A noninvasive procedure for continuous cardiac output monitoring called transtracheal Doppler is presented. It utilizes Doppler ultrasound technology technology in combination with the endotracheal intubation which is routinely used for ventilation in the operating room and critical care unit. To calibrate the procedure for human use, the average angle between the ultrasound beam and the ascending aorta in a set of patients was determined by fitting the aortic diameters determined by transtracheal Doppler to those measured by external echocardiography. This calibration was then tested in a second set of patients by comparing the cardiac output measurements with the output of thermodilution, the current clinical standard. Good agreement was found.
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