Fig 1.
Determination of electrographic intervals from equine ECG recordings and restitution analysis in the temporal and spatial domains.
a) A typical equine ECG trace at a low heart rate (HR) of between 43–45 beats per minute (bpm). The P-wave, QRS-complex, and T-wave are annotated. b) The same trace showing the intervals RR, QRS, TQ and QT that the signal analysis yielded. These gave basic cycle length (BCL), conduction velocity (θ), diastolic interval (DI) and action potential duration (APD) respectively (Horse Eq46: age 7 years, thoroughbred female). c) Temporal patterns of action potentials (AP) over time: with progressively shortened basic cycle lengths (BCL) (i), with (continuous line) or without variations in APD (dashed lines) results in a corresponding dependence of diastolic interval (DI) upon BCL (ii), a unity gradient of the resulting APD(DI) restitution function (dashed tangent) provides an instability criterion relating AP recovery characteristics to alternans (iii). d) Analysis incorporating spatial AP propagation at velocity θ, generates active and resting wavelengths, λ and λ0, each varying with BCL (i and ii respectively). Unity gradients in the derived λ(λ0) plots (iii; dashed tangent) yield instability conditions that additionally incorporate AP propagation contributions. (c and d adapted from [38]).
Fig 2.
Accommodation of ECG waveform to increasing HRs.
ECG traces obtained through a range of low to high HRs covering the normal physiological spectrum encountered in horses. Each trace is displayed as a rhythm strip; the vertical markers on the horizontal line above trace marks the timing of successive R-waves. The range of HRs within each of trace was (a) 40–45 (b) 51–55 (c) 61–62 (d) 73–76 (e) 89–93 (f) 105–108, (g) 118–120 and (h) 140–145 bpm. (Same horse as shown in Fig 1).
Fig 3.
Presence of transient alternans in all horses at both low and high HRs.
(a) Transient alternans recorded at a HR over the range of 103–113 bpm. The trace was taken from an episode of transient alternan, which lasted a total of 14 beats (7.8 secs in duration). (b) A trace recorded from the same horse, in absence of alternans, and over a similar HR, ranging from 105–109 bpm. The QT interval for each PQRST complex is marked below the trace, demonstrating alternations in the QT interval between successive complexes. The vertical markers above each trace mark the timings of consecutive R-waves of each PQRST complex. (c) A histogram showing the number of episodes of transient alternans recorded within each interval HR of 10 bpm (value in brackets is the number of horses from which a HR within that interval was recorded). (Same horse as in Figs 1 and 2).
Fig 4.
Dependence of basic and derived ECG parameters on BCL.
The effect of increasing HR (depicted as a shortening of BCL) on the measured QT interval (a) and QRS duration (b), and on the calculated TQ interval (c), 1/(QRS duration) (d), λ (e), and λ0 (f) values. Data used were obtained from analysis of ECG recordings from the same horse as in Figs 1–3.
Fig 5.
Representative figure for the analysis of QT interval and wavelength restitution plots.
Plots of QT interval (a) and 1/(QRS duration) (b), and λ0 (c) against TQ interval, and λ against λ0 (d).
Fig 6.
Complementary dependencies of λ and λ0 on BCL.
(a)-(f). Plots of λ (i) and λ0 (ii) against BCL in six horses. The solid black line was manually fitted to demonstrate the trend of the data points. The dotted black line shows trends at the higher BCLs only. (a) Eq34 (a 2 year old male thoroughbred), (b) Eq39 (a 3 year old female thoroughbred), (c) Eq44 (a 4 year old female thoroughbred), (d) Eq52 (a 2 year old male thoroughbred), (e) Eq56 (a 2 year old male thoroughbred) and (f) Eq67 (a 3 year old male thoroughbred)).
Fig 7.
Comparison of electrocardiographic parameters at baseline and critical HRs.
(a)–(f) Plots of QT vs. TQ interval ((i)) and lambda vs. lambda0 ((ii)), representing of action potential duration and wavelength restitution respectively, from the same 6 thoroughbred horses as in Fig 6. (a) Eq34 (a 2 year old male thoroughbred), (b) Eq39 (a 3 year old female thoroughbred), (c) Eq44 (a 4 year old female thoroughbred), (d) Eq52 (a 2 year old male thoroughbred), (e) Eq56 (a 2 year old male thoroughbred) and (f) Eq67 (a 3 year old male thoroughbred).
Table 1.
ECG characteristics at baseline and HRcrit, from analysis of QT interval restitution.
Table 2.
ECG characteristics at baseline and HRcrit, from analysis of wavelength restitution.
Table 3.
Comparison of results of QT interval restitution against electrocardiographic parameters at baseline HR.
Table 4.
Comparison of results of wavelength restitution against electrocardiographic parameters at baseline HR.