Medical instrumentation- patient monitoring systems

patient monitoring systems

1. UEEG002 Medical Instrumentation PATIENT MONITORING SYSTEMS Poornima D, AP ( Sr.Gr )/EEE, Sri Ramakrishna Institute of Technology, Coimbatore

2. The objective of patient monitoring is to have a quantitative assessment of the important physiological variables of the patients during critical periods of their biological functions. For diagnostic and research purposes, it is necessary to know their actual value or trend of change. Used for measuring continuously or at regular intervals, automatically, the values of the patient’s important physiological parameters. Critically ill patients recovering from surgery, heart attack or serious illness, are often placed in special units, generally known as intensive care units, where their vital signs can be watched constantly. The long-term objective of patient monitoring is to decrease mortality and morbidity by: ( i ) organizing and displaying information in a form meaningful for improved patient care (ii) correlating multiple parameters for clear demonstration of clinical problems (iii) processing the data to set alarms on the development of abnormal conditions (iv) providing information, based on automated data, regarding therapy (v) ensuring better care with fewer staff members. System Concept

3. The choice of proper parameters is an important issue in patient monitoring- should have high information content Monitoring of the following biological functions is often needed. Electrocardiogram (ECG) heart rate (instantaneous or average) pulse rate blood pressure (indirect arterial blood pressure, direct arterial blood pressure or venous blood pressure) body temperature respiratory rate. In addition to these, electroencephalogram (EEG), oxygen tension (pO2) and respiratory volume also become part of monitoring in special cases. Equipment such as defibrillators and cardiac pacemakers are also connected to this system for acting automatically in emergency situations Trends in monitoring include software control, arrhythmia monitoring, haemodynamics monitoring, monitoring during transportation of the patient and increased user friendliness. Monitoring is generally carried out at the bedside, central station and bedside with a central display. The choice depends upon medical requirements, available space and cost considerations

4. Bedside Patient Monitoring Systems Are available in a variety of configurations from different manufacturers. The common feature amongst all is the facility to continuously monitor and provide non-fade display of ECG waveform and heart rate. Some instruments include pulse, pressure, temperature and respiration rate monitoring facilities. The advent of microcomputers has replaced the traditional monitoring devices with a single general purpose unit capable of recognizing the nature of the signal source and processing them appropriately. The firmware is the hardware needed to do the signal analysis and user interaction and the usual switches, knobs, dials and meters can be replaced by a touch-sensitive character display

5. Block Diagram of Bedside Monitoring System The system is designed to display an electrocardiogram, heart rate with high and low alarms, pulse rate, dynamic pressure or other waveforms received from external preamplifiers Also gives immediate and historical data on the patient for trend information on heart rate, temperature, and systolic and diastolic blood pressures for periods up to eight hours The system basically consists of three circuit blocks: Preamplifier section Logic boards Display part.

6. 01 Preamplifiers Incorporate patient isolation circuits based on optical couplers 03 Display The character generator output is mixed with the Y output for numeric display on the CRT. 02 Logic boards Various amplified signals are carried to a multiplexer and then to an analog-to-digital converter, included in the logic board. The central processing unit along with memory gives X and Y output for the CRT display. The alarm settings, selection switches for different parameters and the defibrillator synchronization system communicate with the CPU. The alarm signals are also initiated under its control .

7. Central Monitoring Systems With central monitoring, the measured values are displayed and recorded at a central station. The signal conditioners are mounted at the bedside and the display and alarms, etc. are located in a central station. The central station monitoring equipment may incorporate multi-microprocessor architecture to display a flexible mixture of smooth waveforms alphanumerics and graphics on a single cathode ray tube. Presents all the information at a glance and generates audible and visual alarms if preset vital sign limits are exceeded. Displays the patient’s vital sign data and by watching this data, the attending staff can detect problems before they reach the alarm stage. Provides a recording of the ECG and sometimes of other parameters, especially of the few seconds just before an alarm, which shows what kind of irregularity led to the alarm

