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It has been widely accepted that determination of cardiac output (CO) is a useful and often lifesaving adjunct in the resuscitation of critically ill patients. Monitoring of these patients typically occurs within the intensive care unit (ICU), operating room, cardiac catheterization lab, or Emergency Department. Cardiac output determination has conventionally been obtained by thermodilution or dye dilution measurements that are invasive and associated with potential risk to the patient. Complication rates up to 7.2 percent using the invasive pulmonary artery (PA) catheter have been reported.1–3 There is wide variability in CO measurements using a PA catheter as well. Stetz and Miller identified error rates between 4 and 10 percent with triplicate CO measurements, while single CO measurements had a wider variability, ranging between 7 and 17 percent.4 Other disadvantages to use of the PA catheter include risk of sepsis, intermittent cardiac output determinations, expense, and restricted use to “monitored” facilities within a hospital (i.e., ICU or operating room).

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Noninvasive devices that measure cardiac output have been slowly gaining acceptance in the medical community and have shown very good agreement with the “gold standard” thermodilution technique. Transesophageal Doppler (TED) and thoracic electrical bioimpedance (TEB) are two modalities that can measure CO noninvasively and continuously. Furthermore, they can measure other hemodynamic variables such as preload, contractility, and systemic vascular resistance in real time. This provides the physician with the ability to follow trends as well as the response to interventions such as the institution of vasopressors or following the administration of a fluid bolus. The measurements obtained require little training, are highly reproducible, and pose little risk to the patient. Another advantage lies in their ability to identify hemodynamic compromise before it becomes clinically apparent and when therapy may be most beneficial. Most Emergency Departments utilize noninvasive devices to measure blood pressure, heart rate, oxygen saturation, and, on rare occasions, central venous pressure. These noninvasive devices may allow the Emergency Physician to monitor the hemodynamic status of a patient more closely and to institute therapy earlier.

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Introduction

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In 1842, Christian Doppler identified that the velocity of a moving object is proportional to the shift in reflected frequency of an optic wave of known frequency. This principle has been adapted to sound waves and is now the basis for Doppler devices that measure the velocity of blood flow and related hemodynamic variables continuously. The first use of Doppler to measure the velocity of red blood cells in humans or animals occurred in 1969.5,6 Cardiac output determination was first conducted via the suprasternal approach, but this was cumbersome, and it was difficult to obtain data continuously.7,8 The device currently in use involves Doppler measurements through a transesophageal approach, directly measuring the blood velocity in the descending aorta. The esophageal Doppler monitor (EDM; CardioQ, Deltex Medical Inc.) is one such device that will display continuous hemodynamic data (cardiac output, peak velocity, and corrected flow ...

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