Thoracic bioimpedance and Doppler cardiac output measurement: learning curve and interobserver reproducibility

Nine previously untrained health professionals learned to measure cardiac output (Qt) by suprasternal continuous-wave Doppler ultrasound (QtDopp) and by thoracic bioimpedance (Qtbi). Each received standardized written, videotaped, and individual instruction. First the novice, then the reference examiner, measured QtDopp or Qtbi in triplicate in an adult male subject. The reference examiner was blind to the novice measurements and the novice was not informed of the reference measurements. Each novice repeatedly measured QtDopp or Qtbi in different subjects until the mean novice QtDopp or Qtbi was within 10% of the corresponding mean reference measurement in three of four consecutive subjects. The novice observers required an average of 12.9 +/- 3.5 trials to learn to measure QtDopp, and an average of 8.4 +/- 4.5 trials to learn to measure Qtbi. The likelihood of novice agreement with the reference improved with experience. The same degree of intraobserver variability as reported for Qt measured by thermodilution (coefficient of variance less than or equal to 10%) was achieved with Qtbi in 150 (99%) of 152 triplicate measurements and QtDopp in 216 (97%) of 222 triplicate measurements. More importantly, interobserver agreement (within 10%) was achieved with both Qtbi and QtDopp. Reproducible noninvasive Qt measurement will allow these techniques to be used to monitor trend changes in Qt.