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The probability that the V-fib is induced is dependent upon the pathway of the body current. To compare the danger of different current paths through the body, standard IEC 60479-1² defines the heart-current factor F [12] (Eq. 1.1).

(1.1)

Iref is the body current that determines V-fib for the path left hand to feet, and Ih is the fibrillation current for different body paths, as shown in Table 1.1.

Table 1.1 Heart-current factor F for different current paths

Path	Heart-Current Factor F
IEC	Simulated
Left hand–feet	1	1
Hands–feet	1	0.85
Left hand–right hand	0.4	0.75
Right hand–feet	0.8	0.88
Hands–seat	0.7	0.84
Left foot–right foot	0.04	0.01
Left hand–right foot	1.00	1.00
Left hand–left foot	1.00	1.00
Right hand–right foot	0.88	–
Right hand–left foot	0.8	0.89

The larger is the heart-current factor; the more dangerous is the current pathway through the body.

As an example, a current of 225 mA hand-to-hand has the same likelihood of producing ventricular fibrillation as a current of 90 mA left hand-to-both feet. Therefore, the hand-to-hand pathway is less hazardous than the left hand-to-both feet.

The IEC does advise that the published heart-current factors must be considered as a rough estimation of the relative danger of the various current paths with regard to ventricular fibrillation. The IEC formulation of the heart-current factor is, in fact, based on experiments on corpses, animals and volunteers, or data from electrical accidents. Trials on animals produce results whose extrapolation to humans may not always be reliable, due to the obvious anatomical differences. In addition, exhaustive information about electrical accidents may not be available; therefore, it may not be possible to evaluate the magnitude of the body current affecting the injured and have reliable data.

A possible alternative computation for F can be obtained through human phantoms, which are computerized models that allow the numerical simulation [13] of the current pathways through the human body when subjected to external stimuli, as shown in the last column of Table 1.1.

The comparison between the two sets of heart-current factors shows that the IEC may underestimate the magnitude of F for pathways involving the right hand, probably due to the extrapolation of the results of measurements on animals to humans.

The same heart current factors are also applicable to d.c. currents.