Impedance Cardiography: Clinical Evaluation of Endotracheal Cardiac Output Monitor (ECOM)

Introduction: Cardiac output is a key physiological parameter. Unfortunately, it is often difficult to measure without using invasive techniques with associated risk. The pulmonary artery catheter has, since its introduction, been considered the gold standard for the measurement of cardiac output in man. Unfortunately, pulmonary artery catheters have been associated with serious complications.1,2 Thoracic electrical bio-impedance (TEB) has been suggested as a possible non-invasive technique for the measurement of cardiac output. The accuracy and reliability of TEB has been evaluated multiple times with some studies demonstrating good correlation with thermodilution3-7 and others with poor correlation.6,8-15. TEB is not reliable in patients after cardiopulmonary bypass8,9, kidney transplants11, congestive heart failure16, pulmonary edema14, sepsis12, pregnancy15, abdominal surgery11,17, or critical illness13,14.

One of the limitations of TEB is the signal to noise ratio. Commonly TEB systems use an alternating current (1 - 4 ma at 20-100 Khz) applied to the skin and have little control over the percentage of current passing through the vascular structures in the chest. Changes in the electrical impedance of the lungs with respiration change the percentage of the total current passing through blood containing structures. Finally, treating all of the blood containing structures in the chest as a single impedance to be measured, does not allow separation of the signal into its various components. A technique which stabilizes the percentage of current delivered to the target structure and records the electrical impedance signals directly from that target structure improves the accuracy and reliability of impedance based measurement of cardiac output.

The Endotracheal Cardiac Output Monitor (ECOM) is a system which records the voltages produced by a current (2 ma 100 Khz) delivered to the tracheal mucosa by electrodes on an endotracheal tube. The proximity of the ascending aorta and trachea allows the design of a device that can optimize the current delivery and signal recording from impedance changes in the ascending aorta. This study will test the accuracy and efficacy of the ECOM system in anesthetized patients.

Significance: A simple, inexpensive, continuous, non-invasive, accurate, reproducible method of measuring cardiac output in patients would reduce the need for pulmonary artery catheter measurements of cardiac output. Such a technique would allow the rapid diagnosis and treatment of hemodynamic instability in the critically ill patient. The maintenance of cardiac output is essential for the prevention of end organ injury which results in morbidity and mortality. A technique that accurately measured cardiac output would greatly assist the management of the critically ill patient.

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