Limb electrodes

Because this is an unsolvable problem, the ideal unipolar lead does not exist. Nonideal solutions are to use one electrode at some remote point (e.g., at a limb or an earlobe). Another solution is to add the voltages picked up by more than one electrode, such as two limb electrodes (augmented leads) or three limb electrodes (Wilson central terminal). 

Figure 10.1 The six limb leads derived from three limb electrodes (both arms and left leg). Here,...

Measurement of the ECG is performed using a differential amplifier. Two limb electrodes at a time are selected for the input to the differential amplifier stage. ECG amplifiers typically have gains of about 1000 and so increase the nominally 1-mV biopotential to about 1 V for driving a strip-chart recorder, cathode-ray-tube display, or computer data-acquisition card. Chart speeds in ECG recording have been standardized at 25 and 50 mm/s. 

Einthoven s electrode placement has been adopted and standardized into the ECG Lead system known as the Einthoven triangle and is shown in Fig. 17.38. Electric potential differences are measured between the three limb electrodes along the line between the electrode placements, and their potentials are called lead I, n, and III such that... 

Other Lead systems are also possible and have been standardized into what is known as the 12-lead system that constitutes the clinical ECG. In addition to leads I, II, and m, there are three others known as the augmented leads aVR, aVL, and aVF. These leads use an electric addition process of the limb-electrode signals to create a virtual signal reference point in the center of the chest known as the Wilson central terminal (WCT). From the WCT reference point, the augmented lead vectors point to the right arm, left arm, and left leg, respectively. 

Step 2 First start by attaching the limb electrodes to the arms and legs. When attaching the electrodes to the legs it helps to place them with the tab facing towards the torso so the cables don t pull (Figs. 1.12 and 1.13). 

The augmented vector lead aVR should be checked after a recording is made. PQRST waves in lead aVR are almost always negatively deflected (point downwards below the baseline). A positive deflection should alert the recorder to the possibility that the cables/leads attached to the limb electrodes could have been applied the wrong way round. This can mimic a condition called dextrocardia and is known as technical dextrocardia . 

The films were then soaked in water and removed from the plates. Portions were mounted in glass cells which were filled with potassium chloride solution two Ag/AgCl electrodes were inserted into the limbs of the cells and the unit was placed in a thermostat. The resistance of the films was determined, from time to time, by connecting the cells in series with a known resistance and applying a potential of 1 V to the combination the potential drop across the standard resistance was measured by means of a valve potentiometer. 

Measurement of muscle activity, usually measured by electrodes placed on the skin. The EMG is used in sleep research to aid in the discrimination of sleep stages, and also as part of diagnosis of sleep disorders such as periodic limb movements and restless legs syndrome. 

In the maximal electric shock (MES) test a supramaximal stimulus is applied bilaterally through corneal or auricular electrodes to induce tonic hind limb extension in rats or... 

Moving-boundary electrophoretic techniques, originally demonstrated by Tiselius in 1937, employ a U-tube with the sample occupying the lower part of the U and the two limbs being carefully filled with a buffered electrolyte so as to maintain sharp boundaries with the sample. Electrodes are immersed in the electrolyte and direct current passed between them. The rate of migration of the sample in the electric field is measured by observing the movement of the boundary as a function of time. For colourless samples, differences in refractive index may be used to detect the boundary. Such moving-boundary techniques are used mainly in either studies of the physical characteristics of molecules or bulk preparative processes. 

The forward half of the CV is identical to a linear-sweep voltammogram. The back half of the CV represents the reverse electron-transfer processes occurring at the working electrode if the peak on the forward limb of the CV represents the oxidation reaction, Fe + -> Fe -I- e, then the reverse limb represents the reduction reaction, Fe e Fe. V/e can gain much information if the peak current of the reverse limb is smaller than the peak current during the forward part of the cycle (see next section). Such information cannot be obtained in a LSV experiment because no reverse limb is traversed. 

Since the 1970s, nerve surrounding electrodes have become the most successful types of implantable biomedical electrodes in clinical use. For example, they have been used for stimulation of the lower limb [12], the urinary bladder [13], and for recording neural signals [14,15]. Some electrodes have been safely in use for over 15 years [16]. They have been investigated intensively and many efforts have been made in respect to spatial selectivity and nerve fiber recruitment. 

If a microsystem should interface a regenerating nerve, some basic demands have to be taken into consideration the system must be absolutely non toxic, it must be stable in the physiologic environment for a long time, and the mechanical load on the severed nerve should be as minimal as possible during muscle contraction and limb movement. The neural prostheses should be equipped with multiple electrodes for recording nerve signals and for stimulation of different portions of the nerve. 

