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Canadian Journal of Anesthesia 49:835-849 (2002)
© Canadian Anesthesiologists' Society, 2002
Cardiothoracic Anesthesia, Respiration and Airway

Mechanisms of hemodynamic changes during off-pump coronary artery bypass surgery

[Les mécanismes de changements hémodynamiques pendant le pontage aortocoronarien à coeur battant]

Pierre Couture, MD FRCPC*, André Denault, MD FRCPC*, Patrick Limoges, MD FRCPC*, Peter Sheridan, MD FRCPC*, Denis Babin, MSc* and Raymond Cartier, MD FRCSC{dagger}

* From the Departments of Anesthesiology, and
{dagger} Surgery Montreal Heart Institute, Montreal, Quebec, Canada.

Dr. Pierre Couture, Department of Anesthesiology, Montreal Heart Institute, 5000 Bélanger Street East, Montreal, Quebec H1T 1C8, Canada. Phone: 514-376-3330, ext. 3732; Fax: 514-376-1355; E-mail: p.couture{at}sympatico.ca


   Abstract
TOP
 Abstract
Introduction
 Methodology
 Results
 Discussion
 Myocardial ischemia
 Reduced preload
 Myocardial dysfunction
 Cardiac compression
 Mitral regurgitation
 Conclusion
 Conflict of interest statement
 References
 
Purpose: To describe the mechanisms of hemodynamic changes during off-pump coronary artery bypass graft surgery (OP-CABG).

Source: Pertinent medical literature in the English and French languages was identified through a Medline computerized literature search and a manual search of selected articles, using off-pump coronary artery surgery, beating heart surgery, hemodynamic, and transesophageal echocardiography as key words. Human and animal studies were included.

Principal finding: Hemodynamic variations in OP-CABG may be due to mobilization and stabilization of the heart, or myocardial ischemia occurring during coronary occlusion. Suction type and compression type stabilizers produce hemodynamic effects through different mechanisms. Heart dislocation (90° anterior displacement) and compression of the right ventricle to a greater extent than the left ventricle are responsible for hemodynamic alterations when using suction type stabilizers. Compression of the left ventricular outflow tract and abnormal diastolic expansion secondary to direct deformation of the left ventricular geometry are proposed mechanisms for hemodynamic derangements with compression type stabilizer. Coronary occlusion during the anastomosis can have additional effects on left ventricular function, depending on the status of collateral flow. The value and limitations of electrocardiographic (ECG), hemodynamic and echocardiographic monitoring modalities during OP-CABG are reviewed.

Conclusions: In summary, hemodynamic changes which can either be secondary to the stabilization technique or to transient ischemia represent an important diagnostic challenge during off-bypass procedures. The mechanism can vary according to the stabilization system. Current monitoring such as ECG and hemodynamic monitoring are used but remain limited in establishing the cause of hemodynamic instability. Transesophageal echocardiography is used in selected patients to diagnose the etiology of hemodynamic instability and can direct therapy, particularly in those with severe myocardial systolic and diastolic dysfunction, mild to moderate mitral regurgitation, or for patients who are unstable during the procedure.


   Introduction
TOP
 Abstract
 Introduction
Methodology
 Results
 Discussion
 Myocardial ischemia
 Reduced preload
 Myocardial dysfunction
 Cardiac compression
 Mitral regurgitation
 Conclusion
 Conflict of interest statement
 References
 
CORONARY artery bypass grafting on the beating heart has become a widely applied procedure. Although off-pump coronary artery bypass grafting (OP-CABG) procedures are quite attractive because of the obvious advantages of avoiding cardiopulmonary bypass-related complications, the main limitations of this approach to surgery are incomplete revascularization and intraoperative hemodynamic instability.

The basic principles of complete revascularization should not be compromised when considering OP-CABG.1 Indeed, in a review of 3372 surgical patients from the Coronary Artery Surgery Study Registry with triple-vessel disease, patients with severe angina (New York Heart Association class III and IV) or left ventricular (LV) dysfunction (ejection fraction < 35%) had a better six-year survival and event-free survival when grafts to three or more vessels were completed.2 However, with off-pump procedures, there has often been a compromise in the completeness of revascularization, with many authors reporting an ungrafted circumflex coronary artery (CX).3 Tasdemir et al.4 identified ungrafted CX stenoses as a risk factor for morbidity and mortality. In order to obtain adequate exposure for complete revascularization, particularly for the difficult lateral CX and posterior branches, midline sternotomy, combined with methods providing appropriate positioning of the heart and adequate mechanical stabilization, remains the most popular approach for OP-CABG.1,5,6

The second problem related to CABG, which is the topic of this review, is hemodynamic instability. Hemodynamic variations in OP-CABG may be due to mobilization and stabilization of the heart, or myocardial ischemia occurring during coronary occlusion. Each type of stabilization device can also produce its own related hemodynamic effects. The purpose of this article is to review the mechanisms of hemodynamic derangements according to the stabilization device used.


