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Ann Thorac Surg 2000;70:1355-1360
© 2000 The Society of Thoracic Surgeons
a Cardiopulmonary Research Science and Technology Institute, Dallas, Texas, USA
Address reprint requests to Dr Mathison, MCVI Research and Outcomes, 8900 N Kendall Dr, Miami, FL 33176
Presented at the Thirty-sixth Annual Meeting of The Society of Thoracic Surgeons, Fort Lauderdale, FL, Jan 31–Feb 2, 2000.
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Abstract |
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Methods. Forty-four patients (35 men, 9 women; mean age, 64.5 ± 9.6 years) underwent off-pump coronary artery bypass grafting. The hemodynamic variables were collected before and after positioning the heart for anastomosis of the left anterior descending, circumflex, and posterior descending coronary arteries.
Results. There was a significant increase in right ventricular end-diastolic pressure during positioning for all vessels, and in left ventricular end-diastolic pressure during positioning for the left anterior descending and circumflex coronary arteries. Positioning for the circumflex artery showed the largest increase of left and right ventricular end-diastolic pressure, resulting in the greatest hemodynamic compromise.
Conclusions. In the clinical setting of diseased human hearts, there is a biventricular contribution to altered hemodynamics. The increase of right ventricular end-diastolic pressure in all positions suggests that the major cause of hemodynamic changes is disturbed diastolic filling of the right ventricle, especially by direct ventricular compression.
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Introduction |
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TopAbstractIntroductionPatients and methodsResultsCommentReferences |
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In the standard on-pump technique, CABG for three-vessel disease is performed through a median sternotomy approach under conditions of global cardiac arrest. It enables access to all coronary artery segments, particularly the circumflex artery and posterior descending artery branches for multiple grafting. On the other hand, in the off-pump technique, for the grafting of the circumflex artery and posterior descending artery branches through sternotomy, anterior displacement of the beating heart needs to occur. This often causes hemodynamic compromise.
Because the most frequently encountered hemodynamic disturbance is systemic hypotension, some studies have proposed that a univentricular left heart assist will result in hemodynamic stability [3–5]. However, other studies have theorized that a right heart assist will achieve this stability [6] because many of the maneuvers performed during beating heart surgical procedures (volume loading, Trendelenburg, right hemisternum elevation, release of restricting right pericardium) are primarily aimed at aiding the right heart.
Thus, it is important to analyze the hemodynamic changes during the periods when the heart is tilted and find a more effective way to access the circumflex artery and posterior descending artery without hemodynamic compromise.
Animal studies [7] investigating the hemodynamic changes during heart displacement have indicated that such displacement has its primary deleterious effects on the right heart. Nierich and associates [8] and Jansen and coworkers [9] reported the experience of the first 100 patients undergoing CABG using the Octopus stabilization, and concluded that moving the heart to reach the target site of anastomosis caused hemodynamic alteration, but that the disturbance could easily be corrected by anesthetic interventions, such as fluid loading and low doses of inotropic agents. Despite this, there are no published studies analyzing the hemodynamic changes in humans during this displacement.
The purpose of this study is to analyze the hemodynamic changes in patients whose hearts are tilted during beating heart surgical procedures.
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Patients and methods |
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TopAbstractIntroductionPatients and methodsResultsCommentReferences |
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After median sternotomy, the heart was suspended in a pericardial cradle. Two 18-gauge catheters were introduced through the right pulmonary vein: one into the left atrium; the other into the left ventricle to monitor the left atrial pressure and left ventricular end-diastolic pressure. The right atrial and right ventricular end-diastolic pressures, and mixed venous oxygen saturation were obtained through the Swan-Ganz catheter. (The catheter was pulled back from the pulmonary artery to the right ventricle just before data collection began.) An ultrasound transit-time flow probe (Transonic Systems, Ithaca, NY) was placed around the aorta for cardiac output measurement.
After hemodynamic stability was obtained (about 3 to 5 minutes), baseline data were collected, including heart rate, arterial blood pressure, left and right atrial pressure, left and right ventricular end-diastolic pressure, cardiac output, and mixed venous oxygen saturation. Then the heart was positioned, and the coronary stabilizer (Octopus Tissue Stabilizer, Medtronic, Inc, Minneapolis, MN) was applied to obtain an optimal view for distal anastomosis. The operating table was always tilted in the head-down position (Trendelenburg maneuver) for circumflex and posterior descending artery anastomosis. Each time the operating table was repositioned, the zero levels for measurements were corrected. The same hemodynamic variables as baseline were collected after positioning.
Measurements for all pressures and cardiac output were taken at the end-inspiration phase during mechanical ventilation.
