Year : 2009 | Volume
: 12 | Issue : 2 | Page : 174-
Trans-esophageal echocardiography in off-pump coronary artery bypass grafting
Poonam Malhotra Kapoor1, Ujjwal Chowdhury2, Banashree Mandal1, Usha Kiran1, Rajendra Karnatak1,
1 Department of Cardiac Anaesthesiology, CN Centre, All India Institute of Medical Sciences, Ansari Nagar, New Delhi - 110 029, India
2 Department of CTVS, CN Centre, All India Institute of Medical Sciences, Ansari Nagar, New Delhi - 110 029, India
The two features of off-pump coronary artery bypass (OPCAB) grafting that lead to haemodynamic instability are, transient occlusion of the coronary arteries during distal anastomosis construction and displacement of the heart to provide access to the distal coronary arteries. The position of the heart as seen by trans-oesophageal echocardiography (TOE) can often provide an indication as to how much compression of the right or left ventricle has occurred. If either chamber is not filling, repositioning of the heart will be necessary. Close observation of the heart with TOE during periods of coronary occlusion may facilitate detection of worsening cardiac function as evidenced by weakening contraction, ventricular dilatation, or increasing mitral or tricuspid regurgitation. Haemodynamic change are more pronounced with displacement of the heart to access posterior than the anterior coronary arteries. Cardiac manipulations along with transient occlusion of coronary arteries during distal anastomosis may cause transient hypotension with increased filling pressures. TOE is helpful in this scenario as it helps to differentiate between cardiac dysfunction secondary to myocardial ischaemia (in which regional wall motion abnormalities will be present) from a much more common scenario where the increase in filling pressure is secondary to extra-cardiac compression and provides the ability to detect mitral regurgitation with a colour flow Doppler as well as assess the right heart function.
Concerns regarding morbidity associated with conventional surgical myocardial revascularization on cardiopulmonary bypass have led to a resurgence of interest in off-pump coronary artery bypass surgery (OPCAB) during the last decade, with the expectation that it would be safer if a cardiopulmonary bypass could be avoided.
The advantages of OPCAB over conventional coronary artery bypass grafting (CABG) using cardiopulmonary pulmonary bypass (CPB), include avoidance of the adverse effects of CPB. ,, The disadvantages are - haemodynamic instability during graft construction and concerns about the long-term patency of bypass grafts constructed on a beating heart. 
Haemodynamic Changes During OPCAB
During distal anastomosis, both displacement of the heart for coronary access and transient occlusion of the coronary arteries contribute to hemodynamic instability.  The ability of the patient to tolerate occlusion of the artery being grafted is critically dependent on both the severity of the lesion in the artery and the presence of collateral flow into or from it.
Coronary occlusion during the anastomosis can have other effects on the left ventricular (LV) function. These depend on the collateral flow. Brown, et al have shown that occlusion of a severely stenosed vessel (greater than 90%) with good collaterals may have less severe ischemia than occlusion of a vessel with only a 60 to 70% stenosis with less collateral flow. Koh, et al using intra-operative tran-oesophageal echocardiography (TOE) in patients undergoing OPCAB, observed that both LV systolic and diastolic functions were depressed in those patients with collaterals during coronary occlusion of the left anterior descending (LAD) artery lasting up to 15 minutes using the Octopus device. All disturbances were normalized within 10 minutes of reperfusion.
Use of TEE Monitoring in OPCAB
During graft construction, it is critical to know precisely the severity and location of the coronary lesions as well as the surgical plan as to which vessels will be occluded in what order and plans for the use of shunts or other means to support the circulation. When multiple grafts are to be performed with OPCAB, the order in which they are performed is important.  Highly obstructed vessels supplied by collateral flow are usually grafted first to provide flow into more critical vessels before they are grafted. To avoid any unnecessary confusion during the operation, the plan should be discussed directly with the surgeon. The cardiac output and perfusion pressure during displacement of the heart can be maintained by augmentation of preload with volume loading or use of Trendelenburg position.  TOE is a good means of assessing the adequacy of volume status before displacement for graft construction is attempted.  It can often provide an indication of the degree of compression of the right or left ventricle as the heart is positioned. If either chamber is not filling, repositioning of the heart will be necessary. Close observation of the heart with TEE, during periods of coronary occlusion may facilitate detection of worsening cardiac function as evidenced by weakening contractions, ventricular dilatation, or increasing mitral or tricuspid regurgitation [Figure 1].
