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Right arrow Coronary disease

Ann Thorac Surg 2004;77:1586-1592
© 2004 The Society of Thoracic Surgeons

Original article: cardiovascular

Endoscopic localization and assessment of coronary arteries by 13 MHz epicardial ultrasound

Ricardo P. J. Budde, MSa, Rudy Meijera, Patricia F. A. Bakker, MD, PhDa, Cornelius Borst, MD, PhDa,a*, Paul F. Gründeman, MD, PhDa

a Heart Lung Center Utrecht, University Medical Center, Utrecht, The Netherlands

Accepted for publication October 10, 2003.

* Address reprint requests to Dr Borst, University Medical Center Utrecht (Room G02.523), Heidelberglaan 100 3584 CX, Utrecht, The Netherlands
e-mail: exp.cardio{at}hli.azu.nl


   Abstract
Top
 Abstract
Introduction
 Material and methods
 Results
 Comment
 Conclusion
 Acknowledgments
 References
 
BACKGROUND: In totally endoscopic coronary artery bypass grafting the target coronary artery is difficult to locate and assess. We explored the capacity of a high-frequency epicardial ultrasound mini-transducer (Aloka, Tokyo, Japan) to endoscopically locate and assess the left anterior descending (LAD), third obtuse marginal (OM3), and right posterior descending (RDP) coronary arteries.

METHODS: In eight pigs, the LAD, OM3, and RDP were endoscopically exposed. The mini-transducer was manipulated by the "da Vinci" telemanipulation system (Intuitive Surgical, Inc, Mountain View, CA) over the unstabilized and stabilized epicardium to identify the target artery, obtain a scout scan, and both transverse and longitudinal images.

RESULTS: In both unstabilized and stabilized conditions, the LAD and RDP were identified within a median of 29 seconds. In stabilized conditions, assessment was complete in 112 seconds (92 to 205) (median with range) for the LAD and 140 seconds (54 to 197) for the RDP. Stabilization of the OM3 was required for identification (16 [5 to 60]) and assessment (111 [82 to 225]). Overall identification was correct in 23 of 24 arteries. The OM branches and RDP became fully exposed endoscopically with stroke volume (SV) and mean arterial pressure (MAP) remaining at 67% ± 11% (mean ± standard error of the mean) and 70% ± 5% of baseline values, respectively. Scanning itself did not augment the decrease in SV and MAP significantly.

CONCLUSIONS: After proper endoscopic exposure and stabilization, robot-assisted epicardial ultrasound scanning enabled endoscopic identification and assessment of major coronary arteries within a median of 169 seconds per artery. Exposure, stabilization, and scanning were accompanied by an acceptable drop in stroke volume and mean arterial pressure.


   Introduction
Top
 Abstract
 Introduction
Material and methods
 Results
 Comment
 Conclusion
 Acknowledgments
 References
 
In totally endoscopic coronary artery bypass surgery (TECAB) several obstacles are encountered. The target coronary artery is often difficult to locate due to the epicardial fibrofatty layer, a tangential angle of view, difficult to interpret anatomical landmarks, and bleeding during dissection of epicardial fat [1–3]. Failure to locate the left anterior descending artery (LAD) endoscopically has resulted in intraoperative conversion to an open-chest procedure in up to 9% of patients undergoing TECAB [3]. There is a risk of grafting the wrong vessel (diagonal branch instead of LAD).

Absence of tactile feedback on the telemanipulation systems used in TECAB prevents palpatory assessment of the target coronary artery in search of the optimal anastomotic site [1–3]. Intraoperative planning may be improved by knowledge of the internal luminal diameter, the presence and extent of plaque, and the existence of nearby side branches which may hamper anastomosis suturing due to torrential backflow [1–3].

High-frequency epicardial ultrasound can accurately locate side branches and septal perforators as well as assess the dimensions and vessel wall quality of coronary arteries in open chest coronary artery bypass grafting (CABG) [4–8]. Experience with the endoscopic use of ultrasound to detect coronary arteries is anecdotic, partly due to the relatively bulky size of most transducers, which prohibits their passage through a port [8, 9]. Recently, a 13 MHz mini-transducer (Aloka, Tokyo, Japan) was developed that is small enough to pass an 11-mm trocar. It can be handled by telemanipulation systems. When used in combination with endoscopic cardiac positioning and stabilization devices [10], ultrasound assessment of coronary arteries on the anterior, lateral, and posterior sides of the heart might prove feasible.

