Biomedical Instrumentation

Archive for the ‘Patient Monitoring’ Category

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.

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.

There are a limited number of situations in which telemetry is practical in the diagnosis and treatment of patients. Most involve measurement of the electrocardiogram. Some common applications are discussed here.

Telemetry for emergency patient monitoring: In many areas ambulances and emergency rescue teams are equipped with telemetry equipment to allow electrocardiograms and other physiological data to be transmitted to a nearby hospital for interpretation. Two-way voice transmission is normally used in conjunction with the telemetry to facilitate the identification of the information and to provide instructions for treatment. Through the use of such equipment, ECGs can be interpreted and treatment begun before the patient arrives at the hospital. Often the data must be transmitted many miles and sometimes from a moving vehicle. To be effective, the system must be capable of providing reliable reception and reproduction of the transmitted signals regardless of conditions. In some cases, an emergency rescue squad can transmit physiological information from a portable transmitter to a receiver in their vehicle. The vehicle which contains a more powerful transmitter and better antenna system is able to retransmit the data to the hospital.

Telemetry of ECGs from extended coronary care patients: Cardiac patient must often be observed for rhyme disturbances for a period of time following intensive coronary care. Such patients are generally allowed a certain amount of mobility. To make monitoring possible, some hospitals have extended coronary care units equipped with patient monitoring systems that include telemetry. In this arrangement, each patient has ECG electrodes taped securely to their chest. The electrodes are connected to a small transmitter unit that also contains the signal conditioning equipment. A telemetry for each monitored patient is usually included as part of the monitoring system. The output of each receiver is connected to one of the ECG channels of the patient monitor. A potential problem in the use of telemetry with free roaming patients concerns being able to locate a patient in case the alarm should sound.

Heart rate is deriving by the amplification of the ECG signal and by measuring either the average or instantaneous time intervals between two R successive peaks. Techniques used to calculate heart rate include.

Average heart rate meters: The heart rate meters, which are a part of the patient monitoring systems, are usually of the average reading type. They work on the basis of converting each R wave of the ECG into a pulse of fixed amplitude and duration and then determining the average current from these pulses. They incorporate specially designed frequency to a voltage converter circuit to display the average heart rate in terms of beats per minute.

Instantaneous heart rate meters: Instantaneous heart rate facilities detection of arrhythmias and permits the timely observation of incipient cardiac emergencies. Calculation of heart rate from a patient’s ECG is based upon the reliable detection of the QRS complex. Most of the instruments are, however, quite sensitive to the muscle noise generated by patient movement. This noise often causes a false high rate that may exceed the high rate alarm. A method to reduce false alarm is by using a QRS matched filter. This filter is a fifteen sample finite impulse-response-filter whose impulse response shape approximates the shape of a normal QRS complex. The filter, therefore, would have maximum absolute output when similarly shaped waveforms are input. The output from other parts of the ECG waveform, like a T wave, will produce reduced output. Other techniques used to calculate heart rate includes:

Average calculation: This is the oldest and most popular technique. An average rate is calculated by counting the number of pulses in a given time. The average method of calculation does not show changes in the time between beats and thus does not represent the true picture of the heart’s response to exercise, stress and environment.

Combination of beat-to-beat calculation with averaging: This is based on a four or six beats average. The advantage of this technique over the averaging techniques is its similarity with the beat-to-beat monitoring system. The normal heart rate measuring range is 0-250 beats/min. Limb or chest ECG electrodes are used as sensors.

Electromyography is an instrument used for recording the electrical activity of the muscles to determine whether the muscle is contracting or not; or for displaying on the CRO and loudspeaker the action potentials spontaneously present in a muscle or those induced by voluntary contractions as a means detecting the nature and location of motor unit lesions; or for recording the electrical activity evoked in a muscle by the stimulation of its nerve. The instrument is useful for making a study of several aspects of neuromuscular function, neuromuscular condition, extent of nerve lesion, reflex responses, etc.

EMG measurements are also important for the myoelectric control of prosthetic devices. This use involves picking up EMG signals from the muscle at the terminated nerve ending of the remaining limb and using the signals to activate a mechanical arm. This is the most demanding requirement from an EMG since on it depends the working of the prosthetic device. EMG is usually recorded by using surface electrodes or more often by using needle electrodes, which are inserted directly into the muscle. The electrodes pick up the signals from the contracting muscle fibers. The signal can then be amplified and displayed on the screen of a cathode ray tube.

The surface electrodes may be disposable, adhesive types or the ones which can be used repeatedly. A trained EMG interpreter can diagnose various muscular disorders by listening to the sounds produced when the muscle potentials are fed to the loudspeaker. Modern EMG machines are PC based available both in console as well as laptop models. They provide full color waveform display, automatic cursors for marking and making measurements and keyboard for access to convenient and important test controls. The system usually incorporates facilities for recording of the EMG and evoked potentials. The simulators are software controlled. For report generation in the hard copy form, popular laser printers can be used.