Hemodynamic changes are more pronounced with displacement of the heart to access posterior coronary arteries than anterior vessels. ,, Thus, the use of TOE during OPCAB surgery might be useful for guiding changes to limit the duration of ischemia in patients who develop marked new segmental wall motion abnormalities (SWMAs), e.g., by inserting an adequately sized intracoronary perfusion catheter  or by repositioning the epicardial stabilizer. Moreover, new SWMAs persisting at the end of surgery may predict a complicated postoperative course. 
The reliability of TOE in monitoring the LV segmental wall motion during OPCAB surgery has been questioned, as the vicinity between the TOE probe and the heart is reduced by pericardial retraction, placement of a lap pad below the heart, and vertical displacement of the heart. , Transgastric views of the LV are not feasible during right coronary artery (RCA) and circumflex artery grafting because of the loss of contact of the heart with the diaphragm, obscuring the transgastric echocardiographic window. The elevation of the heart by placing a lap pad underneath will also obscure the transgastric window. Therefore, changes such as compression of the explain RV with under filling of the LV or aggravation of valvular regurgitation need to be monitored [Figure 1].
Monitoring Methods During OPCAB
During the OPCAB, all monitoring methods have limitations. It is better to rely on their combination rather than a single mode of monitoring to detect myocardial ischemia. As all new wall motion abnormalities will resolve within a few minutes after revascularization, the main interest in intraoperative TOE is after reperfusion.  It is well established that detection of persistent cardiac segmental wall motion (SWMA) abnormalities is associated with higher cardiac enzyme levels, and more clinical problems such as oedema and atrial fibrillation.  Such persistent SWMAs, after revascularization could lead the surgeon to revaluate the patency of the coronary bypass graft.
Segmental Wall Motion Abnormalities and TEE
Each myocardial segment of the LV is analyzed according to its systolic motion and thickness with a focus on any new wall motion abnormalities during each phase of the surgery. New SWMA abnormality is defined as any segmental systolic dysfunction of the LV myocardium occurring during surgery. The heart is divided into 16 segments. According to its status, at the end of the surgery, the new SWMA abnormality is classified in the following manner: 
Total regression: The new SWMA abnormality during surgery returned to the previous status.Partial regression: The new SWMA abnormality returned partially to the initial pattern.Persistency: The new SWMA abnormality did not return to initial pattern.
Segmental wall motion of the LV is analyzed using a 16-segment model and a five-grade scale according to current guidelines.  By considering both endocardial motion and myocardial thickening, the grading system defines score 1 = normokinesis, score 2 = mild hypokinesis, score 3 = severe hypokinesis, score 4 = akinesis, and score 5 = dyskinesis  [Table 1]. At least 50% of the endocardial and epicardial borders have to be visible in a segment graded as normal to be considered adequate for analysis of the wall motion, and approximately 33% have to be visible in a segment graded as abnormal.  For analysis of wall motion, at least 50% of the endocardial and epicardial borders have to be visible in a segment graded as normal and approximately 33% have to be visible in a segment graded as abnormal. If the epicardium is not visible, a segment is still considered adequate for analysis if the endocardial border is almost completely visible (approximately greater than or equal to 90%) throughout the cardiac cycle. Segments not fulfilling these criteria are graded as 0 is equal to no view. Semi-quantitation of LV systolic function is obtained with the wall motion score index (WMSI) as recommended by the American Society of Echocardiography.  The WMSI is obtained by summation of the score of each segment divided by the number of myocardial segments examined. The percentage difference (∆ %) of WMSI during different periods of surgery (coronary artery clamping, end of the surgery) and on the seventh postoperative day, in relation to the beginning of surgery is calculated as follows:
∆ % = WMSI period - WMSI beginning / WMSI beginning X 100
New Ischemia During OPCAB
The incidence of new ischemia during OPCAB surgery is analyzed with digital TOE recordings and paper printouts of the seven-lead ECG. TOE analysis for detection of ischemia is performed by comparing the three mid-oesophageal views obtained at each subsequent study timepoint with the corresponding baseline views obtained after sternotomy. Evidence of ischemia is defined as the worsening of segmental wall motion by two or more grades in two or more segments in the territory vascularized by the target coronary artery. These marked changes in wall motion are required to maintain a high specificity of TOE for diagnosing ischemia in a situation when displacement of the heart and placement of an epicardial stabilizer can complicate the analysis of wall motion.
