doi: 10.1161/CIRCULATIONAHA.105.001644
(Circulation. 2006;114:I-331 – I-338.)
© 2006 American Heart Association, Inc.
Myocardial Protection and Vascular Biology |
Is Blood Superior to Crystalloid Cardioplegia?
A Meta-Analysis of Randomized Clinical Trials
From the Division of Cardiovascular Surgery (V.G., J.O., A.A.A., S.E.F.), Sunnybrook and Women’s College Health Sciences Centre, University of Toronto, Toronto, Canada; Division of Cardiac Surgery (R.W.), Toronto General Hospital, Toronto, Canada.
Correspondence to Veena Guru, Sunnybrook and Women’s College Health Sciences Centre, 2075 Bayview Avenue, H-410, Toronto, Ontario M4N 3M5 Canada. E-mail veena.guru{at}utoronto.ca
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Abstract |
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Background— Many small, randomized, controlled trials have evaluated the effectiveness of blood as compared with crystalloid cardioplegia for myocardial protection during cardiac surgery. Blood cardioplegia provides a closer approximation to normal physiology, which may translate into measurable clinical benefits. This meta-analysis describes the effectiveness of blood cardioplegia in lowering adverse postoperative outcomes.
Methods and Results— MEDLINE, EMBASE, and the Cochrane registry of controlled trials were searched for clinical trials. The search was restricted to peer-reviewed English language publications of randomized controlled trials that primarily compared blood and crystalloid cardioplegia in adult patients. Each trial was blindly assessed and abstracted by 2 reviewers. The primary outcomes were: low output syndrome (LOS), myocardial infarction (MI), and death. Surrogate outcomes included postoperative creatinine kinase MB (CKMB) increase. Random effects summary odds ratio (OR) for binary outcomes, and weighted mean difference for continuous outcomes were calculated. A total of 34 trials were included. The majority of trials were conducted in patients undergoing elective CABG surgery (n=18). The incidence of LOS was decreased significantly with blood cardioplegia (OR, 0.54; 95% confidence interval [CI], 0.34 to 0.84; P=0.006; 879 patients, 10 trials). The incidence of MI and death were similar between treatment groups (MI: OR, 0.78; 95% CI, 0.54 to 1.13; 4316 patients, 23 trials) (death: OR, 0.80; 95% CI, 0.46 to 1.40; 4022 patients, 17 trials). CKMB release after surgery at 24 hours was reduced with blood cardioplegia (5.9 U/L; 95% CI, 1.6 to 10.2; P=0.007; 821 patients, 7 trials).
Conclusions— Blood cardioplegia provides superior myocardial protection as compared with crystalloid cardioplegia, including lower rates of LOS, and early CKMB increase, whereas the incidence of myocardial infarction and death are similar.
Key Words: cardioplegia • meta-analysis
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Introduction |
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Cardioplegia is an important strategy to facilitate cardiac surgery while limiting intraoperative myocardial injury. Initially cardioplegia was introduced as an agent to allow for hypothermic hyperkalemic arrest.1,2 Various additives were explored to try to optimize the myocardium during this time of ischemia.3,4 Blood was then found to be the most logical vehicle for delivery of potassium cardioplegia.5,6 The optimal cardioplegic temperature, timing, and routes of delivery were further explored.7–9 Although cardioplegia is a generally accepted to be a mandatory tool for myocardial protection during on-pump cardiac surgery, there is still controversy regarding various aspects including its composition, temperature, and mode of delivery.
Blood as opposed to crystalloid cardioplegia could potentially improve postoperative cardiac outcomes, because it more closely approximates normal physiology caused in part by its oxygen carrying capacity and less associated hemodilution. Some clinicians have not adopted blood cardioplegia because the evidence to date has frequently focused on surrogate endpoints and has not shown clear clinical benefit. Blood cardioplegia also can impair visualization during construction of coronary anastomoses, and added costs may be necessary for the blood cardioplegic delivery systems compared with crystalloid cardioplegia.
This systematic review attempts to determine whether the use of blood cardioplegia is more beneficial compared with crystalloid cardioplegia in terms of clinical cardiac outcomes and cardiac markers of ischemia.
