Biomedical Instrumentation

Archive for October, 2009

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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.

Applications of digital computer in medicine and related fields are so numerous. Most of these applications, however, utilize a few basic capabilities of the computer which provide an insight to ways in which computer can be used in conjunction with modern medical electronics and instrumentation.

• Data acquisition and storage: The reading of instruments and transcribing of data can be done automatically under control of computer. This not only results in a substantial saving of time and effort, but also reduces the possibilities of number of errors in the data. When data are expected at irregular intervals, the computer can continuously scan all input sources and accept data whenever they are actually produced. The ability of the digital computer to store and retrieve large quantities of data is well known. The biomedical field provides ample opportunities to make use of this capability. In a modern hospital large amounts of data are accumulated from many sources.
  
• Data reduction and transformation: The sequence of numbers resulting from digitizing an analog physiological signal such as ECG or EEG would be quite useless if retrieved from the computer in raw form. To obtain meaningful information from such data some form of data reduction or transformation is necessary to represent the data as a set of specific parameters. These parameters can be analyzed, compared with other parameters, or otherwise manipulated.
  
• Mathematical operation and pattern recognition: Many important physiological variables cannot be measured directly, but must be calculated from other variables that are accessible. If a digital computer is connected on-line with the measuring instruments, the calculated results can often be obtained while the patient is still connected to the instruments. To reduce certain types of physiological data into useful parameters, it is often necessary that important features of a physiological waveform or an image be identified. Digital computer programs are available to search the data representing the ECG signal for certain predetermined characteristics that identify each of the important peaks.
  
• Limit detection and control function: In application involving monitoring and screening, it is often necessary to determine when a measured variable exceeds certain limits. By comparison of measured parameter with each limit of the range, the computer can indicate which parameters exceed the limit and the amount by which they deviate from normal. Digital computers are capable of providing output signals that can be used to control other devices. The computer can also be used to provide feedback to the source of its data.

Instrumentation systems for monitoring patients in intensive and coronary care units are very important. In recent years, especially since the advent of microprocessor, an increasing number of patient-monitoring systems include some form of digital computer. The type of computer involved and the extent of its role in the overall patient monitoring system may vary widely. In some systems, a small computer, usually a microprocessor is used to store a limited amount of data and control a display of the ECG and other variables in an analog system.

The waveforms either move across the screen with uniform brightness or remain stationary until the replaced by new information, which appears to sweep across the screen and replace the old trace. Computer controlled displays of this type usually include on-screen digital readouts of such parameters as systolic and diastolic blood pressures and heart rate. In another type of computerized patient monitoring system, the computer is simply attached to a conventional analog patient monitor to store and analyze information. Except for the interface through which the computer receives its data, the two systems are completely independent.

A computer failure would have no affect whatever on the monitoring of patients. The computer is an integral part of the patient monitoring system and, in addition to storing and analyzing data, takes over many of the functions otherwise performed by analog circuitry, such as the filtering of signals to remove noise and artifacts and the controlling of alarms in case of an emergency. In a few very large hospitals, the patient monitoring system is integrated into a more extensive computer system in which patient records; laboratory test results, pharmacy records, and related information are combined with the ongoing data obtained from the patient monitor.

Such system may also tie in with the operating suite, cardiac laboratory, and other special diagnostic laboratories. By bringing together data from many sources, the computer can provide more completion information to assists the medical staff in their diagnoses and in monitoring the treatment of patients.

A range of electrical safety analyzers are commercially available for testing both medical facility power systems and medical equipment. They vary in complexity from simple volt-ohm-meter to computerized automatic measurement systems that generate hard copies of test results. The facilities available in these testers are given below.

Mechanical testing of electrical outlets: The power delivery point in the patient area usually consists of the outlets in the vicinity of the patient. The outlets should have three-prong wall receptacles that meet the ground retention force requirements as per the relevant standards. These force requirements are important as they ensure that plugs on medical devices do not fall out of the receptacle, possible placing the patient in danger.