8. Wireless telemetry - permits examination under normal conditions and in natural surroundings without any discomfort or obstruction to the subject under investigation. Factors influencing healthy and sick persons during the performance of their daily tasks can be easily recognized and evaluated. Very useful in situations where no cable connection is feasible. Study of active subjects like swimmers, riders athletes, pilots, manual labourers is possible . Most convenient during transportation within the hospital area as well for the continuous monitoring of patients sent to other wards or clinics for check-up or therapy. BioTelemetry-- Wireless Telemetry

9. Components of wireless telemetry The major components of a wirless telemetry system are Modulation systems Radio carrier T ransmitter Receiver

10. Modulation Systems Used for transmitting biomedical signals Makes use of two modulators. a comparatively lower frequency sub-carrier a Very High Frequency (VHF) carrier, which finally transmits the signal from the transmitter. Double modulation gives better interference free performance in transmission and enables the reception of low frequency biological signals. The sub-modulator can be a FM (frequency modulation) system or a PWM (Pulse Width Modulation) system, whereas the final modulator is practically always an FM system.

11. Signal is transmitted by varying the instantaneous frequency in accordance with the signal to be modulated on the wave, while keeping the amplitude of the carrier wave constant. The rate at which the instantaneous frequency varies is the modulating frequency. The magnitude to which the carrier frequency varies away from the centre frequency is called “Frequency Deviation” and is proportional to the amplitude of modulating signal . Frequency Modulation

12. FM signal is produced by controlling the frequency of an oscillator by the amplitude of the modulating voltage. The frequency of oscillation for most oscillators depends on a particular value of capacitance. If the modulation signal can be applied in such a way that it changes the value of capacitance, then the frequency of oscillation will change in accordance with the amplitude of the modulating signal. Figure shows a tuned oscillator that serves as a frequency modulator. The diode is operating in the reverse-biased mode and presents a depletion layer capacitance to the tank circuit. This capacitance is a function of the reverse-biased voltage across the diode and, therefore, produces an FM wave with the modulating signal applied. This type of circuit can allow frequency deviations of 2–5% of the carrier frequency without serious distortion Frequency Modulation

13. Pulse Width Modulation Pulse width modulation is less perceptive to distortion and noise. Transistors Q 1 and Q 2 form a free-running multi-vibrator. Transistors Q 3 and Q 4 provide constant current sources for charging the timing capacitors C l and C 2 and driving transistors Q 1 and Q 2. When Q 1 is ‘off’ and Q 2 is ‘on’, capacitor C 2 charges through R 1 to the amplitude of the modulating voltage e m . The other side is connected to the base of transistor Q 1 and is at zero volt. When Q 1 turns ‘on’ switching the circuit to the other stage, the base voltage of Q 2 drops from approximately zero to - e m . Transistor Q 2 will remain ‘off’ until the base voltage charges to zero volt. Since the charging current is constant at I, the time required to charge C 2 and restore the circuit to the initial stage is: Similarly, the time that the circuit remains in the original stage is  

14. Both portions of the astable period are directly proportional to the modulating voltage. When a balanced differential output from an amplifier such as the ECG amplifier is applied to the input points 1 and 2, the frequency of the astable multi-vibrator would remain constant, but the width of the pulse available at the collector of transistor Q2 shall vary in accordance with the amplitude of the input signal

15. Choice of Carrier Frequency In every country there are regulations governing the use of only certain frequency and bandwidth for medical telemetry. Radio frequencies normally used for medical telemetry purposes are of 37, 102, 153, 159, 220 and 450 MHz. The transmitter is typically of 50 mW at 50 W - gives a transmission range of 1.5 km in the open flat country and much less in built-up areas . Radio waves are used as carriers - they can travel through most non-conducting material such as air, wood, and plaster with relative ease . Radio waves are hindered, blocked or reflected by most conductive material and by concrete because of the presence of reinforced steel. Therefore, transmission may be lost or be of poor quality when a patient with a telemetry transmitter moves in an environment with a concrete wall or behind a structural column. Reception may also get affected by radio frequency wave effects that may result in areas of poor reception or null spots, under some conditions of patient location and carrier frequency.