The phenomenon of electro-osmosis can be studied by using a U-tube [fig. (9)] in which a plug of moist clay (a negative sol) is fixed. The two limbs of the tube are filled with water to the same level. The platinum electrodes are dipped in water and potential applied. It is observed that water level rises on the cathode side, while it falls on the anode side. This motion of the medium towards the negative electrode, shows that the charge on the medium is positive. Similarly, for a positively charged sol, electro-osmosis will occur in the reverse direction. 

The electrocardiogram can be obtaining using standard limb leads and/or precordial leads. A lead should be selected that is stable over time and that has a sharp demarcation at the end of the T wave to facilitate the measurement of the QT interval duration. One can also position a monophasic action potential electrode catheter through the femoral or carotid artery to obtain endocardial monophasic action potentials (see below Modification of the Method). 

Six of the 12 leads of the ECG are placed directly on the chest wall. By international convention, these electrodes are placed in predetermined locations and record activity from sites directly over specific parts of the heart. The other six leads are associated with recordings from the limbs. 

If the system contains three constituents with different mobilities, three boundaries will form a rate of movement equal in magnitude, but opposite in direction, to that of the middle boundary is now imparted to the whole liquid. The fastest moving constituent moves ahead, whereas the slowest constituent is given an apparent negative velocity after electrophoresis has proceeded for some time, one limb of the section C contains the former constituent and the other contains the latter in a pure form. Several devices have been employed to impart a movement to the liquid one method is to withdraw gradually, by means of clockwork, a plunger which fits loosely into one of ihe electrode vessels, while another is to keep one electrode vessel closed, e.g., the left-hand one in Fig. 131, and to force buffer solution into it at the desired rate by means of a syringe operated by a constant speed motor. 

A routine ECG is composed of 12 leads. Sis are called limb leads (I, II, III, aVR, aVL, and aVF), because they are recorded between arm and leg electrodes, and six are called precordial or chest leads (Vj, V2, V3, V4, V5, and Vg) and are recorded across the sternum and left precordium. Each lead records the same electrical impulse but in a different position relative to the heart. Areas of pathology shown on the ECG can be localized by analyzing differences between the tracing in question and what is known to be normal in the 12 different leads. 

In emission spectrography35 the sample is dissolved in a suitable solvent, together with a known weight of zinc chloride. The sample is placed in a U-shaped fulgurator, one limb of which consists of a hollow graphite electrode. The test solution slowly passes to the top of this electrode and forms a film during a spark discharge between the end and a copper counter electrode. The lines of silicon (634.701 nm) and zinc (636.235 nm) are measured. 

A number of apparatuses were constructed for this purpose [257-264], some of which are commercially available. Here we describe the apparatus marketed by LKB (Bromma, Sweden) (see also Ref. 264) (Fig. 6.26). The apparatus consists of a inverted U-shaped tube that is composed of two parts connected by a joint in the upper part. One limb accommodates the electrophoresis column that is surrounded by a covering jacket, the other limb filled with buffer serves to make connection with the electrode vessel. At the lower end of the column a collecting funnel is attached. Liquid connection or disconnection between the two limbs can be done through a ground female joint. The stopcock above this joint connects the system to a buffer reservoir when the column is washed or when the zones are eluted. In the male joint of the top piece there is a hole that allows liquid to pass to the other limb. By turning the liquid stream can be disconnected at this point. The column size is 55 X 2 cm Sephadex or cellulose serve as the supporting material. Separation is run at 500-1000 V and 25-50 mA. 

RNS (at 3 Hz with square-wave pulses 0.3 ms in duration) of the facial nerve to the orbicularis oculi and orbicularis nasalis, accessory nerve to the trapezius, and median nerve to the abductor pollicis brevis muscles can be studied. Hand-held bipolar electrodes and adhesive surface electrodes are used for stimulation and recording, respectively. Routine nerve conduction studies (NCS) of all four limbs, including the VII nerve, can also be performed. 

Computer-brain interfaces can work two ways. Cochlear implants have been developed to detect sound with an external microphone and relay the electrical signal to electrode arrays that directly stimulate inner ear nerve fibers. A visual prosthesis promises to similarly help the blind by applying electrical signals from a camera to an array of microelectrodes implanted into the visual cortex of the brain. Electrical signals from the brain can be used to control prosthetic limbs, computer software, or robots. Electrodes implanted into the pleasure centers of the brains of rats have been used to train rats to respond to investigators commands. 

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