Statistical analysis
Data are presented as mean ± standard deviation (absolute values) or as mean ± standard error of the mean (relative values). For comparison of baseline data with the data obtained during heart displacement, a paired Student’s t test was used.
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Results |
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TopAbstractIntroductionPatients and methodsResultsCommentReferences |
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Positioning for the left anterior descending artery anastomosis
Mean arterial pressure decreased only 4.4% ± 4.2% (p > 0.05) in this positioning, but stroke volume was decreased to 17.5% ± 5.1% (p < 0.05). Right atrial pressure and right ventricular end-diastolic pressure were increased to 43.5% ± 16.0% (p < 0.01 versus baseline) and 67.5% ± 12.4% (p < 0.01 versus baseline), respectively, as were left atrial pressure and left ventricular end-diastolic pressure (32.2% ± 15.0%; p < 0.05 versus baseline for the atrial pressure and 33.6% ± 10.0%; p < 0.05 versus baseline for ventricular pressure; Fig 1; Table 2).
Comparison among three positionings
Every positioning, even for the left anterior descending artery, caused an increase of right ventricular end-diastolic pressure (Fig 1). And the positioning for the posterior descending artery caused an increase of left ventricular end-diastolic pressure, although it was not statistically significant. The positioning for the circumflex artery showed the largest increase of left and right atrial pressures and left and right ventricular end-diastolic pressures, resulting in the largest hemodynamic compromise. The positionings for the circumflex and the posterior descending arteries caused a larger increase of left and right atrial pressures than the positioning for the left anterior descending artery did.
Comparison of hemodynamic changes on the basis of ejection fraction
Patients were divided into those with an ejection fraction of 40% or less and those with an ejection fraction of more than 40%. There were no statistical differences between these two groups of patients for changes of mean arterial pressure and cardiac output with any positioning of the heart, although there was a tendency for mean arterial pressure and cardiac output to decrease more in the group with an ejection fraction of 40% or less for left anterior descending positioning (Table 3).
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Figure 2 shows the preoperative midesophageal four-chamber view of the heart. Without changing the TEE window, Figure 3 demonstrates when the stabilizer was placed for a circumflex anastomosis. It is noticed that the left ventricular cavity is diminished considerably, the left atrium is enlarged, and both right atrium and ventricle are compressed. Figure 4 demonstrates when the stabilizer was placed for a very proximal posterior descending artery anastomosis. It is noticed that the stabilizer considerably compressed the right ventricular cavity just below the tricuspid valve, the right atrium is enlarged, and left atrium and ventricle are not affected.
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Comment |
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TopAbstractIntroductionPatients and methodsResultsCommentReferences |
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Circumflex positioning
Basically, results similar to those from animal experiments by Gründeman and associates [7] were obtained. All pressures in four chambers (right and left atrial pressures and right and left ventricular end-diastolic pressures) were elevated, while mean arterial pressure, cardiac output, and stroke volume were decreased. However, in contrast to the animal studies, Trendelenburg positioning did not restore mean arterial pressure, cardiac output, and stroke volume to normal.
Left anterior descending artery positioning
We collected data for the left anterior descending artery positioning, not because we expected that it would show significant changes, but to compare the difference between the circumflex and left anterior descending artery positioning. Surprisingly, however, the positioning for left anterior descending artery caused an increase of the right atrial pressure and right ventricular end-diastolic pressure. Although left atrial pressure and left ventricular end-diastolic pressure were also elevated, the pressures from the right heart were much more increased.
The possible cause of the increase in the right ventricular end-diastolic pressure, induced by the positioning of the left anterior descending artery, is that the positioning had resulted in right ventricular outflow obstruction or mitral regurgitation. However, TEE did not show any such occurrences with left anterior descending artery positioning, but it showed that left anterior descending artery positioning caused minimal to moderate compression of the left ventricle in every case, and of the right ventricle in several cases.
We believe that the most likely explanation for the elevation of right atrial and right ventricular pressures with left anterior descending artery positioning is right ventricular compression. When the stabilizer is placed on the left ventricle, it is compressed directly. Burfeind and colleagues [10] studied the effects of mechanical cardiac stabilization on hemodynamics and concluded that mechanical stabilization of the left anterior descending coronary artery may temporarily decrease cardiac output. This is not, their study suggests, attributed to impaired contractility or ischemia, but is secondary to direct ventricular compression with reduced stroke volume. Also, Jurmann and coworkers [11] showed that placement of the epicardial stabilizer resulted in a small decrease in left ventricular end-systolic and end-diastolic dimensions. We think that 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 though the left ventricle is compressed, the effect on the right ventricle is greater than that on the left.
