Anesth Analg 2005;100:306-314
© 2005 International Anesthesia Research Society
doi: 10.1213/01.ANE.0000143565.18784.54
CARDIOVASCULAR ANESTHESIA
Segmental Myocardial Wall Motion During Minimally Invasive Coronary Artery Bypass Grafting Using Open and Endoscopic Surgical Techniques
S. Mierdl, MD*,
C. Byhahn, MD*,
V. Lischke, MD*,
T. Aybek, MD
,
G. Wimmer-Greinecker, MD,
S. Dogan, MD,
S. Viehmeyer*,
P. Kessler, MD*, and
Klaus Westphal, MD*
*Department of Anesthesiology, Intensive Care Medicine and Pain Control, Department of Thoracic and Cardiovascular Surgery, J.W. Goethe-University Hospital, Frankfurt, Germany
Address correspondence and reprint requests to Klaus Westphal, MD, Department of Anesthesiology and Intensive Care Medicine, Katharina-Kasper-Kliniken, Richard-Wagner-Str. 14, D-60318 Frankfurt, Germany. Address e-Mail to klaus.westphal{at}em.uni-frankfurt.de.
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Abstract
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Current options for minimally invasive surgical treatment of single-vessel coronary artery disease include beating heart procedures without cardiopulmonary bypass (CPB) via mini-thoracotomy (MIDCAB) and totally endoscopic robot-assisted techniques (TECAB) with CPB. Both procedures are associated with potential myocardial stress before revascularization, such as single-lung ventilation (SLV), temporary coronary artery occlusion, cardiac luxation, intrathoracic carbon dioxide insufflation, and extended CPB and operating time. In this echocardiographic study we sought to evaluate the extent of intraoperative segmental wall motion abnormalities (SWMA) during MIDCAB and TECAB surgery and to identify factors affecting SWMA. Forty-six patients with single-vessel coronary artery disease were studied. Sixteen patients were operated using the MIDCAB technique and 30 patients with TECAB. In both groups sequential transesophageal echocardiograms were recorded during the entire procedure. Hemodynamic data and oxygenation variables were acquired simultaneously. In both groups, mild but obvious perioperative SWMA were identified and noted to increase during the course of the operation. These SWMA were more pronounced in the TECAB group. Independent of operating time, these changes disappeared completely after revascularization. No significant hemodynamic compromise was observed. We conclude that MIDCAB and TECAB techniques are associated with significant perioperative SWMA. The appearance of more profound SWMA in the TECAB group compared with the MIDCAB patients might have been the result of intrathoracic CO2 insufflation, as SLV was used in both groups. No persistent SWMA or post-CPB SWMA were apparent in either group. More extensive intraoperative ventricular SWMA was detected in the TECAB group, suggesting that a more frequent risk for right ventricular dysfunction may exist during TECAB procedures.
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Introduction
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The surgical treatment of the single-vessel coronary artery disease unsuitable for catheter intervention has changed in recent years towards less invasive procedures. These new techniques aim to reduce surgical trauma by limited incisions and avoid cardiopulmonary bypass (CPB) by using beating heart techniques. Grafting of the internal thoracic artery (ITA) onto the left anterior descending artery (LAD) through a small, left anterior thoracotomy (minimally invasive direct vision coronary artery bypass, MIDCAB) without CPB has been demonstrated to be a safe alternative to standard coronary artery bypass grafting (CABG) with sternotomy and CPB (1). The most recent development has been robot-assisted totally endoscopic techniques (totally endoscopic coronary artery bypass; TECAB) (2) usually performed using CPB.
Although both procedures require single-lung ventilation (SLV) during ITA dissection and coronary anastomosis to ensure adequate exposure of the surgical field, the additional carbon dioxide (CO2) insufflation is required for TECAB to improve exposure and instrument movements.
The adverse effects of CPB, artificially augmented intrathoracic pressure, and SLV have been described in several studies (3–6). However, no data are yet available regarding potential adverse effects of these variables or their combination on myocardial wall motion in patients with coronary artery disease. Therefore, biventricular segmental myocardial wall motion was analyzed in patients who underwent MIDCAB or TECAB procedures using transesophageal echocardiography (TEE). Furthermore, creatine kinase (CK), CK-MB, ST segment analysis, oxygenation, and outcome were assessed.