   Methodology
TOP
 Abstract
 Introduction
 Methodology
Results
 Discussion
 Myocardial ischemia
 Reduced preload
 Myocardial dysfunction
 Cardiac compression
 Mitral regurgitation
 Conclusion
 Conflict of interest statement
 References
 
Pertinent medical literature in the English and French languages was identified through a Medline computerized literature search and a manual search of selected articles, using off-pump coronary artery surgery, beating heart surgery, hemodynamic, and transesophageal echocardiography (TEE) as key words. Human and animal studies were included. The Medline search was updated up to January 2002 and references included in this review. In addition, the experience of the Montreal Heart Institute in OP-CABG surgery since 1996 is described.


   Results
TOP
 Abstract
 Introduction
 Methodology
 Results
Discussion
 Myocardial ischemia
 Reduced preload
 Myocardial dysfunction
 Cardiac compression
 Mitral regurgitation
 Conclusion
 Conflict of interest statement
 References
 
Using the above strategy, a total of 80 articles were retrieved. References selected are representative but not all- inclusive. A total of 39 references were excluded because they were not specifically related to the topic. We will also discuss the value and limitations of electrocardiographic, hemodynamic and echocardiographic monitoring modalities during OP-CABG.


   Discussion
TOP
 Abstract
 Introduction
 Methodology
 Results
 Discussion
Myocardial ischemia
 Reduced preload
 Myocardial dysfunction
 Cardiac compression
 Mitral regurgitation
 Conclusion
 Conflict of interest statement
 References
 
Hemodynamic changes during OP-CABG: peculiarities of the stabilization device
Various devices are being marketed for the purpose of restricting regional wall motion: the Diamond Grip Rib Spreader Cardiac Stabilizer (Genzyme Corp., Cambridge, MA, USA), the Origin Cardiac Stabilizer and Stabilizer Foot (Origin, Menlo Park, CA, USA), Mechanical Stabilizer (CTS Inc., Cupertino, CA, USA), the Octopus Tissue Stabilizer System (Octopus, Medtronic Inc., Minneapolis, MN, USA), and the fork-type compression stabilizer developed and currently used at the Montreal Heart Institute (CoroNéo Inc., Montreal, Quebec, Canada).1,7–9 While these devices are different, their stabilizing effects could generally be attributed to a compression effect (CoroNéo Inc., CTS Inc., Cupertino, CA, USA) or to a suction effect (Octopus, Medtronic Inc., Minneapolis, MN, USA).10

1) SUCTION TYPE STABILIZER (OCTOPUS, MEDTRONIC INC.)
Hemodynamic variations occurring during OP-CABG using the Octopus stabilizer have been first studied in animals. Borst et al.10 in Utrecht, The Netherlands, developed a mechanical suction stabilization system (Octopus, Medtronic Inc., Minneapolis, MN, USA) and, using the pig as a laboratory model, were able to show minimal arrhythmogenesis, no hemodynamic deterioration, little superficial histologic change related to suction application, and excellent reproducible reduction of cardiac surface motion to 1 mm by 1 mm. Although Borst et al.10 found no hemodynamic deterioration with this device during grafting of the left anterior descending (LAD) artery and right coronary (RC) artery in a pig model, the main challenge in OP-CABG remains access to the CX territory. Exposing posterior branches by displacing the beating heart (apex points anteriorly) tends to decrease arterial pressure both in the pig model11 and in patients.12 Experimentally, Grundeman,11 and Jansen et al.12 have reported the feasibility of immobilizing the posterolateral cardiac wall with the straight Octopus paddle fixed directly on the ventricle. The apex was progressively raised anteriorly during a two-minute period by pulling on the left ventricle. The heart "dislocation" (phase 3 of Figure 1Go) caused a 26% decrease in mean arterial pressure, a 37% decrease in cardiac output, biventricular failure characterized by a major drop in stroke volume (44%), despite elevation of right ventricular (RV) end-diastolic pressure and unchanged LV end-diastolic pressure (Figure 1Go). Twenty degrees head-down position (Trendelenburg) normalized cardiac output and mean arterial pressure (phase 4 of Figure 1Go). There was also a decrease in coronary blood flow measured by ultrasound flow probe in the LAD artery, the RC artery, and the CX artery by 34%, 25% and 50% respectively, which was restored at 20 head down tilt (Figure 2Go).13 Using the same experimental model,14 these authors further elucidate the mechanism of the biventricular dysfunction by measuring RV and LV dimension with two-dimensional echocardiography (Figure 3Go). They observed that a significant portion of the RV free wall was pressed against the interventricular septum, whereas the RV outflow tract was somewhat narrowed but remained patent, resulting in a decrease in the RV dimension with a smaller decrease in LV dimension. No valvular incompetence was observed. Tilting the whole-body head down in 20 Trendelenburg position normalized mean arterial pressure, stroke volume, LV dimension, while it partially corrected RV dimension (Figure 4Go, study 1). In addition, right heart bypass increased stroke volume and mean arterial pressure by increasing LV preload, in contrast, LV bypass failed to restore systemic circulation (Figure 4Go, study 2). They concluded that the changes accompanying 90 anterior displacement of the beating porcine heart were caused primarily by RV deformation and decreased pump function without signs of valvular incompetence or inflow or outflow obstruction. The displacement prevented normal RV and LV diastolic expansion by pressing the heart against the surrounding tissue, resulting in RV diastolic dysfunction. It is inferred that vertical displacement affects mostly the RV function because the right heart bypass restored LV function whereas left heart bypass failed to restore cardiac output and mean arterial pressure.14 These data are in agreement with a similar protocol using a sheep model15 where right heart ventricular bypass also restored LV function.



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  		FIGURE 1 Relative changes in hemodynamic variables during vertical displacement of the beating porcine heart by the Utrecht Octopus and the effect of head-down tilt phase 1 (BASE) = pericardial control position; phase 2 (FIX) = fixation of the suction tentacles to the posterior cardiac wall; phase 3 [DIS (1)] = displacement of the heart by the Octopus, phase 4 (DIS + TREND) = Trendelenburg maneuver (20 head-down tilt); phase 5 [DIS (2)] = retracted heart with table returned to horizontal position; and phase 6 (FREE) = pericardial position after release of the Octopus. Statistical comparison with control values: *P < 0.05, ** P < 0.001. Reprinted with permission from the Society of Thoracic Surgeons (The Annals of Thoracic surgery, 1997; 63: S90).

 


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  		FIGURE 2 Relative changes in hemodynamic variables during vertical displacement of the beating porcine heart by the Medtronic Octopus stabilizer and the effect of the head-down tilt. Phase 1 (BASE) = pericardial control position; CX = circumflex coronary artery; phase 2 (DIS) = displacement of the heart by the Octopus; phase 3 (DIS + TREND) = Trendelenburg maneuver (20 head-down tilt) while the heart remained 90 retracted; LAD = left anterior descending coronary artery; RCA = right coronary artery; X = mean arterial pressure. Statistical comparison with control values *P < 0.05, **P < 0.01, #P < 0.001, ^P < 0.046 vs combined relative value of LAD and RCA flows. Reprinted with permission from the Society of Thoracic Surgeons (The Annals of Thoracic surgery, 1998; 65: 1350).

 


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  		FIGURE 3 Echographic cross-sectional areas (left panels) and M-mode images (right panels) obtained during 90 anterior displacement (DISPLACED) and subsequent head-down positioning (+ TRENDELENBURG). VS = ventricular septum; RV = right ventricle; LV = left ventricle. The ultrasound probe was positioned between two Octopus tentacles and aligned to the posterior wall (ANATOMIC). Note the severe deformation of the RV on cardiac retraction (arrows, DISPLACED) and the partial recovery of RV cross-sectional area after the Trendelenburg maneuver. Reproduced from reference 14 with permission.

 


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  		FIGURE 4 Relative changes in hemodynamic variables and RV and LV cross-sectional areas during vertical displacement of the beating porcine heart with Octopus tissue stabilizer (study 1) and the effect of mechanical circulatory support on displacement (study 2). Statistical comparison with control values *P < 0.05; **P < 0.01; #P < 0.001; n.m. = not measured; erroneous readings from the ultrasound aortic flow probe caused by turbulent flow from LHB cannula. DIS = displaced; LV = left ventricle; RV = right ventricle; RHB = right heart bypass; LHB = left heart bypass. The measurements were taken during the following periods: study 1: BASE = pericardial control position; DIS = displacement of the heart by the Octopus; DIS + TREND = Trendelenburg maneuver while the heart remained retracted. In study 2, additional measurement periods included: DIS + RHB = heart displaced by the Octopus and supported with RHB and DIS + LHB = heart displaced and supported by LHB. Reproduced from reference 14 with permission.

 
This device has also been used successfully for multiple vessel OP-CABG in human studies12,16,17 with minimal morbidity and mortality. Hemodynamic changes have been less well characterized in humans than in animal models using this device. Jansen et al. have reported their results on the first 100 patients with the Octopus stabilizer.18 They observed that through a median sternotomy, exposure of the anterior wall and inferior wall is well tolerated hemodynamically (Table IGo). Exposure of the posterior wall is also well tolerated, provided that heart dislocation is performed slowly (approximately one minute; Table IGo). However, fluid redistribution (Trendelenburg manoeuver) and dopamine were necessary in 67% of patients to maintain arterial pressure with posterior wall exposure. Recently, Nierich et al.17 showed in a study conducted on 150 patients that the stroke volume was mainly affected during diagonal (DIAG) revascularization (-25%). This was attributed to right heart compression, which is squeezed between the left ventricle and right pericardium.


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  		TABLE I Hemodynamic changes during immobilization and presentation of target vessel by the Octopus tissue stabilizer
 
More recently, Mathison et al. provided the first detailed clinical report during OP-CABG using the Octopus II.19 Circumflex positioning resulted in a decrease in mean arterial pressure of 22.2 ± 4.4% and stroke volume of 28.5 ± 3.5%, an increase in left and right atrial pressure of 59.9 ± 15.9% and 166.7 ± 34.2%, and LV and RV end-diastolic pressure of 59.4 ± 16.6% and 151.4 ± 9.8%. Positioning for the posterior descending artery also resulted in a decrease in stroke volume of 22.4 ± 7.9% with an increase in left and right atrial pressure and RV end-diastolic pressure, with no significant change in LV end-diastolic pressure. Positioning for the left descending artery anastomosis showed the least hemodynamic change, although they observed a decrease in stroke volume of 17.5 ± 5.1% with an increase in right atrial and RV end-diastolic pressure. Using TEE, they observed that, for every positioning, the right ventricle is more affected by the compression, because its wall is thinner, its pressure is relatively low, and it is pressed against the pericardial cradle. Therefore, even if the left ventricle is compressed, the effect on the right ventricle is greater. They also explained that the increases in left and right atrial pressure were related to the anterior positioning of the heart, with the apex pointing upward, by the folding of the heart somewhere close to the atrioventricular valves, causing some flow obstruction. The same authors also have shown that RV assistance provided stable hemodynamic during exposure of the posterior wall.20 Other authors have found LV assist devices to be effective in patients with LV failure.21