16. Choice of Carrier Frequency Another serious problem that is sometimes present in the telemetry systems is the cross-talk or interference between telemetry channels. It can be minimized by the careful selection of transmitter frequencies by the use of a suitable antenna system by the equipment design. The range of any radio system is primarily determined by transmitter output power and frequency. In medical telemetry systems, factors such as receiver and antenna design may make the power and frequency characteristics less significant. The use of a higher-powered transmitter than is required for adequate range is preferable as it may eliminate or reduce some noise effects due to interference from other sources

17. Transmitter The transistor T acts in a grounded base Colpitts R.F oscillator with L1 and C1 and C2 as the tank circuit. The positive feedback to the emitter is provided from a capacitive divider in the collector circuit formed by C1 andC2. Inductor L1 functions both as a tuning coil and a transmitting antenna. Trim capacitor C2 is adjusted to precisely set the transmission frequency at the desired point. In this case, it is within the standard FM broadcast band from 88 to 108 MHz. Frequency modulation is achieved by variation in the operating point of the transistor, which in turn varies its collector capacitance, thus changing the resonant frequency of the tank circuit. The operating point is changed by the sub-carrier input. Thus, the transmitter’s output consists of an RF signal, tuned in the FM broadcast band and frequency modulated by the sub-carrier oscillator (SCO), which in turn is frequency modulated by the physiological signals of interest.

18. Receiver is a common broadcast receiver with a sensitivity of 1mV. The output of the HF unit of the receiver is fed to the sub-demodulator to extract the modulating signal. In a FM/FM system, the sub-demodulator first converts the FM signal into an AM signal. An AM detector demodulates the newly created AM waveform. Output is linear with frequency deviation only for small frequency deviations. In PWM/FM system, a square wave is obtained at the output of the RF unit. Square wave is clipped to cut off all amplitude variations of the incoming square wave and the average value of the normalized square wave is determined. The value is directly proportional to the area Area is directly proportional to the pulse duration Pulse duration is directly proportional to the modulating frequency So output signal is directly proportional to the output voltage of the demodulator. The output voltage of the demodulator is fed to a chart recorder. Demodulated signal can also be stored using tape recorder. Receiver

19. Single Channel Systems Multi-Channel Systems Types of Telemetry Systems

20. S ingle Channel Telemetry Systems A single physiological parameter is monitored We will see ECG Telemetry System Temperature Telemetry System

21. In wireless telemetry monitoring, the parameter which is most commonly studied is the electrocardiogram. ECG Telemetry System Two main parts: • Telemetry Transmitter - consists of an ECG amplifier a sub-carrier oscillator a UHF transmitter dry cell batterie s. • Telemetry Receiver consists of a high frequency unit and a demodulator, a cardioscope to display a magnetic tape recorder to store the ECG a heart rate meter with an alarm facility

22. For distortion-free transmission of ECG, the following requirements must be met: The subject should be able to carry on with his normal activities whilst carrying the instruments without the slightest discomfort. Motion artefacts and muscle potential interference should be kept minimum. The battery life should be long enough so that a complete experimental procedure may be carried out. While monitoring paced patients for ECG through telemetry, it is necessary to reduce pacemaker pulses.

23. Temperature Telemetry Systems Telemetry systems for alternating potentials such as ECG, EEG and EMG are easy to construct. Stable Telemetry systems for temperature, pressure – continuous monitoring for long periods has greater design problems. Information is conveyed as a modulation of the mark/space ratio of a square wave.

24. Temperature is sensed by a thermistor placed in the emitter of transistor T1. T1 and T2 form a multi-vibrator circuit timed by the thermistor, RI + R2 and C1. R1 is adjusted to give 1:1 mark/space ratio at midscale temperature (35–41°C). The multi-vibrator produces a square wave output at about 200 Hz. Its frequency is chosen based on available bandwidth required response time the physical size of the multi-vibrator timing capacitors This is fed to the variable capacitance diode D2 via potentiometer R3. D2 is placed in the tuned circuit of a RF oscillator constituted by T3. Transistor T3 forms a conventional 102 MHz oscillator circuit, whose frequency is stabilized against supply voltage variations by the Zener diode D3 between its base and the collector supply potential. T4 is an untuned buffer stage between the oscillator and the aerial. The aerial is normally taped to the collar or harness carrying the transmitter .