The aim was to identify potential adverse effects on myocardial function and to compare the influence of the MIDCAB technique on the beating heart with the TECAB closed chest technique using CPB and intrathoracic CO2 insufflation. Our hypothesis was that the combination of SLV and intrathoracic CO2 insufflation during TECAB should cause more profound intraoperative segmental wall motion abnormalities (SWMA) than SLV without intrathoracic CO2 insufflation during MIDCAB.
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Methods
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After approval by the IRB and with informed written consent, 50 consecutive patients with symptomatic coronary disease of the LAD scheduled for minimally invasive revascularization were studied during a 1-yr period. After extensive consideration about MIDCAB and TECAB procedures, including benefits and risks, the surgeon chose one of these methods of revascularization. Twenty patients were operated on the beating heart with the MIDCAB technique, whereas 30 patients underwent TECAB surgery using the DaVinci telemanipulation system (Intuitive Surgical, Mountain View, CA) and the Port Access system (Heartport, Redwood City, CA) for CPB. All patients received a standard initial volume loading of 1000 mL of lactated Ringer’s solution and another 1,000 mL of hydroxyethyl starch 6% prior to the initiation of SLV.
MIDCAB was performed on the beating heart with CPB standby. SLV was started immediately before skin incision. A 7–8 cm anterolateral minithoracotomy was then performed in the left fourth intercostal space for preparation of the left ITA and its end to side grafting onto the LAD. Double-lung ventilation (DLV) was resumed after surgical hemostasis was ensured.
For TECAB, the patients were placed supine with the left chest slightly elevated for TECAB. Once SLV was started, the procedure was performed endoscopically through 3 left-sided 1–2 cm incisions. Under continuous CO2 insufflation, the left ITA was dissected, followed by institution of CPB via the femoral vessels. After occlusion of the ascending aorta with an endoaortic balloon catheter and application of antegrade cardioplegia, the left ITA was grafted end-to-side onto the LAD by continuous suture. Separation from CPB was achieved under SLV after coronary reperfusion and rewarming. DLV was reinstituted after surgical hemostasis and CO2 release from the thoracic cavity.
Baseline arterial blood gas tension analysis for Po2 and Pco2 assessments were performed immediately before skin incision and then at 30, 90, and 120 min after institution of SLV and 5 min after DLV was resumed. Simultaneously, intrathoracic CO2 pressures were recorded during TECAB (Fig. 1). Automated ST segment analysis at J + 60 ms for leads I, II, and V5 were recorded (Hellige Marquette Solar 7000/8000 Patient Monitor; Marquette Medical Systems, Milwaukee, WI). ST segment alterations of 
1 mm (0.1 mV) from baseline, persisting for more than 60 s were considered an indication of ischemia.
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Figure 1. Data collection in correlation to intraoperative stages. DLV = double lung ventilation; CP&B = control of perfusion and bleeding; IMA = internal mammary artery;
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All echocardiographic examinations were performed in the transgastric mid short axis view of the left and right ventricles (LV, RV) at the level of mid-papillary muscles and the insertion of tricuspid valve apparatus respectively. A Vingmed System Five echocardiography device with a multiplane 5–7 MHz TEE probe was used (GE Vingmed, Horten, Norway). Device settings included frequency, 6.7 MHz; power, 4 dB; depth, 14 cm; frame rate, 70–80/min. An electrocardiogram (ECG) triggered cine-loop of 3 cardiac cycles of each ventricle was acquired at the above-mentioned intervals and directly stored on the image processing system (Echo PAC v. 6.1, Vingmed, Horten Norway) for off-line analysis. During data acquisition, the patient was disconnected from ventilation at end-expiration. Analysis of segmental wall motion was performed off-line after surgery by dividing the LV in 6 segments according to the guidelines of the American Society of Echocardiography and the Society of Cardiovascular Anaesthesiologists (7). Because no established model for segmentation of the RV exists, the RV was divided into two segments. The posterior segment adjacent to the liver and diaphragm was labeled "diaphragmatic," whereas the anterior segment with no contact to structures other than pericardium was labeled "free." The tricuspid apparatus was used as a landmark for the RV cross-section. The ventricular septum was considered a component of the LV.
Analysis of LV and RV function was based on a qualitative visual assessment of the motion and thickening of a given segment during systole and graded according to a scale for wall motion that has been used extensively in the echocardiography literature. The qualitative grading for wall motion is: 1 = normal (>30% thickening), 2 = mildly hypokinetic (10% to 30% thickening), 3 = severely hypokinetic (<10% thickening), 4 = akinetic (no thickening), and 5 = dyskinetic (paradox movements during systole). Because most of the patients were receiving myocardial depressants, such as ß-adrenergic blocking drugs, and because of negative inotropic effects of anesthesia drugs, grades of 1 and 2 were defined as normal myocardial function and grades of 3, 4, or 5 were classified as myocardial dysfunction.
Left and right ventricular ejection fraction (LVEF, RVEF) were calculated from static two-dimensional images by planimetry using the tools provided by the postprocessing software. The end-diastolic area coincided with the peak of the R-wave, and the end-systolic area was measured at the smallest cross-section after the R-wave. Ejection fraction and fractional diameter shortening (LVFS, RVFS) were calculated according to the established recommendations (8).
Two independent and equally experienced echo-cardiographers according to the guidelines of the German Society of Anesthesiology performed all examinations. The first echocardiographer performed the intraoperative TEE monitoring and therefore was not blinded to the patient’s identity and clinical data. The second echocardiographer had no information regarding hemodynamic or clinical data of the patients and was blinded to the time points but not to the study purpose. Derived from a study by Rouine-Rapp et al. (9), agreement between the investigators was defined as independently assigned grades within the normal (grades 1 and 2) or abnormal (grades 3–5) ranges. When the echocardiographers independently agreed, the classification assigned to segments was considered final. When one investigator classified function as normal and the other abnormal or when the classification was different by 2 or more points, the investigators met and assigned a class of function by consensus. If the investigators could not agree on a consensus classification, the respective segment was examined by a third echocardiographer, and classified according to the majority opinion of all 3 investigators.
All data are presented as mean ± sd. Calculation and data analysis were performed by using a statistical package (GraphPad InStat 3.0, GraphPad Software, San Diego, CA). Data were compared to baseline values, and statistical significance was determined with Friedman test and Dunn’s posttest, one-way analysis of variance with Bonferroni adjustment, or Wilcoxon’s matched pairs test, as appropriate. Differences were considered to be statistically significant if P was <0.05.
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Results
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All patients had isolated coronary single-vessel disease with a proximal subtotal stenosis of the LAD that was not suitable for angioplasty. All patients received continuous medication with ß-adrenergic blocking drugs preoperatively. No patient had a history of myocardial infarction. There were no significant differences between groups regarding age and gender (Table 1).
The MIDCAB technique was used successfully in 16 of 20 patients. Four patients needed conversion to conventional on-pump surgery as a result of hemodynamic instability during LAD exposure (n = 3) and severe arrhythmia during ITA dissection (n = 1). All TECAB cases (n = 30) were completed successfully. Therefore, 16 MIDCAB and 30 TECAB patients underwent TEE analysis. No patient had any SWMA in preoperative fluoroscopy.
On initiation of SLV the Pao2 declined significantly in both groups, persisted at this level throughout SLV and recovered to baseline after resumption of DLV. Paco2 showed only minor changes in the MIDCAB group, whereas continuous intrathoracic CO2 insufflation during SLV in TECAB patients caused a gradual increase of Paco2 that returned to baseline after CPB and CO2 release. (Table 2).
Hemoglobin concentration declined significantly in both groups throughout the procedure with lowest mean values of 10.7 ± 0.9 g/dL (MIDCAB) and 9.3 ± 0.4 g/dL (TECAB), respectively (Table 2).
In both groups episodes of ST segment changes ±1 mV from baseline were observed in leads I during SLV and CO2 insufflation but returned to baseline at the end of surgery.
