Stress Doppler echocardiography of the internal thoracic artery – a new non-invasive approach for functional assessment after minimally inva
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a Faculty of Medicine, University of Witten-Herdecke, Department of Cardiology and Internal Medicine, General Hospital Solingen, Germany
b Department of Cardiac and Thoracic Surgery, Heart Center Voelklingen, Germany
c Department of Thoracic and Cardiovascular Surgery, Heinrich-Heine-University, Düsseldorf, Germany
d Department of Cardiology, Pneumology and Angiology, Heinrich-Heine-University, Düsseldorf, Germany
e Department of Cardiology, Heart Center Wuppertal, University of Witten-Herdecke, Germany
Abstract
Minimally invasive surgery for coronary revascularization using the left internal thoracic artery (ITA) has gained increasing interest. For control of graft function the established transcutaneous color-Doppler echocardiography in combination with a stress-test was performed to test the ability of this novel technique. Twenty-one patients having received a single ITA-graft were evaluated early postoperatively at rest and during isometric stress test with a handgrip exercise. Compared to the right internal thoracic artery, the mainly systolic flow is changed to a wide diastolic component when the left ITA is anastomosed to the coronary artery. The peak systolic/peak diastolic velocity ratio changed from 4.5±1.9 to 1.4±0.47 (P<0.0001). During stress reaction with the isometric handgrip maneuver the grafted ITA showed a significant increase of the mean diastolic flow (29.1±13.3 to 44.3±14.7 cm/s, P<0.0001) and total blood flow (124.8±55.4 ml/min to 176.6±71.7 ml/min), which may demonstrate an efficient bypass function. We conclude, that the noninvasive measurement of ITA-graft function with Doppler-ultrasound may be a clinically useful method to assess the functional status after minimally invasive coronary artery bypass grafting. In combination with the hand-grip test it represents a valid new technique with the potential to estimate graft patency.
Key Words: Internal thoracic artery bypass; Stress Doppler echocardiography; Minimally invasive cardiac surgery
1. Introduction
The aorto-coronary bypass grafting using the internal thoracic artery (ITA) is the method of choice due to the long-term patency rate [1]. The graft is a conduit with a proven functional adaptability, depending upon the actual myocardial demands, by increasing its diameter when flow requirements make this necessary [2]. Besides coronary artery bypass grafting with the use of extracorporal circulation beating heart techniques have gained increasing interest [3]. These efforts in ‘less invasive coronary artery surgery’ lead to the revascularization via a left lateral minithoracotomy with ITA harvesting and anastomosing it predominantly to the left anterior descending (LAD) coronary artery [4]. It has been shown to yield excellent results with a low associated morbidity and mortality [5]. A noninvasive investigation for postoperative assessment of graft function would be helpful.
Ultrasonic methods have been extensively used to measure blood flow in peripheral vessels and more recently in internal thoracic arteries. The examination of this vessel has gained general acceptance in the pre- [6] and postoperative [7] course. The origin and the proximal part are positioned close to the chest wall after a coronary artery bypass operation as well and, therefore, a noninvasive Doppler echocardiographic evaluation is possible. The phenomenon that ITA graft flow changes from a dominant systolic pattern preoperatively to a diastolic pattern postoperatively has been described repeatedly [8]. This method provides not only an accurate noninvasive evaluation of ITA graft function, but also an optimal approach to gain insight into coronary flow properties. Pharmacological stress tests (using adenosine or dubutamine) need some organization, are not easily done, and cannot be repeated often. In order to assess the autoregulatory and vasodilatory capabilities of the ITA-graft we examined the response of ITA flow by color Doppler echocardiography in rest and during exercise with a handgrip-maneuver after minimally invasive direct coronary artery bypass (MIDCAB).
2. Material and methods
2.1. Patient population
The MIDCAB procedures were used in 21 patients (18 male, 3 female) with a high comorbidity. The majority of patients had two-vessel and three-vessel diseases (Table 1). All patients have received a single left ITA-graft. The ultrasound study was performed in accordance with the local Ethics Committee and the patients were investigated in the early postoperative stage within the second to fourth postoperative week. All patients were informed and gave oral consent for the ultrasound investigation.
