LETTER

Ambulatory impedance cardiography: new possibilities

To the Editor: The primary objective of the study by Sherwood et al. (5) was "to assess how a newly developed ambulatory impedance monitor (AIM) would compare with established impedance cardiographic instrumentation." Furthermore, "the study's objective was not to revisit the validation of impedance cardiography as a technique per se but rather to evaluate whether the AIM system, with its unique hardware and electrode configuration, would yield cardiovascular function indexes similar to those that are obtained with standardized impedance cardiographic methodology."

In my opinion, simultaneous, long-time recording of electrocardiogram and the signal reflecting the central hemodynamic activity might bring some further useful diagnostic data, particularly in arrhythmia patients and in pharmacological studies. Considering all limitations of the technique, it seems that electrical impedance cardiography, as a simple method for continuous, noninvasive determination of stroke volume, maximum velocity of ejection, and ejection time, could be used to provide such a signal. Cardiac contractility and the stroke volume indexes might be also used for determination of hemodynamic efficiency in healthy individuals (in sports medicine or work physiology) or in patients during regular pacing and arrhythmic or ischemic events for supplementary diagnosis or for evaluation of pharmacological therapy.

From this point of view, development of the new device and the feasibility study performed by Sherwood et al. (5) seem to be important and very promising. However, these authors, who describe the "unique hardware and electrode configuration" of their AIM system, did not mention previous achievements in this field. According to a search performed using the Ovid Technologies database (which I accessed after publishing my papers on this subject), the idea of impedance cardiography ambulatory monitoring with signal recording on memory chips was introduced in 1985 and 1987 by Webster's group (4, 7) and, with the use of Personal Computer Memory Card International Association (PCMCIA) cards, in 1995 and 1996 by Cybulski et al. (1, 2). These conference papers were published in books by IEEE or Peter Peregrinus. In 1996, Willemsen et al. (6) described the ambulatory monitoring of impedance cardiogram device and the results collected in 25 subjects in various conditions and in 26 subjects during 24-h monitoring. This device, however, enabled only storage of the results of calculations performed on ensemble-averaged signals, so it did not give access to every single heartbeat.

We have constructed an ambulatory-monitoring, four-channel recorder that allows the simultaneous collection of electrocardiogram and impedance cardiography signals from a built-in miniaturized impedance cardiography device on a 20-MB PCMCIA Flash Memory Card (1, 2). This system allows for off-line, beat-to-beat automatic evaluation of cardiac output, stroke volume, ejection time, preejection period, and heart rate. The system was checked in 80 healthy subjects in ambulatory conditions (1-3). During the day, the percentage of properly recognized heart cycles with respect to all above-mentioned parameters varied from 20 to 80% and at night from 75 to 90%. Speaking and vigorous movements markedly lowered this percent. The system was also used in clinical examinations to demonstrate the hemodynamic efficiency of pacing and in 10 arrhythmic patients. A similar study showed the increased variability of stroke volume in patients with atrial fibrillation. Some interesting data were also obtained before and during 8-s asystole in a young man provoked by the tilt test (3). Also in this study (3), hemodynamically efficient and nonefficient cardiac supraventricular ectopics were shown in the same patient. Using the system, we were also able to demonstrate the hemodynamic effects of dynamic exercise on a cycloergometer with the work load up to 200 W (7).

Technically, our system is very flexible and can be easily adapted for recording from various physiological signal sensors, such as a breathing detector or pulse oximeter, which may extend the number of its applications.

I am very pleased that Sherwood et al. (5) successfully developed and initially tested another construction; their work may help in the exchange of ideas on possible applications of their system and the technique per se in the future.