The ICH S7B document describes procedures for the evaluation of delayed ventricular repolarization risk (QT increase). Repolarization can be investigated both in vivo and in vitro. Amongst in vitro techniques, perfused isolated heart models offer a number of advantages:
- Relatively easy implementation compared to in vivo models,
- Relatively simple interpretation, since it does not involve parameters such as baroreceptor reflex, protein binding or effects of metabolites,
- Ability to stimulate at multiple frequencies during a single experiment,
- Ability to study spatial repolarization dispersion at various regions of the heart (base and apex for example),
- Ability to study cardiac transmural repolarization dispersion.
Throughput and translatability position this technique between HERG studies and in vivo models (Figure below). Most protocols require electrical heart stimulation. Stimulation can be applied either in the atrium, or in the ventricle if the atrium is removed.
Scoring current and emerging cardiac safety screening technologies in terms of translatability and throughput reveals their respective strengths and weaknesses. Reproduced with the permission from MJ Killeen: Drug-induced arrhythmias and sudden cardiac death: implications for the pharmaceutical industry. Drug Discov Today. 2009 Jun;14(11-12):589-97.
Case of ventricular stimulation
In this example (figure below), ECG and MAP are acquired in the Langendorff-perfused rabbit heart model. Acquisition of ECG is performed by two electrodes placed on the surface of the heart, in a position similar to lead II. Ventricular stimulation artifact is closely followed by ventricular depolarization signal. The ISO10r module analyses the Q peak (yellow) and R peak (red) and S peak (purple), the J point (black), T peak (light blue) and Tend (dark blue). Spontaneous beat is indicated by a green marker.
MAP are measured on different location on the heart surface and analyzed by MAP30a. The following parameters are evaluated: action potential duration (APD), conduction, instability (indicative of beat to beat APD variability), triangulation (indicative of changes of Phase 3 repolarization), and reverse-use dependency (indicating that the APD is more prolonged at slow heart rates).
ECG and MAP analysis in the Langendorff-perfused rabbit heart model with ventricular stimulation
Case of atrial stimulation
When stimulation is atrial, stimulation artifact is followed by atrial depolarization signal (P wave). In the following example, ECG is acquired on a guinea pig’s isolated heart. Perfused pressure signal and left ventricular pressure (LVP) are also acquired and dP/dtmax is indicated with dark blue tags.
To improve analysis, it is recommended to remove the stimulation artifact with the TAR10s module (triggered artifact remover).
ECG morphology can vary quiet dramatically from one data file to another depending on the positioning of the electrodes. Therefore the ECG51a analyzer, which uses a file-specific library of marked beats, is particularly adapted in this case (figure below).
LVP and ECG analysis in the Langendorff-perfused rabbit heart model with atrial stimulation. In the ECG channel the signal (in black) from which the artifact was removed by TAR10s is overlaid on signal with artifact (in red).
Transmural repolarization index (TRI)
This technique allows to identify a relation between prepolarization dispersion (apico-basal and transmural) and proarrhythmic risk in the whole isolated heart.
Transmural Repolarization Indexes (TRI) is acquired using three pins placed in the base, in the apex and in the middle of the heart and each containing 8 electrodes evenly spaced on needles in the ventricle wall’s width.
In the following example acquired on a rabbit, out module TRI10a includes detection of depolarization onset (green cross), peak (blue cross) and end (red cross) of repolarization.
TRI analysis in rabbit. End Repol: Duration is the time difference between repolarization peak and repolarization end