Continuous ECG monitoring of the hospitalized patient usually is accomplished by using a simplified system of three electrodes placed on the right and left upper chest and the left hip. The electrodes function as bipolar leads that measure the potential between two points, one positive and one negative. In so doing, they record ECG leads I, II, and III, which by standard convention are calibrated to a 1-mV deflection equaling 10 mm on the strip-chart recorder with paper speed set at 25 mm/second. The electrode pads themselves should be moist and not outdated. They should be placed on skin that has been cleaned to remove
all sources of electrical resistance. Electrodes may need to be moved if an adequate ECG tracing is not obtained. The lead that gives the best visualization of the P, QRS, and T waves should be chosen for display, especially when monitoring for arrhythmias. Most bedside ECG systems feed to a standard oscillographic display at the bedside and to a remote display screen at a central location for additional surveillance. Monitors in the intensive care unit (ICU) should allow permanent graphic recording of the ECG.

A common error in the interpretation of monitoring ECGs is to diagnose atrial enlargement from an oscilloscopic tracing displaying data from a single lead, without realizing that the tracing has a high sensitivity that gives the P waves an exaggerated height. Similarly, ST-segment abnormalities sometimes are diagnosed erroneously from these electrographic strip recordings. Artifact waveform recordings caused by loose or broken leads, leads placed on bony prominences, or patient movement (particularly during rhythmic chest physiotherapy) also can produce misleading ECG tracings. Whenever the morphologic characteristics of a monitor recording are in question, a standard 12- or 15-lead ECG should be obtained for precise diagnosing. Continuous ECG monitoring is extremely helpful in detecting arrhythmias such as sinus tachycardia, sinus bradycardia, supraventricular tachycardia, and ventricular tachycardia.

The routine monitoring of the respiratory rate and depth of respiration is performed using transthoracic bioimpedance. This technique involves placing one electrode on either side of the thorax and a reference electrode over the hip or the apex of the heart. In pediatric ICUs, the same electrodes and oscilloscope used for ECG surveillance are used to monitor respiratory parameters. Changes in both intrathoracic lung and blood volumes cause proportional changes in electrical resistance. This change in resistance causes the bioimpedance changes that are used to measure the respiratory rate and depth of respiration. Alarm systems are incorporated to identify respirations that are abnormally high, low, or absent. This technology has gained acceptance for monitoring apnea in both the hospital and home settings. Like oscilloscopic ECG monitors, respiratory monitors are subject to mechanical interference by ventilator breaths, patient movements, and loose or inappropriate lead placement.