Biomedical Instrumentation

Archive for the ‘Modern Instrumentation’ Category

More than a half million people fall victim to heart attacks every year and thousands more are critically injured in accidents. Taking care of these patients in special care units of hospitals involves the use of several types of specialized equipment, among which are cardiac pacemakers and defibrillators. Defibrillators and cardiopulmonary resuscitation equipment are also required away from the hospital, in an ambulance or at the scene of emergency. In the past few years electronic pacemaker system have become extremely important in saving and sustaining the lives of cardiac patients whose normal pacing functions have become impaired. Depending on the exact nature of a cardiac dysfunction, a patient may require temporary artificial pacing during the course of treatment or permanent pacing in order to lead an active, productive life after treatment. The rhythmic action of the heart is initiated by regularity recurring potentials originating in the natural pacemakers present in the heart.

External Pacemakers: External pacemakers are used on patients with temporary heart irregularities, such as those encountered in the coronary patient, including heart blocks. They are also used for temporary management of certain arrhythmias that may occur in patients during critical postoperative periods and in patients during cardiac surgery, especially if the surgery involves the valves or septum .External pacemakers usually consist of an externally worn pulse generated connected to electrodes located on or within the myocardium. External pacemakers, which include all types of pulse generators located outside the body, are normally connected through wires introduced into the right ventricle via a cardiac catheter.

Internal Pacemakers: Internal pacemaker systems are implanted with the pulse generator placed in a surgically formed pocket below the right or left clavicle, in the left sub-coastal area, or, in women, beneath left or right major pectorals muscle. Internal leads connect to electrodes that directly contact the inside of the right ventricle or the surface of myocardium. The exact location of the pulse generator depends primarily on the type of electrode used, the nature of the cardiac dysfunction, and the method of pacing may be prescribed.

Related to the measurement of the blood flow is the measurement of volume changes in any part of the body that results from the pulsation the diagnosis of arterial obstructions as well as for pulse-wave velocity measurements of these volume changes, or phenomena related thereto, is called plethysmography. True plethysmography is one that actually responds to changes in volume. Such an instrument consists of a rigid cup or chamber placed over the limb or digit in which the volume changes are to be measured. The cup is tightly sealed to the member to be measured so that any changes of volume in the limb or digit reflect as pressure changes inside the chamber. Either fluid or air can be used to fill the chamber.

Plethysmography may be designed for constant pressure or constant volume within the chamber. In either case, some form of pressure or displacement transducer must be included to respond to pressure changes and to provide a signal that can be calibrated to represent the volume of the limb or digit. The base line pressure can be calibrated by use of a calibrating string. This type of plethysmography can be used in two ways. If the cuff, placed upstream from the sealed, is not inflated, the output signal is simply a sequence of pulsations proportional to the individual volume changes with each heart beat .The plethysmograph can be used to measure the total amount of blood flowing into the limb or digit being measured.

By inflating the cuff to a pressure just above venous pressure, arterial blood can flow past the cuff, but venous blood cannot leave. The result is that the limb or digit increases its volume with each heart beat by the volume of the blood entering during that beat. Another device that quite closely approximates a true plethysmograph is the capacitance plethysmography. In this device, which is generally used on either the arm or leg, the limb in which the volume is being measured becomes one plate of capacitor. The other plate is formed by a fixed screen held at a small distance from the limb by an insulating layer.

Dear friends! Hearing loss has many forms. The most common is related to the body aging process and to long term cumulative exposure of the ear to sound energy. As one grows older, it becomes more difficult to hear. The ear becomes less sensitive to sound, less precise as a sound analyzer and less effective as a speech processor. Loss of hearing differs greatly in different individuals. Changes in the ear occur gradually over time. However, by the time the changes are manifested, it is estimated that approximately 30 to 50 percent or more of the sensory cells in the inner ear have suffered irreparable structural damage or are missing.

Conventional Hearing Aid: Modern hearing aids have evolved from single-transistor amplifiers to modern multi-channel designs containing hundreds and even thousands of transistors. The basic functional parts include a microphone and associated preamplifier, an output transducer or receiver. The total circuitry works on a battery. The use of multiple channels in this design provides different compression characteristics for different frequency ranges. Typically, the crossover frequencies of the channels and the compression characteristics can be adjusted with potentiometers. The register outputs are used to switch resistor networks that control various analog circuits. The active filters are adjusted to generally provide for low-frequency attenuation. This is because most hearing aid wearers require high frequency gain.

Digital Hearing Aid: The major parts are the microphone, an analog-to-digital converter (ADC), the digital signal processor (DSP), the digital-to-analog converter (DAC), the receiver and a two port memory. Essentially, sound waves picked up by the microphone and transformed into electrical signals are converted into digital form by an analog to digital converter. A typical microphone will have an internal noise of 20 db sound pressure level when referred to the input and maximum undistorted output corresponding to a signal of about 90 db sound pressure level. Allowing some margin for peak performance, the total dynamic range required of the analog digital converter is 80 db. This requirement can be achieved with a 14 bit analog to digital converter. My dear friends! I am sure this article will enhance your knowledge regarding the digital and conventional hearing aids.