Electrical testing of electrical outlets: Electrical testing of a wall receptacle should be made to determine whether power is available at the receptacle and if its polarity is correct. Proper polarity of the receptacles means that the hot, neutral and ground wires are connected to their correct positions. Miswiring of an outlet can happen during the original construction of the area or when broken outlet is replaced.

Patient safety: Hospitals are confronted with the difficult problem of creating a safe electric environment for the care and comfort of the patients. Electric shocks, burns and fire hazards result from the careless use of electricity. When electricity is relied upon to support life with devices like external pacemakers, respirators, and etc power failure is a continuous threat. Shock resulting from electric power is a common experience. Disruption of physiologic function by leakage current applied internally remains sometimes hidden and mysterious. Electric current can flow through the human body either accidentally or intentionally.

Electrical currents are administered intentionally in the following cases such as high frequency currents are also passed through the body for therapeutic and surgical purposes and also when recording signals like ECG and EEG, the amplifiers used in the preamplifier stage may deliver small currents themselves to the patient. Accidental transmission of electrical current can take place because of a defect in the equipment; excessive leakage currents due to defect in design; operational error and simultaneous use of other equipment on the patient which may produce potentials on the patient circuit.

Direct method: The direct method of pressure measurement is used when the highest degree of absolute accuracy, dynamic response and continuous monitoring is required. The method is also used to measure the pressure in deep regions inaccessible by indirect means. For direct measurement, a catheter or a needle type probe is inserted through a vein or artery to the area of interest. Two types of probes can be used. One type is the catheter tip probe in which the sensor is mounted on the tip of the probe and pressures exerted on it are converted to the proportional electrical signals. The other is the fluid-filled catheter type, which transmits the pressure exerted on tits fluid-filled column to an external transducer. This transducer converts the exerted pressure to electrical signals. The electrical signals can then be amplified and displayed or recorded. Catheter tip probes provide the maximum dynamic response and avoid acceleration artefacts whereas the fluid-filled catheter type systems require careful adjustment of the catheter dimensions to obtain an optimum dynamic response.

Indirect method: The classical method of making an indirect measurement of blood pressure is by the use of a cuff over the limb containing the artery. Initially, the pressure in the cuff is raised to a level well above the systolic pressure so that the flow of blood is completely terminated. Pressure in the cuff is then released at a particular rate. When it reaches a level, which is below the systolic pressure, a brief flow occurs. If the cuff pressure is allowed to fall further, just below the diastolic pressure value, the flow becomes normal and uninterrupted. The problem here finally reduces to determining the exact instant at which the artery just opens and when it is fully opened. The method is based on the sounds produced by flow changes is the one normally used in the conventional sphygmomanometers. The sounds first appear when the cuff pressure falls to just below the systolic pressure. They are produced by the brief turbulent flow terminated by a sharp collapse of the vessel and persist as the cuff pressure continues to fall. The sound s disappears or changes in character at just below diastolic pressure when the flow is no longer interrupted. These sounds are picked up by using a microphone placed over an artery distal to the cuff. The syphygmomanometric technique is an ausculatory method; it depends upon the operator recognizing the occurrence and disappearance of he sounds with variations in cuff pressure.

Information obtained from a sensor or transducer is often in terms of current intensity, voltage level, frequency or signal phase relative to a standard. Voltage measurements are the easiest to make, as the signal from the transducer can be directly applied to an amplifier having high input impedance. However, most of the transducers produce signal in terms of current, which can be conveniently converted into voltage by using operational amplifiers with appropriate feedback. To make an accurate measurement of voltage, it is necessary to arrange that the input impedance of the measuring device must be large compared with the output impedance of the signal source.

This is to minimize the error that would occur, if an appreciable fraction of the signal source. This is to minimize the error that would occur, if an appreciable fraction of the signal source were dropped across the source impedance. Conversely, accurate measurement of current source signals necessitates that the source output impedance be large compared with the receiver input impedance. Ideally, a receiver that exhibits zero input impedance would not cause any perturbation of the current source. Therefore, high-impedance would not cause any perturbation of the current source. Therefore, high-impedance current sources are more easily handled than low-impedance current sources.

