Interact CardioVasc Thorac Surg 2005;4:222-226. doi:10.1510/icvts.2004.098517
© 2005 European Association of Cardio-Thoracic Surgery
Work in progress report - Experimental |
Do cardiac stabilizers really stabilize? Experimental quantitative analysis of mechanical stabilization
Massimo Lemma1,*,
Andrea Mangini1,
Alberto Redaelli2 and
Fabio Acocella3
1 Department of Cardiovascular Surgery, Luigi Sacco Hospital, Via G.B. Grassi 74, 20157, Milan, Italy
2 Department of Bioengineering, Politecnico di Milano, Milan, Italy
3 Department of Veterinary Surgery and Radiology, Faculty of Veterinary Medicine, Milan, Italy
Received 17 September 2004;
received in revised form 3 February 2005;
accepted 7 February 2005
 Presented at the joint 18th Annual Meeting of the European Association for Cardio-thoracic Surgery and the 12th Annual Meeting of the European Society of Thoracic Surgeons, Leipzig, Germany, September 12–15, 2004.
*Corresponding author. Tel.: +39-239042333; fax: +39-239042652.
E-mail address: m.lemma{at}hsacco.it (M. Lemma).
 |
Abstract
|
|---|
In order to assess the three-dimensional movement of the coronary arteries both during normal cardiac activity and after mechanical stabilization, a polypropylene black marker was placed in 10 pigs on the middle portion of the three main coronary branches. Marker motion was recorded for 10 s using two TV-digital cameras and was estimated with a precision of 50 µm. After stabilization with three different mechanical stabilizers (Medtronic, Genzyme, CTS-Guidant), a remnant coronary artery excursion of about 1.5–2.4 mm was found. There is a significant residual coronary artery motion after mechanical stabilization, which could affect the quality of anastomosis, especially in unfavourable situations.
Key Words: Coronary artery bypass graft; Off pump myocardial revascularization; Mechanical stabilizer
|
1. Introduction
|
|---|
Conventional coronary artery bypass graft (CABG) surgery uses cardiopulmonary bypass (CPB) to allow cardiac surgeons to operate on a motionless heart that has been arrested by means of cardioplegic solution infusion. This technique has become a well established procedure for patients with coronary artery disease. However, CPB has some deleterious effects that are primarily related to a systemic inflammatory response, which results when blood comes into contact with the surface of the extracorporeal circuit of the CPB machine [1,2]. Avoiding CPB, off pump CABG (OPCAB) has been proposed as a safer technique for myocardial revascularization [3]. However, working on the beating heart provides a constantly moving target that threatens to compromise the precision necessary for distal anastomosis. Thus, for the cardiac surgeon, OPCAB is generally considered more technically demanding than CABG on the arrested heart [4].
The present experimental study was designed to assess the extent of cardiac surface motion during normal cardiac activity and after placement of three different mechanical stabilization devices, the performances of which were compared.
|
2. Materials and methods
|
|---|
2.1. Anaesthesia and data acquisition
Ten pigs weighing 74±4 kg were used. All animals received human care according with the Guide for Care and Use of Laboratory Animals published by the National Institute of Health and in accordance with the European Convention on Animal Care. The study protocol was approved by the Animal Experimentation Committee of the State University of Milan.
Premedication was given 30 min before anaesthesia induction. All animals received ketamine (2.0 mg/kg), droperidol plus fentanyl citrate (0.05 mg/kg) and scopolamine (0.5 mg) intramuscularly. Anaesthesia was induced with intravenous pentothal (3 mg/kg). Endotracheal intubation positive pressure ventilation was then started at a volume of 10 ml/kg, with a mixture of oxygen 50% and isofluorane 0.5–1.0%. Pancuronium bromide was given intravenously as a muscle relaxant (0.1 mg/kg). Anaesthesia was maintained by continuous infusion of fentanyl (0.05 mg/kg). Cardiac activity was continuously recorded by electrocardiogram. After manual sternotomy the pericardium was opened longitudinally. Partial anticoagulation was obtained by an intravenous bolus of heparin (1.5 mg/kg). Continuous hemodynamic monitoring was achieved by catheter insertion in the right and left atrial appendage, pulmonary artery and ascending aorta.
