Optimized Positioning of ECG Electrodes for WSN Applications - Application and Multidisciplinary Aspects of Wireless Sensor Networks - page 182

Rearranging in the expected form and using the definitions of limb leads we
obtain:

1

( )

aVR = −− 2V +V +V

RA LA LL

2
1

(9.4)

( )

= − V − V +V − V

LA RA LL RA

2

= − (I + I I) / , 2

as expected.

9.2.2 Differential Lead

In the rest of this chapter we will use mainly measurements from the multichannel
ECG, where each channel represents a unipolar lead, because its electrode potential
is referenced to the Wilson's central terminal potential. The differential lead (DL)
is defined as a difference between two MECG leads, which is equivalent to the
potential difference between the two electrodes.
The wireless electrode (WE) is a device composed of two close body surface
electrodes connected by a conducting wire, with appropriate electronics for digitali-
zation and transmission of the measured data. Its measurements are bipolar and can
be emulated by differential leads. The WE can be implemented in an adhesive strip-
like form including electrodes, signal conditioning electronics, radio, and power sup-
ply. In some cases two WEs can share a common electrode, resulting in reduced
number of required electrodes. The WE is constrained significantly by the transmis-
sion range and power consumption [ 9 ] on the one hand, and by the signal-to-noise
ratio, because of the proximity of electrodes [ 10 ], on the other. Several WEs can be
connected in a BWSN which eliminates the need for additional wires and therefore
improves the applicability of the proposed technology [11]. Besides WSN use in clini-
cal ECG devices, the proposed approach is also useful for personal and mobile ECG
devices because of increased mobility and comfort due to the absence of wires [ 12 ].
The number of used WEs should be as small as possible because of limited data
bandwidth and in order to limit power consumption. Most of the eventually redun-
dant information needs to be extracted. We will focus in the following on methods
for reducing the number of WEs and attempt to optimize their positions on the
monitored person.

9.3 Alternative Methods for Positioning ECG Electrodes

Before explaining the different approaches to reducing the number of leads, we
will illustrate some alternative positioning of electrodes currently used in
electrocardiography.