This theory can explain the hemodynamic changes that occur at every positioning. When the stabilizer is placed for circumflex anastomosis, although the left ventricle is compressed, the right ventricle is more compressed, thus causing a disturbance in diastolic distension. When it is applied to the posterior descending artery anastomosis, the right ventricular end-diastolic pressure is increased because the right ventricle is compressed. Left ventricular end-diastolic pressure is only slightly elevated because the compression of the right ventricle does not significantly affect the left ventricle.
Atrial pressures
Another interesting result is the changes in atrial pressures. In the positioning for circumflex artery, the increase of right atrial pressure was larger than the increase of right ventricular end-diastolic pressure (166.7% versus 151.4%). Also, in the positioning for posterior descending artery, the increases in atrial pressures were larger than ventricular end-diastolic pressures (149.0% versus 82.4% on the right side and 55.4% versus 9.8% on the left side; Fig 1). On the other hand, in the positioning for left anterior descending artery, the increase of right atrial pressure is smaller than the increase of right ventricular end-diastolic pressure (43.5% versus 67.5%).
We think that this is related to the difference of the positioning. The heart needs to be displaced anteriorly, and the apex of the heart points upward during the positioning for circumflex or posterior descending artery. The heart is folded, usually somewhere close to the atrioventricular valves, possibly causing some flow obstruction. However, that kind of displacement is not necessary for the positioning for left anterior descending artery. The elevations of left and right atrial pressures in the left anterior descending artery positioning seem like the reflection of the elevations of ventricular end-diastolic pressures.
Limitations of the study
Unable to separate the effect of displacement from the corrective measures
For circumflex and posterior descending artery anastomosis, the operating table was always tilted for Trendelenburg maneuver. Therefore, our data for hemodynamic changes reflect the effect of this maneuver. Also, before positioning, volume loading was sometimes performed. As far as inotropic use is concerned, it was given for 2 left anterior descending artery anastomoses (4.8%), for 12 circumflex anastomoses (23.1%), and for 3 posterior descending artery anastomoses (12.5%). Norepinephrine (10 to 20 µg intravenously) or dobutamine (2 µg · kg-1 · min-1) was used as an intropic agent. However, because all inotropic agents were given after positioning and after data collection, the use of these agents does not have any effect on the data.
Fluid-filled catheter use
For measurement of the pressures, we used fluid-filled catheters. Compared with the Millar catheter, the fluid-filled catheter is position-dependent. Although each time the operating table was repositioned, the zero levels for the measurements were corrected, hydrostatic differences are still created between the ventricular lumen and the left and right atria when the apex of the heart points upward for circumflex or posterior descending artery. There is a possibility that left and right ventricular end-diastolic pressures were measured slightly higher than the exact value for circumflex or posterior descending artery positioning.
Participation of ischemia in hemodynamic changes during displacement of the heart
Gründeman and associates [12] measured coronary flow in healthy animals and found that displacement of the heart decreased coronary flow, but augmentation of preloads by the Trendelenburg maneuver restored coronary flow parallel to the recovery of cardiac output and mean arterial pressure while the heart remained displaced. They concluded that coronary blood flow was not mechanically obstructed during displacement of the heart.
In our study, displacement of the diseased human heart for circumflex positioning caused decreases of cardiac output and arterial pressure despite use of the Trendelenburg maneuver. Myocardial ischemia might have occurred secondarily owing to the decreased coronary flow caused by decreased cardiac output.
Conclusions
We conclude that the main cause of hemodynamic instability during off-pump CABG is the disturbance of ventricular diastolic filling by direct ventricular compression. The increased filling pressures, decreased stroke volume, and TEE findings all support this theory. This also explains why the effects are more pronounced on the thinner right ventricle. Thus, the right heart assist may be an additional adjunct for beating heart CABG to achieve more stable hemodynamics during positioning for the far posterior area of the heart. Although it was in cases of healthy animals, Gründeman and colleagues [13] reported that right heart bypass was effective for normalizing hemodynamics during exposure of circumflex, and left heart bypass failed to restore systemic circulation. Our next step is to evaluate the effectiveness of the right heart assist for clinical beating heart CABG.
Although our study suggests that the right heart assist is the choice for stable hemodynamics, we also showed that the left ventricular diastolic filling is also disturbed. Therefore, it will be critical to have the optimal flow in any right heart assist device, because too much flow in the right side might result in pulmonary edema, especially if the patient has impaired left ventricular function.
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References |
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TopAbstractIntroductionPatients and methodsResultsCommentReferences |
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