The Monitored ECG

There are two primary applications where a patient’s ECG is continuously monitored. Intensive care units within a hospi­tal often monitor the ECG of critically ill patients to observe the patient’s rate and rhythm. When the patient is suspected of having a life-threatening arrhythmia, it is best to monitor the patient in an environment where a rapid response and therapeutic intervention can be lifesaving. The other applica­tion of the monitored ECG is in the ambulatory, outpatient setting. The patient’s ECG can be monitored by a belt-worn device. In the 1950s Norman J. Holter (13), an American physicist, demonstrated that the ECG could be monitored while the subject was physically active. However, the technol­ogy of the day resulted in a very heavy backpack device weighing 85 lb and was impractical for routine use. Recording devices and their associated electronics and batteries eventu­ally became small enough to allow for a belt-worn tape re­corder using originally reel-to-reel technology, but now rely­ing exclusively on cassettes. These tape devices still have many inherent limitation, such as poor noise figures, low dy­namic range, and limited frequency response. If one is merely recording the patient’s rate and rhythm, then the tape tech­nology is adequate. However, high-resolution ECGs might also be useful when obtained from the ambulatory patient, and the tape technology is definitely limited for this appli­cation.

Newer digital recorders are currently available whereby the ECG is digitized and stored in either high density mem­ory chips or on actual hard disk drives. Current versions of the latter have removable drives with capacities exceeding 500 Mbytes. Depending on the application, the ECG may be sampled between 250 Hz and 1000 Hz. Originally, only one ECG lead was recorded on tape-based systems, but the new systems are not limited by the poor frequency response of tape systems or the physical size constraints of magnetic re­cording heads. With digital systems the number of simultane­ous (or near simultaneous) recordings is not particularly lim­ited, but three or four ECG leads are a practical number. Electrode positions for these monitored leads do not follow the conventions of the 12-lead ECG and are often similar to the bipolar limb leads, where the electrodes are placed a few inches apart over the chest, creating several lead fields through the heart.

For hospital monitoring, where the patient is being evalu­ated for a critical cardiac condition, only a few leads are re­corded, but a full 12-lead ECG is periodically recorded. In some cases the patients, although not acutely ill, are given the freedom to walk about the hospital and their ECG is tele­metered via radio frequencies by an antenna/receiver net­work. In such cases the goal is to monitor the patient’s rhythm in ‘‘normal’’ activities. The Monitored ECG

The massive amount of ECG information obtained during continuous monitoring is overwhelming. In the hospital ap­proach, the ECG signals are usually fed to a large system of monitor screens where specially trained technicians view the actual recording of 10 to 50 patients. In conjunction with com­puter-based software the high risk situations are quickly identified with appropriate communication to the medical staff, for example, ‘‘code blue.’’ This is not the case with outpa­tient monitors where the patient returns to the hospital 1 or 2 days later. Then the entire record is scanned with an inter­active software analysis program. Often just a compressed printout of the continuous ECG can be quickly inspected for an abnormal rhythm. An example of this ‘‘full disclosure’’ mode is shown in Fig. 11. A 7.5 min recording is shown in this format. Note that the first several minutes have a normal rhythm (there are several other abnormalities in this tracing, but they are beyond the scope of this article). The abnormal beats begin to appear in groupings of two or three. A condi­tion known as nonsustained ventricular tachycardia appears in the fourth trace from the bottom.

The event recorder is an extension to ambulatory re­cordings. In this case the patient wears a recorder for many days or even weeks. When the patients experience a symp­tom, such as chest palpitations or dizziness, they push an event button on the recorder which causes the recorder to save 1 min to 2 min of data before and after the event. The patient can call the physician office and transmit these data via a modem for rapid interpretation. The most advanced ver­sion of the event recorder is an implantable device that moni­tors the ECG for months or years and uses special monitoring software to record suspicious events without patient activa­tion. This type of unit can be interrogated at regular intervals over the phone or during regular visits.

Leave a Reply

Your email address will not be published. Required fields are marked *

Confirm that you are not a bot - select a man with raised hand: