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Ann Thorac Surg 1999;68:1974-1977
© 1999 The Society of Thoracic Surgeons
a Division of Cardiothoracic Surgery, Department of Surgery, East Carolina University School of Medicine, Greenville, North Carolina, USA
Address reprint requests to Dr Chitwood, Department of Surgery, East Carolina University School of Medicine, Greenville, NC 27858;
e-mail: chitwood{at}brody.med.ecu.edu
Presented at the International Symposium on Myocardial Protection From Surgical Ischemic-Reperfusion Injury, Asheville, NC, Sept 21–24, 1997.
Abstract
Background. Minimally invasive heart operation differs from traditional cardiac operations through the omission of a sternotomy, cardiopulmonary bypass, or both. Current concerns with minimally invasive operation include: operative safety, learning curve, operative times, arrest times, and adequacy of myocardial protection. While many of the protective strategies used for traditional procedures may be applied to minimally invasive cardiac operations, the safe applications of minimally invasive operations require unique techniques of myocardial protection.
Methods and Results. Omission of extracorporeal perfusion may benefit patients through attenuation of systemic inflammatory response, decrement in neurologic insults, and reduced bleeding complications. As a counterbalance, surgeons must consider long-term operative quality and level of myocardial protection provided during beating heart coronary operation. Current issues that must be addressed include: pharmacologic management, coronary collateralization and ischemic preconditioning, the utility of intraluminal coronary shunts, and technical adequacy of the anastomosis. Nonsternotomy cardiopulmonary bypass methods utilize alternative incisions and "port-access" technology, and may render more rapid patient recovery including: decreased pain, shortened hospital stay, and more rapid return to work. Altered strategies of myocardial protection in a closed chest environment must address: method of cannulation, technique of aortic occlusion, rapidity and maintenance of cardiac arrest, and cardiac de-airing techniques.
Conclusions. Previous obstacles to minimally invasive cardiac operations included limitations in operative exposure, inadequate perfusion technology, and inability to provide myocardial protection. Recent advances in videoscopic visualization and evolving mechanisms of myocardial protection may justify the expanding application of minimally invasive techniques.
Innovations in minimally invasive cardiac operation have arrived like a great tsunami. Operative systems being developed relate either to avoidance of sternotomy, avoidance of cardiopulmonary bypass, or both. Alternative incisions (ie, hemisternotomy, mini-thoracotomy) may render more rapid patient recovery including: decreased discomfort, shortened hospital stay, more rapid return to work, reduction in cost, and better cosmesis. Similarly, omission of extracorporeal perfusion may benefit patients through avoidance of the systemic inflammatory response manifest by fewer bleeding, pulmonary, renal, and neuro-embolic complications. As a counterbalance, surgeons must consider operative safety and learning curve, potential prolonged bypass and arrest times, level of myocardial protection, and long-term result. To date, few comparative data are available.
The question arises: Should myocardial protective strategies be different for minimally invasive operations? The basic tenets of myocardial protection still relate to myocardial oxygen delivery/consumption, reperfusion modification, and endogenous bio-protection. Each parameter must be considered when performing these new operations; however, ideal cardioprotective strategies remain in evolution even for traditional operations. Current minimally invasive coronary artery bypass techniques include both beating heart methods, termed "MIDCAB" or "keyhole" procedures, and "closed chest" cardiopulmonary bypass approaches. In both beating heart and nonsternotomy cardiopulmonary bypass methods, the myocardial protective environment has been altered, compared with traditional sternotomy approaches. Beating heart minimally invasive coronary surgery is different from conventional approaches for the following reasons: potentially ischemic region, possibility of arrhythmias and instability, lessened exposure and access, moving target for anastomosis, possibility of incomplete revascularization, and lessened surgeon and anesthesiologist comfort. Closed-chest, arrested heart minimally invasive cardiac surgery is different from traditional approaches for the following reasons: more instrument conflicts, increased ventricular warming, inferior ventricular visualization, decreased ventricular decompression, greater challenges in cannulation, potentially, longer operative and cross-clamp times, and intracardiac air removal challenging.
Beating heart coronary bypass operation
When considering beating heart coronary bypass operation, potential endothelial injury and myocardial ischemia during proximal arterial occlusion may occur. Previous studies have demonstrated coronary artery endothelium to be more susceptible to vasospasm after ischemia and reperfusion [1]. In addition, electron microscopic studies have demonstrated platelet adhesion and aggregation, endothelial denudation with disruption of the intercellular junctions, subendothelial matrix edema, and vesiculation of the smooth muscle cells in the reperfused vessel wall [2]. Likewise, myocardial stunning is a well-described phenomenon after a transient period of ischemia, which may render the ventricle more susceptible to arrhythmias [3]. The absence of a quiescent ventricle presents special challenges for surgeons undertaking beating heart coronary operation, and special protective strategies are therefore necessary to prevent endothelial or myocardial injury.
