Changes in transthoracic electrical impedance during submaximal treadmill exercise in patients with ischemic heart disease--A preliminary report

Bioimpedance-derived Differences in Cardiac Physiology during Exercise Stress Testing in Low-risk Chest Pain Patients

BACKGROUND

Little has been written about the utility of thoracic electrical bioimpedance (TEB)-derived cardiac physiologic variables in evaluating patients with low-risk chest pain syndromes. Noninvasive bioimpedance can monitor cardiac physiology while a patient is performing an exercise stress test. In addition, the demographics of patients with chest pain, the incidence of coronary artery disease (CAD), and the methods used for evaluation have well-documented sex differences.

OBJECTIVE

The objectives are to show that there are different cardiac physiologic responses to exercise stress test in Chest Pain Evaluation Unit patients with and without true CAD that could be used to stratify patients and that there is a sex difference in TEB results.

METHODS

Patients 18 to 65 years of age with low-risk chest pain were eligible. Patients were attached to the TEB throughout the exercise stress test procedure. Heart rate (HR) was monitored. Primary dependent variables were TEB-measured cardiac output (CO, L/min) and stroke volume (SV, ml) at peak exercise. Secondary variables were TEB-measured ejection fraction (%), end-diastolic volume (EDV, ml), ventricular ejection time (ms), and thoracic fluid index (omega) at peak exercise. Outcome variables were either proved CAD or patient sex. CAD was proved by angiography, stress scintigraphy, or stress echocardiogram. Results were compared using a Student's t test assuming equal variances, with significance considered at a P < 0.05, and 95% confidence intervals were calculated for significant results.

RESULTS

Nine patients had proved CAD, 82 patients did not. Forty-three women and 48 men were included in the study. At peak exercise, patients with CAD had a significantly smaller increase in EDV than patients without CAD (32.8 +/- 59.5 ml versus 89.3 +/- 101.8 ml) without a significant change in CO, SV, or HR. At peak exercise, women had a significantly smaller increase in CO and SV without a significant change in HR. In addition, women had a significantly smaller increase in EDV.

CONCLUSION

When compared with patients without CAD, patients with CAD have a significantly smaller increase in EDV and a trend toward the same effect in CO and SV. Women have significantly smaller increases CO, SV, and EDV compared with men. Because there were no differences in HR, using HR as the sole end point would miss these differences. TEB is a practical means of measuring these variables.

Application of impedance cardiography during exercise

Impedance cardiography has been used over the last 30 years to measure stroke volume on a beat-by-beat basis. Cardiac output has been successfully measured with either upper or lower body exercise during light or moderate workloads. With strenuous exercise, movement artifacts severely limit the acquisition of a quality impedance cardiogram. Advances in computer technology and signal conditioning techniques have created the next generation of impedance cardiograph systems. The purpose of this study was to evaluate such a system, the noninvasive continuous cardiac output monitor (NCCOM3-R7), at rest and during submaximal upright cycle exercise. In addition, the relationships between thoracic impedance (Z(o)), first derivative of the change in thoracic impedance (dZ/dt) and posture were evaluated using the NCCOM3-R7 and the Minnesota impedance cardiograph 304B (MIC). Twenty-eight healthy men and women participated. The Z(o) progressively increased when moving from the supine to seated to standing position with both instruments. However, the NCCOM3-R7 yielded lower Z(o) values and higher dZ/dt values compared with the MIC for all postures. Z(o) and dZ/dt values appear to be dependent upon factors such as posture, gender, electrical current, and characteristics of the instrumentation. Exercise cardiac output values seemed reasonable for most subjects, although population subsets exist where the accuracy must be questioned. The general consensus supported by the impedance literature and reaffirmed by the present observations is that impedance cardiography provides a reasonable estimate of the directional changes in stroke volume and cardiac output during exercise and can be used to monitor changes in thoracic fluid balance. As this technology evolves and is further refined, it will undoubtedly play an increasing role in environmental medicine, exercise stress testing, cardiac rehabilitation, and sports medicine.

A practical, cost-effective, noninvasive system for cardiac output and hemodynamic analysis

Impedance cardiography is a relatively inexpensive, noninvasive technique for measuring cardiac output on the basis of resistive changes in the thorax to electrical current flow. In conjunction with blood pressure monitoring and physiologic maneuvers, the technique may be used to monitor thoracic and total body fluid volume and express a variety of contractility indexes as well as relative and absolute measurements of stroke volume. We have tested hemodynamics in our laboratory by using a cost-effective, powerful microcomputer-based portable noninvasive technique, which makes possible the ensemble averaging of impedance cardiographic waveforms. In conjunction with physiologic maneuvers, the technique has been implemented at our institution and has provided helpful information in our experience in evaluating volume overload, hypertension, hypotension, shock, and heart failure. It is hoped that this noninvasive, relatively cost-effective approach will be more widely appreciated in the future, given the economic realities of medicine today.