From these observations in animals and in humans, it appears that hemodynamic changes are primarily caused by changing the normal position of the heart, particularly with dislocation (90 anterior displacement) of the heart and compression of the right ventricle to a greater extent than the left ventricle. The Octopus system, when positioned on the anterior surface of the heart, suspends the anterior wall and does not seem to impede LV diastolic filling, although right heart compression can occur.17 This is in contrast with access to the obtuse marginal and distal RC artery branch access, which may lead to diastolic filling abnormalities of the heart. This is thought to be secondary to the Octopus articulating arms and tissue stabilizers which immobilize the heart by pressure, instead of suspension.

Finally, coronary occlusion during the anastomosis can have other effects on LV function, and these effects depend on collateral flow.22 Brown et al. showed that occlusion of a severely stenosed vessel (> 90%) with good collaterals may lead to a less severe myocardial ischemia than the occlusion of a vessel with only a 60 to 70% stenosis with less collateral flow.23 Koh et al.22 using intraoperative TEE in patients undergoing OP-CABG, observed that both LV systolic and diastolic function were depressed in those patients without collaterals compared to only diastolic dysfunction in patients with collaterals during coronary occlusion of the LAD artery lasting up to 15 min using the Octopus device. All disturbances normalized within ten minutes of reperfusion. However, the hemodynamic consequences of the displacement of the heart to facilitate the procedure are probably not influenced by the degree of coronary artery stenosis.24

2) COMPRESSION TYPE STABILIZER (THE FORK-TYPE COMPRESSION STABILIZER) [CORONÉO INC., MONTREAL, QUEBEC, CANADA]
At the Montreal Heart Institute, OP-CABG surgery began in September 1996.9 Our experience with beating heart surgery now includes close to 740 cases and represents over 2100 distal anastomoses.9 The conversion rate to cardiopulmonary bypass is 0.4% (Dr. R. Cartier, personal communication). Based on the first 500 patients, an average of 3.10 grafts/patient were completed and 72% of the patients had either triple, quadruple or quintuple bypass.9 The CX artery was grafted in 73% of cases; in 7%, two grafts were completed on the CX network. The detailed surgical technique has been published1 but in summary, all surgeries were performed with a full median sternotomy. Anesthesia technique was left to the discretion of the attending anesthesiologist. Phenylephrine and nitroglycerine infusions were administered as required to normalize hemodynamics. Target artery immobilization was achieved through mechanical stabilization with a specifically designed surgical apparatus capable of ergonomically accessing all coronary arteries (Cor-Vasc system; CoroNéo Inc., Montreal, Quebec, Canada; Figure 5Go). A light-weight titanium retractor serves as the platform for this system. The system consists of four distinct coronary stabilizers, "push" (Figure 5AGo) and "pull" (Figure 5BGo) types, each optimized for specific artery exposure and immobilization. A bloodless surgical field is ensured by silastic bands (Retract-O-Tape; Quest, Allen, TX, USA), which isolate and occlude the target artery at the arteriotomy site. A heart "verticalizing" technique, was developed to expose the posterior circumflex territory through pericardial traction.1 Four deep pericardial sutures are placed at the base of the heart, interspersed in a fan-shape arrangement between the left superior pulmonary vein and the inferior vena cava. The "verticalizing" technique enables extraction of the apex, with minimal distortion of the ventricle while preserving hemodynamics. The surgical table, rotated rightward (30) and set in Trendelenburg position, assists in exposing the posterior coronary territory and helps maintain RV preload. The coronary artery was stabilized using a pull type stabilizer. Exposure of the LAD and DIAG coronary arteries used the same setting except two traction sutures were usually used and the table was positioned in a reverse Trendelenburg position.



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  		FIGURE 5 Example of the stabilizing system (Cor-Vasc System; CoroNéo Inc., Montreal, Quebec, Canada) used at the Montreal Heart Institute for the left anterior descending artery (A) and the marginal artery anastomosis (B).

