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Department of Anesthesiology and Critical Care Medicine, Gifu University School of Medicine, Japan
Address correspondence and reprint requests to Shigeru Akamatsu, MD, Department of Anesthesiology & Critical Care Medicine, Gifu University School of Medicine, 40 Tsukasa-machi, Gifu City, Gifu 500-8705, Japan. Address e-mail to akamatsu{at}cc.gifu-u.ac.jp
Abstract
Automated cardiac output measurement (ACOM), which integrates digital color Doppler velocities in space and in time, has been validated using transthoracic echocardiography but has not been tested using transesophageal echocardiography (TEE). Therefore, we determined the feasibility of the ACOM method by TEE in 36 patients undergoing cardiovascular surgery. Regions of interest for ACOM were placed within a color sector across the main pulmonary artery (PA), the mitral annulus, and the left ventricular outflow tract. Cardiac output was determined from the PA flow, the mitral flow, and the left ventricular ejection flow at each view using the ACOM method. We compared mea-surements of cardiac output derived from the ACOM method with measurements simultaneously obtained by thermodilution (TD). In the mitral flow analysis, the values derived from ACOM correlated well with those from TD (R2 = 0.85; mean difference = 0.01 ± 0.58 L/min in the 2-chamber view; R2 = 0.78; mean difference = –0.10 ± 0.68 L/min in the 4-chamber view). In the PA flow analysis, the values derived from ACOM did not correlate with those from TD (R2 = 0.30). In the left ventricular outflow tract analysis, it was very difficult to obtain the optimal view (44%) in which color Doppler flow signals adequately appeared. Using the ACOM method, we obtained good correlation and agreement for cardiac output measurements in the mitral flow analysis compared with TD. The ACOM method is a practical and rapid method to measure cardiac output by TEE analysis of mitral flow.
IMPLICATIONS: Automated cardiac output measurement by transesophageal color Doppler echocardiography is a practical and rapid method to measure cardiac output. This technique is a promising new approach to echocardiographic quantification in the intraoperative setting.
Accurate measurement of cardiac output is important in assessing critically ill patients and monitoring the impact of therapeutic maneuvers. Various techniques for measurement of cardiac output are used to diagnose and guide the treatment of cardiovascular disorders. The thermodilution (TD) technique is easier to perform, but the need for catheterization of the right side of the heart has limited its applicability. A variety of noninvasive echocardiographic methods have been reported to determine cardiac output. Although echocardiographic methods have been reported to be accurate in carefully controlled studies, these methods require distinct two-dimensional echocardiographic images and meticulous care in measurement (1–4).
Transesophageal echocardiography (TEE) is frequently used during surgery for evaluation of regional wall motion abnormalities and ventricular volume status (5–9). Although cardiac output measurement by TEE has been reported using pulsed-wave or continuous-wave Doppler echocardiography (10–14), a widely adopted method for clinical practice has not evolved. These methods are technically off-line and are highly operator dependent and time consuming.
A new Doppler echocardiographic technique has been developed using a color Doppler technique for automated cardiac output measurement (ACOM) that assumes neither a flat flow velocity profile nor collinearity with the scan line. The ACOM method, using spatial and temporal integration of color Doppler velocity profiles, provides an accurate and quick automated flow volume measurement without the limitations inherent in pulsed-wave or continuous-wave Doppler echocardiography (15–18). However, there have been no reports of the application of the ACOM technology using TEE. The purpose of the present study, therefore, was to determine the feasibility of the ACOM method by TEE in an intraoperative setting.
Methods
After IRB approval, written informed consent was obtained from each patient. The study included 40 patients in sinus rhythm undergoing cardiac or vascular surgery while monitored by TEE and pulmonary artery (PA) catheterization. Patients with either a preoperative history or intraoperative TEE evidence of cardiac valvular disease, septal hypertrophy, or dysrhythmias were excluded. Anesthetic management was at the discretion of the anesthesiology care team.