The aim of this study was to assess the feasibility of endoscopic exposure and subsequent localization and assessment by a 13 MHz mini-transducer of the LAD, third obtuse marginal branch (OM3), and right descending posterior artery (RDP) in the pig.


   Material and methods
Top
 Abstract
 Introduction
 Material and methods
Results
 Comment
 Conclusion
 Acknowledgments
 References
 
Animals
Eight female Dutch landrace pigs (weight range, 55 to 85 kg) were used. The animals received humane care in compliance with the Guide for the Care and Use of Laboratory Animals prepared by the Institute of Laboratory Animal Resources, National Research Council, and published by the National Academy Press (revised 1996). The study was approved by the Animal Experimentation Committee of the Utrecht University.

Anesthesia
Anesthesia was induced by ketamine (10 mg/kg), midazolam (0.2 mg/kg), and atropine (0.04 mg/kg) intramuscularly, and thiopental sodium (4 mg/kg) intravenously. Loading doses of midazolam (0.5 mg/kg), sufentanil citrate (6 µg/kg), and pancuronium bromide (0.1 mg/kg) were administered intravenously. Subsequently, the animal was connected to a positive pressure ventilator.

Anesthesia was maintained by a mixture of oxygen and air (FiO2 = 0.5) with added halothane (0% to 1.0%), and a continuous intravenous infusion of midazolam (0.7 mg · kg–1 · h–1), sufentanil citrate (2 µg · kg–1 · h–1), and pancuronium bromide (0.1 mg · kg–1 · h–1). To obtain a heart rate below 70 beats per minute and reduce cardiac irritability, propranolol was administered intravenously to a maximum of 25 mg. Electrocardiogram, phasic and mean arterial blood pressure (MAP), end-tidal CO2, nasal temperature, and cardiac output (CO) were monitored continuously. At the end of the procedure animals were sacrificed by pentobarbital sodium (200 mg/kg) intravenously.

Surgery
To obtain access to the aorta and right atrium, partial median sternotomy was performed and the pericardium was opened. A calibrated transit time flow probe (20 to 24S), connected to a calibrated flow meter (model T208, Transonic Systems, Inc, Ithaca, NY), was placed around the ascending aorta for continuous measurement of CO. After a bipolar pacing lead was sutured to the right atrial appendage, pacing was started at a fixed rate of 80 beats per minute. Stroke volume (SV) was calculated by dividing CO by heart rate (pacing rate of 80). A fluid manometer catheter was placed in the left femoral artery to measure phasic arterial blood pressure and MAP.

Trocars were placed as follows: one trocar (Ø 12 mm) subxiphoidally for the stereoscope; two trocars (Ø 11 mm) for the "da Vinci" (Intuitive Surgical, Inc, Mountain View, CA) instruments [1] approximately 5 cm lateral to the camera port at the left and right sides; one trocar (Ø 15 mm) for the endoscopic cardiac positioner [10] on the anterior axillary line in the left second intercostal space; one trocar (Ø 15 mm) for the endoscopic cardiac stabilizer [11]; and one trocar (Ø 15 mm) for the ultrasound mini-transducer in the left and right fifth intercostal spaces, respectively, each on the midclavicular line. The chest was closed and a hook, attached to a table rail mounted lifting device, was inserted under the xiphoid process, hoisting the sternum ventrally approximately 5 cm [10].

The slave unit of the da Vinci computer enhanced telemanipulation was positioned at the head side of the operating table, and docked to the trocars [1].

A modified, articulating version of the Starfish cardiac positioner (Medtronic, Minneapolis, MN), named the "EndoStarfish," has been developed by us for endoscopic use [10]. The EndoStarfish enabled appropriate cardiac displacement to expose the OM branches and RDP. The EndoStarfish was fixed to the apex (– 400 mm Hg) and by subsequently hoisting the apex ventrally, cranially and to the right, the OM branches of the circumflex coronary artery were exposed. To expose the RDP, the apex of the heart was hoisted ventrally, cranially, and to the left.

A single Octopus-1 Tissue Stabilizer arm (Medtronic, Minneapolis, MN) was attached to the heart (– 400 mm Hg) to stabilize the target area [11]. During displacement, CO and MAP were continuously monitored to determine the optimal position for presentation of the target vessel with the least decrease in SV and MAP. No Trendelenberg positioning was employed.

