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Ann Thorac Surg 2000;70:212-217
© 2000 The Society of Thoracic Surgeons

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Original articles: Cardiovascular

Transit time flow measurement: experimental validation and comparison of three different systems

^a Department of Cardiovascular Surgery, University Hospital Insel, Bern, Switzerland
^b Division of Cardiology, University Hospital Insel, Bern, Switzerland

Address reprint requests to Dr Walpoth, Department of Cardiovascular Surgery, University Hospital Insel, CH-3010, Bern, Switzerland
e-mail: beat.walpoth{at}insel.ch

	Abstract

Top Abstract Introduction Material and methods Results Comment References

 
Background. Bloodflow measurements are of major clinical importance for quality control in vascular surgery. They allow detection of low-flow situations which may influence outcome adversely. The purpose of the present study was to validate three different flow systems for measuring absolute blood flow.

Methods. Measurements were performed in an experimental flow model using arteries and veins and blood or saline at two different temperatures. As a reference method true flow was measured by volume sampling.

Results. Correlation coefficients between transit time flow and true flow measurements ranged between 0.71 and 0.92. Systematic overestimation and underestimation of transit time flow were observed, but after second-order correction all correlations were excellent, ranging from 0.93 to 0.95 irrespective of flow medium and temperature.

Conclusions. Transit time flow measurements are exact and reproducible. Second-order correction yields good accuracy and high precision, with minimal differences among the three systems evaluated.

	Introduction

Top Abstract Introduction Material and methods Results Comment References

 
Assessment of coronary blood flow is of great clinical value in determining perfusion abnormalities in cardiovascular disease. In surgical patients, it is of particular importance to assess blood flow before, during, and after bypass surgery. Ascer and coauthors [1, 2] showed a prognostic value of flow and resistance measurements for determining long-term graft patency. Therefore, transit time blood flow measurement may be an important tool for quality control in cardiovascular surgery.

Over the past years Doppler and electromagnetic flow meters have been used to assess coronary blood flow in small bypass grafts [3]. Louagie and coworkers showed the clinical value of a pulsed Doppler flowmeter in a large study [4]. However, several limitations and problems affect the reliability of these methods for assessing vascular blood flow. The Doppler technique is influenced by probe position (angle), motion artifacts, sample size, flow velocity profile, and vessel diameter [5]. Electromagnetic flow measurements may be compromised by the fitting of the probe, zero-line drift, and hematocrit level [6, 7].

Recently, the transit time flow method has been used with increasing frequency in cardiovascular surgery. Application of transit time flow measurement is fast and easy even under surgical conditions, and artifacts affect results less severely than in other techniques [8–12].

The purpose of the present study was to validate transit time flow measurements using three different systems under various conditions (arterial or venous graft material, saline or bloodflow medium, and room or body temperature).

	Material and methods

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Methodologic principle of transit time flow measurement
The transit time method uses two piezoelectric crystals transmitting ultrasound through the blood vessel toward a reflector on the other side of the vessel (Fig 1). Volume flow is calculated by measuring the difference between transit times upstream and downstream in the blood vessel [13 –15].

View larger version (16K): [in this window] [in a new window]   Fig 1. Principle of transit time flow measurement. Arrowsrepresent ultrasound signal pathway.

System 1. CardioMed Flowmeter (CM 4008, Medi-Stim AS, Oslo, Norway)

System 2. Transonic Flowmeter (T 206, Transonic Inc, Ithaca, NY)

System 3. Triton Small Vessel Flowmeter (System 6, model 257, Triton Technology Inc, San Diego, CA)

Although each manufacturer provides several different probe sizes, in the current study we used only 4-mm probes with an insonication angle of 45 degrees.

Flow model
The flow model consisted of a roller pump producing a pulsatile flow pattern that allowed continuous flow variations from 25 to 350 ml/min. Because the tubing system was shorter than 40 cm, full pulsatility on the flow curve was obtained. The circuit included a thermoregulated reservoir and a water-filled tank, kept at room temperature, for optimal ultrasound signal coupling. True flow was measured by sampling in a special reservoir that was weighed on a high-precision scale (Mettler 2000E, Mettler-Toledo Inc, Greifensee, Switzerland). An integrator for flow and time was used for assessing blood flow over time for the three systems. Because blood has a higher specific weight than saline, volume calculations were corrected for hematocrit.

Study protocol
Flow was measured at ten different flow levels starting at 25 ml/min and ending at 350 ml/min. For each set of experiments the order of the flow probes was changed randomly.

Three sets of variables were compared: type of graft, flow medium, and temperature. We evaluated flow through both veins and arteries, using fresh porcine carotid arteries (90 measurements in 4 arteries) or human saphenous vein grafts (114 measurements in 5 grafts left over from cardiac surgery patients). The two flow media were 0.9% saline (103 measurements) and outdated human whole blood at a hematocrit of 30% (101 measurements). We compared flow at two temperatures: 25°C, or room temperature (164 measurements), and 37°C, or body temperature (40 measurements).