A left anterolateral minithoracotomy with a supramammary incision was performed through the fourth intercostal space. After opening the pericardium longitudinally the LAD was identified and inspected. A rib elevator was used for ITA harvesting under direct vision. Proximal ITA dissection was continued, usually up to the second rib, until sufficient pedicle length allowed a tension-free curved course of the ITA; side branches were clipped and cut. Local immobilization at the anastomosis was achieved with a stabilizer. Under direct vision, the ITA to LAD anastomosis was performed using routine techniques.
2.2. Color Doppler ITA flow evaluation
The technique of color Doppler echocardiography was carried out in the way as previously described by us [9]. During Doppler echocardiography the patient lies in a supine position. Transthoracic imaging and Doppler spectra derived from the ITA were obtained with a real-time two-dimensional echocardiographic unit using a 7.5 MHz phased array transducer connected to a Hewlett Packard sonographic system (Sonos 2500, Andover, MA).
In the most cases the detection of the graft was possible in the first intercostal space; in the case of failure the supraclavicular position was used. The colorflow facility was used to identify easier the origin and the course of the vessel. After detection of the ITA in longitudinal line the diameter was measured with the transducer. In order to obtain an optimal flow velocity signal the highest peak velocities and a well-defined spectral morphology was required after minimizing the angle between vessel and Doppler beam.
The mean systolic velocity (MSV), mean diastolic velocity (MDV), mean velocity (MV), peak systolic velocity (PSV) and peak diastolic velocity (PDV) were achieved. Total blood flow, peak systolic to diastolic velocity ratio (PSDVR) and mean systolic to diastolic velocity ratio (MSDVR) were calculated. Measurements were performed according to the internal analytic package of the ultrasound machine. The different flow velocities were measured by tracing out the contour of the Doppler velocity tracing. Values for each parameter were obtained by averaging measurements from three to five cardiac cycles. The investigations at rest were made on both sides for comparison of the ungrafted right ITA with the left ITA as a control parameter for initial data.
After baseline measurements at rest, patients undertook a handgrip-maneuver with a balloon dynamometer (Martin, Stuttgart, Germany). The flow in the ITA was monitored throughout the maneuver with the Doppler-probe in place. The squeezing of the balloon was performed with the right hand and sustained his grip at 40–50% of his maximal voluntary contraction for 3 min. According to the constitution of the patient, the pressure in the balloon ranged between 0.2 and 0.3 kg/cm2. Isometric exercise was used to produce an increased cardiac load to aid the assessment of the ITA-graft. The heart rate and the blood pressure were monitored at each period. The patients had to speak during the pressure recording to prevent any inadvertent performance of Valsalva maneuver.
2.3. Statistical analysis
Results are expressed as mean value±S.D. (standard deviation) unless otherwise indicated. Statistical analysis comparing the same between the right and the left side and between rest and stress conditions were performed with a nonparametric test (Wilcoxon signed-rank test) for paired observations. A probability value <0.05 was considered significant. Statistical analyses were performed on the SPSS (Version 8.0 for personal computer, Chicago, IL) statistical software program.
3. Results
After surgery in all patients clinical symptoms of angina pectoris were clearly improved. The diameter of the ITA-graft was measured and flow data recorded. The flow pattern in a non-grafted vessel has a biphasic or triphasic wave form with a large systolic peak, sometimes followed by small reversed and then by a diastolic flow component. The flow in the grafted ITA was detected in all patients. Compared to the ungrafted right ITA, the mainly systolic flow is changed to a meanly wide diastolic component in the left ITA anastomosed to the coronary artery. The flow in the residual proximal branches finds ifself in the systolic Doppler signal.
3.1. Rest
The mean diameter was 2.7±0.3 mm for the grafted left ITAs and 2.6±0.4 mm for the ungrafted right ITAs (P=0.001). Comparing the grafted left to the ungrafted right ITA the mean systolic flow velocity decreased significantly (P<0.0001) and the diastolic flow velocity increased significantly (P<0.005) to a dominant diastolic pattern. Median total flow was not different in ungrafted right ITA and the grafted left ITA (Table 2). There was no significant difference in resting flow rates between grafted and ungrafted vessels (P>0.05). The hemodynamic parameters were on the same level during the investigation on both sides in rest. The ratio of peak systolic velocity/peak diastolic velocity (PSDVR) changed from 4.5±1.9 to 1.4±0.5 (P<0.0001) and the mean systolic velocity/mean diastolic velocity ratio (MSDVR) from 3.9±1.7 to 1.3±0.4 (P<0.0001).
