Systemic Temperature During CPB
Systemic hypothermia has traditionally been used during cardiac surgical procedures to provide a degree of myocardial, cerebral, and somatic protec-tion. In the past few years, however, there has been growing recognition that moderate degrees of systemic hypothermia (ie, 28°C) may not produce additional benefit over mild hypothermia (ie, 34°C) in terms of cerebral or cardiac preservation. As a result, there has been a trend toward the use of mild hypothermia during most cardiac surgical procedures, unless deep hypothermia (ie, <20°C) is needed for circulatory arrest. The single randomized, controlled trial of mild vs moderate hypothermia in CABG patients reported significantly less postoperative AF in the mild hypothermia group. Postoperative AF was identified only retrospectively from chart review, which leaves open the possibility that some arrhythmias were not detected, however. The data from the current trial confirm the pilot data reported by this same group of investigators in a much smaller group of patients. The authors speculated that significant increases in sympathetic nerve activity during rewarming may have been responsible for an increased frequency of postoperative AF in patients who experienced moderate degrees of hypothermia. There have been no studies, however, that address this possibility directly.
In the year 2002, > 20% of all CABG operations were performed using beating-heart techniques without the use of CPB. A variety of beating-heart techniques for bypass surgery have been described, including minimally invasive direct coronary artery bypass with a small anterior thoracotomy incision, OPCAB using a conventional sternotomy incision, and port-access and robotically assisted CABG.> These techniques have been popularized based on the assumption of technically equivalent results coupled with improved short-term outcomes.
Because at least some portion of postoperative AF has been thought to be due to intraoperative atrial ischemia, there was some reason to believe that the beating-heart approach to CABG (ie, without arrest of the heart) would result in less postoperative AF. Several authors have reported a reduced incidence of postoperative AF in patients undergoing beating-heart CABG. But other authors have reported no difference. In the current evidence review, two randomized, controlled trials and one large-scale concurrent cohort study were identified. Ascione et al reported a significantly lower rate of postoperative AF in the OPCAB group than in the on-pump CABG group. In contrast, though, Van Dijk et al reported no difference in AF frequency for the two surgical approaches in a randomized, prospective study of similar size. The large-scale concurrent cohort study reported by Hernandez et al found a small but statistically significant difference in the frequency of postoperative AF between on-pump CABG patients and OPCAB patients favoring the OPCAB group.
In conventional CABG operations with CPB and an arrested heart, the CABGs are created after cardioplegic arrest. As an alternative, intermittent aortic cross-clamping can be used to arrest the heart for short periods of time to allow for a quiet operative field during the creation of distal anastomoses. Some percentage of postoperative atrial arrhythmias are thought to be due to inadequate myocardial preservation during cardioplegic arrest. Several investigators have studied the effects of various methods of myocardial protection on the frequency of postoperative atrial arrhythmias, but no convincing benefit for any one particular protection strategy has been identified in retrospective reports, either in terms of postoperative AF or other conduction ab-normalities. In four of the five identified randomized, controlled trials related to cardioplegia or myocardial protection strategy, Butler et al identified no difference between cold potassium cardioplegia and intermittent aortic cross-clamping, and Fontan et al found no benefit to any particular cardioplegic technique. Hynninen et al found no benefit to insulin-enhanced cardioplegia, and Wand-schneider et al found no benefit to blood cardioplegia vs crystalloid cardioplegia. In a small study, Pehkonen et al found a lower incidence of postoperative AF in the group that received cold crystalloid cardioplegia.
The (3-adrenergic receptor antagonists have been shown in most randomized, controlled studies to reduce the incidence of postoperative AF in cardiac surgery patients. It has been postulated that increased sympathetic tone may be one mechanism that is responsible for AF. There has been interest in the use of TEA as an adjunct to conventional anesthesia because it has been shown to decrease both heart rate and BP variability, suggesting effective sympathetic blockade. Nonrandomized studies of TEA have produced conflicting results. Two randomized, controlled studies have also reported conflicting results. Jideus et al reported no difference in the rate of postoperative AF, but Scott et al reported a significant reduction in the rate of postoperative AF with the use of TEA.
In most CABG operations, the pericardium is usually opened longitudinally in its anterior aspect. This opening provides unobstructed access to the underlying heart and proximal great vessels. The pericardium is usually left open, although some surgeons choose to close a portion of the pericardium. A second, or auxiliary, incision in the posterior pericardium has been used to facilitate the drainage of blood into the chest cavity where it can be evacuated with chest tubes. This technique has been shown in nonrandomized trials to reduce the incidence of both postoperative pericardial effusion and postoperative supraventricular tachycardia. In contrast, Asimakopoulos et al found no association between the use of posterior pericardiotomy and the incidence of postoperative AF. In the single randomized, controlled trial identified in the evidence review, Kuralay et al reported a significant reduction in postoperative AF for patients who underwent posterior pericardiotomy. Although this study reports a benefit, it is important to keep in mind that there are multiple other reasons why this effect could be a surrogate for other (and unidentified) intraoperative factors.
