Many different cardiac abnormalities, especially those
that damage the heart muscle itself, often cause part
of the heart to remain partially or totally depolarized
all the time. When this occurs, current flows between
the pathologically depolarized and the normally polarized
areas even between heartbeats. This is called a
current of injury. Note especially that the injured part
of the heart is negative, because this is the part that is
depolarized and emits negative charges into the surrounding
fluids, whereas the remainder of the heart is
neutral or positive polarity.

Some abnormalities that can cause current of injury
are
(1) mechanical trauma, which sometimes makes
the membranes remain so permeable that full repolarization
cannot take place;
(2) infectious processes
that damage the muscle membranes; and
(3) ischemia of local areas of heart muscle caused by local coronary occlusions, which is by far the most common cause of
current of injury in the heart. During ischemia, not
enough nutrients from the coronary blood supply are
available to the heart muscle to maintain normal membrane
polarization.

Systolic injury current:
During electrical systole (QT interval) the ischemic myocardium is less positive than the healthy myocardium (due to less amplitude of the action potential. This causes the electrical current to run from the healthy myocardium (more positive) to the ischemic myocardium. This is known as the systolic injury current. It is reflected in the ECG tracing as ST-segment elevation or depression according to the thickness and location of the ischemic area. If the ischemia affects the subendocardial area then the systolic injury current will be running from epicardium towards the endocardium (i.e. away from the body surface). The result will be ST-segment depression in the ECG leads corresponding to the ischemic territory. If the injuried area is whole thickness (transmural), then the systolic injury current will be running from the neighboring healthy myocardium towards the injured area. The summation vector of the resultant current will be directing outwards and causes ST-segment elevation in the leads representing the affected area.

Diastolic injury current:
The theory of diastolic current of injury is somewhat different. It is based on the fact that the resting membrane potential in the ischemic area is less negative in comparison with the healthy areas. This generates the diastolic injury current during the electrical diastole (TQ-interval). The direction of this current is from the ischemic area towards the healthy area. Thus it causes elevation of the TQ-segment in case of subendocardial infarction and depression of of TQ-segment in transmural infarction. But the TQ-segment is representing the base line for the ECG recording. So the net result will be apparent ST-segment depression and elevation respectively.