underestimate the QT interval when using the telemetry-derived ECGs, and
is consistent with other studies that have attempted to identify
alternative means to monitor patients during the COVID pandemic. When
comparing the use of a handheld ECG device to a standard 12-lead ECG to
assess QT intervals, Cheung, et al. concluded that while QT was similar
when the device was used across multiple positions, interpretation of a
single lead consistently led to underestimation of the QT
interval.10
Expert guidance has suggested that the risk of treatment with
hydroxychloroquine and azithromycin may outweigh the benefit in patients
with a QT interval longer than 500msec.11 Given a mean
baseline QTc interval of 467msec, variance in the measured QT interval
of the magnitude suggested in our cohort may lead to inappropriate
treatment with QT-prolonging medications.
Other studies have suggested that mobile cardiac outpatient telemetry
may be used for QT and arrhythmia monitoring during the COVID
pandemic.12, 13 However, these studies did not compare
data to the standard 12-lead ECG, and our findings are consistent with a
prior study comparing QTc measurements using single lead portable ECG
devices and 12-lead ECG.14 A multi-lead tracing may
allow for increased diagnostic accuracy. Prior comparisons of
telemetry-derived ECGs and 12-lead ECGs also found moderate correlation
between QTc measurements using these modalities, though had conflicting
conclusions as to whether or not the telemetry-derived ECGs offered an
acceptable alternative to the standard ECG.15, 16 None
of these comparisons assessed the ability of these alternative ECG
monitors to assess other ECG parameters such as morphology or ST segment
changes.
In our analysis, there was acceptable agreement in diagnoses among the
different ECG modalities, though with some notable limitations. Although
in general there was agreement in the QRS axis, 3 (9.09%) subjects with
normal axis were incorrectly identified as having a rightward or
northwest axis using the 7-lead derived ECG. This may be attributable to
inconsistent telemetry electrode placement leading to systematic error
in the mathematical derivation.
There was a suggestion of decreased sensitivity for identifying low
voltage QRS complexes on the 7-lead and single lead ECGs, which may be
due to the lack of precordial leads for interpretation. Conversely, a
larger number of subjects were identified as having T wave inversion or
ST segment changes on the 7-lead or single-lead ECG than on the standard
12-lead ECG. This finding is not consistent with prior comparisons of
the ESAI configuration with a standard 12-lead ECG, which found
comparable incidence and location of ST segment changes in the setting
of acute MI.17
Our findings suggest that single lead tracings are quite limited and may
not be an adequate replacement for traditional ECG monitoring given the
inability to define axis or morphology, a high likelihood of
underestimating the QT interval, and overestimation of ST segment
changes. Although utilizing a 7-lead tracing may allow additional
accuracy, there were still significant limitations in interval
measurement and the identification of morphologic changes. We did not
evaluate serial changes on telemetry derived ECGs, however our results
call into question whether changes in QT intervals can be reliably
monitored with telemetry-derived ECGs. Based on our results, it appears
prudent that a baseline 12-lead ECG should be performed, as well as a
new 12-lead after any dose change of QT prolonging drug or significant
clinical event.
Our study has several strengths in its methodology. We rigorously
evaluated the ECG measurements by having two independent readers with
disagreements adjudicated by a third reader. This study was performed in
a real-world setting; we included consecutive admissions to two cardiac
care units and did not exclude any 12-lead ECGs or telemetry ECGs if
they were readable. Our statistical approach used multiple validated
methods for assessing the agreement between our measurements, including
use of the Bland-Altman agreement plots to quantify and visualize
differences between pairs of measurements. While this study was
motivated by a novel use of the ICU telemetry system during the time of
a pandemic, it has broader implications for the use of the telemetry
system as a replacement for routine electrocardiograms in a wide array
of cardiovascular and acute care conditions.
There are several limitations to this study. Though attempts were made
to collect ECG and telemetry strips at close intervals, not all of the
tracings were performed simultaneously, which may lead to minor
differences in morphology and intervals among ECG and telemetry strips.
Additionally, variation in telemetry electrode placement may lead to
error in the mathematical formulas used to derive the telemetry ECG, and
correct electrode placement could not be verified although ICU staff are
well trained in correct lead placement. Finally, the current study only
assesses for differences between ECG parameters at one point in time. We
did not assess serial ECGs in the same patient, thus our results cannot
be extrapolated to the utilization of telemetry-derived ECGs to monitor
for changes in intervals or morphology.