Both endogenous and exogenous protective methods must be considered. Myocardial oxygen consumption may be decreased pharmacologically during proximal artery occlusion by utilizing ß-adrenergic antagonists. Use of these agents has demonstrated a reduction in oxygen demand with subsequent reduction in the accumulation of deleterious metabolic substrates responsible for initiating myocardial reperfusion injury. In controlled studies, nontreated ventricles have been shown to impair left ventricular stroke volume and stroke work [4]. In addition, ß-adrenergic antagonists provide negative chronotropy, which facilitates technical aspects of the operation.
There is compelling clinical and experimental evidence that the human heart can be ischemic preconditioned. During preconditioning, a brief ischemic challenge applied to the ventricle before a period of ischemia is protective against future ischemia/reperfusion injury. Ischemic preconditioning has been shown to be associated with beneficial metabolic effects including reduced rate of high-energy phosphate catabolism and deleterious metabolite accumulation during prolonged periods of ischemia, the additive result of which enhances postischemic myocardial function [5].
Preconditioning may be elicited by transient episodes of ischemia, hypoxia, catecholamine release, and myocardial stretch. Similarly, preconditioning can be mimicked pharmacologically with alpha-adrenergic, cholinergic, and adenosine receptor agonists. Orwall and associates have demonstrated that adenosine offers the clinical promise of postbypass myocardial stunning and infarction attenuation [6]. Although the exact salutary mechanism remains unclear, both preconditioning and adenosine may have important clinical roles in beating heart operation.
With the development of better mechanical stabilization devices, however, the pharmacological methods described using ß-blockers and/or adenosine have largely become unnecessary from a technical standpoint. Because their cardioprotective properties are more theoretical than clinical, many surgeons have abandoned these techniques. One can assume that hearts with severe coronary stenoses have been undergoing "ischemic preconditioning" for quite some time. Therefore, it is not surprising that in these individuals, one can occlude coronary arteries in the left system for extended periods without significant left ventricular anterior wall dysfunction on transesophogeal echocardiogram.
The advent of coronary grafting without cardiopulmonary bypass has caused a resurgence in the interest of intraluminal coronary shunts to maintain distal perfusion during anastomosis. In addition to prevention of distal myocardial ischemia, advocates of this technique note technical advantages of reduced visual obstruction from bleeding with prevention of accidental entrapment of the posterior coronary wall. Review of series in which this technique is utilized during off-pump coronary artery bypass report low rates of perioperative infarction (1.5%) and graft patency rates comparable with those obtained by conventional cardiopulmonary bypass techniques [7]. While used regularly by a minority of surgeons, they have not yet achieved widespread popularity. The right coronary artery is less forgiving, however, in that proximal occlusion may result in hypotension and bradycardia. Thus, these protective strategies become more important in right coronary grafts, and the surgeon must be aware of all these techniques.
Critics of beating heart operations suggest decreased graft patency and increased perioperative morbidity. Clearly, the best myocardial protection is excellent acute and chronic graft patency. The importance of the left internal mammary artery (LIMA) to the left anterior descending artery (LAD) graft on survival mandates that the patency rates of minimally invasive direct coronary artery bypass anastomosis must be as good or better than those obtained with traditional methods. Our group has adopted a method of selective angiography of the LIMA graft via the left radial artery catheter site to assure adequate anastamosis before closure of the chest [8]. The technique involves cannulating the LIMA orifice with a 4F internal mammary artery catheter (Cordis Europa, Roden, The Netherlands) and, under flouroscopic visualization, a forceful injection of approximately 8 mL of contrast is introduced. In Elbeery and associate’s series of 50 patients, the IMA orifice was cannulated in 98% of the patients within 15 minutes, and the images obtained were satisfactory to excellent in 94% of cases. Intervention based on angiographic results was required for 4 patients (8%), 2 of whom required sequential LAD anastamoses. There were no perioperative myocardial infarctions or deaths [9].
Other groups are evaluating the utility of intraoperative duplex scanning to assess the adequacy of the anastamosis. Criteria for adequacy are based upon graft flow velocity, anastomotic velocity step-up, and B-mode visualization. Potential drawbacks of this technique include difficulty in scanning a moving vessel, and concerns that graft manipulation could disrupt the anastamosis. Van Son and associates have used transthoracic duplex scanning to assess LIMA graft velocity and have demonstrated it to be a satisfactory method of evaluating graft patency [10].
Close intraoperative patient monitoring and communication with a knowledgeable anesthesiologist is paramount during beating heart coronary operation. Because the heart is not afforded the metabolic advantages of cardiopulmonary bypass, both the surgeon and the anesthesiologist must protect against the hemodynamic parameters associated with increased myocardial oxygen demand (tachycardia, increased wall tension). Finally, careful patient selection must be emphasized. Incomplete revascularization secondary to ungrafted diagonal branches should be avoided, as residual ischemia is a poor long-term protectant of the heart.
Arrested heart minimally invasive cardiac operation
While the "minithoracotomy" approach appears to provide adequate direct visualization of the anterior surface of the heart, off-pump procedures remain limited to the epicardium. Port-access cardiac operation using an occlusive aortic balloon may provide the benefits of minimally invasive operation without sacrificing the advantages of cardiopulmonary bypass cardiac arrest and myocardial preservation. The unique situation of cardiac arrest in the closed chest environment requires altered myocardial protective strategies during both "port-access" coronary artery bypass procedures and all minimally invasive valve operations.