Ultrasound equipment
A linear array mini-transducer (Aloka, Tokyo, Japan), with an imaging frequency of 13 MHz in B-mode and 7 MHz in color-Doppler, was used. It measures 15 mm in length, 6 mm in width, and 9 mm in height, has an image scan width of 10 mm, and an image depth of approximately 4 cm. A custom made snap-on metal probe holder enabled manipulation of the transducer by the end-effectors of the da Vinci instruments. The transducer was placed in a gel-filled protective cover (Ultracover, International Medical Products, Inc, Zutphen, The Netherlands), originally intended for use on a transesophageal echocardiography probe, which acted as a stand-off sleeve to facilitate scanning and improve image quality by limiting near-field transducer artifacts (Fig 1).



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  		Fig 1. The Aloka 13 MHz mini-transducer (top) and as it was used in this study, wrapped in sleeve with probe holder (bottom). The transducer can be passed through an 11-mm trocar.

 
The probe was connected to an SSD 5000 Prosound ultrasound system (Aloka, Tokyo, Japan) by a flexible cable. An experienced sonographer (RM) operated the ultrasound system. To enable retrospective analysis, selected images were stored on videotape and on a personal computer. The ultrasound image was displayed picture-in-picture on the master console of the da Vinci system, providing the operator with the real-time ultrasound image while scanning (Fig 2). All vessels were scanned by the same investigator (RPJB).



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  		Fig 2. Surgeon's view on master console of the posterior side of the heart (apex pointing to right upper corner of image) during stabilized (single Octopus-1 arm, large arrowhead) scanning of a RDP. (Inset) Ultrasound image (inferior caval vein, small arrowheads). (ART. = artery; RDP = right posterior descending coronary artery.)

 
Ultrasound scanning and hemodynamic measurement protocol
Cardiac output and MAP were recorded at baseline and during the scanning process. Baseline control values comprised 3 minutes of baseline measurements with the heart freely beating in its anatomical or displaced position depending on the target artery.

During the unstabilized scan, the target coronary artery was scanned with the mini-transducer. First, the exposed target area was visually inspected to locate the approximate position of the artery. Second, with the end-effectors of the da Vinci system instruments, the mini-transducer, orientated perpendicular to the expected vessel course, was manipulated over the epicardial surface utilizing the transducer's full image scanwidth of 10 mm to locate the artery. Third, to familiarize the operator with the course of the artery and spot side branches and septal perforators, a transverse scout scan was performed in the up-stream and down-stream directions of the vessel over a distance of about 6 cm (LAD), 3 cm (OM3), and 4 cm (RDP) (Fig 3). Fourth, a transverse image (vessel outline round, no signs of compression) and a longitudinal image were obtained (vessel visible over entire length of scan image) at the approximate mid part of the section scanned during the scout scan. In the transverse image, the internal coronary artery diameter was measured both top to bottom and side to side. In the longitudinal image, it was measured top to bottom. The time required to finish each scanning step was recorded.



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  		Fig 3. Schematic drawing of the porcine coronary anatomy. Arrows indicate the part of the target arteries scanned during the scout scan. Vessel diameter was measured in the mid portion of this section. (CX = circumflex coronary artery; OM3 = third obtuse marginal coronary branch; LAD = left anterior descending coronary artery; RCA = right coronary artery; RDP = right posterior descending coronary artery.)

 
During the stabilized scan, the previous scanning steps were repeated on the stabilized heart. The vessel location was marked by placing a microvascular clip on the epicardial surface.

Angiography
After the animals were sacrificed, the heart was excised and roentgenogram markers were placed over the microvascular clips. The coronary arteries were visualized by selectively injecting the left main stem and right coronary artery with contrast medium (C-arm BV27, Philips, Eindhoven, The Netherlands). By comparing positions of the roentgenogram markers and the coronary angiogram, coronary artery identification was scored as correct or incorrect.

Statistical analysis
Values are presented as median with range (scanning times) and mean ± standard error of the mean (SEM, hemodyamic data).

A paired Student's t test was used to compare scanning times, to evaluate the effect of ultrasound scanning on SV and MAP compared to stabilized and unstabilized target artery exposure, and OM and RDP exposure alone compared to baseline. A post hoc Bonferroni correction was applied to adjust for multiple testing. The Wilcoxon signed ranks test was used to evaluate the time needed to obtain longitudinal and transverse images. A p value of p less than or equal to 0.05 was considered significant.


