If a current of sufficient amplitude is passed between a pair of surface electrodes then muscles will contract. They contract because a stimulus is being introduced to the nerve fibers, which supply the muscles. If we use a short pulse of current, a graph can be drawn in which the current needed to cause stimulation is plotted against the duration of the current pulse. This graph shows that, unless an excessively high current is to be applied, a pulse lasting at least 50μs, and preferably as long as 2ms, is needed to stimulate the nerve fibers. The stimulation only occurs underneath one of the surface electrodes. The stimulation occurs underneath the electrode at which the pulse is seen as a negative pulse. We can explain this by considering what is actually happening to a nerve fiber when a current is applied. A nerve will be stimulated when the transmembrane potential is reversed by an externally applied current. If, where the current enters the nerve, the externally applied current flows from positive to negative, then the transmembrane potential will be increased, and where the current leaves the nerve, the transmembrane potential will be reduced. Stimulation will occur where the current leaves the nerve. If the positive electrode is called the anode and the negative the cathode, then stimulation starts underneath the cathode. An astable multivibrator is needed to produce such a pulse but will have very low current at output. Thus a complimentary Darlington pair is used in second stage to achieve high current gain. Then the output is passed through a step up transformer to step up the pulse to desired voltage.
Design and implementation of ECG and EMG signal acquisition circuits.
-The Electrocardiography (ECG) is the process of visualizing the electrical activity of the heart over a period of time. These electrical activities result from the depolarization of the myocardium in a certain pattern. The electrical activities are monitored using surface electrodes over the body.The ECG has an amplitude range of 0.5 mV – 3.0 mV and a frequency range of 1Hz to 50Hz. To effectively monitor the ECG, an amplifier is required with a gain of approximately 1000. For this, an Instrumentation amplifier with a high common mode rejection ratio is used. For patient safety, optical isolation is needed. In addition, a bandpass filter is also needed to suppress the contribution of the DC biasing and high frequency noisy signals. The 50Hz default supply line frequency in mainly rejected due to the Common mode effect of the Instrumentation amplifier. For proper filtering, the bandpass filter should have a lower cutoff of 0.1Hz and a higher cutoff of 500Hz (one decade more than the desirable frequency range on both sides). - EMG stands for Electromyography, which means visualization of the electrical signals of muscles. A muscle is usually controlled by nerve signals (nerve action potentials) sent from the brain. When nerve action potentials reach the muscle, muscle action potentials are initiated through the neuromuscular junctions which gives rise to EMG. Such an EMG is also called voluntary EMG since it is the result of a voluntary action of the person.The body is a volume conductor, consisting of electrolytes, having both positive and negative free ions. Therefore, within the body the dipole vectors give rise to an external current which change with time. When a current flows through a resistor, a potential drop occurs across it. Similarly, due to the external current distribution in the volume conductor, potential differences are produced between any two points. This potential difference can be picked up using two electrodes placed at two suitable points outside, which when plotted against time gives the well-known EMG.The EMG has an amplitude range of 0.5 mV – 5 mV and a frequency range of 1Hz to 5 kHz. To effectively monitor the EMG, an amplifier is required with a gain of approximately 500. For this, an Instrumentation amplifier with a high common mode rejection ratio is used. For patient safety, optical isolation is needed. In addition, a bandpass filter is also needed to suppress the contribution of the DC biasing and high frequency noisy signals. The 50Hz default supply line frequency in mainly rejected due to the Common mode effect of the Instrumentation amplifier. For proper filtering, the bandpass filter should have a lower cutoff of 0.1Hz and a higher cutoff of 10 kHz
Design and implementation of a Tetra-polar Impedance Measurment (TPIM) System.
Electrical bioimpedance is the opposition to a flow of current by biological tissues. In bioimpedance measurements, usually an alternating current of constant amplitude is applied to a biological tissue through a pair of electrodes and the resulting voltage is measured across another pair of electrodes; commonly known as tetra-polar impedance measurement technique (TPIM). The ratio of the measured voltage to the applied current is the transfer impedance, quantified as ohms. Impedance of a particular tissue depends on its dielectric properties: conductivity and permittivity. Bioimpedance can reflect different physiological conditions and events like transthoracic impedance changes during breathing because the electrical impedance of lung tissue changes as a function of air content. Therefore monitoring of anatomical structures and physiological processes and characterization of tissues by electrical bioimpedance techniques have attracted scientists as it is noninvasive, nonionizing and the instrumentation is relatively simple. The impedance of the body can be measured by applying a small electric current via a pair of electrodes and picking up the resulting small voltage with another pair of electrodes
Derivative based QRS detection using MATLAB.
The QRS complex has the largest slope (rate of change of voltage) in a cardiac cycle because of the rapid conduction and depolarization characteristics of the ventricles. As the rate of change is given by the derivative operator, the d/dt operation is the most useful basis to develop an algorithm to detect the QRS complex. The derivative operator enhances the QRS complex and suppress the slow P and T waves. However the derivative based operator have noisy nature which requires significant smoothing.The squaring operation makes the result positive and emphasizes large differences resulting from QRS complexes. The small differences arising from P and T waves are suppressed. The high-frequency components in the signal related to the QRS complex are further enhanced.The thresholding procedure adapts to changes in the ECG signal by computing running estimates of signal and noise peaks. A peak is said to be detected whenever the final output changes direction within a specified time interval. In the following discussion, SPKI represents the peak level that the algorithm has learned to correspond to QRS peaks. NPKI represents the peak level related to non-QRS events (such as noise, EMG and various artifacts).
Filtering out common artifacts of ECG signal using MATLAB.
An ECG signal can be corrupted by powerline interference (around 200Hz) and random high Frequency (1000Hz) or low frequency (100Hz) noises. For accurate analysis and understanding of any acquired ECG signal, first all types of noise must be filtered out.An 8th order 60Hz "bandstop" filter can be incorporated to remove the powerline interference.An 8th order 70Hz "lowpass" filter removes high frequency noise.A 2nd order 2Hz "highpass" filter removes low frequency artifact. A lower order highpass filter is used because higher order highpass filter significantly distorts the low frequency components of the ECG (for example, P wave, T wave, ST segment.)