3.2. Changes during exercise
All patients completed the exercise protocol and satisfactory measurements were obtained during exercise, no patient had symptoms of angina pectoris. The stress test induces an increase in pulse rate, systolic and diastolic blood pressure and additionally in graft flow, the double product increased significantly (8828 to 12,666, P<0.001).
At rest, both peak and mean diastolic were often, but not always, higher than the corresponding systolic velocities. During the handgrip maneuver, both the systolic and diastolic peak and mean velocities increased, whereby the rising of the peak and mean diastolic velocity was more pronounced (peak systolic velocity 16.3% compared to peak diastolic velocity 59.5%, P<0.001, mean systolic velocity 24.8% to mean diastolic velocity 63.7%, P<0.001). The increase of the mean velocity was highly significant (Table 3, Fig. 1).
The stress test induces a significant decrease of PSDVR and MSDVR. This finding demonstrates that an increasing diastolic flow contributes essentially to the overall increase in averaged peak and mean. The hemodynamic changes by exercise leads to a significant increase of total blood flow (124.8±55.4 ml/min to 176.6±71.7 ml/min, P<0.001) in the ITA-graft (Fig. 2).
Five patients of the series had an angiographic visualization of the region of anastomosis with digital subtraction angiography (DSA) for the exclusion of a graft dysfunction because of clinical reasons. The anastomosis presented no signs of obstruction. Between the two patient groups (with and without DSA) there was no difference in the estimated flow parameters in rest and under stress conditions.
4. Discussion
Myocardial revascularization by minimally invasive coronary bypass surgery offers an alternative concept to conventional coronary artery bypass grafting (CABG) and excellent clinical results are reported with the MIDCAB procedure [4]. It can be performed in patients at high risk when conventional CABG would be used [10]. One important aspect of this procedure during the postoperative course is the functional assessment of the graft. Ideally every patient should have postoperative angiography to visualize the region of anastomosis, but this is an invasive approach. In our study group five patients had an angiographical control, the estimated flow patterns at rest and during stress conditions were comparable in both patient groups.
The difference in diameter between the ungrafted RIMA and the LIMA is a question certainly only of a computational difference, in clinical importance is this surely unimportant. The determined vessel diameters lay in a comparable order of magnitude to anatomical [11] and angiographic studies [12]. An intact internal thoracic artery graft shows a characteristic biphasic flow velocity profile in its proximal part only seen in patients with ITA grafts. The Doppler flow pattern of an in-situ ITA resembles that of other medium-sized systemic arteries, such as the external carotid artery. Intramyocardial blood vessels differ from those in other muscle areas because they exhibit a low resistance during diastole and an elevated resistance during systole due to the increased wall tension particulary in the left ventricular wall. Flow in the distal epicardial coronary arteries occurs almost entirely during diastole, as described in intracoronary flow studies [13]. The ultrasound measurements of the ITA concern the proximal part of an elastic vessel anastomosed distally to the coronary artery. The systolic part of the flow pattern represents flow into a distensible but effectively blind-ended tube and to patent large nonligated side-branches. An obstruction of the graft would affect the diastolic part but not the systolic flow [5,14].
Flow velocity assessment at rest gives incomplete information, as the diastolic flow does not exclude a possible stenosis of the anastomosis. Pharmacologic stress testing is difficult to perform repeatedly. Therefore, the hand-grip exertion was used because of minimal thoracal movements during the stress testing in comparison to treadmill and bicycle ergometer making continuous Doppler signals recordings impossible. Furthermore, this stress test can easily be performed and repeated.