   Results
Top
 Abstract
 Introduction
 Material and methods
 Results
Comment
 Conclusion
 Acknowledgments
 References
 
Surgery
All animals survived the entire procedure without the need to defibrillate or administer inotropic drugs. Lifting the sternum increased the distance between the posterior side of the sternum and anterior wall of the heart by approximately 5 cm. Endoscopic displacement of the heart by the EndoStarfish enabled adequate exposure of the OM branches with SV remaining at 69% ± 8% (mean ± SEM, p = 0.016) and MAP at 70% ± 5% (p = 0.002) of baseline values, and the RDP with SV remaining at 67% ± 11% (p = 0.022) and MAP at 85% ± 7% (p = 0.154) of baseline values. Full access for ultrasound scanning was obtained. Compared to target vessel exposure only, epicardial scanning by the mini-transducer did not result in significant hemodynamic consequences except for a 2 mL decrease in SV during unstabilized LAD scanning (p = 0.011). This decrease was of no clinical importance.

Ultrasound scanning protocol
Endoscopic manipulation of the mini-transducer by the handling tool was easily achieved by the end-effectors of the da Vinci system instruments; it was not restricted by the transducer cable. Due to the gel-filled sleeve, the near field of the ultrasound image was easy to interpret. The picture-in-picture displayed ultrasound image provided sufficient detail to adequately spot side branches and septal perforators from the robot master console.

During scanning, arteries and veins were easily discriminated by using color-Doppler imaging or on the basis of the vessel outline, because veins collapse when mild pressure is applied with the transducer (Fig 4). Scanning times are listed in Table 1. Obtaining optimal longitudinal images was significantly more time consuming than optimal transverse images for the stabilized LAD (p = 0.025), OM3 (p = 0.017), and RDP (p = 0.012), but not for the unstabilized LAD (p = 0.128) and RDP (p = 0.075). Vessel diameter measured in longitudinal and transverse (mean of both measurements) images were 1.9 ± 0.4 mm (mean ± standard deviation) and 2.1 ± 0.5 mm for the LAD, 1.4 ± 0.4 mm and 1.6 ± 0.6 mm for the OM3, and 1.4 ± 0.4 mm and 1.6 ± 0.4 mm for the RDP, respectively.



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  		Fig 4. Transverse Doppler image of a RDP with two adjacent veins. Blue depicts arterial flow and yellow/red depicts venous flow. (ART. = artery; RDP = right posterior descending coronary artery.)

 

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  		Table 1. Time (Seconds) Required for Scanning Procedure Subsets

 
Scanning of LAD
In both unstabilized and stabilized conditions, the LAD was identified within 30 seconds in all animals. During the scout scan, septal perforators and side branches were spotted easily. In stabilized condition the scan procedure was completed within a median of 132 seconds.

Scanning of third OM branch
In six animals, completion of all scanning steps on the unstabilized heart was impossible due to excessive motion of the target area. After stabilization, local cardiac motion was sufficiently reduced to adequately visualize the third OM branch (identification within 60 seconds in all animals) and complete all scanning steps within a median of 137 seconds. During the stabilized scout scan, the third OM branch could be visualized up to its origin from the circumflex coronary artery (Fig 5).



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  		Fig 5. B-mode image of circumflex artery (between arrows) with OM3 (arrowheads). (OM3 = third obtuse marginal coronary branch.)

 
Scanning of RDP
Cardiac motion of the RDP target area was less compared to the OM region, enabling completion of all scanning steps in both unstabilized and stabilized conditions. In stabilized condition, the RDP was identified within 84 seconds in all animals. The scan procedure was completed within a median of 169 seconds. Septal perforating branches were spotted easily (Fig 6).



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  		Fig 6. Split screen longitudinal B-mode (left) and power Doppler flow (right) image of a RDP with septal perforating branch. Note the side branches coming off the septal perforator running parallel to the RDP (arrow). (RDP = right posterior descending coronary artery.)

 
Angiography
Angiography confirmed that both the LAD and RDP were correctly identified in all eight animals. In one animal, a diagonal branch was mistaken for the OM3.


