L2 Cardiac Monitor PDF

Biomedical. Eng. Dept. 5th stage Lec. 2 Cardiac Monitor Introduction: A cardiac monitor is a device designed to display the electrical activity of the heart as a wave pattern on a monit...

Biomedical. Eng. Dept. 5th stage Lec. 2 Cardiac Monitor Introduction: A cardiac monitor is a device designed to display the electrical activity of the heart as a wave pattern on a monitor screen. The device continuously monitors the cardiac activity and allows the detection of changes in heart rate, rhythm, and conduction. It is usually used as a bedside monitor in cardiac care and intensive care units and is an essential equipment for the detection of life-threatening arrhythmias. The cardiac monitors are available in many forms, which range from simple handheld devices for remote monitoring to sophisticated patient monitoring systems used in hospitals. There are several clinical situations in which continuous observation of the ECG and heart rate is important to the care of the patient. Continuous observation of the ECG during the administration of anesthesia helps doctors monitor the patient condition while he or she is undergoing medical procedures and during recovery from anesthesia. Constant monitoring of the ECG and heart rate of the myocardial infarction patient during the danger period of several days following the initial incident has made possible the early detection of life-threatening cardiac arrhythmias. Continuous monitoring of the fetal heart rate during labor may help in the early detection of complications. These and other clinical application of continuous monitoring of the ECG and heart rate are made possible by cardiac monitors. Principle: The most important physiological parameters monitored of critical care patients in the intensive care unit or otherwise are the heart rate and the shape of the electrical waveform, commonly known as electrocardiogram (ECG), produced by the heart. 1 Biomedical. Eng. Dept. 5th stage Lec. 2 The information on these parameters can lead to detection of the presence of arrhythmias or changes in the heart rate that might be indicative of a serious condition. Thus, a cardiac monitor is specifically useful for monitoring patients having some cardiac problem. Cardiac monitoring is different from hemodynamic monitoring, which monitors the pressure and flow of blood within the cardiovascular system. Both types of monitoring are usually performed simultaneously on critical heart patients. A typical cardiac monitor comprises of the following: Electrodes: Disposable type pre-gelled Ag/AgCl electrodes used with conductive gel to pick up the ECG signal. These electrodes have average impedance of ≤2 kΩ. Signal processing: Amplifier and an LCD for the amplification and display of the ECG waveform and heart rate, which allows direct observation of these parameters. Heart rate computation: A heart rate meter to indicate average heart rate in numerical form with audible beep or flashing light or both with each beat. Alarm system: An alarm system to produce signal in the event of abnormalities occurring in the heart rate. The cardiac monitor is basically similar to the conventional oscilloscope used for the display of waveforms in electronic laboratories. They have the usual circuit blocks like vertical and horizontal amplifiers and the time base for display of time varying ECG with respect to time. However, they differ in two important aspects as compared with the conventional oscilloscopes. The ECG is a low frequency signal, and therefore, the cardiac monitor operates at slower sweep speeds. The monitor also includes memory so as to display a continuously moving ECG trace. Most of the present-day cardiac monitors are designed to be used at the bedside. They are also available as portable units that can operate on storage batteries. The display screen, and the unit is usually mounted on one side of the bed at a height at which it is possible to conveniently observe the waveforms from a distance. 2 Biomedical. Eng. Dept. 5th stage Lec. 2 Cardiac monitors with a smaller screen size are often mounted on anesthesia trolleys for continuous monitoring of the ECG of anaesthetized patients. The cardiac monitor provides a visual display of the patient’s heart rhythm, which is particularly useful information during heart attacks, when patients can develop lethal cardiac arrhythmias. The monitor sounds an alarm if the patient’s heart rate goes above or below a predetermined number. Computerized ECG monitoring and analysis are now an integral part of bedside monitors. They normally make use of multiple microprocessors that run sophisticated arrhythmia analysis software. However, most of the present-day monitors include automatic monitoring of blood pressure, temperature, pulse oximeter, and respiration rate and are appropriately called vital signs monitors. Figure.1 illustrates a block diagram of a general ECG monitoring instrument. It comprises of several integrated circuits that can be divided into the following sections: Analog front end (AFE) Power management Communications Software algorithms The AFE captures the ECG signal, carries out necessary signal conditioning, and digitizes the signals. The rest of the system analyses, displays, prints a hard copy, stores, and transmits the data via radio frequency (RF) link. 3 Biomedical. Eng. Dept. 5th stage Lec. 2 Figure 1: Block diagram of a cardiac monitor. 