Statistics
Linear regression analysis was performed by the least squares method. Correlation coefficients and the regression equation were calculated for each system. A second-order correction was performed to correct for deviation from the line of identity. The correction was achieved by multiplying the individually measured flow values with the regression equation. By doing this, the correlation was more or less unchanged but the slope of the regression equation approached 1 and the intercept 0.

Univariate analysis was used to assess the influence of the various variables (vessel type, flow medium, temperature) on flow measurements

Bland-Altman plotting was used to assess agreement between the systems. The mean absolute difference (designating accuracy) and standard deviation of the mean absolute difference (designating precision) [12] were calculated for all three systems and displayed graphically.

	Results

Top Abstract Introduction Material and methods Results Comment References

 
Transit time versus absolute flow measurements
Linear regression analysis of pooled data (Fig 2) showed a good correlation among all three systems. System 3, however, significantly overestimated true flow in arteries but underestimated flow in veins. Systems 1 and 2 showed similar tendencies, but they were not significant. After second-order correction all three systems showed an excellent correlation that ranged from 0.93 to 0.95. Evaluation of the standard error of the estimate of the mean yielded 15% for system 1, 13% for system 2 and 16% for system 3. The other variables (saline versus blood and room versus body temperature) had no effect on the correlation between transit time and true flow in any of the three systems (Fig 3). The second-order correction for artery and vein use had only minimal effect with regard to medium and temperature.

View larger version (37K): [in this window] [in a new window]   Fig 2. Correlations between Doppler and true flow measurements. Given are uncorrected (left) and second-order corrected (right) data for the three systems. All show some differences in flow between veins and arteries; these are minor in system 1 and 2 but significant in system 3. After second-order correction, the differences are negligible.

View larger version (38K): [in this window] [in a new window]   Fig 3. Correlations between Doppler and true flow measurements. The effects of flow media (saline or blood, left). temperature (25° or 37°C, right). All data are corrected for differences in graft material (arteries or veins). Flow media had only a minor effect on Doppler flow measurements (not significant), whereas temperature produced larger deviations, particularly in system 3, although they are not significant.

View larger version (37K): [in this window] [in a new window]   Fig 4. Bland-Altman analysis of three systems. Plotted are true flow (x-axis) versus the differences in Doppler minus true flow measurements (y-axis). System 3 shows the largest deviations, which diminish after second-order correction.

	Comment

Top Abstract Introduction Material and methods Results Comment References

Comparison of flowmeters
Overall correlations between transit time flow and true flow in the three flowmeters are shown in Figure 2. Both systems 1 and 2 showed excellent correlation. System 3 significantly overestimated arterial flow but underestimated venous flow, probably because of the missing side lock with partial volume effect. After second-order corrections all three systems showed excellent correlation not only for arterial and venous grafts but also for saline and blood perfusion at both normothermic and hypothermic (room temperature) perfusion (Fig 3). Conversely, the Bland-Altman analysis (Fig 4) showed reduced variability after second-order correction.

Previously Hartman and associates compared electromagnetic and transit time flow measurements and found a high correlation between the two methods (r = 0.98) although measurements were carried out in the canine ascending aorta. Correlation with true flow (exsanguination) yielded a correlation coefficient of 0.93 [9]. Laustsen and coworkers carried out a clinical validation study using system 1 and reported a high correlation coefficient compared with true flow (exsanguination) in arteries (internal thoracic artery, r = 0.99) and veins (saphenous vein grafts, r = 0.99) [10]. In a previous study in patients undergoing coronary artery bypass grafting, we were able to show a correlation of 0.89 between exsanguination and transit time measurement [11]. Our in vitro data confirm these clinical findings.

Clinical implications
Transit time flow measurements are accurate and precise but results may depend on graft material. Our results may have important implications both for comparative studies using arterial and venous grafts and for low-flow situations to help rule out graft failure and thus prevent adverse clinical outcomes. The findings support the need for proper validation studies and the use of second-order corrections for improving accuracy and precision.

The clinical relevance of accurate, intraoperative flow assessment during vascular or coronary bypass surgery has been demonstrated in several studies [11, 12, 17–20]. Ascer and colleagues reported a significantly higher occlusion rate and worse clinical outcomes in autologous peripheral grafts when high vascular resistance and low flow were documented at the time of surgery [18]. Recently, transit time flow measurements were proposed for quality control in patients undergoing minimally invasive bypass surgery. Jaber and coworkers predicted differences in mean graft flow in patients with severe stenotic anastomoses [19]. Similarly, graft failure was detected by transit time flow measurement in patients during coronary artery bypass grafting [20].

Of the three transit time flow systems tested in this study, systems 1 and 2 had excellent correlation with true flow, whereas system 3 demonstrated significant deviations for arteries and veins. Proper validation with mathematical corrections yields reliable measurements for system 3. Because of its easy, fast application and excellent reproducibility, transit time flow measurement appears to be an accurate and reliable method for intraoperative flow measurements. That information may be important for quality control and outcome studies [14, 16].

	Acknowledgments

 
This study was partly supported by the Swiss Heart Foundation.

	References

Top Abstract Introduction Material and methods Results Comment References

 

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