   Comment
Top
 Abstract
 Introduction
 Material and methods
 Results
 Comment
Conclusion
 Acknowledgments
 References
 
The principal results of this study are: (1) Epicardial 13 MHz ultrasound scanning enabled endoscopic identification and assessment of the LAD, OM3, and RDP in all eight animals within a maximum of 281 seconds; (2) identification of the target vessel was correct in 23 of 24 arteries; (3) RDP and OM branches could be fully exposed endoscopically without too great a drop in SV and MAP; (4) and local stabilization was required to visualize the OM3 properly.

Endoscopic epicardial ultrasound scanning
In open-chest CABG, epicardial ultrasound for visualization of the coronary artery [5, 12] and quality assessment of the coronary anastomosis [13] did not gain wide acceptance in the 1980s and 1990s due to technical limitations. The relatively low frequency resulted in too low a resolution. The large size of the available transducers limited their use to the anterior side of the heart. The recent development of both (minimally invasive) off-pump CABG and small high-frequency transducers renewed interest in the technique, as epicardial ultrasound can aid in choosing the optimal anastomotic site with little or no coronary pathology or septal perforators (backbleeding) [5–8]. The latter is particularly important in TECAB, where bleeding from the anastomotic site is difficult to control and may be a reason for conversion [2].

Despite the small size of the current probes, visualization of arteries and anastomoses on the lateral and posterior sides of the heart (especially parts of the left circumflex branch) is still difficult with most transducers [4, 6]. To allow endoscopic use of a transducer, it must be small enough to pass through a port, have some kind of handling tool to allow it to be manipulated by endoscopic instruments, and it should be connected to a very flexible cable that does not restrict positioning of the probe. To our knowledge, the mini-transducer employed in this study is the only transducer currently available that fits all these requirements. All major target sites for bypass surgery were visualized properly.

Endoscopic scanning of the LAD and RDP did not require stabilization of the target area as the transducers image scanwidth of 10 mm was sufficient to keep the target artery visualized in transverse imaging during each phase of the cardiac cycle. This facilitated subsequent first time correct placement of the stabilizer. For the OM3, stabilization was necessary to scan the artery.

One artery, identified as an OM3 during scanning, proved to be a diagonal branch during the ex vivo angiography. The coronary anatomy of this specific heart was unusual as the diagonal branches coming off the LAD were prominently extending to the left lateral side of the heart. Knowledge of the coronary anatomy before scanning based on a preoperative angiogram, as is routinely performed in the clinical setting, would probably have been helpful in this specific case.

Due to its small size and flexible cable, the mini-transducer was easily manipulated inside the chest. Inability to scan certain segments of the vessels was mainly due to limitations in endoscopic exposure achieved with the current endoscopic cardiac positioners and not related to the size and(or) maneuverability of the mini- transducer.

Complete scanning of the stabilized artery required from 74 to 281 seconds, with a median of 132, 137, and 169 seconds for LAD, OM3, and RDP, respectively. Endoscopic epicardial scanning did not significantly affect SV and MAP. Thus, it is a safe diagnostic technique.

Hemodynamic effects of endoscopic exposure of OM branches and RDP
Using the same tools as described before [10], the OM branches and RDP could be properly exposed endoscopically, making them fully accessible for ultrasound scanning. In stabilized condition, MAP remained on average above 70 mm Hg without the use of inotropic drugs or Trendelenburg positioning.

Limitations
The coronary anatomy of the pig is somewhat different from that in the human, sometimes with less epicardial fat surrounding the coronary arteries, making them easier to spot. In this study, however, the porcine epicardial tissue gradually developed edema after opening of the pericardium, making the coronary arteries more difficult to locate visually. Clinically, the smaller diameter of diseased coronary arteries might make them less easy to spot from the master console. With growing experience and use of color-Doppler, however, this should not be a limiting factor. We successfully visualized the LAD and OM branches with plaques and calcifications using the mini-transducer in one patient undergoing on-pump CABG via sternotomy access. Hemodynamic effects in patients with depressed left ventricular function may be clinically significant and require support with inotropic drugs and(or) Trendelenburg positioning.


   Conclusion
Top
 Abstract
 Introduction
 Material and methods
 Results
 Comment
 Conclusion
Acknowledgments
 References
 
After proper endoscopic exposure and stabilization, robot-assisted epicardial ultrasound scanning enabled endoscopic identification and assessment of major coronary branches within a median of 169 seconds per artery. Exposure, stabilization, and scanning were accompanied by an acceptable drop in SV and MAP that did not require the Trendelenburg position or administration of inotropic drugs.