Diagnosis of Hemodynamic Derangements
Hemodynamic instability during OPCAB can be secondary to ischemia, reduced preload, cardiac compression, myocardial dysfunction, mitral regurgitation, or a combination of these. Cardiac manipulations during OPCAB lead to hemodynamic instability. This along with distal anastomosis causing transient occlusion of the coronary arteries may cause transient hypotension with increased filling pressures. TOE is most helpful in this scenario. In these patients, TOE helps to differentiate between cardiac dysfunction secondary to myocardial ischemia, as evidenced by RWMA from a much more common scenario where the increase in filling pressure is secondary to extra-cardiac compression and also its ability to detect MR with colour-flow Doppler, as well as assessment of right heart function [Figure 2] [Table 2].
Approach to Intra-Operative Echocardiography During OPCAB
Intra-operative monitoring with TEE during OPCAB is prognostic and useful. It is useful to have a systematic approach, as information obtained before, during, and after graft construction is important. A complete examination is recorded as a baseline for later comparison. A detailed examination of LV function, regional wall motion abnormalities (RWMA), right heart function, and the valves must be done. The approach to be followed is shown in [Table 3].
Mitral Regurgitation (MR) and Tricuspid Regurgitation (TR) to be Documented
Occasionally, significant acute mitral valve dysfunction can precipitate hemodynamic instability following heart positioning or coronary artery clamping.
Patients who are most at risk of developing severe mitral valve regurgitation are those with pre-existing myocardial dysfunction or mild to moderate mitral regurgitation. When an increase in pulmonary artery pressure (PAP) and central venous pressure (CVP) are observed, a color Doppler TEE of the mitral valve can make the diagnosis. Inferior vena cava clamping has been used to control an acute increase in PAP unresponsive to usual treatment.  Mitral valve repair or replacement can be considered if persistent after revascularization.
TEE Examination of Ascending Aorta Epiaortic TEE
The aorta is next assessed for the presence of atherosclerosis. Epiaortic echocardiography is the best way to detect atherosclerosis in the ascending aorta.  Certain features of aortic plaque morphology detected by TEE may prove to have prognostic and therapeutic significance. 
TEE for Effects of Displacement of Heart
Gaining access to the coronary arteries: if the TEE imaging plane is properly oriented to pass simultaneously through the middle of the MV annulus and the LV apex and held in that position by a clamp, the entire LV can be examined very quickly by just rotating the multiplane angle from zero to 180. A color-flow Doppler is then activated and the angle is decreased back to zero to quickly assess changes in MR. The RV and TR are then examined in a four-chamber view completing the examination of the heart. 
TEE and IABP Insertion
Intra-aortic balloon counter-pulsation has been used to support the cardiovascular system during graft construction in high-risk OPCAB patients with left main coronary artery disease, unstable angina, and/or poor ventricular function.  TEE can be used to facilitate insertion of the balloon pump by ensuring that the guide wire is in the thoracic aorta before attempting to advance the balloon catheter and position the tip of the catheter just distal to the left sub-clavian artery.
TEE for Reduced Preload
Hypotension secondary to hypovolemia is usually associated with a decrease in PAP and CVP. Fluid loading and Trendelenburg position restores cardiac output and LV preload. A TEE can help 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.  .
Cardiac Compression- Use of Stabilizer in OPCAB
During left anterior descending and diagonal 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. Using the Octopus stabilizer, [Figure 2] the main causes of hemodynamic disturbance during positioning are thought to be decreased by RV filling and to a lesser extent LV filling. Volume loading, Trendelenberg position, and vasopressor infusion usually correct these derangements, although an RV assist device has been proposed for unsuitable patients.  TEE is indicated in patients who do not respond to the above treatment and helps to differentiate between cardiac dysfunction and extra-cardiac compression.
Suction Type-Octopus System
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. This is in contrast with access to the obtuse marginal and distal right coronary 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 that immobilize the heart by pressure instead of suspension [Figure 2].