A polypropylene black marker (0.5 mm diameter) was placed in sequence on the middle portion of the left anterior descending coronary artery (LAD), obtuse marginal branch (OM) and posterior descending coronary artery (PDA). Marker motion was recorded for 10 s using two digital TV-cameras (Sony DCR-PC2E, Sony Corporation, Tokyo, Japan) with an image frequency of 25 frames per second, both during free heart beating and after placement of three different mechanical stabilization devices (Medtronic Octopus, Medtronic Inc. Minneapolis, MN, USA – CTS Axius Guidant Stabilizer System, Guidant Corporation, Santa Clara, CA, USA – Genzyme OPCAB Immobilizer, Genzyme Corporation, Cambridge, MA, USA). A deep pericardial retraction suture was used to present the target coronary artery on the lateral and inferior wall of the left ventricle. This suture was kept in place during the evaluation for each device. The heart was placed in its ‘rest’ position between each stabilization until the pre-stabilisation hemodynamic parameters were recovered. A total of 12 recordings, 3 basal and 3 for each stabilizer on LAD, OM and PDA, were performed for each animal. The sequence of the stabilizers and the coronary sites investigated were changed for each pig. The pigs were discarded at the end of surgery.
2.2. Data extraction and analysis
The camera system was calibrated using an array of partially known points [5,6], with an estimated precision of 50 µm within the region of interest (approximately 10x10x10 cm). The absolute reference coordinate system was placed at the optical centre of the left TV-camera. The images were in RGB format; only the red channel was selected and converted into grey scale. For each camera image array, the location of the marker was estimated by selecting the marker pixel with a threshold filter and calculating the average values of the x, y pixel locations in the local reference frame. Through the calibration parameters, the three-dimensional coordinates of the marker were calculated from the two-dimensional data arrays taken with the two TV-cameras. The marker displacement was calculated with respect to its average position (Fig. 1). Further, the marker motion was triggered with respect to the respiratory frequency and data relative to three complete respiratory cycles were considered.
View larger version (29K):
[in this window]
[in a new window]
|
Fig. 1. Marker displacements in the Cartesian coordinate system (x,y,z) were calculated with respect to the average marker location (xavg, yavg and zavg). Data refer to animal n° 7, LAD stabilization by Genzyme stabilizer. LAD = left anterior descending coronary artery; avg = average.
|
|
Two indexes were extracted from the row data: (1) the three-dimensional average distance of the samples with respect to the average marker position (MD-3D) and (2) the systolic to diastolic motion (SDM). In order to calculate the MD-3D index, the average position of the marker was first calculated throughout the investigated time lapse, then the distance between each recorded point of the trajectory of the marker and the average marker position was computed. The MD-3D index is the mean value of the calculated distances. The SDM index is defined as the average distance between the systolic (diastolic) marker location and the subsequent diastolic (systolic) marker location (Fig. 2). In the first cycle, the end-diastolic marker location is selected manually and the end-systolic location is the farthest within the subsequent beat; therefore, the diastolic to systolic distance (dd–s) is calculated as:
| (1) |
where j indicates the considered cycle, x(i), y(i) and z(i) are the marker coordinates at each recorded frame within the heart beat in question and xd, yd and zd are the end-diastolic coordinates. Accordingly, the diastolic to systolic distance (ds–d) is:
| (2) |
where xs, ys and zs, are the end-systolic coordinates.
View larger version (35K):
[in this window]
[in a new window]
|
Fig. 2. Three-dimensional marker motion reconstruction. Data refer to animal n° 2, OM stabilization by CTS stabilizer. The two arrows point out the distance between the systolic (diastolic) marker location and the subsequent diastolic (systolic) marker location, which have been used to calculate the systolic to diastolic motion (SDM) index. i and i+1 are two consecutive marker location. OM = obtuse marginal branch; avg = average.
|
|
The new co-ordinates are then used as the end-diastolic marker location of the next cycle. SDM is calculated as:
| (3) |
where Nc is the number of cardiac cycles.
2.3. Statistical analysis
Data are presented as mean ± standard deviation. Statistical analysis was performed using Student's t-test for data with normal distributions and with the Wilcoxon signed rank test and the Mann–Whitney rank sum test for non-normal distributions.
|
3. Results
|
|---|
Motion analysis of LAD, OM and PDA showed a significant reduction of both MD-3D and SDM indexes after stabilization (Table 1). Un-stabilized cardiac wall excursion was up to a mean of 12.5±2.9 mm for the LAD, 10.2±4.5 mm for the OM and 9.6±3.4 for the PDA. The three mechanical stabilization devices reduced this motion significantly. However, there was a remnant coronary artery excursion after stabilization of about 1.5–2.4 mm.
Positive pressure ventilation had an important influence on cardiac surface stabilization. Fig. 3 depicts the three-dimensional marker motion before and after stabilization with each of the three different stabilizers. The influence of mechanical ventilation on cardiac motion can easily be pointed out in the four panels.