1. Analog Front End (AFE) AFEs in the cardiac monitor share the same basic requirements as that of ECG machine but differ in the number of leads, fidelity of signal, interference for rejection, and so on. While the cardiac monitors have facilities for display of 12-leads, but are generally operated as a 5-, 3-, or 1-lead systems. The essential requirements of AFE include the need to reject interference from strong RF sources, pace signals, lead-off signals, respiration signals, common-mode line frequency, signals from other muscles, and electrical noise. All this is performed on several channels simultaneously. Besides, most cardiac monitors are required to recover quickly from a defibrillation event, which can saturate the front end and charge capacitors. 4 Biomedical. Eng. Dept. 5th stage Lec. 2 Input Circuit The complete input circuit used in present-day cardiac monitors has three main circuit blocks: (i) Low pass filter circuit, to suppress RF interference from electrosurgery machines, (ii) High voltage protection circuit, similar to electrocardiographs, to provide voltage clamp in the presence of defibrillator pulses, (iii) Over voltage protection circuit (iv) Leads off detection. Electrosurgery Interference Electrosurgery machines generate RF signals within a range of (0.4–5) MHz with peak-to-peak amplitudes of 100-1000 V, pulse modulated at rates from (1.5 to 25) kHz for coagulating or 120 Hz for cutting. Cardiac monitors are often used in operation theatres, where the RF is applied through a pointed scalpel at the point of incision, and the return path for the current is through a wide area electrode on the opposite side of the patient’s body. RF interference occurs due to any one of two possible modes. The first and usually the most severe is due to conduction, i.e. the RF energy is actually carried via the patient into the monitor. The second is radiation, by which the RF energy is transmitted through the air and is induced into the circuits of the monitoring instrument and its leads and cables. The ECG signal, on the other hand, is of the order of 1mV with frequency components below 100 Hz. The ECG input amplifier, the ECG electrode-skin interface, and the scalpel-tissue interface are the main sites where interference occurs. Moreover, the common mode RF signal gets converted into a normal mode signal by an imbalance in the capacitance between input to ground and the differential amplifier input. 5 Biomedical. Eng. Dept. 5th stage Lec. 2 In order to reduce interference caused by electrosurgery and RF emissions, it is very essential to use filters at the input of leads of the monitoring instruments. To minimize this problem, a guard is used, which besides shielding the input circuits from electromagnetic radiation also helps in equalizing the capacitance from input to ground of each amplifier input. Leads Off Detector The ‘leads off’ detector circuit usually works on the principle that loss of body contact of electrode causes a high impedance change at the electrode/ body contact surface, consequently causing a loss of bias at the appropriate amplifier input. This sudden change makes the amplifier to saturate, producing maximum amplitude waveform. This waveform is rectified and applied to a comparator that switches on an alarm circuit (leads off) when the waveform exceeds a certain amplitude. Common-Mode Rejection Ratio (CMMR) The ability of the amplifier to reject common voltages on its two input leads is known as common-mode rejection (CMR) and is specified as the ratio of common-mode input to differential input to elicit the same response. In addition to the millivolt level ECG signal, there is a DC offset signal that is hundreds of millivolts, with channel-to-channel common- mode voltages differing by over a volt. Some of the 50/60 Hz common-mode interference can be cancelled with a high-input-impedance instrumentation amplifier, which removes the AC line noise common to both inputs. To further reject line power noise, the signal is inverted and driven back into the patient through the right leg by an amplifier. Only a few microamps or less are required to achieve significant CMR improvement. In addition, 50/60 Hz digital notch filters are used to reduce this interference further. 