25. Medical measuring problems often involve the simultaneous transmission of several parameters. Multi-channel telemetry is particularly useful in athletic training programs Several physiological parameters of the person monitored simultaneously. ECG and heart rate, respiration rate, temperature, intravascular and intra-cardiac blood pressure. Number of sub-carriers used are the same as the number of signals to be transmitted. Each channel has its own modulator. The RF unit—the same for all channels—converts the mixed frequencies into the transmission band Receiver unit contains the RF unit and one demodulator for each channel Multi Channel Telemetry Systems

26. For multi-channel radiotelemetry, various channels of information are combined into a single signal- multiplexing . There are two basic methods of multiplexing. Frequency–division multiplexing makes use of continuous-wave sub-carrier frequencies. Signals frequency–modulate multiple subcarrier oscillators Does not overlap the frequency spectra of the other modulated signals. Signals from all channels are added together through a summing amplifier Gives a composite signal in which none of the parts overlap in frequency. This signal modulates the RF carrier of the transmitter and is broadcast. Time–division multiplexing M ultiple signals are applied to a commutator circuit. Circuit rapidly scans the signals from different channels. It samples each signal for an instant of time Gives a pulse train sequence corresponding to input signals. A frame reference signal is also provided as to make it easy to recognize the sequency and value of the input channels.

27. Telemetry of ECG and Respiration Respiration is detected by using the same pair of electrodes that are used for the ECG. A 10 kHz sinusoidal constant current is injected through electrodes E1 and E2 attached across the subject’s thoracic cavity. The carrier signal is generated by a phase shift oscillator The varying thoracic impedance associated with respiration produces an ac voltage whose amplitude varies with a change in impedance. The amplitude varying carrier is amplified by an amplifier A1. An amplifier filterA3 recovers the respiration signal by using rectifiers and a double pole filter.

28. Telemetry of ECG and Respiration ECG detected by electrodes E1 and E2 is amplified in A1 along with the respiratory signal. Passed through a low-pass Butterworth filter stage A2 which passes the ECG signal but blocks respiratory signal. The amplified ECG signal is then summed up with the preprocessed respiration signal in A4

29. Cardiac Output Measuring Systems

30. Cardiac output - quantity of blood delivered by the heart to the aorta per minute Major determinant of oxygen delivery to the tissues. Introduction

31. A fall in cardiac output may result in Low blood pressure Reduced tissue oxygenation Acidosis Poor renal function and shock Stroke volume of blood pumped from the heart with each beat at rest varies among adults between 70 and 100 ml, while the cardiac output is 4 to 6 l/min. Direct method of estimating the cardiac output - measuring the stroke volume by the use of an electromagnetic flow probe placed on the aorta and multiplying it by the heart rate. Involves surgery and not preferred in routine applications .Another method is the Fick’s Method Determines the cardiac output by the analysis of the oxygen-keeping of the body Complicated, difficult to repeat, necessitates catheterization Not considered as a solution to the problem, though it is practised at many places even now. The most popular method is indicator dilution Dye dilution Thermal dilution

32. I ndicator Dilution Method if we introduce into or remove from a stream of fluid a known amount of indicator and measure the concentration difference upstream and downstream of the injection (or withdrawal) site, we can estimate the volume flow of the fluid. q(t) is instantaneous blood flow , c(t) is concentration as function of time. Employs several different types of indicators. Two methods are generally employed for introducing the indicator in the blood stream it may be injected at a constant rate it may be injected as a bolus. Assumes complete mixing of blood and indicator, No loss of indicator between place of injection and place of detection  

33. Continuous infusion suffers from the disadvantage that most indicators recirculate, and this prevents a maxima from being achieved. In the bolus injection method , a small but known quantity of an indicator such as a dye or radioisotope is administered into the circulation I njected into a large vein or preferably into the right heart itself. After passing through the right heart, lungs and the left heart, the indicator appears in the arterial circulation. The presence of an indicator in the peripheral artery is detected by a suitable (photoelectric) transducer and is displayed on a chart recorder. This way we get the cardiac output called the dilution curve.