View larger version (31K):
[in this window]
[in a new window]
|
Fig. 3. Marker displacements in the Cartesian coordinate system (x,y,z) before stabilization (panel a), using CTS stabilizer (panel b), Medtronic stabilizer (panel c) and Genzyme stabilizer (panel d). Data refer to animal n° 8.
|
|
As far as the performance of each of the three mechanical stabilizers is concerned, comparison showed that significant differences were present only for Genzyme on the LAD using the SDM index. As a matter of fact, stabilizations obtained by CTS and Medtronic were significantly better (Table 1).
|
4. Discussion
|
|---|
The results of this study confirm that OPCAB should be considered more demanding or technically more challenging than conventional CABG using CPB. Three important surgical rules warn that any operating field should be well exposed, steady and bloodless. During OPCAB, cardiac surgeons voluntarily break these rules. It has often happened that performing a coronary anastomosis on a beating heart in a bloody field has turned out to be a discouraging and frustrating adventure. However, to date, no available mechanical stabilization device has been able to achieve a steady, bloodless field comparable to that of a cardioplegically arrested heart. Thus, OPCAB raises doubts about the quality of coronary anastomoses: can (average) cardiac surgeons guarantee the same results in terms of patency rate both during on-pump and off-pump surgery? Up to now there is no completely clear evidence. In 2001, Puskas et al. [7] reported an impressive OPCAB patency rate of 98% at hospital discharge, while more recently, Khan et al. [8] have reported the results of a prospective randomized trial with a patency rate at three months of 98% for on-pump versus 88% for off-pump patients (P=0.002). The learning curve seems to play an important role in OPCAB, and many series have been published demonstrating higher patency rates and less anatomic stenosis [9,10].
Motion of the heart occurs in the three dimensions of space and can be described as a smoothly varying combination of sinusoidal waves. There are only a few reports that quantify cardiac surface motion. In a porcine model, Borst et al. [11] computed the two-dimensional area (x and y axis) covered by a reference point on the epicardium during free heart beating and after placement of a vacuum mechanical stabilizer, showing a significant reduction of motion (from 73±43 mm2 to 1.3±0.5 mm2) during heart stabilization. Koransky et al. [12] have analyzed the three-dimensional motion of the LAD coronary artery in pigs, using sonomicrometry crystals. Motion and velocity were analyzed on a beat-to-beat basis in the x, y and z planes using triangulation theory before and after placement of a vacuum-assisted stabilizer. Stabilization resulted in a significant reduction of excursion (11.36±1.74 vs. 5.99±1.30 mm; P<0.05), maximum Cartesian velocity (141.80±29.73 vs. 86.55±29.45 mm/s; P<0.05) and average Cartesian velocity of the LAD (44.30±7.02 vs. 21.46±4.54 mm/s; P<0.05). More recently, Cattin et al. [13] have shown a significant reduction of maximal excursion, speed of excursion and motion trajectory excursion after Octopus stabilization of the LAD in three pigs. These authors used an 8-bit high-speed black and white video camera (50 frames/s) coupled with a laser sensor (60 µm resolution) to capture heart wall motion around LAD in all three dimensions. In their experiments, remnant coronary artery excursion after stabilization was about 300–330 µm in the x and y plane and about 2–2.6 mm in the z plane.
In the present study, we tried to quantify the three-dimensional motion of the three main branches of the coronary artery tree both during unrestrained cardiac activity and after cardiac stabilization with three different mechanical stabilization devices. To the best of our knowledge, this is the first analysis of the three-dimensional motion of the inferior and lateral heart wall. It has been demonstrated that the geometric accuracy (the accuracy by which a target in a Cartesian coordinate system can be approached) of humans is between 100–200 µm [14]. This limit can be reduced to 50 µm under ideal conditions (rested elbows and microscopic view). However, during OPCAB, environmental conditions are usually anything but ideal, and it can be argued that a residual coronary artery excursion after stabilization of about 1.5–2.4 mm could cause a loss of surgical precision, especially in presence of small, diffusely diseased coronary arteries.
4.1. Limitations of the study
This is an experimental study on pig hearts. Ten animals of about 70 kg were selected in order to have comparable heart dimension with human beings of the same weight. However, the pig heart is usually smaller, has less epicardial fat and presents a coronary tree with a different spatial orientation. We can assume that the same analysis on human beings could produce different results.