 
The hemodynamic changes associated with the use of this stabilizing system have been reported recently by Do and Cartier25 in 31 patients undergoing OP-CABG monitored with a Swan-Ganz catheter (n = 25) or an Oximetric catheter (n = 6). The target coronaries were clamped proximally and distally to the anastomosis site without preconditioning. They observed a decrease in mean systemic arterial pressure (SAP) during the procedure, which differed according to the grafted coronary artery: LAD (-11 ± 19%), DIAG (-13 ± 27%), CX (-19 ± 17%) and RC or posterior descending artery (PDA; -17 ± 14%). The mean pulmonary artery pressure (PAP) increase was maximal with the DIAG (+47 ± 84%) and was more important during LAD (+30 ± 36%) and CX (+21 ± 48%) than RC/PDA revascularization (+10 ± 24%). There was no significant change in mixed venous oxygen saturation (SvO2). These changes occurred during the stabilization period before vessel occlusion, and were well tolerated by all patients, in whom inotropic support was rarely needed. They concluded that using a "fork-type" stabilizer, the mobilization and stabilization of the heart were responsible for a decrease in SAP and an increase in PAP, rather than clamping of the coronary. The marked elevation in PAP during revascularization of the DIAG and LAD territory may be explained by a compression of the LV outflow tract, as illustrated in Figure 6Go. To further define the hemodynamic changes associated with the use of this "fork-type" stabilizer, we studied an additional 53 patients undergoing OP-CABG, in whom we also measured the cardiac output and SvO2.26 TEE was used in five of these patients to assess the systolic and diastolic function, to detect regional wall motion abnormalities and mitral regurgitation during the procedure. The left ventricle was divided in 16 segments according to the recommendation of the American Society of Echocardiography.27 Mitral inflow velocities were measured at the tip of the mitral leaflets on three consecutive heartbeats at the end of expiration. The parameters recorded were the following: peak velocity of the early diastolic filling wave (E), late diastolic filling wave (A), and a ratio of these two velocities (E/A). Our results are in agreement with those reported earlier.25 The SAP decrease was found after stabilization of the LAD (-8.6 ± 22.1%), DIAG (-14 ± 25.2%), CX (-16.3 ± 13.8%) and RC (-15.1 ± 15.6%). The anteroposterior compression of the beating heart was responsible for the most important changes in PAP. Indeed, PAP increase was only significant during DIAG (+37.7 ± 73.6%) and was more important on LAD (+23.9 ± 35.2%) than CX (+12.5 ± 35.9%) or RC revascularization (+12.7 ± 23.8%). Changes in SvO2 were < 10% and the decrease in cardiac index was more important for DIAG revascularization (-14.8 ± 16.8%). Cardiac wall motion and mitral valve function were unaffected even when the heart is displaced at 90 for CX exposure (unpublished data). Also, from our results, changes in LV mitral inflow patterns that could be suggestive of LV diastolic dysfunction28 were shown to be significant and more important than systolic regional dysfunction (unpublished data). The E/A ratio used to describe such dysfunction was greatly impaired following DIAG and LAD stabilizations (Figure 7Go). It is inferred that compression of the beating heart for stabilization of the DIAG and LAD prevented normal diastolic expansion by direct deformation of the LV geometry (Figure 6Go). Such a problem may be prevented by applying minimal compression force on the heart to obtain good exposure, especially when dealing with the DIAG arteries. Other studies using a compression type stabilizer also showed transient hemodynamic changes which recovered after the heart returned to anatomic position.29,30



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  		FIGURE 6 Schematic representation of the heart compression for the left anterior descending and diagonal coronary revascularization during beating heart coronary artery bypass graft surgery. DG = diagonal artery; LAD = left anterior descending artery; LV = left ventricle; RV = right ventricle. Reproduced from reference 25 with permission.

 


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  		FIGURE 7 The effect of clamping the diagonal artery on the mitral E and A wave. The occlusion of the diagonal was associated with a reduction of the E velocity with an increase in the A velocity compatible with ischemia or preload reduction through compression of the LV outflow tract.

 
One study evaluated cardiac function during OP-CABG with a compression type stabilizer using invasive hemodynamic and TEE monitoring. In this study, Jurmann et al.31 assessed 28 patients undergoing single bypass grafting to the LAD artery with the internal thoracic artery through a small left anterior thoracotomy. They used the CTS MIDCAB system (Cardiothoracic Systems Inc., Cupertino, CA, USA) and applied gentle pressure until sufficient immobilization of the anastomotic site at the level of the LAD was achieved. They reported that the application of the epicardial stabilizer resulted in a minor decrease in LV end-diastolic and systolic diameter, unchanged fractional area change, while the cardiac index, stroke volume index and pulmonary capillary wedge pressure (PCWP) remained unchanged. After ten minutes of coronary artery occlusion, there was a compromise in LV regional wall motion as represented by the development of new hypokinetic segments or the aggravation of preexisting hypokinetic segments, an increase in LV systolic and diastolic dimension, a decrease in fractional area change (Table IIGo). Diastolic function was not evaluated. In general, this compromise in LV function does not reach critical levels and can easily be compensated through appropriate pharmacologic intervention. The differences, as compared to our results, could be attributed to the operative technique, the use of a different stabilizing device, or a difference in patient populations, as our patients had multivessel disease, with different collateral networks, which are known to affect LV function during vessel occlusion.22


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  		TABLE II Left ventricular dimensions and function from intraoperative transesophageal echocardiography minimally during invasive direct coronary artery bypass grafting*
 
