Rapid screening of Electrocardiograms (ECGs)
For additional details and results, see Chu et al. 2001 and www.MouseSpecifics.com
Prior to these tests, mice were evaluated in a Comprehensive Lab Animal Monitoring System (CLAMS) at the Jackson Laboratory by Dr. Kevin Seburn (data available). Mice were shipped to Boston immediately after CLAMS. After a 3-day acclimation period, mice were subjected to ECG.
ECG recording. ECGs were non-invasively recorded in 17 strains of conscious mice according to a recently published method [Chu]. Briefly, mice were gently removed from their cages and positioned on an AnonyMOUSE ECG recording platform (Mouse Specifics, Inc., Boston, MA), which included an array of gel-coated ECG electrodes (Red Dot; 3M, St. Paul, MN) embedded in the floor of the platform spaced to provide contact between the electrodes and animals' paws [Chu]. The spacing between electrodes was approximately 3 cm. Filter paper, with openings for the electrodes, prevented mouse urine from short-circuiting the signals. The electrodes were connected to an amplifier (HP78901A, Hewlett-Packard, Andover, MA) by a shielded 3-electrode lead set (M1605A Snap, Hewlett-Packard, Andover, MA). Because even modest handling of mice may induce alterations in heart rate [Desai], each mouse was permitted to acclimatize for 10 min prior to collection of baseline data. The signals were digitized with 16-bit precision (DI-220, DATAQ Instruments, Inc., Akron, OH) at a sampling rate of 2500 samples/second. When mice were sitting or otherwise positioned such that the paws were not in contact with three electrodes, the output from the amplifier was discarded. Only data from continuous recordings of 20-30 ECG signals were used in the analyses. Data were transmitted to the www.mousespecifics.com internet site (Mouse Specifics, Inc., Boston, MA) using standard file-transfer protocols for ECG signal analyses by e-MOUSE [Chu].
ECG analyses. Each signal was analyzed using
, an internet-based physiologic waveform analysis portal. e-MOUSETM
incorporates Fourier analyses and linear time-invariant digital filtering of
frequencies below 2 Hz and above 100 Hz to minimize environmental signal
disturbances. The software uses a peak detection algorithm to find the peak of
the R-waves and to calculate heart rate (see Figure 1).
Heart rate variability was calculated
as the mean of the differences between sequential heart rates for the complete
set of ECG signals. Subsequently, determination of 1st and
2nd derivatives and algebraic "if-thens" search the ECG
signals for probable P-wave peaks and onset and termination of QRS complexes.
Because the T-wave is not separate from the QRS in rodent
ECGs [Bestetti,Richards], there have been discrepancies in the definition of the QT interval and
reported values [Mitchell]. In accord with Mitchell et al., we routinely included
the inverted and/or biphasic portions of the T-wave in our calculations of the
QT interval. We defined the end of the T-wave of each signal as the point
where the signal returned to the isoelectric line [Wang,Kirchhoff] [the mean voltage
between the preceding P-wave and QRS interval]. The QT intervals were rate
corrected (QTc) by application of the equation recommended by Mitchell et al.
[Mitchell] for use in mice. The software plots its interpretation of P, Q, R, S, and
T for each beat so that spurious data resulting from unfiltered noise or motion
artifacts may be rejected. e-MOUSE
then calculates the mean of the ECG time intervals for each set of