Heart rate, cardiac output, blood pressure and stroke work all increase during handgrip exercise [15]. This causes an increase of blood flow to the muscular circulation (systolic phase) and to the coronary circulation (diastolic phase). The amount of the diastolic increase represents the function of the anastomosis. At rest patients with normal anastomosis have both peak and mean diastolic velocities similar to the systolic ones, as well as the corresponding ratios. During the handgrip-maneuver the increase of the diastolic flow patterns were higher than the systolic flow patterns. The mean systolic to diastolic velocity ratio (MSDVR) was between 1.0 to 1.5 in rest and it decreased to 1.0 or less during handgrip-maneuver. In this judgment the increase of total blood flow has to be recognized. This seems to be the result of measurement of a normal graft function.
In conclusion, the aim of our present study was to describe the feasibility of this novel technique for measurement of the ITA-graft flow after MIDCAB-procedure and to estimate the function of the graft with interpretations of the Doppler findings. The handgrip-maneuver may be an investigation to test graft patency. Additionally, this technique would be suitable for all patients with coronary artery bypass grafting using the internal thoracic artery also with sternotomy.
Acknowledgements
We wish to thank G. Plehn, MD for critical revising of the manuscript and Mrs M. Marx for preparing the manuscript.
References
Loop FD, Lytle BW, Cosgrove DM, Stewart RW, Goormastic M, Williams GW, Golding LAR, Gill CC, Taylor PC, Sheldon WC, Proudfit WL. Influence of the internal mammary artery graft on 10-year survival and other cardiac events. New Engl J Med 1986; 314:1–6.
Morita R, Kitamura S, Kawachi K, Kawata T, Mizuguchi K, Kameda Y, Hasegawa J, Yoshida Y. Exercise coronary flow reserve of bilateral internal thoracic artery bypass grafts. Ann Thorac Surg 1993; 55:883–887.
Buffolo E, Andrade JC, Branco RJN, Teles CA, Aguiar LF, Gomes WJ. Coronary artery bypass grafting without cardiopulmonary bypass. Ann Thorac Surg 1996; 61:63–66.
Subramanian V, Stelzer P. Clinical experience with minimally invasive coronary artery bypass grafting. J Eur Cardio-thorac Surg 1996; 10:1058–1063.
Calafiore AM, Gallina S, Iaco A, Teodori G, Iovino T, di Giammarco G, Mazzei V, Vitolla G. Minimally invasive mammary artery Doppler flow velocity evaluation in minimally invasive coronary operations. Ann Thorac Surg 1998; 66:1236–1241.
Canver CC, Armstrong VM, Nichols RD, Mentzer RM. Color-flow Duplex Ultrasound assessment of internal thoracica artery graft after coronary bypass. Ann Thorac Surg 1995; 59:389–392.
Krijne R, Lyttwin RM, Hltgen R, Heinrich KW, Marx R, Sons H. Combined two-dimensional and Doppler sonographic examination of the internal mammary artery grafts from the supraclavicular fossa. Int J Card 1992; 37:61–64.
Bono de DP, Samani NJ, Spyt TJ, Hartshorne T, Trush AJ, Evans DH. Transcutaneous ultrasound measurements in internal mammary artery to coronary artery grafts. Lancet 1992; 339:379–381.
Marx R, Jax T, Schoebel FC, Schannwell CM, Plehn G, Leschke M, Strauer BE. Arteria thoracica interna-Bypass-Grundlagen der Dopplersonographie für die pr- und postoperative Diagnostik. Z Kardiol 1998; 87:Suppl II, 80–86.
Mehran R, Dangas G, Stamou SC, Pfister AJ, Dullum MK, Leon MB, Corso PJ. One-year clinical outcome after minimally invasive direct coronary artery bypass. Circulation 2000; 102:2799–2802.
Nemes A, Sotonyi P, Balogh A, Nagy SJ. Use of the internal mammary artery for myocardial revascularization. Acta Chir Acad Sci Hun 1977; 18:123–128.
Singh RN. Radiographic anatomy of the internal mammary arteries. Cathet Cardiovasc Diagn 1981; 7:373–386.
Kern MJ, Bach RG, Donohue TJ, Caracciolo EA, Wolford T, Aguirre FV. Role of large pectoralis branch artery in flow through a patent left internal mammary artery conduit. Cathet Cardiovasc Diagn 1995; 34:240–244.