After the induction of general anesthesia and tracheal intubation, a 5-MHz multiplane TEE probe (Toshiba, Tochigi, Japan) was inserted, and a routine echocardiographic evaluation was performed with the use of a commercially available machine (SSA-550A, Toshiba) equipped with software for ACOM (15). An observer blinded to hemodynamic data performed echocardiographic imaging. Doppler measurements were obtained from two-dimensional recordings with color Doppler signals of the PA in the ascending aortic short axis view (PA analysis), the mitral annulus in the 4-chamber view and the 2-chamber view (mitral flow analysis), and the left ventricular outflow tract (LVOT) in the deep transgastric long axis view (LVOT analysis). Each view was obtained in accordance with the American Society of Echocardiography and the Society of Cardiovascular Anesthesiologists guidelines for TEE (9). The transesophageal 2-chamber view was obtained from the same probe position of the 4-chamber view, by rotating the image array to approximately 90 degrees.
The principle of the ACOM method has been described in experimental and clinical studies (15–18). The ACOM method uses digital color Doppler velocities within a box positioned over a region of interest in the two-dimensional images with color Doppler signals and relies on the spatial integration assuming hemiaxial symmetry and temporal integration during a selected time interval of the instantaneous flow components. Stroke volume is calculated by a temporal integration of flow volume rate in each frame throughout the systolic or diastolic period (Fig. 1). Cardiac output was then calculated by multiplying stroke volume by heart rate, obtained automatically from electrocardiogram (ECG). In the present study, frame rate was set at 18–27 frames/s with a 30- to 50-degree color sector. The pulse repetition frequency was 4.5 kHz. To avoid aliasing, the color baseline (Doppler 0 shift) was shifted until no aliasing occurred in the region of interest. The cut-off frequency of the wall filter was set high enough to eliminate the clutter signals from the moving tissue (cut-off frequency 900 Hz). An optimal gain setting was obtained without random color noise in the nonflow areas by maximizing the gain level. On a selected beat, the systolic or diastolic period was manually defined by a trigger mark based on the ECG and the appearance and disappearance of the color flow. The measurements from 3 beats were averaged.
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We collected data during four study periods: (a) after the induction of anesthesia; (b) approximately 10 min before pericardiectomy or aortic cross-clamping; (c) approximately 20 min after termination of cardiopulmonary bypass or removal of the cross-clamp; and (d) during closure of the incision. We obtained four matched data from each patient.
Two investigators, blinded to each other’s measurements, independently performed ACOM on 10 randomly selected subjects. The values of the two investigators were correlated. The mean and SD of the difference of these measurements are reported both in absolute terms and normalized to the average of the two measurements.
All values are expressed as mean ± SD. Least-squares linear regression analysis was used for obtaining correlation coefficients between the values derived from the ACOM method and TD method in the PA analysis, the mitral flow analysis, and the LVOT analysis. The mean and SD of the difference of two methods (with TD measurement subtracted from ACOM) are reported both in absolute terms and normalized to the average of the two measurements (19). Additionally, the difference in the two methods is plotted against their means to better reflect variability in the measurement as a function of measurement magnitude. A P value <0.05 was considered statistically significant.
Results
Four of 40 patients were subsequently excluded from the study because of the development of significant ventricular arrhythmia during operation. The remaining 36 patients who completed the study protocol included 22 men and 14 women (age range, 47–77 yr; mean, 65 yr). The procedures performed included coronary artery bypass grafting (n = 20) and peripheral vascular surgery (n = 16).
Cardiac output was successfully determined by using the ACOM method in all patients using the mitral flow analysis. ACOM was successfully determined in 24 of 36 patients (67%) in the PA analysis and in only 16 of 36 patients (44%) in the LVOT analysis. We obtained matched data of 144 epochs in the mitral flow analysis: 96 epochs in the PA analysis, and 64 epochs in the LVOT analysis. It was difficult in the LVOT analysis to obtain the adequate two-dimensional images of the deep transgastric long-axis view and fill the LVOT with adequate color Doppler signals during systole. Technical difficulties in the PA analysis included interference from air in the left main stem bronchus, resulting in poor image quality.