Dr Borst discloses that he has a financial relationship with Medtronic.

 


   Acknowledgments
Top
 Abstract
 Introduction
 Material and methods
 Results
 Comment
 Conclusion
 Acknowledgments
References
 
The authors acknowledge the contributions of John Dries, Elly van Zwol, Cees Verlaan, Merel Schurink, and Dr Wim-Jan van Boven. Special thanks to Thomas C. Dessing, MS, for assistance in data acquisition.


   References
Top
 Abstract
 Introduction
 Material and methods
 Results
 Comment
 Conclusion
 Acknowledgments
 References
 

  1. Loulmet D., Carpentier A., d'Attellis N., et al. Endoscopic coronary artery bypass grafting with the aid of robotic assisted instruments. J Thorac Cardiovasc Surg 1999;118:4-10.[Abstract/Free Full Text]
  2. Dogan S., Aybek T., Andreßen E., et al. Totally endoscopic coronary artery bypass grafting on cardiopulmonary bypass with robotically enhanced telemanipulation: report of forty-five cases. J Thorac Cardiovasc Surg 2002;123:1125-1131.[Abstract/Free Full Text]
  3. Kappert U., Schneider J., Cichon R., et al. Development of robotic enhanced endoscopic surgery for the treatment of coronary artery disease. Circulation 2001;104(Suppl I):I102-I107.
  4. Xing Y.Q., Khabbaz K.R., Karia D., et al. Intraoperative epicardial coronary artery imaging: rationale, methods and applications. Echocardiography 2002;109:735-741.
  5. Oda K., Hirose K., Fukutomi T., Yamashiro T., Ogoshi S. Intraoperative detection of embedded coronary arteries in MIDCAB using a color Doppler microprobe. Ann Thorac Surg 1999;68:263-264.[Abstract/Free Full Text]
  6. Suematsu Y., Takamoto S., Ohtsuka T. Intraoperative echocardiographic imaging of coronary arteries and graft anastomoses during coronary artery bypass grafting without cardiopulmonary bypass. J Thorac Cardiovasc Surg 2001;122:1147-1154.[Abstract/Free Full Text]
  7. Haaverstad R., Vitale N., Williams R.I., Fraser A.G. Epicardial colour-Doppler scanning of coronary artery stenoses and graft anastomoses. Scand Cardiovasc J 2002;36:95-99.[Medline]
  8. Eikelaar J.H.R., Meijer R., van Boven W.J., Klein P., Gründeman P.F., Borst C. Epicardial 10-MHz ultrasound in off-pump coronary bypass surgery: a clinical feasibility study using a minitransducer. J Thorac Cardiovasc Surg 2002;124:785-789.[Abstract/Free Full Text]
  9. Falk V., Fann J.I., Grünenfelder J., Burdon T.A. Endoscopic Doppler for detecting vessels in closed chest bypass grafting. Heart Surgery Forum 2000;4:331-333.
  10. Gründeman P.F., Budde R., Mansvelt Beck H., Boven van W.J., Borst C. Endoscopic exposure and stabilization of posterior and inferior branches using the Endo-Starfish cardiac positioner and the Endo-Octopus stabilizer for closed-chest beating heart multivessel CABG: hemodynamic changes in the pig. Circulation 2003;108(Suppl I):I34-I38.
  11. Borst C., Jansen E.W.L., Tulleken C.A.F., et al. Coronary artery bypass grafting without cardiopulmonary bypass and with-out interruption of native coronary flow using a novel anastomosis site restraining device ("Octopus"). J Am Coll Cardiol 1996;27:1356-1364.[Abstract]
  12. Hiratzka L.F., McPherson D.D., Brandt B., III, Lamberth W.C., Jr, Marcus M.L., Kerber R.E. Intraoperative high-frequency epicardial echocardiography in coronary revascularization: locating deeply embedded coronary arteries. Ann Thorac Surg 1986;42(6 Suppl):S9-11.
  13. Hiratzka L.F., McPherson D.D., Lamberth W.C., Jr, et al. Intraoperative evaluation of coronary artery bypass graft anastomoses with high-frequency epicardial echocardiography: experimental validation and initial patient studies. Circulation 1986;73:1199-1205.[Abstract/Free Full Text]



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