Continuous noninvasive monitoring of left ventricular function during exercise by thoracic impedance cardiography-automated derivation of systolic time intervals

Systolic time intervals (STI) obtained during exercise are useful as a method of estimating global left ventricular function. The conventional method, however, which requires a carotid pulse tracing as well as a phonocardiogram of high quality, is technically difficult under conditions of exercise. We have validated a new method of obtaining STI which employs the first derivative of thoracic electrical impedance (DZ/DT). The DZ/DT was recorded using surface electrodes and a microcomputer for automated signal processing. The new method was studied in 20 male normal subjects (aged 18 to 53 years) at rest and during increasing levels of upright exercise. Heart rate ranged from 61 to 173 beats/min. Results obtained simultaneously by both techniques showed no significant difference. Thus impedance cardiography allows continuous monitoring of STI during exercise and may prove to be a valuable addition to multistage stress testing.

A comprehensive cardiac exercise stress processor for environmental health effects studies

We have shown that an interactive microcomputer system using noninvasive cardiovascular measurements during exercise is both possible and practical. Experimental use of the system has verified our choice of variables as appropriate for automatic generation of a cardiovascular data base, but additional studies are required to determine the system's sensitivity for assessing health-effect decrements.

Evaluation of transthoracic electrical impedance in the diagnosis of pulmonary edema

To evaluate the clinical usefulness of measuring transthoracic electrical impedance in patients with pulmonary edema, we studied 27 normal subjects and 33 patients. In normal subjects, impedance increased when body position changed from supine to standing (p less than 0.01) and when lung volume increased from residual volume to total lung capacity (p less than 0.01). Impedance values vari7 omega) when measured hourly without removing the electrodes. In only five of 11 patients with clinically severe pulmonary edema and five of eight with radiologically severe pulmonary edema were impedance values outside the normal range. Changes in impedance values correlated with changes in clinical and radiographic indexes of pulmonary edema. We conclude that (1) single measurements of impedance are useless in diagnosing pulmonary edema, and (2) impedance changes as predicted as pulmonary edema increases and decreases, and these changes are readily detected by simpler methods of evaluation.

Monitoring of pulmonary fluid volume and stroke volume by impedance cardiography in patients on hemodialysis

Changes of intrathoracic fludi volume during maintenance therapy with hemodialysis were studied in ten patients with the aid of impedance cardiography. It was found that there was a statistically significant correlation between the decrease in body weight and the increase in basic thoracic impedance in the individual patients, but the values differed considerably from one patient to another. Therefore, comparison of sequential measurements is feasible only in one and the same patient. Out of 16 patients with overhydration undergoing dialysis, the stroke volume could not be determined by impedance cardiographic studies in eight, owing to the abnormal morphology of the dZ/dt curve. Moreover, because the basic thoracic impedance was not constant during treatment, the calculated changes in stroke volume did not appear to be reliable. This has to be taken into account when repeated measurements are made under clinical conditions. In some of the patients, the dZ/dt curve displayed interesting anomalies, probably resulting from hemodynamic alterations. The diagnostic meaning of these features is not yet known.

Changes in transthoracic electrical impedance at high altitude

Mean transthoracic electrical impedance (impedance) which is inversely related to intrathoracic extravascular fluid volume was measured in 121 normal healthy volunteers at sea-level and at 3658 metres altitude. Fifty (group A) reached the high altitude location after an hour's journey in a pressurised aircraft. Twenty-five (group D) underwent slow road ascent including acclimatisation en route. Thirty permanent residents (group B) and 16 temporary residents at high altitude (group C) were also studied. Serial studies in the 30 subjects of group A who developed symptoms of high altidue sickness showed a significant decrease of impedance up to the fourth day of exposure to high altitude which later returned to normal. The 4 volunteers who developed severe symptoms showed the largest drop in impedance. A case of acute pulmonary oedema developing at 4300 metres showed an impedance value of 24-1 ohms on admission. After effective treatment the impedance increased by 11-9 to 36-0 ohms. Twenty asymptomatic subjects of group A and 25 of group D showed a small average increase in impedance values at high altitude. These obstructions suggest that measurement of transthoracic electrical impedance may be a valuable means of detecting incipient high altitude pulmonary oedema.