The preoperative CK level was 32.2 ± 4.7 (MIDCAB) or 29.4 ± 5.8 U/L, respectively (TECAB, not significant.), and increased to 142.8 ± 58.0 (MIDCAB, P < 0.0001 versus preoperative CK) or 1.228.5 ± 1.020.5 U/L (TECAB, P < 0.0001 versus preoperative CK, P = 0.0001 versus MIDCAB) in the postoperative period. Preoperative CK-MB was not assessed because of small CK concentrations <100 U/L.
Maximum postoperative CK-MB levels were 5.4 ± 1.3 U/L (MIDCAB) or 31.2 ± 26.9 U/L (TECAB, P < 0.05) and thus remained less than 10% of total CK. Serum lactate was not increased during SLV and before CPB (MIDCAB baseline level: 9.5 ± 3.7 mg/dL; TECAB baseline level: 11.4 ± 2.7 mg/dL; not significant) but was 11.1 ± 4.6 (MIDCAB, not significant versus Baseline) and 32.3 ± 26.7 mg/dL (TECAB, P = 0.0106 versus Baseline; P = 0.0033 versus MIDCAB) after revascularization and weaning from CPB in the TECAB group, respectively.
Except for a continuous dopamine infusion (3 µg · kg–1 · min–1) as standard treatment started at the beginning of the procedure in any patient undergoing CABG, no patient needed additional inotropic or vasopressor support or antiischemic treatment either intraoperatively or postoperatively.
The postoperative course was uneventful in all TECAB patients but one who underwent reexploration of the chest to control postoperative bleeding and developed respiratory failure with prolonged artificial ventilation. There was no significant difference in the postoperative need for analgesia (Dipyrone, nonsteroidal antirheumatics, or opioids) between MIDCAB or TECAB patients. All patients of both groups except the one mentioned above were discharged from the intensive care unit within the first 24 h after admission.
A total of 1840 segments were assessed. In the MIDCAB group, 160 RV segments (two segments analyzed at each of the five measurement points in 16 patients) and 480 LV segments (six segments analyzed at each point) were recorded and analyzed. In the TECAB group, 300 RV and 900 LV segments underwent TEE analysis.
In both groups LVEF and LVFS remained almost unchanged. A significant increase of SWMA during SLV was observed in TECAB patients in the inferior, septal, anteroseptal, and anterior segments of the LV, whereas this occurred only in the anteroseptal and posterior segments during MIDCAB. When comparing both groups after 90 and 120 min of SLV, SWMA in the septal segment were significantly more pronounced in TECAB patients. After 30 min of SLV, more MIDCAB than TECAB patients showed an increase in SWMA score of at least 2 points from baseline. In contrast, after 90 and 120 min of SLV, an SWMA score increased by 2 or more points was more likely in the TECAB cohort (Table 3).
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Table 3. Scores of Segmental Wall Motion Abnormalities (SWMA) of the Left Ventricle (Mean ± sd) and Number of Patients with an Increase in SWMA Score of at Least 2 Points from Baseline
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Mild hypokinesis (score = 2) of the anteroseptal, septal, and inferior segments occurred significantly more often in the TECAB group. Although severe hypokinesis (score = 3) did not occur during MIDCAB, it was detected in TECAB patients, especially in the posterior segment. Akinesis (score = 4) was detected in a small number of MIDCAB patients but not in TECAB patients (Table 4). Dyskinesis did not occur in either group.
A remarkable increase of both RVEF and RVFS was observed during MIDCAB but not during TECAB. However, TECAB patients had a higher degree of SWMA in both segments of the RV. These findings also included SWMA of the highest degree (i.e., SWMA 2 points from baseline). A significant increase of SWMA was observed in MIDCAB patients as well, but it was limited to the free wall. No SWMA score increases of 2 or more points from baseline were observed in MIDCAB patients during SLV, but such increases occurred in two MIDCAB cases after reinstitution of DLV (Table 5).
Segments showing an SWMA score increase of 3 or more points from baseline were observed significantly more often in TECAB than in MIDCAB patients. Particularly severe hypokinesis and akinesis were more likely to occur in this group (Table 6).
