ORIGIN AND SHAPE OF THE OSBORN/J-WAVE IN THE ECG INVESTIGATED WITH A
REALISTIC COMPUTER MODEL OF THE HUMAN HEART

M. Potse and R. Cardinal

Canadian Cardiovascular Congress 2005

ref: M. Potse and R. Cardinal.  Origin and shape of the Osborn/J-wave
in the ECG investigated with a realistic computer model of the human
heart.  Can. J. Cardiol., 21 Suppl. C:128C, 2005.


INTRODUCTION  The so-called "Osborn" or "J" wave in the ECG
appears as a small positive wave directly after the QRS
complex. This wave is usually encountered in hypothermia,
and is believed to be related to the negative notch in the
action potential (AP) of epicardial cells, due to a further
increase in transient outward current (Ito) which is denser
in epicardial cells.


METHODS  Propagating action potentials were computed using a
12-million-node monodomain model of the human heart
including endocardial to epicardial rotation in fiber
direction. Membrane ionic currents at each node were
represented either by an adapted Priebe-Beuckelmann or
Bernus model of the human ventricular myocyte. Both models
distinguish endocardial, epicardial, and M-cells. The models
differ in the expression of Ito, which lacks a fast
inactivation process in Bernus' case. Surface
electrocardiograms were computed from the distribution of
membrane potentials using an inhomogeneous torso model.
Increased levels of conductivity for Ito in epicardial cells
were used to obtain a larger transmural dispersion in this
current.


RESULTS  A two-fold or larger conductivity of epicardial Ito
led to notable differences in action potentials and to
typical J-waves. Although these waves appear at first sight
as mere notches in the ECG, subtraction of normal and
modified ECG showed a long triangular wave extending from 50
to to 250 ms after QRS onset, and rising to 0.5 mV at 90ms
after QRS onset. The Bernus model does not include a fast
inactivation of Ito and therefore does not produce the
typical notch in the epicardial AP. In opposition to current
belief, this did not affect the ECG noticeably. In
particular, Bernus' model produced the same notch-like
J-wave in the ECG.


DISCUSSION  We have shown that J-waves can be explained by
hypothesizing only a change in epicardial Ito
conductivity. However, there is no clear relation between
the negative notch in epicardial AP and the notch-like
J-wave. The wave's appearance as a small notch seems to be
due to the end of the S deflection in the ECG being lifted
up above the baseline. That is, this notch is already
present in the ECG but changes appearance due to the
superposition of the enhanced Ito contribution. This
contribution itself is much longer and higher than the
apparent notch.