Screening for QTc prolongation during acute exercise in childhood acute lymphoblastic leukemia survivors

Funding Acknowledgements

Type of funding sources: Public grant(s) – National budget only. Main funding source(s): Institute of Cancer Research (ICR) of the Canadian Institutes ofHealth Research (CIHR), C17 Council

Background

Anthracycline-related cardiotoxicity is a major cause of mortality and morbidity in childhood acute lymphoblastic leukaemia (ALL) survivors. Electrophysiologic complications and cardiac autonomic dysfunction are both known to be developed by childhood ALL survivors. Current methods for detection of cardiotoxicity have limitations, particularly due to their lack of sensitivity for early detection of subclinical cardiac dysfunction. Early detection of cardiac dysfunction remains a cardiologist’s challenge and is essential to allow optimal therapeutic intervention.

Purpose

This study aimed to observe ventricular repolarization during a maximal cardiopulmonary exercise test (CPET) in childhood ALL survivors. We hypothesized that cancer treatments lead to changes in ventricular repolarization that persist over time, and that the use of CPET allows the unmasking of electrophysiological abnormalities.

Methods

A total of 250 childhood ALL survivors underwent a maximal CPET on an ergocycle, and their direct oxygen uptake was measured. All survivors were monitored continuously during the test using a 12-lead electrocardiogram. Measurements of the QT interval were completed at rest, at the end of each stage of the CPET, and during recovery. The QT interval was defined as the period from the onset of the Q-wave to the end of the T-wave, measured linearly. Values were corrected (QTc) using a specific group equation. To compare the effect of cardiorespiratory fitness on QTc during CPET, participants were divided in two groups according to the median of survivors’ cardiorespiratory fitness (group A: <32.0 mL.kg-1.min-1; group B: ≥32.0 mL.kg-1.min-1).

Results

All survivors (median age: 21 years, 51.5% male) included in the final analysis (n=200) performed a validated maximal CPET. At rest, the QTc interval was 379.1±32.2ms. None of the participants had a prolonged QTc during exercise (371.5±16.1ms, range 310.7-416.1ms). The mean QTc interval during CPET was not different between groups A and B (370.1±17.6ms and 373.0±14.4ms, p=0.218). Group A had a longer QT interval at low to moderate exercise intensities. During recovery, the QTc interval was 373.4.1±16.2ms.

Conclusions

Cancer and anthracycline treatments have an impact on the cardiorespiratory system. Low cardiorespiratory fitness in childhood ALL survivors is associated with longer ventricular repolarization during exercise. These differences may be an indicator of altered cardiac function. This shows the importance of studying the response to exercise to improve early cardiac dysfunction detection, as well as documenting the autonomic nervous system response to exercise between survivors with lower and higher physical fitness.

Fully automated initiation of simulated episodes of atrial arrhythmias

AIMS

To develop computational tools for automatically initiating a large number of independent episodes of atrial arrhythmias in electro-anatomical computer models of the atria and therefore facilitating the design of in silico experiments.

METHODS AND RESULTS

A biophysical model of the atria was constructed from segmented medical images of the human atria of a patient with atrial fibrillation (AF). A set of 40 initial conditions were generated based on a priori knowledge about wavefront propagation and the number and location of reentries (1-6 randomly distributed over the atrial epicardium). Simulations were run from each of these initial conditions in three substrates representing different forms of AF dynamics (stable rotors; multiple unstable meandering wavelets; and wavelets broken by repolarization heterogeneities). To demonstrate the applicability of the initiation method for testing clinical of therapeutic interventions, the channel I(Kr) was blocked after 2 s of simulation and its effect on the number of functional reentries was documented. The use of pre-computed initial conditions enabled to successfully generate episodes of simulated AF in each substrate. Blockade of I(Kr) channel prolonged action potential duration, resulting in a reduction of the number of functional reentries. In the substrate with unstable spiral waves, the effect was sufficiently large to terminate AF in about two-thirds of the cases. In the two other substrates, the effect was minor.

CONCLUSION

These new simulation tools may help investigate in computer models therapeutic interventions in different substrates in order to identify substrate-specific optimal therapy.

Evaluation of ablation patterns using a biophysical model of atrial fibrillation

Atrial fibrillation (AF) is the most common form of cardiac arrhythmia. Surgical/Radiofrequency (RF) ablation is a therapeutic procedure that consists of creating lines of conduction block to interrupt AF. The present study evaluated 13 different ablation patterns by means of a biophysical model of the human atria. In this model, ablation lines were abruptly applied transmurally during simulated sustained AF, and success rate, time to AF termination and average beat-to-beat interval were documented. The gold standard Cox's Maze III procedure was taken as reference. The effectiveness of twelve less invasive patterns was compared to it. In some of these incomplete lines (entailing a gap) were simulated. Finally, the computer simulations were compared to clinical data. The results show that the model reproduces observations made in vivo: (1) the Maze III is the most efficient ablation procedure; (2) less invasive patterns should include lines in both right and left atrium; (3) incomplete ablation lines between the pulmonary veins and the mitral valve annulus lead to uncommon flutter; (4) computer simulations of incomplete lines are consistent with clinical results of non-transumural RF ablation. Biophysical modeling may therefore be considered as a useful tool for understanding the mechanisms underlying AF therapies.

Study of atrial arrhythmias in a computer model based on magnetic resonance images of human atria

The maintenance of multiple wavelets appears to be a consistent feature of atrial fibrillation (AF). In this paper, we investigate possible mechanisms of initiation and perpetuation of multiple wavelets in a computer model of AF. We developed a simplified model of human atria that uses an ionic-based membrane model and whose geometry is derived from a segmented magnetic resonance imaging data set. The three-dimensional surface has a realistic size and includes obstacles corresponding to the location of major vessels and valves, but it does not take into account anisotropy. The main advantage of this approach is its ability to simulate long duration arrhythmias (up to 40 s). Clinically relevant initiation protocols, such as single-site burst pacing, were used. The dynamics of simulated AF were investigated in models with different action potential durations and restitution properties, controlled by the conductance of the slow inward current in a modified Luo-Rudy model. The simulation studies show that (1) single-site burst pacing protocol can be used to induce wave breaks even in tissue with uniform membrane properties, (2) the restitution-based wave breaks in an atrial model with realistic size and conduction velocities are transient, and (3) a significant reduction in action potential duration (even with apparently flat restitution) increases the duration of AF. (c) 2002 American Institute of Physics.