Akasaka T, Yoshida K, Maeda K, Hozumi T, Tagaki T, Kaji S, Kawamoto T, Morioka S, Nasu M, Yoshikawa J. Flow dynamics of angiographically no-flow patent internal mammary artery grafts. J Am Coll Cardiol 1998; 31:1049–1056.
Manolas J. Vergleich von Handgrip-Apexkardiogramm-Test und Belastungs-EKG zur Erfassung von Patienten mit koronarer Herzkrankheit. Herz 1993; 18:256–266.(Roger Marx, Gerhard Kalwe)
b Department of Cardiac and Thoracic Surgery, Heart Center Voelklingen, Germany
c Department of Thoracic and Cardiovascular Surgery, Heinrich-Heine-University, Düsseldorf, Germany
d Department of Cardiology, Pneumology and Angiology, Heinrich-Heine-University, Düsseldorf, Germany
e Department of Cardiology, Heart Center Wuppertal, University of Witten-Herdecke, Germany
Abstract
Minimally invasive surgery for coronary revascularization using the left internal thoracic artery (ITA) has gained increasing interest. For control of graft function the established transcutaneous color-Doppler echocardiography in combination with a stress-test was performed to test the ability of this novel technique. Twenty-one patients having received a single ITA-graft were evaluated early postoperatively at rest and during isometric stress test with a handgrip exercise. Compared to the right internal thoracic artery, the mainly systolic flow is changed to a wide diastolic component when the left ITA is anastomosed to the coronary artery. The peak systolic/peak diastolic velocity ratio changed from 4.5±1.9 to 1.4±0.47 (P<0.0001). During stress reaction with the isometric handgrip maneuver the grafted ITA showed a significant increase of the mean diastolic flow (29.1±13.3 to 44.3±14.7 cm/s, P<0.0001) and total blood flow (124.8±55.4 ml/min to 176.6±71.7 ml/min), which may demonstrate an efficient bypass function. We conclude, that the noninvasive measurement of ITA-graft function with Doppler-ultrasound may be a clinically useful method to assess the functional status after minimally invasive coronary artery bypass grafting. In combination with the hand-grip test it represents a valid new technique with the potential to estimate graft patency.
Key Words: Internal thoracic artery bypass; Stress Doppler echocardiography; Minimally invasive cardiac surgery
1. Introduction
The aorto-coronary bypass grafting using the internal thoracic artery (ITA) is the method of choice due to the long-term patency rate [1]. The graft is a conduit with a proven functional adaptability, depending upon the actual myocardial demands, by increasing its diameter when flow requirements make this necessary [2]. Besides coronary artery bypass grafting with the use of extracorporal circulation beating heart techniques have gained increasing interest [3]. These efforts in ‘less invasive coronary artery surgery’ lead to the revascularization via a left lateral minithoracotomy with ITA harvesting and anastomosing it predominantly to the left anterior descending (LAD) coronary artery [4]. It has been shown to yield excellent results with a low associated morbidity and mortality [5]. A noninvasive investigation for postoperative assessment of graft function would be helpful.
Ultrasonic methods have been extensively used to measure blood flow in peripheral vessels and more recently in internal thoracic arteries. The examination of this vessel has gained general acceptance in the pre- [6] and postoperative [7] course. The origin and the proximal part are positioned close to the chest wall after a coronary artery bypass operation as well and, therefore, a noninvasive Doppler echocardiographic evaluation is possible. The phenomenon that ITA graft flow changes from a dominant systolic pattern preoperatively to a diastolic pattern postoperatively has been described repeatedly [8]. This method provides not only an accurate noninvasive evaluation of ITA graft function, but also an optimal approach to gain insight into coronary flow properties. Pharmacological stress tests (using adenosine or dubutamine) need some organization, are not easily done, and cannot be repeated often. In order to assess the autoregulatory and vasodilatory capabilities of the ITA-graft we examined the response of ITA flow by color Doppler echocardiography in rest and during exercise with a handgrip-maneuver after minimally invasive direct coronary artery bypass (MIDCAB).