In addition to the small incision, methods of cannulation for minimally invasive operations are somewhat variable depending on the procedure performed. The femoral artery and vein may be utilized with satisfactory cardiopulmonary support. However, the longer, smaller caliber, venous cannula frequently requires the use of a kinetic pump in the venous drainage line to augment return to the heart-lung machine. Likewise, smaller arterial cannulas may require utilization of special guidelines and management parameters to optimize bypass.
In early cases, we utilized variable component cardioplegia systems in an attempt to maintain myocardial quiescence. In these cases, potassium concentration was varied to maintain arrest. However, in the closed chest environment, we found that lower myocardial temperatures were superior, and now use constant potassium component cardioplegia [11]. In the 43 mitral patients in which assisted bypass techniques have been utilized during minimally invasive valve procedures at our institution, no right or left ventricular failure has been noted. Our current preferences utilize either femoral artery or direct aortic arch arterial cannulation. Venous drainage is via a 23F to 25F single-stage, right atrial cannula, and has been quite successful in decompressing the venous system, while providing a mechanism by which to retract the right atrium medially to expose the intraatrial groove [11].
Cardiopulmonary bypass has been effective, allowing adequate cardiac arrest and myocardial protection with a quiescent, bloodless field. Aortic occlusion has been performed by either a percutaneous endovascular intraluminal balloon in the port-access system or transthoracic aortic clamp. The endovascular technology utilizes a triple-lumen catheter with an inflatable balloon at its distal end. Antegrade cardioplegia is delivered through a central lumen, which also acts as an aortic root vent. A second lumen is used as an aortic root pressure monitor, and a third lumen is used for balloon inflation to provide aortic occlusion. The system allows endovascular aortic occlusion, cardioplegia delivery, and left ventricular decompression [12]. Both experimental and clinical evidence have successfully demonstrated the feasibility of port-access coronary bypass grafting and valve procedures [13].
Our group has developed a different approach to aortic occlusion utilizing direct, thoracoscopic visualization and a specially designed transthoracic aortic cross-clamp with a "pincer"-like mechanism (Fig 1). The clamp is inserted via a separate 5-mm transthoracic incision, and takes less than 3 min to insert and apply. The advantages of its application include: (1) direct visualization and selection of clamp application site; (2) avoidance of percutaneous insertion of balloon under fluoroscopic guidance; (3) security of aortic occlusion; and (4) reduced product cost. The primary disadvantage of the transthoracic cross-clamp is the requirement for a standard aortic root cardioplegia vent catheter. We perform this using thoracoscopic visualization and suturing techniques, or under direct vision when possible.
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Deairing of the heart becomes a major concern when one has limited access to the left ventricle when weaning from cardiopulmonary bypass. Moreover, the right coronary artery is susceptible to air introduction with the patient in the mini-thoracotomy position. During the procedure, we utilize continuous, low-flow carbon dioxide insufflation into the chest cavity to displace intracardiac and pulmonary vein air. Carbon dioxide is rapidly absorbed, thereby facilitating air displacement. In addition, before removal of the aortic clamp, we rigorously turn the patient from side to side while aggressively ventilating the lungs to remove residual air. The aortic root cannula allows direct venting of the aorta before and after cross-clamp release. Transesophageal echocardiography is invaluable in monitoring the de-airing process.
Despite our attempts to remove cardiac air, we have observed transient ST elevation in 5 of 43 patients undergoing minimally invasive mitral valve procedures at our institution. In these patients, weaning from cardiopulmonary bypass was postponed until the electrocardiogram normalized. In spite of the significantly longer cross-clamp time to perform minimally invasive mitral valve procedures at our institution, we have observed no cases of significant postoperative left ventricular dysfunction [11].
Conclusions
A less invasive approach to cardiac operation has been propelled by recent advances in instrumentation and technology. Previous obstacles to minimally invasive cardiac operations included limitations in operative exposure, inadequate perfusion technology, and inability to provide adequate myocardial protection. With recent advances in videoscopic visualization, and evolving mechanisms of myocardial protection, beating heart coronary artery bypass operation, and closed chest cardiopulmonary bypass are safe, and may justify expanding applications.
The cardiovascular surgeon in the future will likely have three-dimensional video assistance, new methods of valve insertion and fixation, innovative articulated instruments, and remote tactile feedback devises. The minimally invasive approach must not, however, compromise results when compared with traditional methods. Appropriate patient selection, close transesophageal echocardiographic monitoring, and meticulous attention to detail remain paramount.
References
This article has been cited by other articles:
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  S. Muraki, C. D. Morris, J. M. Budde, R. N. Otto, Z.-Q. Zhao, J. D. Puskas, R. A. Guyton, and J. Vinten-Johansen Preserved myocardial blood flow and oxygen supply-demand balance with active coronary perfusion during simulated off-pump coronary artery bypass grafting J. Thorac. Cardiovasc. Surg., January 1, 2002; 123(1): 53 - 62. [Abstract] [Full Text] [PDF]
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