Perioperative GIK Solution Infusion
Metabolic substrate enhancement with glucose or other energy sources during periods of myocardial ischemia and reperfusion has been proposed as one strategy to limit myocardial necrosis. As early as 1965, Sodi-Pollaris et al noted that GIK solutions administered to patients experiencing acute myocardial infarction limited the subsequent ECG changes. In addition, animal models of myocardial ischemia/ reperfusion have indicated that GIK solution infusion limited tissue necrosis, resulted in less myocardial acidosis, reduced the frequency of ventricular arrhythmias, and resulted in fewer wall motion abnormalities. Two randomized, controlled trials of GIK solution infusion in patients undergoing heart surgery were identified in the evidence review. Wistbacka et al reported that in patients undergoing CABG surgery there was no difference in the rate of postoperative AF associated with the use of GIK solutions and that the use of GIK solutions was associated with serious adverse effects such as hypoglycemia. In contrast, Lazar et al found in unstable patients with angina who were undergoing CABG that GIK infusion enhanced myocardial performance and also resulted in less postoperative AF.
Heparin-Coated CPB Circuits
CPB has long been associated with a variety of deleterious systemic inflammatory effects mediated by a generalized systemic inflammatory response. Heparin-coated CPB circuits have been developed to reduce the systemic inflammatory response associated with CPB, as measured by less complement activation, less leukocyte activation, a reduction in the release of cytokines, and the need for less systemic anticoagulation therapy. Proponents have postulated that the use of heparin-coated CPB circuits would result in less postoperative bleeding and fewer thromboembolic complications, but the reported results have been mixed. Two randomized, controlled trials of the use of heparin-coated circuits were identified. The data in these studies that addressed the impact on bleeding and neurologic injury were mixed, but both of these reports documented at least some evidence for a reduction in the rate of postoperative AF with the use of a heparin-coated circuit. Ovrum et al reported significantly less postoperative AF in the heparin-coated circuit group (a reduction of approximately 50%). Sven-marker et al studied different types of heparin-coated circuits and found that one type was associated with less postoperative AF. Both groups of investigators questioned whether their observation of less postoperative AF in the group of patients in whom heparin-coated circuit had been used was truly due to the type of circuit or to other” unmeasured factors.
It is important to keep in mind that our recommendations are based on a relatively small number of studies. In the case of posterior pericardiotomy and the use of mild hypothermia” our recommendations are based on only single randomized controlled studies. We understand that” when making clinical decisions for the individual patient” the reader must place our recommendations in the proper context.
AF remains a significant complication following cardiac surgery. This arrhythmia is associated with an increased hospital length of stay” increased costs” and an increased risk for thromboembolic complications. On the basis of the findings of this and the other reports in this series” we suggest that a multifactorial approach” involving appropriate prophylaxis and treatment for this arrhythmia” will best serve our cardiac surgery patients. Below are the recommendations for the management of intraoperative interventions. A summary of these clinical recommendations and grades of evidence is presented in Table 2.
1. We recommend the use of mild, rather than moderate, hypothermia to reduce the
frequency of postoperative AF (strength of recommendation” A; evidence grade” fair; net benefit” substantial).
2. Posterior pericardiotomy may be a useful adjunct to help reduce the incidence of postoperative atrial arrhythmias; however, this recommendation is based on a single, small-scale randomized, controlled trial. Posterior pericardiotomy is not currently standard of care and is not widely used as an adjunct to reduce postoperative AF
(strength of recommendation” B; evidence grade” fair; net benefit” intermediate).
3. OPCAB cannot be recommended to decrease postoperative AF because of conflicting results reported from randomized, controlled trials or large-scale concurrent cohort studies (strength of recommendation” I; evidence grade” fair; net benefit” conflicting).
4. No specific recommendations can be made regarding which type of cardioplegia (or intermittent aortic cross-clamping) best reduces the incidence of postoperative AF
(strength of recommendation” I; evidence grade” good; net benefit” none).
5. No recommendation can be made regarding the use of TEA as an adjunct to conventional general anesthesia to prevent postoperative AF after cardiac surgery
(strength of recommendation” I; evidence grade” fair; net benefit” conflicting).
6. We cannot recommend GIK solution infusion to prevent postoperative AF because of conflicting results from the identified randomized, controlled trials (strength of recommendation” I; evidence grade” fair; net benefit” conflicting)
7. We recommend the use of heparin-coated circuits to reduce the rate of postoperative AF (strength of recommendation” B; evidence grade” fair; net benefit” intermediate).
Table 2—Summary of Recommendations for Intraoperative Interventions
|Patients, Total No.||Does Therapy Reduce Postoperative AF?||Strength of Recommendation||
|Net Benefit to Patient|
|Systemic temperature during CPB*||1||65||Yes||A||Fair||Substantial|
|Beating heart CABG||2||8,348||Inconclusive||I||Fair||Conflicting|
|Myocardial protection technique||5||1,029||No||D||Good||None|
|Thoracic epidural anesthesia||2||549||Inconclusive||I||Fair||Conflicting|
|GIK solution infusion||2||62||Inconclusive||I||Fair||Conflicting|
|Heparin-coated CPB circuit||2||517||Yes||B||Fair||Intermediate|