Compression Type Stabilizer-CTS Midcab System (Cardiothoracic Systems Inc, Cupertino, CA, USA)
Application of this epicardial stabilizer resulted in a minor decrease in LV end diastolic and systolic diameters and unchanged fractional area change while the cardiac index, stroke volume index, and pulmonary capillary wedge pressure (PCWP) remained unchanged. Comparing the two types of stabilizers,-suction and compression type, it is important to note that each type of stabilizer produces a different hemodynamic profile. With the fork-type compression stabilizer, the compression of the beating heart for stabilization of the diagonal and LAD prevented normal diastolic expansion by direct deformation of the geometry. 
Anterior Displacement of Heart and Effect on Hemodynamics
Changes accompanying 90-degree anterior displacement of the beating heart were caused primarily by right ventricular deformation and decreased pump function without signs of valvular incompetence, inflow, or outflow obstruction. The displacement prevented normal right ventricular and left ventricular diastolic expansion by pressing the heart against the surrounding tissue, resulting in RV diastolic dysfunction. 
In a study conducted by Grundeman, et al. on 150 consecutive patients undergoing OPCAB with Octopus Tissue Stabilization System, stroke volume (SV) was significantly reduced by dislocation at all target sites; 6% at LAD, 25% at the diagonal branch artery, 14% at RCA, and 21% at obtuse marginal. The application of head-down positioning (LAD-56%; D 74%; RCA 90%; OM 96%) increased not only surgical exposure but also pre-load, producing correction of ventricular filling pressures and output.  Fluid redistribution was sufficient to correct cardiac output.
Myocardial Dysfunction and TEE in OPCAB
Hemodynamic instability related to severe systolic dysfunction is characterized by an increase in PAP and CV 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 extra-cardiac compression. TEE may be considered in patients with known preoperative systolic dysfunction, or in patients who remain hypotensive despite intravenous nitroglycerine and inotropic support [Figure 3]. Pulmonary venous flow velocity, besides contractility of ventricles and ejection fraction, are good guides to systolic function [Figure 3].
It is dependent on a large number of inter-linked factors such as geometry of ventricles, tissue elastance relaxation, and pressure-volume curve . The issue of the importance of diastolic function evaluation during cardiac surgery has recently been raised.  The role of diastolic function evaluation during OP-CABG surgery has not been reported in literature. We, at AIIMS, are currently using Doppler to evaluate both right and left diastolic function during OP-CABG. Such an evaluation not only allows us to understand the hemodynamic changes occurring during this procedure, but also remains an investigative tool.
TEE: Measurement of Diastolic Dysfunction in OPCAB
Mitral inflow velocities are measured at the tip of the mitral leaflets on three consecutive heartbeats at the end of expiration. The variables recorded include: peak velocity of early diastolic filling wave (E) and late diastolic filling wave (A), the ratio of these two velocities (E/A), and deceleration time (dt). Peak systolic (S), diastolic (D), and atrial reversal (Ar) pulmonary vein flow velocities were measured in the left and the right upper pulmonary veins. The DD patterns have been classified into three groups with the following general criteria: impaired relaxation (E/A, 1.0, dt. 240 ms, and S,D), pseudonormal (E/A . 1.0 -1.5, dt 160-200 ms, and S, D, with Ar. 35 cm/s), and restrictive (E/A . 1.5, dt, 160 ms, S, D, and Ar. 35 cm/s). The diastolic dysfunction (DD) patterns are often correlated with the age of the patient.  Diastolic dysfunction is an early marker of ischemia as relaxation of the myocytes in diastole is a process that is energy dependent and thus sensitive to impaired perfusion [Figure 4].
Myocardial Ischemia Detection with TEE During OPCAB
During OPCAB, myocardial ischemia (MI) detection is difficult. Often, when the heart is displaced, the voltage of the ECG is often too low to provide useful information regarding myocardial ischemia because of loss of contact of the heart with the chest wall. In this situation, TEE may be used to detect a new RWMA suggesting ischemia. At least a limited view of the LV can be developed in most patients from the mid-esophageal window even when the heart is displaced by directing the imaging plane through the left atrium towards the left ventricle.  There are some patients, however, in whom no usable TEE views of the heart can be developed once the heart is displaced. Detecting signs of ischemia with ECG or TEE during vessel occlusion is not surprising. The main point is to observe resolution of these changes by the end of the case. Failure of these changes to clear, quickly, once flow is restored should be a cause for concern, and a graft occlusion should be considered.