Although suction and compression type stabilizers have not been compared directly, the available literature suggests that each type of stabilizer produces a different profile of hemodynamic changes. With the Octopus system, circumflex positioning demonstrated the largest hemodynamic compromise, because the right ventricle is more compressed than the left ventricle, causing a disturbance in diastolic distention.19 Moreover, with anterior dislocation of the heart, the heart is folded causing a flow obstruction. Contrary to this technique, the verticalization method used at the Montreal Heart Institute does not rely on the dislocation of the heart by the stabilizer. The fan-shape distribution of pericardial stay sutures implanted well below the phrenic nerve line allows reorientation of the apex without manual mobilization of the heart, thus avoiding distortion of the ventricle’s geometry.1 In our series, CX revascularization was associated with a minimal drop in SAP and cardiac index. Likewise, heart stabilization and compression in other territories carried the same decrease in SAP and cardiac index.25 On the other hand, Jensen et al.18 found minimal hemodynamic effect during revascularization of the anterior and inferior wall using the Octopus stabilizer. Curiously, in our experience, PAP elevations are much more marked when the heart is not vertically displaced during DIAG and LAD revascularization.25 This is particularly true for patients with severe myocardial dysfunction or with mild to moderate mitral regurgitation. To temporarily obviate this problem, a simple snaring technique of the inferior vena cava had been used32 with success. This disturbance is thought to be due to the direct compression of the LV outflow tract which leads to abnormal diastolic expansion secondary to direct deformation of the LV geometry.26 Minimizing compression forces on the heart may prevent these hemodynamic variations.

Value and limitations of monitoring modalities
MONITORING MYOCARDIAL ISCHEMIA
In order to decrease myocardial dysfunction related to each coronary artery territory, our strategy consists of starting with the dominant lesion. This vessel is normally the most collateralized and consequently is less likely to induce myocardial ischemia. We normally avoid bypassing the distal RC artery if the proximal section is not occluded or critically (> 90%) stenotic. Instead, we focus on the posterior descending artery, decreasing the risk of ischemia and arrhythmia, because the atrioventricular artery is bypassed. The circumflex artery is normally revascularized at the end of the procedure.1

The common method of intraoperative myocardial ischemia monitoring during CABG is electrocardiography, using the combination of lead II and V5 for ST-segment changes (Figure 8Go).33 However, during OP-CABG, the heart is frequently mobilized, particularly for the CX and posterolateral coronary arteries, which often results in microvoltages in the monitored leads. The value of electrocardiography and ST-segment monitoring under these circumstances has not been well explored. TEE is a sensitive method for detecting myocardial ischemia through the observation of segmental wall motion abnormalities during CABG with cardiopulmonary bypass (CPB).34,35 However, few studies have demonstrated the role of this technique during CABG without CPB. Moisés et al. addressed this question in a study evaluating 27 patients undergoing OP-CABG monitored with TEE.36 Without the use of a stabilizer, they performed a total of 48 anastomoses: 26 to the LAD, three to the DIAG, and 19 to RC. They observed 31 (64%) new segmental wall motion (SWM) abnormalities during coronary occlusion defined as a change in score of 1, as recommended by the American Society of Echocardiography.27 For each segment, a score was assigned as follows: normal = 1; hypokinetic = 2, akinetic = 3, and dyskinetic = 4. At the time of chest closure, complete recovery occurred in 16 (50%) segments, partial recovery in ten (33%), and no recovery in five (17%). On the seventh postoperative day, the new SWM abnormalities persisted in all five segments without recovery at the end of the surgery and in two of ten (20%) segments with partial recovery. Electrocardiographic ST-segment changes suggestive of myocardial ischemia occurred during only 9/48 (19%) anastomoses confirming its lower sensitivity compared to TEE. In this study, TEE was more predictive of persistent SWM abnormalities in the postoperative period.36 A study by Kotoh et al.37 uses TEE with colour kinesis to facilitate the evaluation of regional wall motion. Colour kinesis can automatically determine the endocardial excursion by acoustic quantification and demonstrate the change in wall motion in colour layers on a single end-systolic frame. From a total of 34 patients undergoing beating heart CABG, they observed new SWM abnormalities in four patients (12%) during the anastomosis of the left internal mammary artery-LAD. Only one of these patients developed electrocardiographic abnormalities. However, electrocardiographic abnormalities were recorded without wall motion abnormalities in three patients. In OP-CABG, the pericardium was displaced to facilitate anastomosis, especially during anastomosis to the CX artery or RC artery. The echocardiographic views of the left ventricle were sometimes limited after pericardial traction during that time. In these cases, TEE may be insufficient to evaluate wall motion suggestive of myocardial ischemia, as is the case for electrocardiography. On the other hand, monitoring of PAP, along with the PCWP has been shown to be insensitive to detect myocardial ischemia in patients undergoing CABG under CPB.35,38 The use of PCWP as a surrogate is based on the relationship between ischemia and a decrease in ventricular compliance, resulting in an increase in filling pressure as reflected by a rise in PAP and PCWP. van Daele et al.38 found a sensitivity of 33% for an increase in PCWP of 3 mmHg over the baseline to detect myocardial ischemia compared to abnormal wall motion detected by TEE in patients undergoing CABG. All patients with an increase in mean PCWP had inferior wall myocardial ischemia and possible transient papillary muscle dysfunction and mitral valve regurgitation.



