Comparison of esophageal Doppler monitor generated minute distance and cardiac output in a porcine model of ventricular fibrillation
Introduction
Cardiac output (CO) measurement is used frequently in the clinical management of patients with significant cardiac dysfunction and hemodynamic instability. With the use of the pulmonary artery catheter, CO measurement is unreliable in low flow states of cardiopulmonary resuscitation (CPR). For these reasons, an easy-to-use, non-invasive, quick and reliable indicator of CO can be of considerable value in the management of a patient during CPR. Esophageal doppler ultrasound has been used as a non-invasive method for monitoring trends of changes in CO with conflicting results [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. Esophageal doppler ultrasound monitoring of blood flow velocity and CO have been described in detail in previous studies [17], [18]. In brief, when an ultrasound beam is directed at a column of blood flow in the aorta, the reflected sound wave will have a shift in frequency. The magnitude of this shift is directly proportional to the velocity of blood flow in the aorta. The main problems with doppler ultrasound methodology when used to measure CO from aortic blood flow include: inaccurate assessment of aortic diameter and negating its variation with mean aortic pressure, angle deviation between ultrasound beam and vessel axis, and failure to precisely determine the proportion of blood flow through the ascending aorta [19].
The esophageal doppler monitor (EDM) uses a 4-MHz frequency transducer to measure descending aortic blood flow, giving a beat by beat display of a waveform. The area within the waveform is the stroke distance (SD) defined as the distance traveled by a column of blood passing down the descending aorta with each left ventricular stroke (Fig. 1). The SD multiplied by the heart rate is equal to the minute distance (MD), which is the distance traveled by a column of blood in 1 min of time. The EDM converts the MD to an estimated CO using a nomogram scale in adult human patients. Accuracy of the nomogram scale has yet to be defined, but benefits in time and ease of measurements are welcomed. The technique we describe for use in our porcine model (outside nomogram scale) uses blood velocity measurement alone without estimation of cross sectional area in the descending aorta to compare to CO.
Data regarding the accuracy of using EDM to monitor CO in healthy, critically ill and surgical patients are conflicting [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. Although it has been proposed that doppler CO is suitable for monitoring changes in CO [1], [2], [3], [4], [5], [6], [7], [8], [9], [20], there is, however, no published data using the EDM during CPR. The purpose of this study was to establish that simultaneous trends of change in MD (EDM) and CO (florescent microsphere technique) agree in experimentally induced ventricular fibrillation in a porcine model before and during CPR.
Section snippets
Anesthesia and instrumentation
Twenty domestic pigs weighing between 27 and 31 kg, 5–6 months of age, were fasted overnight and anesthetized with an intramuscular injection of telazol (6 mg/kg) and xylazine (6 mg/kg). Tracheal intubation was performed and anesthesia maintained with a concentration of 0.5–1.5% isoflurane. Animals were ventilated using a volume-cycled ventilator. Ventilation rate and end-tidal CO2 were not controlled.
Surgical cutdowns were performed on the right and left femoral vessels. Micromanometer tipped
Results
Mean animal weight was 28.9±2.1 kg.
Microsphere CO during baseline ranged from 2.09 to 8.94 (4.94±2.17 ml/min). Doppler MD during baseline ranged from 790 to 2643 (1734±712 cm/min).
Microsphere CO during CPR ranged from 0.04 to 1.81 (0.457±0.42 ml/min). Doppler MD during CPR ranged from 140 to 611 (260±100 cm/min).
The correlation between CO and MD at all time-points (96 measurements) is shown in Fig. 3. r2 for this comparison was 0.96.
The correlation between CO and MD during baseline (11
Discussion
Cardiac output is the ultimate expression of cardiovascular function, however its measurement is not feasible in the clinical setting of CPR. This relates to the difficulties in placement and inherent risks of the pulmonary artery catheter, and unreliable measurement during low flow states of CPR. Although data regarding the accuracy of using doppler MD to monitor trend of change of CO are conflicting [1], [2], [3], [4], [5], [21], [12]. Doppler ultrasound monitoring of MD may be a
Acknowledgements
The authors are very grateful to Dr Rebecca Kirby for her advice and support concerning preparation of this report. We are also grateful to Deltex Medical, Inc. for their equipment support.
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