In the mitral flow analysis, the values derived from ACOM correlated well with those from TD in the 2-chamber view (y = 1.06 x –0.29; R2 = 0.85; P < 0.0001; SEE = 0.04 L/min; mean difference = 0.01 ± 0.58 L/min) (Fig. 3) and in the 4-chamber view (y = 1.00 x –0.09; R2 = 0.78; P < 0.0001; SEE = 0.04 L/min; mean difference = –0.10 ± 0.68 L/min)(Fig. 4). In the PA analysis, the values derived from ACOM did not correlate with those from TD and tended to overestimate (R2 = 0.30; mean difference = 1.15 ± 1.50 L/min) (Fig. 5A). In addition to difficulties to obtain the optimal view in which color Doppler flow signals adequately appeared, the values derived from ACOM did not correlate with those from TD and tended to underestimate (R2 = 0.41; mean difference = –1.47 ± 1.15 L/min) in the LVOT analysis (Fig. 5B).
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Discussion
The present study demonstrates that automated analysis of color Doppler velocities across the mitral annulus can accurately estimate cardiac output by TEE. Our results support the mitral flow analysis, but not the PA and the LVOT analysis, by the TEE ACOM technique as an alternative and readily obtained method for intraoperative measurement of cardiac output. TEE reveals high-quality images of the heart, especially the mitral valve area in near distance, compared to transthoracic echocardiography. The transesophageal 4-chamber view and 2-chamber view are easily obtained in routine practice. The two-dimensional images of the mitral valve and the mitral annulus are clear and of high-resolution because of the inherent characteristics of TEE. TEE color Doppler also provides high-quality flow signals of the mitral flow. This provided high-fidelity Doppler measurements across the mitral annulus. Muhiudeen et al. (10) reported that cardiac output determined from the mitral valve diastolic flow velocity did not correlate with TD-derived cardiac output using transesophageal pulsed-wave Doppler echocardiography. Pulsed-wave Doppler measurements have inherent constraints. There are the assumptions that the mitral annulus is circular and that the cross-sectional area is constant throughout the cardiac cycle (20). The flow velocity profile may not be flat at the sampling site (21). These factors may introduce a potential source of error in pulsed-wave Doppler measurements. In contrast, the ACOM method does not require measuring the area of flow tract such as the mitral annulus. The edge of the color profile is detected as the width of the flow tract in each frame. The flow volume is automatically calculated by double integration of Doppler signals in space (across the full width of the flow tract) and in time. This frame-by-frame tracking function of the velocity profile enabled us to measure cardiac output accurately.
Our results demonstrate an excellent correlation between TD measurement and ACOM in the mitral flow analysis but little, if any, relationship in the PA analysis or the LVOT analysis. These findings differ from those previously obtained by transesophageal pulsed-wave Doppler or continuous-wave Doppler echocardiography (10–14). Previous studies reported that cardiac output determined from the PA analysis or the LVOT analysis provided a fair estimate of intraoperative cardiac output but not the mitral flow analysis. Changes of the mitral annulus and the mitral flow velocity profile might introduce errors in pulsed-wave Doppler measurements. The ACOM method can track these changes and provide a fair estimate in the mitral analysis by TEE. Continuous-wave and pulsed-wave Doppler echocardiography are inherently more sensitive and precise in velocity measurement than color Doppler echocardiography, especially in sampling distal locations such as LVOT. Thus, the ACOM method based on color Doppler echocardiography did not demonstrate accurate estimate of cardiac output in the PA analysis and the LVOT analysis.