In general, the frequency response of the system should be compatible with the operating range of the signal being measured. To process the signal waveform without distortion, the bandpass of the system must encompass all of the frequency components of the signal that contribute significantly to signal strength. The range can be determined quantitatively by obtaining a Fourier analysis of the signal. The bandpass of an electronic instrument is usually defined as the range between the upper and lower half-power frequencies. The results of a measurement in medical instruments are usually displayed either on analog meters or digital displays.

The primary purpose of medical instrumentation is to measure or determine the presence of some physical quantity that may some way assist the medical personnel to make better diagnosis and treatment. Accordingly, many types of instrumentation systems are presently used in hospitals and other medical facilities. The majority of the instruments are electrical or electronic systems, although mechanical systems such as ventilators or Spiro meters are also employed. Certain characteristic features, which are common to most instrumentation systems, are also applicable to medical instrumentation systems. In the broadest sense, any medical instrument would comprise of the following four basic function components.

Measurand and Display system: The physical quantity or condition that the instrumentation system measures are called the measurand. The source for the measurand is the human body which generates a variety of signals. The measurand may be on the surface of the body or it may be blood pressure in the chambers of the heart. Display Systems Provides a visible representation of the quantity as a displacement on a scale, or on the chart of a recorder, or on the screen of a cathode ray tube or in the numerical form.

Transducer or sensor: A transducer is a device that converts one form of energy to another. Because of the familiar advantages of electric and electronic methods of measurement, it is usual practice to convert into electrical quantities all non –electrical phenomenons associated with the measurand with the help of a transducer. Another term sensor is also used in medical instrumentation systems. Basically, a sensor converts a physical measurand to an electrical signal. The sensor should be minimally invasive and interface with the living system with minimum extraction of energy.

Signal conditioner: Converts the output of the transducer into an electrical quantity suitable for operation of the display or recording system. Signal conditioners may vary in complexity from a simple resistance network or impedance matching device to multi-stage amplifiers and other complex electronic circuitry. Signal conditioning usually include functions such as amplification, filtering analog-to-digital and digital-to-analog conversion or signal transmission circuitry. They help in increasing the sensitivity of the instruments by amplification of the original signal or its transduced form.

Biomedical signals are those signals which are used primarily for extracting information on a biological system under investigation. The process of extracting information could be simple as feeling the pulse of a person on the wrist or as complex as analyzing the structure of internal soft tissues by an ultrasound scanner. Biomedical signals originate from a variety of sources such as:

Bioelectric signals: These are unique to the biomedical systems. They are generated by nerve cells and muscle cells. Their basic source is the cell membrane potential which under certain conditions may be excited to generate an action potential. The electric field generated by the action of many cells constitutes the bio-electric signal. The most common examples of bioelectric signals are the ECG and EEG.

Bio-acoustic signals and Biomechanical signals: The measurement of acoustic signals created by many biomedical phenomena provides information about the underlying phenomena. The examples of such signals are: flow of blood in the heart, through the heat’s valves and flow of air through the upper and lower airways and in the lungs which generate typical acoustic signal. These signals originate from some mechanical function of the biological system. They include all types of motion and displacement signals, pressure and flow signals etc. The movement of the chest wall in accordance with the reparatory activity is an example of this type of signal.

Biochemical signals and Bio-magnetic signals: The signals which are obtained as a result of chemical measurements from the living tissue or from samples analyzed in the laboratory. The examples are measurement of partial pressure of carbon-di-oxide, partial pressure of oxygen and concentration of various ions in the blood. Extremely weak magnetic fields are produced by various organs such as the brain, heart and lungs. The measurement of these signals provides information which is not available in other types of bio-signals. Magneto-encephalograph signal is the example for it.