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Discussion
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In our study we demonstrated that new biventricular SWMA occurred during both techniques of minimally invasive CABG surgery with the onset of SLV and thus before myocardial revascularization. A number of potential causes for SWMA exist in the pre-revascularization period. The SLV that was used in both groups and artificially augmented intrathoracic pressure from CO2-insufflation during TECAB appear to be the most likely contributing factors. In addition, the potential influence of arterial hypercarbia during TECAB and positioning of the heart in MIDCAB procedures, along with temporary LAD occlusion later during the SLV period, must be considered. Because the SMWA returned to baseline at the end of surgery regardless of the type of surgical procedure, the use of CPB was likely an insignificant variable compared with other factors.
Because the anesthetic regimen during SLV was identical in both groups and the Pao2 did not differ significantly between TECAB and MIDCAB patients, it is unlikely that the observed differences in the extent of SWMA between groups were attributable to SLV alone. Although it cannot be excluded that the reduced Pao2 during SLV might contribute to the development of new SWMA, disturbances in oxygen transport can be eliminated as a reason for SWMA. Despite a statistically significant hemodilution in both groups throughout the procedure and although hemodilution was significantly higher in the TECAB than in the MIDCAB group, hemoglobin concentrations in both groups were far above any level considered critical (10). This is consistent with findings showing that mild isovolemic hemodilution does not affect myocardial function assessed by echocardiographic standard variables and by SWMA analysis in patients with even severe multivessel coronary artery disease (11,12).
The occurrence of less pronounced RV SWMA in the MIDCAB group compared with the TECAB group suggests that intrathoracic CO2 insufflation plays a major role. An artificially created tension capnothorax leads to mediastinal shift, decreased venous return to the right heart, and direct RV compression, as observed with TEE in the present study. At an early stage after starting TECAB at our institution before the start of this present study, we often experienced massive hemodynamic instability right after the onset of CO2 insufflation. Therefore, every TECAB patient was initially administered at least 1000 mL of lactated Ringer’s solution and another 1000 mL of hydroxyethyl starch 6% before the start of insufflation. For methodical reasons, this intravascular fluid administration was also performed in the MIDCAB cohort in the present setting. Improved cardiac preload may explain the hemodynamic stability during CO2 insufflation in the present cohort despite significant SWMA. Because the SWMA were of no clinical significance and regressed completely after revascularization, we consider intrathoracic CO2 insufflation with a pressure of approximately 10 mm Hg safe in patients with LAD disease.
In contrast to MIDCAB procedures, significant arterial hypercarbia occurred and progressed during the course of CO2 insufflation in TECAB patients. An experimental study disclosed coronary steal phenomena resulting from arterial hypercarbia in swine with artificially created chronic LAD stenosis (13). In our study we could not see any lasting effects from hypercarbia.
The heart is exposed to substantial stress during MIDCAB surgery. To gain optimal exposure of the LAD, mechanical positioning, along with pericardial sutures, is essential. During the suturing of the coronary anastomosis, the surgical field must be kept stable. In addition, the target vessel must be occluded temporarily. Myocardial ischemia as reflected by SWMA seems highly likely during these manipulations. The first SWMA were observed immediately after the onset of SLV in the MIDCAB group (i.e., before the heart was positioned). Interestingly, we observed new akinetic segments in significant number in the LV of four MIDCAB patients. These SWMA developed in the inferior and septal segments 30 minutes after the onset of SLV. In two patients, SWMA persisted over the next hour. This corresponds with the end of internal mammary artery dissection and surgical exposure for coronary anastomosis. We conclude that the exposure and immobilization of the myocardial surgical site led to these alterations. This is in contrast to another study reporting only minor LV hemodynamic changes during surgical exposure for LAD revascularization (14). The patients in that study suffered from multivessel coronary artery disease and were operated on via median sternotomy, whereas our MIDCAB patients had a small anterolateral thoracotomy. The latter approach requires deep pericardial sutures and an important rightward rotation of the LV. TEE data provided by Mathison et al. (14) described RV and LV compression but did not provide a systematic analysis of SWMA. We explain the observed akinetic events with the important changes in cardiac topography during myocardial stabilization for anastomosis.