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Methods |
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MEDLINE was searched (1966 to July 2005) using search terms "CABG" or "valve" and "cardioplegia." This was repeated for the following citation indices: EMBASE, PubMed, and Cochrane registry of controlled trials. The search was restricted to peer-reviewed English language publications of human subjects. Reports that only appeared as abstracts were ineligible. The review included only randomized controlled trials that primarily compared blood and crystalloid cardioplegia in adult patients. Each trial was blindly assessed and the data abstracted by 2 reviewers.10 Details of trial design and outcome definitions were clarified by contacting authors when required.
The primary outcomes of interest were: low output syndrome (LOS) (which includes inotropic and/or intra-aortic balloon pump support), myocardial infarction (MI) (which includes both ECG-defined and/or enzyme-defined MI), and in-hospital death. Surrogate outcomes analyzed were creatinine kinase MB (CKMB) increase at 7, 24, and 48 hours after surgery. Random effects summary odds ratio (OR) for binary outcomes and weighted mean difference (WMD) for continuous outcomes and 95% confidence intervals (CI) were calculated using Review Manager version 4.2. A meta-regression of temperature (cold versus warm/tepid) and delivery route (antegrade versus retrograde) was also conducted. The statistical analyses were completed with SAS software (version 8.2; SAS Institute Inc, Cary, NC). Statistical significance was defined as P<0.01.
Statement of Responsibility
The authors had full access to the data and take full responsibility for their integrity. The authors have read and agree to the manuscript as written.
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Results |
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The abstracts of 269 studies were identified using a MEDLINE search strategy limited to randomized controlled trials (CABG and cardioplegia: n=258, valve and cardioplegia: n=51); 45 studies were found to have trial arms that compared blood and crystalloid cardioplegia. Eleven studies were excluded because the data were included in >1 publication or the outcomes of interest were not reported. The characteristics of the 34 trials are listed in Table 1
.11–45 We were able to contact 20 authors to get more details regarding the trial methodology and results (Table 1). For a majority of trials there was little description of the exact randomization protocol, and the blinding of the intervention to patients and/or outcome assessors. The definitions used for LOS and MI in the individual studies are provided in Table 2.
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Altogether 2582 patients received blood cardioplegia and 2462 patients received crystalloid cardioplegia. Most of the studies were small; only 4 trials included >100 patients per arm, of which only 2 had >1000 patients in total. The majority of trials were conducted in the setting of elective CABG surgery (n=18). Two trials involved patients undergoing valve procedures. Fourteen trials involved CABG patients with a range of clinical urgency. The trials included blood cardioplegia across a range of temperatures from cold to warm, whereas crystalloid cardioplegia was delivered cold (Table 1
). In 11 trials at least 1 arm had both antegrade and retrograde delivery of cardioplegia (Table 1). The mean pump times across trials ranged from 49 to 146 minutes, with aortic cross-clamp times ranging from 32 to 102 minutes (Table 1). Most trials assessed at least 1 co-intervention including temperature (warm versus cold), timing (intermittent versus continuous), and/or route of delivery (antegrade, antegrade/retrograde).
The incidence of low output syndrome after surgery was reported in 10 trials involving 879 subjects in total. Low output syndrome was reduced in patients who received blood cardioplegia (OR, 0.54; 95% CI, 0.34 to 0.84; P=0.006). The individual and aggregated trial results for LOS are presented in Figure 1. In the individual trials, blood cardioplegia was neutral in 1, harmful in 1, and beneficial in 8 relative to crystalloid cardioplegia, with no evidence of heterogeneity.
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The incidence of myocardial infarction was similar in 2206 patients receiving blood cardioplegia (OR, 0.78; 95% CI, 0.54 to 1.13; P=0.19, 23 studies) as compared with 2111 patients receiving crystalloid cardioplegia (Figure 2). The incidence of death was similar in 2044 patients receiving blood cardioplegia (OR, 0.80; 95% CI, 0.46 to 1.40; P=0.44; 17 studies) as compared with 1978 patients receiving crystalloid cardioplegia (Figure 3). Figure 4 shows that funnel plot analysis of trials for the outcome of MI (most measured outcome in trials resulting in the largest patient comparison) demonstrates no evidence of a publication bias.
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The weighted mean difference of CKMB release was significantly reduced with blood cardioplegia at 7 and 24 hours after surgery (see Table 3 for complete details).
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A meta-regression demonstrated no relationship between the outcomes of myocardial infarction and death and temperature, and/or route of delivery.
