Methods
We identified individuals with respiratory failure due to COVID-19
infection who were undergoing telemetry monitoring in the ICU and were
planned to undergo evaluation with a 12-lead ECG. Subjects were included
in the study if they had 12-lead and 7-lead ECGs obtained within a
single 24-hour period and confirmed positive SARS-CoV-2 RT-PCR nasal
swab. The study was approved by the Institutional Review Board at
Columbia University Irving Medical Center.
Each subject received a 12-lead ECG at standard (25mm/sec) speed and
normal voltage, utilizing standard lead placement (GE MAC 5500, GE
Medical Systems, Milwaukee, WI). A 10 second 7-lead ECG and single lead
(lead II) tracing were also obtained from the central telemetry system
(Philips Intellivue, Koninklijke Philips Electronics, Netherlands). This
system uses the EASI System that derives up to 7 leads from 5 electrodes
positioned on the torso, including all 3 limb leads, 3 augmented leads,
and 1 precordial lead (typically lead V1).7
Each 12-lead standard ECG, 7-lead telemetry-derived ECG, and Lead II
tracing was interpreted and manually-measured by 2 independent
cardiologists. Disagreements in reads were adjudicated by an independent
third reader. The qualitative assessment or quantitative measurement
that was agreed upon by at least 2 of the cardiologists was recorded as
the final interpretation. Diagnoses evaluated included underlying
rhythm, QRS axis, cardiac intervals (RR interval, PR interval, QRS
interval, and QT interval), conduction abnormalities, the presence of T
wave or ST segment abnormalities, and the presence of low voltage
complexes. Automated measurements by the ECG computer system for rate
and common intervals were also recorded. The QT interval was measured in
3 consecutive cycles on leads II, III, and the precordial leads (V5, V6
when available) with the maximal interval taken. The end of the T wave
was defined as the tangent from the steepest last limb of the T wave to
its intersection with the baseline. For irregular rhythms, a weighted
average RR interval was calculated.8 QT interval
correction was calculated using commonly-used formulae: Bazett,
Fridericia, Framingham, and Hodges. QT dispersion was calculated as the
difference between the longest and shortest measured QT intervals on
available leads. Significant ST segment changes were considered to be
more than 1mm of elevation or depression. Low voltage was considered any
ECG where the QRS was less than 5mm in all of the limb leads or 10mm in
the precordial leads. Clinical information was obtained from the
electronic medical record including demographics, comorbid conditions,
laboratory parameters, and pertinent therapies.
Statistical analysis was performed using SAS version 9.4 (SAS Institute
Inc., Cary, NC, USA). For continuous variables, means and standard
deviations were calculated. For categorical variables, frequency and
percentage were calculated. The differences in QT duration between the
12-lead ECG and both the derived 7-lead ECG and single-lead telemetry
were calculated using students t-test. Pearson’s correlation
coefficients were calculated between ECG measurement methods and scatter
plots were generated. Bland-Altman analyses were performed to evaluate
agreement between measurement methods.9