6 Biomedical. Eng. Dept. 5th stage Lec. 2 Frequency Response The frequency range of interest for the ECG varies somewhat with the application but is usually around 0.05–100 Hz. Some monitors have two selectable frequency response modes, namely, monitor and diagnostic. In the ‘monitor’ mode or ‘filter-in’ mode, both the low and high frequency components of an electrocardiogram are attenuated. It is used to reduce baseline wander and high frequency noise. The monitor mode bandwidth is generally 0.4– 50 Hz (3 dB points). In the ‘diagnostic’ mode, the instrument offers expanded bandwidth capability of 0.05–100 Hz. Pacemaker Signal Detection Filters after the instrumentation amplifiers remove the pacemaker signals before the ECG signal is digitized by the analog-to-digital converter (ADC). The pace signal is passed through a high pass filter, amplified, and detected by a comparator circuit. Multiplexer A separate ADC is used for each lead, or one ADC can be multiplexed with simultaneous sampling to digitize multiple leads. Powerful ADCs with a resolution of about 20–24 bits at a rate of 200 kbps are used to simultaneously digitize the signals on all nine electrodes. When multiple ADCs are used, all the ADC channels must be tightly matched. Microprocessor/DSP A microprocessor or preferably a digital signal processor (DSP) is used to calculate the signal for each lead, isolate the pace signal, isolate the lead-off/respiration signals, and filter out unwanted frequencies. The DSP also calculates values for a digital-to-analog converter (DAC) driving the right leg drive (RLD) electrode. The output from the DSP goes to LCD display, a hard copy printer, and an RF link. In addition, ECG may be duplicated at a central console at nursing monitoring station via a wired or wireless connectivity. 7 Biomedical. Eng. Dept. 5th stage Lec. 2 Although most cardiac monitors are commonly used in emergency rooms and critical care areas, cardiac monitoring at central station allows for continual observation of several patients simultaneously. Cardiac monitors are also available, which display more than one channel of information such as ECG and pulse waveform with digital heart rate display and alarm facility. The display should be minimum for a four second period. They use time-multiplexing of complete sweeps to avoid the excessive bandwidth introduced by electronic switching or the chopper technique. Each channel, in these instruments, can be independently controlled for updating or freezing the information. Texas Instrument has developed ADS1298 integrated circuit, which provides standard 12- lead ECG and integrated AFE along with 24-bit ADC. The ADS1298 reduces component count and power consumption by up to 95% and, thus, is very useful for portable ECG monitors. The Analog Devices AD8232 is another integrated signal conditioning block for cardiac monitors. It is designed to amplify and filter small biopotential signals in the presence of noisy conditions, such as those created by motion or remote electrode placement. The AD8232 can implement a two-pole high pass filter for eliminating motion artefacts and the electrode half-cell potential. The user can select the frequency cut-off of all filters to suit different types of applications. To improve CMR of the line frequencies in the system and other undesired interferences, the AD8232 includes an amplifier for driven lead applications, such as RLD. 2. Connectivity Modern cardiac monitors have provisions for Ethernet and USB connectivity to meet the networking requirements of hospitals and clinics. Wired connectivity is perhaps the preferred method for connecting to the hospital network; however, the trend to go wireless is growing. Wi-Fi is the most widely used wireless standard in the hospitals due to its widespread availability making device integration relatively easy, but due to higher power 8 Biomedical. Eng. Dept. 5th stage Lec. 2 consumption of Wi-Fi, cardiac monitoring devices working on solely on batteries include lower power wireless technologies such as Bluetooth, Bluetooth low energy, and ZigBee. 3. Software Algorithms Most of the present-day cardiac monitors incorporate computerized software that recognizes life-threatening cardiac arrhythmias that aid in the automatic analysis of the ECG signals and detect arrhythmias. Although detection of cardiac arrhythmias is the job of a highly trained clinician, algorithms that enable higher degrees of diagnosis accuracy and precision have now been developed. The software automatically identifies and labels the significant features of the ECG complex. It also calculates the amplitudes and timing intervals of various cardiac events and displays the results on the monitor. The heart rate variability (HRV) is analyzed with the online R–R calculator and automated HRV analysis especially before, during, and after exercise or dosing. Portable ECG monitors are now available as personal devices that record user’s cardiac function in single lead and display the waveform. They are useful for daily health check. This device is intended for self-testing by adult users who might experience transient symptoms that may suggest cardiac abnormality so that irregularities in the heart may be detected and managed at an early stage. The pocket-sized device weighs only 100 g and can be used at home or outside for recording ECG. The stored recordings can then be shown to the doctor, who can examine and use the information to derive accurate diagnosis based on the symptoms. 9 Biomedical. Eng. Dept. 5th stage Lec. 2 10