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  		FIGURE 8 Ischemic changes during off-pump bypass cardiac surgery. Significant ST-segment elevations were observed in lead II (top panel) during the left anterior descending (LAD) and right coronary artery (RCA) occlusion. Significant ST-segment elevation occured during LAD occlusion and ST depression during RCA occlusion in lead V5 (middle panel). These ischemic changes were accompanied with an increase in heart rate (HR; lower panel).

 
Therefore, during OP-CABG, all the monitoring methods have limitations, and we tend to rely on their combination rather than a single mode of monitoring to detect myocardial ischemia. Because most new wall motion abnormalities will resolve within a few minutes after revascularization,22,36,39 the main interest in intraoperative TEE is after reperfusion. It is well established that detection of persistent SWM abnormalities is associated with postoperative cardiac SWM abnormalities, higher cardiac enzyme levels, and more clinical problems such as pulmonary edema and atrial fibrillation.36 Such persistent SWM abnormalities after revascularization could lead the surgeon to reevaluate the patency of the coronary bypass graft.

DIAGNOSIS OF HEMODYNAMIC DERANGEMENTS
At the Montreal Heart Institute, patients undergoing OP-CABG are generally monitored with a ST-segment analysis system, a radial artery catheter and a pulmonary artery catheter. The value of continuous cardiac output monitoring in OP-CABG has not been well evaluated. Our experience with TEE during cardiac surgery has been published.40 TEE has been mainly used for patients with severe myocardial systolic and diastolic dysfunction, mild to moderate mitral regurgitation, or in patients who developed hemodynamic instability during the procedure.

Hemodynamic instability during OP-CABG can be secondary to ischemia, reduced preload, cardiac compression, myocardial dysfunction, mitral regurgitation, or a combination of these causes.


   Myocardial ischemia
TOP
 Abstract
 Introduction
 Methodology
 Results
 Discussion
 Myocardial ischemia
Reduced preload
 Myocardial dysfunction
 Cardiac compression
 Mitral regurgitation
 Conclusion
 Conflict of interest statement
 References
 
As discussed above, all the monitoring methods for myocardial ischemia have limitations. We found TEE most useful in cardiac manipulations during which hypotension is associated with an increased filling pressure (Figure 9Go). In this situation, TEE can help to differentiate between cardiac dysfunction secondary to myocardial ischemia, in which regional wall motion abnormalities will be present from a much more common scenario where the increase in filling pressure is secondary to extracardiac compression (Dr. A. Denault, personal communication). We found the two-dimensional transesophageal four- and two-chamber view to be the most useful, since the transgastric short axis view is difficult to obtain during OP-CABG, particularly during circumflex and RC artery anastomoses.



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  		FIGURE 9 A) Effect of removal of the "fork-type" stabilizer positioned over the left anterior descending artery on hemodynamic variables. An increase in systemic blood pressure, a reduction in pulmonary artery pressure and in central venous pressure followed the removal of the stabilizer. B) Off-pump bypass cardiac surgery associated with elevated central venous pressure and pulmonary artery pressure followed by hypotension secondary to compression of the left ventricle during left anterior descending coronary artery anastomosis. This was not associated with any regional wall motion abnormalities but secondary to extracardiac compression.

 

   Reduced preload
TOP
 Abstract
 Introduction
 Methodology
 Results
 Discussion
 Myocardial ischemia
 Reduced preload
Myocardial dysfunction
 Cardiac compression
 Mitral regurgitation
 Conclusion
 Conflict of interest statement
 References
 
Hypotension secondary to hypovolemia is usually associated with a decrease in PAP and CVP. Fluid loading and Trendelenburg position restores cardiac output by increasing venous hydrostatic pressure and subsequently, LV preload. If these maneuvers are ineffective, we consider the administration of phenylephrine or norepinephrine. TEE can be useful to confirm hypovolemia and fluid responsiveness if the patient remains hypotensive. Using the fork-type stabilizer, exposure of the circumflex and posterior descending arteries necessitates verticalization of the heart, which may occasionally impede atrial preload by distortion of the right atrium and inferior vena cava (Dr. R. Cartier, personal communication).


   Myocardial dysfunction
TOP
 Abstract
 Introduction
 Methodology
 Results
 Discussion
 Myocardial ischemia
 Reduced preload
 Myocardial dysfunction
Cardiac compression
 Mitral regurgitation
 Conclusion
 Conflict of interest statement
 References
 
Systolic function
Hemodynamic instability related to severe systolic dysfunction is characterized by an increase in PAP, CVP, along with a decrease in CO. TEE monitoring is particularly useful to differentiate between systolic dysfunction associated with regional wall motion abnormalities from cardiac compression where the increase in filling pressure is secondary to extracardiac compression (Dr. A. Denault, personal communication). TEE may be considered in patients with known preoperative systolic dysfunction, or in patients who remain hypotensive despite iv nitroglycerine and inotropic support.

Diastolic dysfunction
Our group has recently raised the issue of the importance of diastolic function evaluation during cardiac surgery.41 The role of diastolic function evaluation during OP-CABG surgery is not reported in the literature. We are currently using Doppler to evaluate both left (Figure 10Go) and right diastolic function (Figure 11Go) during OP-CABG. Such an evaluation allows us to better understand the hemodynamic changes occurring during this procedure, but remains an investigative tool.