2. Material and methods
2.1. Patient population
The MIDCAB procedures were used in 21 patients (18 male, 3 female) with a high comorbidity. The majority of patients had two-vessel and three-vessel diseases (Table 1). All patients have received a single left ITA-graft. The ultrasound study was performed in accordance with the local Ethics Committee and the patients were investigated in the early postoperative stage within the second to fourth postoperative week. All patients were informed and gave oral consent for the ultrasound investigation.
A left anterolateral minithoracotomy with a supramammary incision was performed through the fourth intercostal space. After opening the pericardium longitudinally the LAD was identified and inspected. A rib elevator was used for ITA harvesting under direct vision. Proximal ITA dissection was continued, usually up to the second rib, until sufficient pedicle length allowed a tension-free curved course of the ITA; side branches were clipped and cut. Local immobilization at the anastomosis was achieved with a stabilizer. Under direct vision, the ITA to LAD anastomosis was performed using routine techniques.
2.2. Color Doppler ITA flow evaluation
The technique of color Doppler echocardiography was carried out in the way as previously described by us [9]. During Doppler echocardiography the patient lies in a supine position. Transthoracic imaging and Doppler spectra derived from the ITA were obtained with a real-time two-dimensional echocardiographic unit using a 7.5 MHz phased array transducer connected to a Hewlett Packard sonographic system (Sonos 2500, Andover, MA).
In the most cases the detection of the graft was possible in the first intercostal space; in the case of failure the supraclavicular position was used. The colorflow facility was used to identify easier the origin and the course of the vessel. After detection of the ITA in longitudinal line the diameter was measured with the transducer. In order to obtain an optimal flow velocity signal the highest peak velocities and a well-defined spectral morphology was required after minimizing the angle between vessel and Doppler beam.
The mean systolic velocity (MSV), mean diastolic velocity (MDV), mean velocity (MV), peak systolic velocity (PSV) and peak diastolic velocity (PDV) were achieved. Total blood flow, peak systolic to diastolic velocity ratio (PSDVR) and mean systolic to diastolic velocity ratio (MSDVR) were calculated. Measurements were performed according to the internal analytic package of the ultrasound machine. The different flow velocities were measured by tracing out the contour of the Doppler velocity tracing. Values for each parameter were obtained by averaging measurements from three to five cardiac cycles. The investigations at rest were made on both sides for comparison of the ungrafted right ITA with the left ITA as a control parameter for initial data.
After baseline measurements at rest, patients undertook a handgrip-maneuver with a balloon dynamometer (Martin, Stuttgart, Germany). The flow in the ITA was monitored throughout the maneuver with the Doppler-probe in place. The squeezing of the balloon was performed with the right hand and sustained his grip at 40–50% of his maximal voluntary contraction for 3 min. According to the constitution of the patient, the pressure in the balloon ranged between 0.2 and 0.3 kg/cm2. Isometric exercise was used to produce an increased cardiac load to aid the assessment of the ITA-graft. The heart rate and the blood pressure were monitored at each period. The patients had to speak during the pressure recording to prevent any inadvertent performance of Valsalva maneuver.
2.3. Statistical analysis
Results are expressed as mean value±S.D. (standard deviation) unless otherwise indicated. Statistical analysis comparing the same between the right and the left side and between rest and stress conditions were performed with a nonparametric test (Wilcoxon signed-rank test) for paired observations. A probability value <0.05 was considered significant. Statistical analyses were performed on the SPSS (Version 8.0 for personal computer, Chicago, IL) statistical software program.
3. Results
After surgery in all patients clinical symptoms of angina pectoris were clearly improved. The diameter of the ITA-graft was measured and flow data recorded. The flow pattern in a non-grafted vessel has a biphasic or triphasic wave form with a large systolic peak, sometimes followed by small reversed and then by a diastolic flow component. The flow in the grafted ITA was detected in all patients. Compared to the ungrafted right ITA, the mainly systolic flow is changed to a meanly wide diastolic component in the left ITA anastomosed to the coronary artery. The flow in the residual proximal branches finds ifself in the systolic Doppler signal.