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Discussion |
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We performed a meta-analysis of clinical trials of blood and crystalloid cardioplegia in an attempt to identify whether there was evidence in the aggregated data supporting one form of myocardial protection on hard clinical outcomes. Most of the studies we reviewed were small, focused on surrogate or proxy measures, and lacked sufficient power to identify differences in clinical endpoints. Of the 34 individual studies included in this analysis, only 1 demonstrated a significant difference in low output syndrome,15 without any of the individual studies demonstrating differences in the endpoints of myocardial infarction or death. In the pooled data involving 10 trials and almost 900 patients, there emerged a fairly clear message that low output syndrome was reduced a moderate amount in patients who received blood cardioplegia. In the aggregated data from &4000 patients, the incidence of MI, and death did not differ statistically between patients receiving blood or crystalloid cardioplegia. Although the treatment estimates from the meta-analysis for MI (OR, 0.78) and death (OR, 0.80) favored blood cardioplegia, it is likely that the cumulative number of myocardial infarctions (145/4317, 3.4%) and deaths (54/4022, 1.3%) were insufficient unless the treatment effects of blood cardioplegia were very large. Large trials are required to test for low frequency events.
The qualitative limitations of this meta-analysis includes the fact that the detailed description of the randomization techniques were not provided for all trials, the blinding of patients as well of outcome assessors were not always mentioned, and the presence of co-interventions made it difficult to analyze the primary intervention of interest. Another important limitation, for which we have not adjusted, is that different composition of blood and crystalloid cardioplegia solutions were used by different investigators. Furthermore, the studies were uniformly single center investigations.
The observations in our study may reflect the use of 2 cardioplegic solutions by a large number of surgeons with a variety of surgical conditions, but this study did not adequately evaluate any particular blood cardioplegic formulation, temperature of cardioplegia, or route of administration. In fact previous studies suggest that inadequate blood cardioplegic delivery, particularly at warm temperatures may induce severe cardiac injury, and subsequent low output syndrome.46 Therefore, the beneficial influence of adequate blood cardioplegic protection may have been offset by incomplete blood cardioplegic protection.47 Cold crystalloid cardioplegia offered the benefit of uniform and consistent cooling and therefore more uniform beneficial results would be anticipated with this technique.48 However, despite uniform cooling, cold crystalloid cardioplegia was associated with delayed recovery of myocardial metabolism and ventricular function necessitating the use of the balloon pump and/or inotropes.47
Are the results generalizable to the real world? For one, patients were recruited to these investigations over a span of 25 years; multiple changes have occurred in the practice of cardiac surgery during this time. Patient demographics have also changed substantially, with greatly increased proportions of patients with advanced age, diabetes, and female gender presenting for urgent surgery.49 A wide range of outcome rates were observed across trials, which may suggest varying cardiac surgical experience representative of the real world. The most recent of the individual trials, and the largest, did not favor blood cardioplegia.45 Almost all of the studies were performed in patients undergoing coronary bypass surgery. Strictly speaking, we cannot extrapolate any benefits seen with CABG surgery to patients undergoing other cardiac operations.
In general, the trials included low-risk patients. Trials that focused on higher-risk patients may have been more likely to show differences between groups. In fact, a second study from the University of Toronto limited to subjects with unstable angina15 revealed significant differences in low output syndrome, whereas only differences in surrogate outcomes were observed in an earlier trial performed in elective patients.14 We expected that differences between groups would be more evident for studies that demonstrated longer cross-clamp times. We found there was no relationship between the average cross-clamp time and outcome.
The principal findings from this analysis were a reduction in low output syndrome and decrease in CKMB release with blood cardioplegia without a significant change in the incidence of MI or in-hospital death. Does this represent a clinically important benefit and offset the disadvantages of blood cardioplegia such as the increased costs or impaired visualization? For one, a reduction in low output syndrome may translate into a shortened length of postoperative hospital stay. Second, earlier studies from this institution emphasized that nonfatal perioperative cardiac events (LOS, MI) influenced late mortality.9 Perioperative myocardial protection is therefore crucial, not only for early recovery but also for late survival. On balance, and despite the limitations of this investigation, the evidence supports the use of blood rather than crystalloid cardioplegia for coronary surgery.
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Acknowledgments |
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Sources of Funding
V.G. was supported by a salary fellowship for the Canadian Institutes of Health Research (CIHR). S.E.F. is a Professor, Department of Surgery and Head Division of Cardiovascular Surgery, Sunnybrook and Women’s College Health Sciences Centre.
Disclosures
None.
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Footnotes |
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Presented at the American Heart Association Scientific Sessions, Dallas, Tex, November 13–16, 2005.
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