TEE for Off Pump Robotic Surgery
TEE is also useful for intra-operative quantification of lesions during anesthetic management of robotic off pump CABG.  At our center at AIIMS (with 54 robotic OPCAB), 54 robotic assisted off pump CABG have been performed so far. We used routine TEE for following purposes:
Cannulation of SVC and IVCEndo-Aortic Clamp monitoring (EAC)De-airing after cross clamp removalConfirming optimum surgical correction [Figure 5]
TEE for Post-Operative Complications, Following OPCAB
TEE is very useful in the diagnosis of post-OPCAB complications like pulmonary embolism, which, following OPCAB is a rare complication, reported in literature.  Following a raised CVP, TEE in the postoperative period is done to exclude cardiac temponade. TEE examination may reveal preserved systolic function and an under-filled left ventricle. In contrast, the right ventricle will be dilated and akinetic. Trivial tricuspid regurgitation may be present. The position of permanent pacing lead should be noticed. In addition to a dilated right ventricle, the main and right branches of the pulmonary artery may be dilated. There may be no evidence of tamponade; however, a large collection of blood may be seen within the left pleural cavity.
The differential diagnosis for an acute increase in right ventricle after-load includes pulmonary embolism or reactive pulmonary hypertension after ischemia-reperfusion injury.
TEE Risks in OPCAB
A TEE performed under general anesthesia has its limitations. It is well appreciated that TEE may significantly underestimate the severity of mitral valve regurgitation, , which is caused by the decrease in pre-load and after-load associated with general anesthesia. Attempts to replicate loading conditions in the awake state, using fluid and phenylephrine, have been evaluated in patients under general anesthesia.  Even with these interventions, it is not fully accepted that the evaluation of mitral regurgitation under general anesthesia is as accurate as that in a conscious patient.  When aortic valve regurgitation is evaluated, the agreement between preoperative trans-thoracic echocardiography and TEE is modest.  The decision to convert an OPCAB surgery to an on-pump CABG surgery because of the detection of a patent foramen ovale is controversial. 
It is well known that TEE has the potential to injure patients. Damage to the mouth, esophagus, and stomach are reported to be in the 0 to 1% range. , Esophageal perforation has a high morbidity and mortality rate.  Post-operative swallowing dysfunction is also associated with TEE use.  A swallowing dysfunction can lead to postoperative pneumonia, the need for a tracheostomy, increased intensive care unit stay, and increased duration of hospitalization. Gastro-intestinal complications may present more than 24 hours postoperatively. ,
That being said, the use of TEE during OPCAB surgery carries only a Class IIb recommendation.  Until more evidence-based medicine is available, all OPCAB surgeries do not need to include a TEE assessment.
Future of OPCAB
Conventional CABG using CPB was first performed over 35 years ago  and is one of the most extensively studied operations in history. It has, very clearly, defined results and risks. Numerous uncontrolled studies in scientific literature suggest that OPCAB can be performed with at least similar results, mainly indicating that OPCAB has decreased morbidity and costs compared with conventional CABG.  A few small, but well-controlled trials, comparing the two procedures have recently been published. To date, there has been no clear advantage proven in mortality and freedom from subsequent cardiac events for OPCAB, [Table 4] but long-term, large, and well controlled studies will be needed to conclusively settle the issue. In the meantime, it is likely that OPCAB will continue to be performed on a large number of patients. Echocardiography can be an important tool for managing these patients during surgery.
"Mythology captivates the gullible, logic impresses simple Science, the gnawing of the inquisitive mind relies on proof. Both mythology and logic motivated pioneer physicians but Science guides current practice. OPCAB has journeyed through the past and now, by Science and TEE shall take its place."
TEE should be used in all OPCABs for the following reasons: (1) to ensure proper diagnosis and make sure no other lesions have been missed; (2) to ensure that the procedure was successful; (3) to aid in prompt diagnosis and management of hemodynamic instability and myocardial ischemia; (4) to guide the use of vasopressors, inotropes, and fluid administration, and (5) to ensure the safest manipulation of the ascending aorta. The benefits far outweigh the risks, and it is likely that TEE will be shown to improve outcome in patients undergoing on- and off-pump CABG because it allows for more rapid and accurate diagnosis of the causes of hemodynamic instability and therefore, more rapid and accurate treatment.