3.1. Rest
The mean diameter was 2.7±0.3 mm for the grafted left ITAs and 2.6±0.4 mm for the ungrafted right ITAs (P=0.001). Comparing the grafted left to the ungrafted right ITA the mean systolic flow velocity decreased significantly (P<0.0001) and the diastolic flow velocity increased significantly (P<0.005) to a dominant diastolic pattern. Median total flow was not different in ungrafted right ITA and the grafted left ITA (Table 2). There was no significant difference in resting flow rates between grafted and ungrafted vessels (P>0.05). The hemodynamic parameters were on the same level during the investigation on both sides in rest. The ratio of peak systolic velocity/peak diastolic velocity (PSDVR) changed from 4.5±1.9 to 1.4±0.5 (P<0.0001) and the mean systolic velocity/mean diastolic velocity ratio (MSDVR) from 3.9±1.7 to 1.3±0.4 (P<0.0001).
3.2. Changes during exercise
All patients completed the exercise protocol and satisfactory measurements were obtained during exercise, no patient had symptoms of angina pectoris. The stress test induces an increase in pulse rate, systolic and diastolic blood pressure and additionally in graft flow, the double product increased significantly (8828 to 12,666, P<0.001).
At rest, both peak and mean diastolic were often, but not always, higher than the corresponding systolic velocities. During the handgrip maneuver, both the systolic and diastolic peak and mean velocities increased, whereby the rising of the peak and mean diastolic velocity was more pronounced (peak systolic velocity 16.3% compared to peak diastolic velocity 59.5%, P<0.001, mean systolic velocity 24.8% to mean diastolic velocity 63.7%, P<0.001). The increase of the mean velocity was highly significant (Table 3, Fig. 1).
The stress test induces a significant decrease of PSDVR and MSDVR. This finding demonstrates that an increasing diastolic flow contributes essentially to the overall increase in averaged peak and mean. The hemodynamic changes by exercise leads to a significant increase of total blood flow (124.8±55.4 ml/min to 176.6±71.7 ml/min, P<0.001) in the ITA-graft (Fig. 2).
Five patients of the series had an angiographic visualization of the region of anastomosis with digital subtraction angiography (DSA) for the exclusion of a graft dysfunction because of clinical reasons. The anastomosis presented no signs of obstruction. Between the two patient groups (with and without DSA) there was no difference in the estimated flow parameters in rest and under stress conditions.
4. Discussion
Myocardial revascularization by minimally invasive coronary bypass surgery offers an alternative concept to conventional coronary artery bypass grafting (CABG) and excellent clinical results are reported with the MIDCAB procedure [4]. It can be performed in patients at high risk when conventional CABG would be used [10]. One important aspect of this procedure during the postoperative course is the functional assessment of the graft. Ideally every patient should have postoperative angiography to visualize the region of anastomosis, but this is an invasive approach. In our study group five patients had an angiographical control, the estimated flow patterns at rest and during stress conditions were comparable in both patient groups.
The difference in diameter between the ungrafted RIMA and the LIMA is a question certainly only of a computational difference, in clinical importance is this surely unimportant. The determined vessel diameters lay in a comparable order of magnitude to anatomical [11] and angiographic studies [12]. An intact internal thoracic artery graft shows a characteristic biphasic flow velocity profile in its proximal part only seen in patients with ITA grafts. The Doppler flow pattern of an in-situ ITA resembles that of other medium-sized systemic arteries, such as the external carotid artery. Intramyocardial blood vessels differ from those in other muscle areas because they exhibit a low resistance during diastole and an elevated resistance during systole due to the increased wall tension particulary in the left ventricular wall. Flow in the distal epicardial coronary arteries occurs almost entirely during diastole, as described in intracoronary flow studies [13]. The ultrasound measurements of the ITA concern the proximal part of an elastic vessel anastomosed distally to the coronary artery. The systolic part of the flow pattern represents flow into a distensible but effectively blind-ended tube and to patent large nonligated side-branches. An obstruction of the graft would affect the diastolic part but not the systolic flow [5,14].
Flow velocity assessment at rest gives incomplete information, as the diastolic flow does not exclude a possible stenosis of the anastomosis. Pharmacologic stress testing is difficult to perform repeatedly. Therefore, the hand-grip exertion was used because of minimal thoracal movements during the stress testing in comparison to treadmill and bicycle ergometer making continuous Doppler signals recordings impossible. Furthermore, this stress test can easily be performed and repeated.