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  		FIGURE 10 Mitral valve and pulmonary vein Doppler inflow during baseline, following left anterior descending (LAD) and marginal artery occlusion. During LAD occlusion, a restrictive pattern appears with a predominant E wave and a reduced S to D wave on the pulmonary vein Doppler signal. The application of the stabilizer probably caused a restrictive pattern because of the compression of the LV. This pattern was rather a relaxation abnormality during marginal occlusion, sometimes observed during myocardial ischemia.

 


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  		FIGURE 11 Hepatic vein Doppler signal at baseline, following left anterior descending (LAD) occlusion and release. During LAD occlusion, a reversible restrictive pattern on the hepatic vein Doppler signal appears with a reversal of the S wave signal, which suggests a right ventricular compression.

 

   Cardiac compression
TOP
 Abstract
 Introduction
 Methodology
 Results
 Discussion
 Myocardial ischemia
 Reduced preload
 Myocardial dysfunction
 Cardiac compression
Mitral regurgitation
 Conclusion
 Conflict of interest statement
 References
 
During anterior descending and DIAG artery positioning with the compression type stabilizer, minimal pressure is applied by the stabilization device to avoid direct compression of the LV outflow tract leading to abnormal diastolic expansion. This may lead to an increase in PAP and CVP. Using the Octopus stabilizer, the main causes of hemodynamic disturbance during positioning are thought to be decreased RV filling, and to a lesser extent, LV filling, by direct ventricular compression. Volume loading, Trendelenburg position, and vasopressor infusion usually correct these derangements, although the RV assist device has been proposed for unstable patients.19 TEE is indicated in patients who are not responsive to the above treatment and helps to differentiate between cardiac dysfunction and extracardiac compression.


   Mitral regurgitation
TOP
 Abstract
 Introduction
 Methodology
 Results
 Discussion
 Myocardial ischemia
 Reduced preload
 Myocardial dysfunction
 Cardiac compression
 Mitral regurgitation
Conclusion
 Conflict of interest statement
 References
 
In occasional cases, significant acute mitral valve dysfunction can precipitate hemodynamic instability following heart positioning or coronary artery clamping. Patients who are most at risk to develop severe mitral valve regurgitation are those with preexisting myocardial dysfunction or mild to moderate mitral regurgitation (Dr. R. Cartier, personal communication). When increases of PAP and CVP are observed, colour Doppler TEE of the mitral valve can make the diagnosis. In these cases, we administer iv nitroglycerin and inotropic agents. Inferior vena cava clamping has been used to control an acute increase in PAP unresponsive to usual treatment.32 Mitral valve repair or replacement can be considered if persistent after revascularization, and thus will lead to cancellation of the OP-CABG.

Figure 12Go summarizes our approach to diagnose hemodynamic derangement during OP-CABG.



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  		FIGURE 12 *Transesophageal echocardiography (TEE) = evaluation of ventricular dimension with TEE. The left and right ventricle cavity size will be decreased in hypovolemia and compression of the cardiac chambers, and will be increased in myocardial dysfunction and severe mitral regurgitation. TEE is suggested if the patients remain unresponsive to the usual treatment. PAP = pulmonary artery pressure; CVP = central venous pressure; CO = cardiac output; MV Doppler (restrictive) = restrictive pattern of the mitral valve inflow measured with pulsed Doppler; colour Doppler (MR) = mitral regurgitation as evaluated with colour Doppler; IV nitro = iv nitroglycerine; IVC snaring = snaring of the inferior vena cava to control pulmonary hypertension; HR = heart rate; PVR: peripheral vascular resistance.

 

   Conclusion
TOP
 Abstract
 Introduction
 Methodology
 Results
 Discussion
 Myocardial ischemia
 Reduced preload
 Myocardial dysfunction
 Cardiac compression
 Mitral regurgitation
 Conclusion
Conflict of interest statement
 References
 
In summary, hemodynamic changes secondary to the stabilization technique or transient ischemia represent an important diagnostic challenge during OP-CABG. The mechanism can vary according to the stabilization system. Current monitoring such as ECG and hemodynamic monitoring are used but their usefulness in establishing the mechanism of hemodynamic instability remains limited. In our experience, TEE is used in selected patients, particularly those with severe myocardial systolic and diastolic dysfunction, mild to moderate mitral regurgitation, or for patients who are unstable during the procedure.


   Conflict of interest statement
TOP
 Abstract
 Introduction
 Methodology
 Results
 Discussion
 Myocardial ischemia
 Reduced preload
 Myocardial dysfunction
 Cardiac compression
 Mitral regurgitation
 Conclusion
 Conflict of interest statement
References
 
Dr. Raymond Cartier has a consultantship agreement with CoroNéo Inc.

Received for publication June 10, 2002. Accepted for publication January 4, 2002.


   References
TOP
 Abstract
 Introduction
 Methodology
 Results
 Discussion
 Myocardial ischemia
 Reduced preload
 Myocardial dysfunction
 Cardiac compression
 Mitral regurgitation
 Conclusion
 Conflict of interest statement
 References
 
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