Dear friends! An important development in the field of defibrillators has been the development and successful use of smart automatic or advisory external defibrillators (AEDs) which are capable of accurately analyzing the ECG and of making reliable shock decisions. They are designed to detect ventricular fibrillation with sensitivity and specificity comparable to that of well trained paramedics, then deliver or recommend an appropriate high energy defibrillating shock.

AEDs require self-adhesive electrodes instead of hand-held paddles for two reasons. Firstly, the ECG signal required from self-adhesive electrodes usually contains less noise and higher quality; hence, it allows a faster and more accurate analysis of the ECG and, therefore, facilitates better shock decisions. Secondly, hands-off defibrillation is a safer procedure for the operator, especially if the operator has little or no training.

It was initially though that different self-adhesive electrode designs were needed for defibrillation, pacing, and monitoring. A critical factor in the safety and performance of an automatic external defibrillator is the ability device to accurately assess the patient’s heart and make an appropriate therapy decision. The defibrillator performs this evaluation by sensing electrical signals from the patient’s heart via electrodes and using a computerized algorithm to interpret the electrical signals. An automatic external defibrillator optimized for infrequent used by both first responders and untrained bystanders have been introduced by the agilient technologies, USA.

It is small, light, and virtually maintenance free while it is on standby for long periods, the device automatically self-tests its electronic circuitry everyday and periodically performs an internal discharge and recalibration. The device is powered by a long life disposable lithium battery. It uses a low energy biphasic waveform with dynamic compensation which accommodates a wide range of patient impedances. Multi function self-adhesive electrodes are now commonly used with defibrillator monitor pacer instruments. My dear friends! I am sure this article will enhance your knowledge regarding the smart automatic or advisory external defibrillators.

Stress test or exercise electrocardiography is used when the diagnosis of coronary arterial disease is suspected or to determine the physical performance characteristics of a patient. The test involves the recording of the electrocardiogram during dynamic or occasionally isometric exercise. The diagnostic value of exercise testing primarily concerns either depression or elevation of the ST segment present in myocardial ischemia. Dynamic exercise is performed by the patient who walks on a treadmill on which the speed and elevation can be adjusted, manually or automatically, to suit a variety of graded exercise protocols. Alternatively, the patient may be asked to pedal an electrically braked bicycle ergo meter. Both the treadmill and ergo meter can be used as stand –alone devices for testing physical fitness. Advanced ergo meters and treadmills can store and display activity data, transfer it to a PC and download patient data from the PC.

Treadmill test: There are two basic kinds of exercise protocols used in treadmill tests. The Balk-ware Protocol uses a constant speed of 3.3 miles/ hour, with progressive increments in the load every 2 minutes. This is achieved by increasing the grade or incline of the motor-driven treadmill. The exercising subject therefore walks up hill and his own body weight serves as the load. The Bruce Protocol uses simultaneous increments in both speed and tread mill grade at intervals of 3 minutes. Both the protocols are satisfactory for most clinical purposes. However, they may have to be modified to suit the condition of the individual bring subjected to exercise testing.

Bicycle test: In this test, the speed is usually kept constant and incremental resistance is applied either mechanically or electrically. The work load can be precisely measured in terms of kilograms per meter per minute. Physiological measurements are more easily obtained with the subject comfortable seated and the torso relatively immobile. I am sure this article will enhance your knowledge regarding the exercise stress testing.

Dear all! In order to describe and analyze behavior accurately, data must be recorded in terms other than the subjective report of an observer. Especially for a mathematical analysis, numerical values must be assigned to some aspects of behavior. For behavior involving motor response and motor skills, special testing devices have been developed to obtain a numerical rating. Other test required the completion of some manual or mental task in which the time required for completion is measured.

Sometimes the number of errors is also used to compare the performance of individuals. Many basic behavioral experiments are performed with animals as subject .These experiments are made in neutral environment provided by a sound proof enclosure, often called a skinner box in which the animal is isolated from the uncontrolled environmental stimuli. Each experiment must be designed in such a way that the behavior is well defined and can be measured automatically. In specially instrumented cages, the activity of animals can be quantified. Behavior emitted by organisms to interact with and modify their environment is called instrumental behavior.

Such behavior, which is controlled by central nervous system rather than by the automatic nervous system, can also be conditioned but in a way that differs from classical conditioning. Operant behavior that is positively reinforced tents to occur more frequently in the future but the behavior that is negatively reinforced decreases in frequency. In animal experiments, positive reinforcement is usually administered in the form of food or water given to animals that had been deprived of these commodities. This reinforcement can be administered easily by automatic dispensing devices.

Negative enforcement is in the form of harmless, but painful, electric shocks administered through isolated grids bars that serve as the floor of the cage. With suitable reinforcement, the animal can be conditioned to emit certain behavior such as pressing of bar, in response to a certain stimulus. From changes in the behavior that can occur under the influences of drugs, or when the stimulus is modified, valuable insight into the mechanisms of behavior can be obtained. My dear friends! I am sure this article will enhance your knowledge regarding instrumentation for the experimental analysis of behavior.