Heart rate, cardiac output, blood pressure and stroke work all increase during handgrip exercise [15]. This causes an increase of blood flow to the muscular circulation (systolic phase) and to the coronary circulation (diastolic phase). The amount of the diastolic increase represents the function of the anastomosis. At rest patients with normal anastomosis have both peak and mean diastolic velocities similar to the systolic ones, as well as the corresponding ratios. During the handgrip-maneuver the increase of the diastolic flow patterns were higher than the systolic flow patterns. The mean systolic to diastolic velocity ratio (MSDVR) was between 1.0 to 1.5 in rest and it decreased to 1.0 or less during handgrip-maneuver. In this judgment the increase of total blood flow has to be recognized. This seems to be the result of measurement of a normal graft function.
In conclusion, the aim of our present study was to describe the feasibility of this novel technique for measurement of the ITA-graft flow after MIDCAB-procedure and to estimate the function of the graft with interpretations of the Doppler findings. The handgrip-maneuver may be an investigation to test graft patency. Additionally, this technique would be suitable for all patients with coronary artery bypass grafting using the internal thoracic artery also with sternotomy.
Acknowledgements
We wish to thank G. Plehn, MD for critical revising of the manuscript and Mrs M. Marx for preparing the manuscript.
References
Loop FD, Lytle BW, Cosgrove DM, Stewart RW, Goormastic M, Williams GW, Golding LAR, Gill CC, Taylor PC, Sheldon WC, Proudfit WL. Influence of the internal mammary artery graft on 10-year survival and other cardiac events. New Engl J Med 1986; 314:1–6.
Morita R, Kitamura S, Kawachi K, Kawata T, Mizuguchi K, Kameda Y, Hasegawa J, Yoshida Y. Exercise coronary flow reserve of bilateral internal thoracic artery bypass grafts. Ann Thorac Surg 1993; 55:883–887.
Buffolo E, Andrade JC, Branco RJN, Teles CA, Aguiar LF, Gomes WJ. Coronary artery bypass grafting without cardiopulmonary bypass. Ann Thorac Surg 1996; 61:63–66.
Subramanian V, Stelzer P. Clinical experience with minimally invasive coronary artery bypass grafting. J Eur Cardio-thorac Surg 1996; 10:1058–1063.
Calafiore AM, Gallina S, Iaco A, Teodori G, Iovino T, di Giammarco G, Mazzei V, Vitolla G. Minimally invasive mammary artery Doppler flow velocity evaluation in minimally invasive coronary operations. Ann Thorac Surg 1998; 66:1236–1241.
Canver CC, Armstrong VM, Nichols RD, Mentzer RM. Color-flow Duplex Ultrasound assessment of internal thoracica artery graft after coronary bypass. Ann Thorac Surg 1995; 59:389–392.
Krijne R, Lyttwin RM, Hltgen R, Heinrich KW, Marx R, Sons H. Combined two-dimensional and Doppler sonographic examination of the internal mammary artery grafts from the supraclavicular fossa. Int J Card 1992; 37:61–64.
Bono de DP, Samani NJ, Spyt TJ, Hartshorne T, Trush AJ, Evans DH. Transcutaneous ultrasound measurements in internal mammary artery to coronary artery grafts. Lancet 1992; 339:379–381.
Marx R, Jax T, Schoebel FC, Schannwell CM, Plehn G, Leschke M, Strauer BE. Arteria thoracica interna-Bypass-Grundlagen der Dopplersonographie für die pr- und postoperative Diagnostik. Z Kardiol 1998; 87:Suppl II, 80–86.
Mehran R, Dangas G, Stamou SC, Pfister AJ, Dullum MK, Leon MB, Corso PJ. One-year clinical outcome after minimally invasive direct coronary artery bypass. Circulation 2000; 102:2799–2802.
Nemes A, Sotonyi P, Balogh A, Nagy SJ. Use of the internal mammary artery for myocardial revascularization. Acta Chir Acad Sci Hun 1977; 18:123–128.
Singh RN. Radiographic anatomy of the internal mammary arteries. Cathet Cardiovasc Diagn 1981; 7:373–386.
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