There was no change in biventricular EF and FS in patients who underwent TECAB. In MIDCAB patients, RVEF and RVFS increased significantly during the operation, even when SWMA became more and more apparent. This is consistent with findings from other studies of patients undergoing conventional CABG. We agree with other investigators that changes in EF and FS do not adequately reflect acute myocardial ischemia (15).
Although we observed statistically significant SWMA in both groups of patients, these were not accompanied by specific ST segment changes. This agrees with studies demonstrating that these ST segment changes rarely correlate with intraoperative SWMA and therefore may not always reflect myocardial ischemia even associated with hemodynamic changes. The influence of the observed SWMA in both groups on the outcome might be small or even nonexistent, as it has been shown that only new SWMA after revascularization are significantly associated with adverse clinical outcome (16,17). We consider the SWMA as highly sensitive markers of myocardial ischemia that precede ST segment alterations. Therefore we cannot exclude that these statistically significant SWMA reflected intermittent myocardial ischemia undetected by ECG (18).
Serum lactate concentration as evidence of tissue hypoxia or ischemia remained stable throughout the observation period despite occasional low Pao2 during SLV. It increased only in TECAB patients after weaning from CPB. Twelve hours postsurgery a significant increase of the CK was observed in both groups but no accompanying increase in CK-MB was observed, therefore excluding relevant myocardial damage. The large increase in CK after TECAB procedures was probably a result of limb ischemia after femoral cannulation for CPB with the Port Access system (19).
The study was not designed in randomized fashion because the surgical approach was decided individually for each patient by the surgeon. Another important limitation was that no invasive hemodynamic data were obtained. However, placement of a pulmonary artery catheter is almost impossible during TECAB because of the pulmonary vent required for CPB. In MIDCAB procedures, an insertion of a pulmonary artery catheter poses significant additional risk to an otherwise low risk population suffering from single vessel disease and was not approved by the IRB.
Although the accuracy of SWMA is based on interdisciplinary guidelines, allowing an exact interpretation of the extent of SWMA as an indirect scale of ischemia (7), it is a subjective method that is dependent on the observer’s experience and has the risk of false positives or negatives. Furthermore, nonischemic causes of SWMA have been described. Conduction abnormalities, anesthetics, acute changes in adrenergic tone, or hypovolemia can cause false positive results (20). In addition, the hibernating or stunned myocardium can show impaired systolic function even though the ischemic event is over (21). More sensitive and specific methods for the assessment of myocardial function are technically complicated and time-consuming and therefore not feasible in the perioperative setting of cardiac surgery.
To assess SWMA, we only performed the transgastric mid short axis view. Rouine-Rapp et al. (9) found that 43% of SWMA might be missed by using this view alone without additional transverse or longitudinal planes. We were aware of this problem, but keeping the probe in stable position during the entire study period was thought to generate the most reliable TEE results. Furthermore, the transgastric mid-short axis view allows the evaluation of all myocardial regions perfused by each of the major coronary arteries.
Only CK/CK-MB, but not Troponin T, known as one of the most sensitive markers of myocardial ischemia, was examined in our study. However, it has been shown in patients undergoing cardiac surgery that no marker can distinguish injury resulting from acute infarction from the obligatory injury associated with the procedure itself (22). Therefore, we did not consider Troponin T levels essential to the study.
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Conclusion
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We conclude that MIDCAB and TECAB procedures are accompanied by significant SWMA before myocardial revascularization. However, as SWMA rapidly return to baseline after revascularization and do not cause persistent hemodynamic instability requiring inotropic or vasopressor support, the perioperative patient risk for myocardial ischemia can be estimated to be minimal in both methods. Nevertheless, the RV impact of intrathoracic CO2 pressure on SWMA in TECAB procedures is more important than the mechanical exposure of the LAD in the MIDCAB approach. There might have been a higher risk of limb ischemia in the TECAB group because of the specific endovascular CPB access in addition to other complications associated with the endovascular system as discussed elsewhere (23). Finally, TECAB may be associated with a higher risk for perioperative RV dysfunction when compared with MIDCAB procedures.
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Footnotes
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Accepted for publication August 13, 2004.
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Abstract
Introduction
