Putting the ECG data acquisition subsystem into a Component Monitoring System parameter module mandates high-density packaging and low power consumption, and was only possible by implementing major elements of the circuit in a large mixed analog-digital ASIC.

Nearly everyone is familiar with one of the most important medical parameters-the electrocardiogram, or ECG. The electrical voltages created by the heart have been well-known to the medical community for nearly a century. In the beginning the ECG was measured by sensitive galvanometers with the patient’s hands and feet placed in vessels filled with saline solution. Improvements in the assessment of diagnostic ECG signals have been closely related to the evolution of electronics, great strides being made when amplifying devices such as vacuum tubes and later transistors became available. Today, low-noise operational amplifier circuits are state-of-the-art for ECG signal processing.

Physiologically, the polarization and depolarization of the heart’s muscle mass creates a three-dimensional electrical field that changes with time. As a result, voltages can be measured on the surface of the body that represent the pumping cycle of the myocardium. A strong effort has been made to standardize the points at which the voltages should be measured. The most widely used are three differential voltages: From right arm (RA) to left arm (IA), from LA to left leg (LL), and from LL to RA. These voltages are known as ECG leads I, II, and III. The right leg electrode (RL) acts as the neutral pole in this system.

ECG Signal Characteristics

The amplitude of the ECG signal as measured on the skin ranges from 0.1 mV to 5 mV. The frequency extends from 0.05 Hz to 130 Hz. Physiological signals like the ECG differ from artificial signals in that they are not reproducible from one time segment to another. They are more statistical in nature and have larger variations in signal characteristics than, say, a signal generator output. This places additional requirements on the measurement system, especially the analog input stages. Although the average amplitude is only around 1 mV, there are large dc offset voltages because of electrochemical processes between the electrode attached to the patient and the patient’s skin. These can be as high as +/-500 mV. Also, the contact resistance between an electrode and the body surface can vary widely and reach values around I Mil. In addition, the system must be capable of detecting that an electrode has fallen off the patient. Perhaps the largest constraint is the presence of 60-Hz or 50-Hz power line noise. The human body acts like the midpoint of a capacitive divider between one or more power lines and ground. Thus, common-mode voltages as high as 20V p-p can be superimposed on the body. Eliminating this source of noise is one of the major tasks of an ECG amplifier. Fortunately, the ECG signals are differential signals while the power line voltages are common-mode, so the noise can be reduced with differential amplifiers.

Another requirement results from artificial pace pulses used to stimulate the heartbeat of some patients. Pacemaker devices are implanted into the chest, generating small pulses up to 1V p-p at frequencies in the kilohertz range. Pace pulses are superimposed on the ECG signal and have to be detected to differentiate them from the peak value of the ECG signal, called the QRS complex. This is important because the heartrate measurement is based on this QRS signal, and an incorrect interpretation would result in an incorrect value.

In emergencies when the heart stops beating ventricular fibrillation), a commonly used procedure is to apply a voltage pulse of about 5 kV p-p with a 5-ms duration to synchronize the neural stimulus of the heart’s muscle mass and bring it back to normal operating conditions. Because of the high voltages needed to defibrillate a patient, the inputs of the ECG circuit must be protected. Other sources of noise are electrosurgery devices, which are used in operating rooms as electronic scalpels. These devices contain high-frequency currents in the megahertz range. The high current density at the tip of the electrode coagulates the protein, thereby stopping bleeding. The ECG module must provide additional filtering against this high-frequency noise.

Integrated Solution

The design goals for the Component Monitoring System ECG module included reduced cost, reduced size, minimum power consumption, and increased reliability arid functionality compared to the current patient monitor generation.

The target size was a single-width parameter module. This module measures only 99.6 mm by 36 mm by 97.5 mm (3.9 in by 1.4 in by 3.8 in). It was therefore obvious that we would have to use surface mount technology to meet the size objective. In addition, it soon became apparent that a large percentage of the electronic circuit would have to be integrated in silicon if the entire device was to fit into a single-width module. This and the need for cost reduction on such a high-volume parameter module as the ECG module clearly indicated that an application-specific integrated circuit (ASIC) would be the appropriate solution.