In an intraoperative study of the PA flow, adequate imaging could not be obtained in 24% of patients (11). The deep transgastric long-axis view positions the Doppler beam near parallel to aortic flow. A shortcoming of this approach is that it requires that the TEE probe be positioned at the transgastric, mid-papillary position to be advanced, fully flexed anteriorly, shifted leftward, and then slowly withdrawn until the transgastric long-axis view is imaged. Obtaining this view is technically challenging and cannot be achieved in as many as 12% of patients (13). In a large subset of patients, it is not possible to visualize the LVOT filling with adequate color signals transgastrically to apply ACOM. In addition to not requiring these probe manipulations, adequate images of the transesophageal 4-chamber view and 2-chamber view are easily obtained and suitable for intraoperative monitoring. The frequency of emitted ultrasound in TEE is higher than that in transthoracic echocardiography. High frequency of emitted ultrasound provides high-quality images and Doppler signals in proximal location; however, it causes attenuation of ultrasound so that signal-noise ratio decreases in sampling distal locations. We found it necessary to maximize the gain setting to fill the tract with color signals in PA and LVOT analysis.
There are some advantages in using the ACOM method for the evaluation of cardiac output in comparison with pulsed-wave Doppler echocardiographic measurement. First, cardiac output calculation by the ACOM method can be performed without tracing the Doppler waveform to measure the velocity-time integral. Second, we do not need to measure the area of the flow tract. Finally, the most important advantage of the ACOM method is its ease of application. It calculates cardiac output from one window, which allows for acquisition of data in a fast manner and avoids the problem of nonsimultaneously recorded flow and area data.
The ACOM method has recently been validated for quantification of flow volume and calculation of hemodynamic variables (16–18,22,23). Trindade et al. (18) validated the ACOM method in the LVOT analysis using transthoracic echocardiography. They reported the underestimation of cardiac output when comparing the ACOM method to other methods. This finding of the underestimation agrees with our results in the LVOT analysis using TEE. A possible explanation for these findings is the fact that there is interference with acquisition of adequate color signal filling in the LVOT because of the distance between the transducer and the sampling site, especially in TEE with high frequency of emitted ultrasound. TEE examination is capable of acquiring high-quality 4-chamber or 2-chamber images and excellent color signal filling.
Limitations of our study design include the use of TD as the reference standard. TD may be relatively inaccurate. This technique is susceptible to error but remains the clinical standard for determining cardiac output (24). The ACOM method avoids the assumption of a flat velocity profile, but it does assume hemiaxial symmetry that likely is not strictly true. Applying the assumption of hemiaxial symmetry might introduce errors in mitral analysis because of the complex geometry of the mitral annulus. Frame-by-frame tracking function of effective flow orifice and acquisition of excellent color signals might minimize the errors in TEE. The ACOM method calculates only forward flow during the systolic or diastolic period. This calculation is not applicable to cardiac output in the presence of valvular regurgitation. Finally, machine settings, including frame rate, color gain, and color filter, can affect the accuracy of the ACOM method because flow volume is integrated from time-sequential color flow images (15). In addition, Sun et al. (16) reported that the ACOM method is influenced by the color gain, although reliable cardiac output values can be obtained by optimizing the color gain. Adequate color gain setting is required to obtain accurate measurement by this method.
In conclusion, the ACOM method by TEE at the mitral annulus is a practical and rapid method to measure cardiac output. This technique is a promising new approach to echocardiographic quantification in the intraoperative setting.
Acknowledgments
Supported, in part, by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture (No. 09557126), Japan.
We gratefully acknowledge the assistance of Hiroyuki Tsujino for reviewing the engineering aspects of the manuscript.
References
This article has been cited by other articles:
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  R. Melchior, E. Darling, B. Terry, G. Gunst, and B. Searles A novel method of measuring cardiac output in infants following extracorporeal procedures: preliminary validation in a swine model Perfusion, December 1, 2005; 20(6): 323 - 327. [Abstract] [PDF]
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