We could not precisely identify the direction of coronary artery excursion in three-dimensional space. The absolute reference point (x,y,z) used to estimate the three-dimensional marker coordinates was placed in the optical centre of the left camera, with the two-dimensional surface (x/y axis) parallel to the image surface and the z-axis corresponding to the camera optical axis. The three investigated coronary branches (LAD, OM, PDA) lie on three different ‘surfaces’ of the heart. As a consequence, the three-dimensional coordinates (x,y,z) of the marker absolute reference point of the three coronary arteries cannot be compared.
In conclusion, working on the beating heart is a complex, multi-dimensional tracking task during which the surgeon tries to have the controlled system follow the changes of the target system.
There is significant residual coronary artery motion after mechanical stabilization, which can affect the quality of anastomosis, particularly in unfavourable situations (suboptimal coronary artery exposure, bloody surgical field, small coronary artery or diseased arterial wall). Better understanding of residual coronary motion characteristics after stabilization will be useful to improve mechanical stabilization.
|
References
|
|---|
- Kirklin JK. Prospects for understanding and eliminating the deleterious effects of cardiopulmonary bypass. Ann Thorac Surg 1991;51:529–531.[Medline]
- Picone AL, Lutz CJ, Finck C, Carney D, Gatto LA, Paskanik A, Searles B, Snyder K, Neiman G. Multiple sequential insults cause post-pump syndrome. Ann Thorac Surg 1999;67:978–985.[Abstract/Free Full Text]
- Mack MJ. Pro: beating-heart surgery for coronary revascularization: is it the most important development since the introduction of the heart-lung machine? Ann Thorac Surg 2000;70:1774–1778.[Free Full Text]
- Ngaage DL. Off pump coronary artery bypass grafting: simple concept but potentially sublime value. Med Sci Monit 2004;10:RA 47–54.[Medline]
- Heikkila J. Geometric camera calibration using circular control points. IEEE Transactions on Pattern analysis and machine intelligence October 2000;22:.
- Lenz RK, Tsai RY. Techniques for calibration of the scale factor and image centre for high accuracy 3D machine vision metrology. IEEE Transactions on Pattern analysis and machine intelligence 1988;10:731–720.[CrossRef]
- Puskas JD, Thourani VH, Marshall JJ, Dempsey SJ, Steiner MA, Sammons BH, Brown III WM, Got JP, Weintraub WS, Guiton RA. Clinical outcomes, angiographic patency and resource utilization in 200 consecutive off-pump coronary bypass patients. Ann Thorac Surg 2001;71:1477–1484.[Abstract/Free Full Text]
- Kan NE, De Souza A, Mister R, Flather M, Clague J, Davies S, Collins P, Wang D, Sigwart U, Pepper J. A randomized comparison of off pump and on pump multivessel coronary-artery bypass surgery. N Engl J Med 2004;350:21–28.[Abstract/Free Full Text]
- Jegaden O, Micaeloff P. Off pump coronary artery bypass surgery. The beginning of the end? Eur J Cardiothorac Surg 2001;19:237–238.[Free Full Text]
- Carthey J, De Leval MR, Reason JT. The human factor in cardiac surgery: errors and misses in a high technology domain. Ann Thorac Surg 2001;72:300–305.[Abstract/Free Full Text]
- Borst C, Jansen EW, Tulleken CAF, Grundeman PF, Mansvelt Beck HJ, van Dongen JW, Hodde KC, Bredee JJ. Coronary artery bypass grafting without cardiopulmonary bypass and without interruption of native coronary flow using a novel anastomosis site restraining device (‘Octopus’). J Am Coll Cardiol 1996;27:1356–1364.[Abstract]
- Koransky ML, Tavana ML, Yamaguchi A, Kown MH, Miniati DN, Nowlin W, Robbins RC. Quantification of mechanical stabilization for the performance of off-pump coronary artery surgery. Heart Surg Forum 2003;6:224–231.[Medline]
- Cattin P, Dave H, Grünenfelder J, Szekely G, Turina M, Zundt G. Trajectory of coronary motion and its significance in robotic motion cancellation. Eur J Cardiothorac Surg 2004;25:786–790.[Abstract/Free Full Text]
- Taylor RH, Jensen M, Whitcomb L. A steady-hand robotic system for microsurgical augmentation. Robot Res 1999;12:1201–1221.
This article has been cited by other articles:
|
|
|
|
 
T. Watanabe, S. Omata, M. Odamura, M. Okada, Y. Nakamura, and H. Yokoyama
Three-dimensional quantification of cardiac surface motion: a newly developed three-dimensional digital motion-capture and reconstruction system for beating heart surgery.
J. Thorac. Cardiovasc. Surg.,
November 1, 2006;
132(5):
1162 - 1171.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
Top
Abstract
1. Introduction