34. During the first circulation period, the indicator would mix up with the blood and will dilute just a bit. When passing before the transducer, it would reveal a big and rapid change of concentration - rising portion of the dilution curve. Circulation system is a closed one Fraction of the injected indicator would once again pass through the heart and enter the arterial circulation. A second peak appears. Curve becomes parallel with the time axis when the indicator is completely mixed up with blood The amplitude of this portion depends upon the quantity of the injected indicator and on the total quantity of the circulating blood. For calculating the cardiac output from the dilution curve, M = quantity of the injected indicator in mg Q = cardiac output then,   Dilution Curve

35. Dye Dilution Method The most commonly used indicator substance is a dye. Indocyanine green ( cardiogreen ) dye is usually employed for recording the dilution curve Preferred because of its property of absorbing light in the 800 nm region of the spectrum Reduced and oxygenated haemoglobin have the same optical absorption in the region The concentration of cardiogreen can be measured with the help of infra-red photocell transducer. The curve is traced by a recorder attached to the densitometer Problems relating to the use of the indicator indocyanine green Above a dye concentration of approximately 20 mg/ml of blood, the optical density rises less with an increase in dye concentration than below this level

36. Used for the quantitative measurement of dye concentration The photometric part consists of a source of radiation a photocell arrangement for holding the disposable polyethylene tube constituting the cuvette. An interference filter permits only infrared radiation to be transmitted. It is the isobestic wavelength for haemoglobin at various levels of oxygen saturation. A flow rate of 40 ml/min is preferred Gives as short a response time as possible for the sampling catheter. The output of the photocell is connected to a low drift amplifier. Has a high input impedance and low output impedance. The amplification is directly proportional to the resistance value of the potentiometer R. A potentiometric recorder records the amplifier signa l Densitometer

37. A thermal indicator of known volume introduced into either the right or left atrium will produce a resultant temperature change in the pulmonary artery or in the aorta respectively, the integral of which is inversely proportional to the cardiac output. a multi-lumen thermistor catheter, Swan-Ganz triple lumen balloon catheter is used for this   Thermal Dilution Techniques

38. The balloon, located at or near the tip, is inflated during catheter insertion Carries the tip through the heart into the pulmonary artery. carries a thermistor proximal to (before) the balloon. One lumen terminates at the tip Used to measure the pressure during catheter insertion. A second lumen typically terminates in the right atrium Used to the monitor right atrial pressure (central venous pressure) Injects the cold solutions for thermal dilution. A third lumen is used to inflate the balloon. Thermistor is encapsulated in glass and coated with epoxy to insulate it electrically from the blood. The wires connecting the thermistor are contained in a fourth lumen

39. Solution of 5% Dextrose in water at room temperature is injected as a thermal indicator into the right atrium. It mixes in the right ventricle Detected in the pulmonary artery by means of a thermistor mounted at the tip of a miniature catheter probe. Temperature difference between the injectate and the blood circulating in the pulmonary artery is measured. The reduction in temperature is integrated with respect to time and the blood flow in the pulmonary artery is then computed electronically A meter provides a direct reading of cardiac output Thermal Dilution Set-up

40. The integrator responds to a DT signal corresponding to a maximum temperature change Circuit holds the integral obtained to display the computed cardiac output The summing and multiplication/division operations are performed by a simple analog computing circuit. The timer control unit generates the switching signals necessary for the proper integration of the DT signal and for the display of the computed value of the cardiac output Computing System of Thermal Dilution Method The linearizing amplifier works on the principle that when a fixed resistance of suitable value is placed in parallel with a thermistor, a virtually linear relation between the temperature and resistance of the parallel combination can be obtained over a limited range of operation.

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