higher, but the phases are still random to reduce the duty cycle of individual SMUs. It is this asynchronicity that makes strong EMGs look like noise on a CRT display.

1.4.3EMG Amplifiers

The amplifiers used for clinical EMG recording must meet the same stringent specifications for low-leakage currents as do ECG, EEG, and other amplifiers used to measure human body potentials (see Chapter 8). EMG amplifier gains are typically X1000 and their bandwidths reflect the transient nature of the SMU action potentials. An EMG amplifier is generally reactively coupled, with low and high −3-dB frequencies of 100 and 3 kHz, respectively. With an amplifier having variable low and high −3-dB frequencies, one generally starts with a wide-pass bandwidth, e.g., 50 to 10 kHz, and gradually restricts it until individual EMG spikes just begin to round up and change shape. Such an ad hoc adjusted bandwidth will give a better output signal-to-noise ratio than one that is too wide or too narrow.

EMGs can be viewed in the time domain (most useful when single fibers or SMUs are being recorded), in the frequency domain (the FFT is taken from an entire, surface-recorded EMG burst under standard conditions), or in the time–frequency (TF) domain (see Section 3.2.3 of Northrop, 2002). In the latter case, the TF display shows the frequencies in the EMG burst as a function of time. In general, higher frequency content in the TF display indicates that more SMUs are being activated at a higher rate (Hannaford and Lehman, 1986). TF analysis can show how agonist–antagonist muscle pairs are controlled to perform a specific motor task.

Still another way to characterize EMG activity in the time domain is to pass the EMG through a true RMS (TRMS) conversion circuit, such as an AD637 IC. The output of the TRMS circuit is a smoothed, positive voltage proportional to the square root of the time average of x2(t). The time averaging is done by a single time-constant, low-pass filter. For another time domain display modality, the EMG signal can be full wave rectified and lowpass filtered to smooth it.

1.5The Electrocardiogram

1.5.1Introduction

One of the most important electrophysiological measurements in medical diagnosis and patient care is that of the electrocardiogram (ECG or EKG). Because the heart is an organ essentially made of muscle, every time it contracts during the cardiac pumping cycle, it generates a spatio–temporal

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electric field coupled through the anatomically complex volume conductor of the thorax and abdomen to the skin, where a spatio–temporal potential difference can be measured. The amplitude and waveshape of the ECG depends on where the measuring electrode pair is located on the skin surface.

Before electronic amplification was invented, Willem Einthoven measured the ECG in 1901 using a magnetic string galvanometer. The galvanometer was connected to the patient by two wires connected to two carbon rods immersed in two jars of saline solution in which the patient placed either two hands or a hand and a leg (Northrop, 2002). With the advent of electronic amplification in 1928, it was quickly discovered that many interesting features of the ECG could be revealed by using different electrode placements (e.g., AV and precordial leads, and the Frank vector cardiography lead system) (see Chapter 10 through Chapter 12 in Guyton, 1991; Section 4.6 in Webster, 1992; and Section 4.4 in Northrop, 2002).

Figure 1.4 illustrates schematically the important pacemaker, cardiac muscle and conduction bundle transmembrane potentials in the normal human heart and their relation to the classic, Lead III ECG wave. Note that, following atrial contraction, excitation is conducted to the AV node and then to the ventricles by a complex network of specialized muscle cells forming the conduction bundle system. Propagation delay through the bundles and Purkinje fibers allows the ventricles to contract after the atrial contraction has had time to fill them with blood. The QRS spike in the ECG is seen to be associated with the rapid rate of depolarization of ventricular muscle just preceding its contraction. The P wave is caused by atrial depolarization and the T wave is associated with ventricular muscle repolarization.

1.5.2ECG Amplifiers

Wherever recorded, the ECG QRS spike can range from a 400-μV to 2.5-mV peak. Its amplitude depends on the recording site and the patient’s body type; thus the gain required for ECG amplification is approximately 103. ECG amplifiers are reactively coupled with standardized −3-dB corner frequencies at 0.05 and 100 Hz. If ECG bandwidth were not standardized, ECG interpretation would be difficult and confusing. Most ECG amplifiers allow the operator to switch in a 60-Hz notch filter to attenuate 60-Hz interference that can appear at the output in spite of differential amplification. The notch filter causes little distortion of the raw ECG output signal.

A further requirement of all ECG amplifiers is that they have galvanic isolation (see Chapter 8), which is required to protect the patient from electroshock accidents. Galvanic isolation places a very high impedance between the patient, the ECG electrodes, and ECG amplifier input ground, and the ECG amplifier output and output ground. This limits any current that might flow through the patient to the single microamps if the patient accidentally makes contact with the power mains while connected to the ECG system

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