DISPLAY, RECORDING, AND INTERPRETATION
The American Heart Association (AHA)[23]
has published instrumentation and practice standards for electrocardiographic monitoring
in special care units. They consist of performance standards, performance requirements,
and disclosure requirements. They include recommendations that monitors be able
to simultaneously display and analyze multiple leads and that users be informed of
minimally acceptable accuracy in a standardized manner. Many of the principles detailed
in these standards are also applicable to intraoperative monitoring.
Basic Requirements
The function of the electrocardiographic monitor is to detect,
amplify, display, and record the electrocardiographic
Figure 34-12
Diagram of tracheal tube electrocardiograph system.
(From Narang J, Thys DM: Electrocardiographic monitoring. In
Ehrenwerth J, Eisenkraft JB [eds]: Anesthesia Equipment: Principles and Applications.
St. Louis, Mosby-Year Book, 1992, p 284.)
signal. The ECG is usually displayed on an oscilloscope. Several monitors offer
nonfade storage oscilloscopes to facilitate wave recognition. All electrocardiographic
monitors for use in patients with cardiac disease should also have paper recording
capabilities. The recorder is needed to make accurate diagnoses of complex arrhythmias
and to allow careful analysis of all the electrocardiographic waveforms. The recorder
allows the examiner to differentiate real electrocardiographic changes from artifacts.
Oscilloscope Displays
Most modern oscilloscopes are high-resolution monochrome or color
monitors, similar to those used in computer technology. They frequently allow considerable
flexibility in screen configuration, including waveform positions, colors, and sweep
speeds. The norm in modern technology is to display three electrocardiographic channels
simultaneously. These usually consist of two limb leads and one unipolar precordial
lead. In addition to the waveforms, average heart rates and optional arrhythmia
and ST-segment information are displayed in alphanumeric format (discussed later).
Standard Electrocardiographic Recordings
The ECG is normally recorded on special paper consisting of grids
of horizontal and vertical lines. Distances between vertical lines represent time
intervals, whereas distances between horizontal lines represent voltages. The lines
are 1 mm apart, with every fifth line intensified. The speed of the paper is standardized
to 25 mm/second. On the horizontal axis, 1 mm represents 0.04 second, and 0.5 cm
represents 0.20 second. On the vertical axis, 10 mm represents 1 mV. On every recording,
a 1-cm (1-mV) calibration mark should indicate that the ECG is appropriately calibrated.
The user should follow the manufacturer's recommended calibration procedure for
each monitoring episode. Strip chart recorders that are part of an electrocardiographic
monitoring system should meet all the standards of time-based accuracy, frequency
response, linearity, and so forth, proposed for conventional electrocardiographic
recording systems.[24]
[25]
