Intermittent Vs Continuous Cardioplegia 13 Minutes
Summary
Much controversy persists regarding the optimal techniques for myocardial protection during heart surgery. Numerous studies have compared warm cardioplegia with cold cardioplegia for myocardial preservation, but the outcomes were inconclusive. The aim of this meta-analysis of randomised controlled trials (RCTs) was to compare the beneficial and harmful effects of warm and cold cardioplegia during heart surgery. Electronic databases and manual bibliographical searches were conducted. A meta-analysis of all RCTs comparing warm cardioplegia to cold cardioplegia perfusion during cardiac surgery was performed. Data for clinical events (in-hospital death, myocardial infarction (MI), low output syndrome, postoperative use of intra-aortic balloon pump, stroke and atrial fibrillation), postoperative cardiac index, postoperative creatine kinase-MB (CK-MB) and cardiac troponin release were extracted, and we summarised the combined results of the data of the RCTs as relative risk (RR), with 95% confidence intervals. A total of 41 RCTs including 5879 patients were assessed in this study. We found that there was no statistical difference between patients receiving warm cardioplegia and cold cardioplegia in the incidences of clinical events. Warm cardioplegia was associated with improved postoperative cardiac index. CK-MB and cardiac troponin concentrations after surgery were significantly lower in the warm group as compared with the cold group. Using warm cardioplegia for myocardial protection during heart surgery resulted in similar incidences of clinical events, significant improvement in postoperative cardiac index and reduction in postoperative enzyme release as compared with cold cardioplegia.
1 Introduction
Perioperative myocardial damage is one of the most common causes of morbidity and mortality after heart surgery. The improvement of technique of myocardial preservation has contributed greatly to significant advances in cardiac surgery. However, serious questions remain regarding use of warm versus cold cardioplegia, blood versus crystalloid cardioplegia, antegrade versus retrograde delivery and intermittent versus continuous perfusion.
The debate over the optimal temperature of cardioplegia during cardiac surgery has been one of the most important aspects of myocardial protection. Early cardioplegic techniques used cold crystalloid solutions to initiate and maintain cardiac arrest during heart surgery, and it remained as a cornerstone of cardiac surgical practice since its introduction in the early 1950s. Although it could lower myocardial oxygen demands and the risk of ischaemic damage, cold cardioplegia might inhibit myocardial enzymes and result in the delay in metabolic and functional cardiac recovery after surgery. In the hope of maximising intra-operative myocardial protection, warm blood cardioplegia was first introduced in 1970s [1], and the technique of warm induction followed by cold cardioplegia or terminal warm cardioplegia reperfusion (hot-shot) was clinically applied and found to be effective for myocardial protection [2,3]. Thereafter, continuous and intermittent perfusions of warm blood cardioplegia were introduced in 1980s and proved to provide excellent myocardial protection during heart surgery.
Numerous randomised controlled trials (RCTs) have been conducted to compare warm cardioplegia with cold cardioplegia for myocardial protection, but the outcomes of these trials remained inconclusive. The objective of this study was to systematically review RCTs in which warm cardioplegia was compared with cold cardioplegia for heart surgery.
2 Methods
2.1 Identification and selection of studies
Relevant studies were identified and selected by searching the databases, Medline (1966–March 2009), Embase (1980–March 2009), Cochrane controlled trials register (Cochrane Library Issue 1, 2009) and PUBMED (updated to March 2009) under the search words 'cardioplegia' as well as 'randomised controlled trial'. We scanned bibliographies in relevant articles and conference proceedings. Search strategy was restricted to include articles in English language only.
The following selection criteria were applied: (1) RCT; (2) adult patient; (3) trials comparing warm cardioplegia (including lukewarm cardioplegia) versus cold cardioplegia as both initial induction and reperfusion solutions in patients receiving cardiac surgery; (4) the trials should report at least one of the outcomes we needed; (5) a given patient population was used only once; if the same population appeared in other publications, the article that provided the most complete data was selected; (6) the trials in which warm cardioplegia was only applied in induction stage or used as terminal warm reperfusion (hot-shot) were not included; and (7) any trial in which participants underwent heart transplantation procedures was excluded.
2.2 Data extraction
Data were independently abstracted from each study with a predesigned review form, and disagreement was resolved by consensus. We extracted data on study characteristics, patient clinical characteristics and demographics; primary outcomes including all causes in-hospital death, perioperative myocardial infarction (MI, proven by electrocardiogram (ECG) and/or enzyme); additional outcomes including low output syndrome (LOS), postoperative use of intra-aortic balloon pump (IABP), stroke, atrial fibrillation (AF), cardiac index and concentrations of CK-MB and cardiac troponin (cTn).
2.3 Quality of methodology
The quality of each fully published trial was assessed by means of Jadad score (Table 1 ) [4]. Any disagreement was resolved by consensus. The overall quality score was based on the number of criteria met (score range, 2–10).
Table 1
Methodologic quality assessment (Jadad score).
2.4 Statistical analyses
Data analysis was performed using the random-effects mode with Review Manager Software (RevMan 5, Cochrane Collaboration, Oxfordshire, UK). We tested heterogeneity between trials by using the Cochrane chi-squared and I 2 tests, with p ≤ 0.1 or I 2 ≥ 50% indicating significant heterogeneity [5]. For all dichotomous data (death, MI, LOS, IABP, stroke and AF), relative risk (RR) and 95% confidence intervals (CIs) were calculated for each independent study and for the summary statistic, with values of ≪1 favouring warm cardioplegia. The relative risk for each clinical event was considered as significant if p ≤ 0.05 (two sided). The weighted mean difference (WMD) and 95% CIs were calculated for continuous data (postoperative cardiac index, cTn and CK-MB concentrations). Meta-regression analysis was done by using the statistical software package Stata/SE 8.0 (Stata Corp, College Station, TX, USA). We performed sensitivity analyses, subgroup analyses and meta-regression to explore important clinical differences among trials that might affect our results. Publication bias was estimated with the weighted regression test of Egger.
3 Results
3.1 Description of the selected studies
Using the search strategy, we identified 57 RCTs comparing warm cardioplegia with cold cardioplegia as both induction and reperfusion solution for myocardial preservation during heart surgery. As many as 16 studies were further excluded: seven reported no result which we needed and nine because they were preliminary reports or subgroup analyses. A total of 41 trials completely fulfilled the criteria for consideration in the meta-analysis [6–46] (Table 2 ). All of the studies included were published as peer-reviewed articles.
Table 2
Baseline characteristics of trials included in meta-analysis.
The meta-analysis involved 5879 patients in total: as many as 2944 patients were randomised to the warm cardioplegia group and 2935 to the cold cardioplegia group (cold blood: 2007, cold crystalloid: 928). Most of the studies were conducted in the setting of coronary artery bypass grafting (CABG) surgery, and seven trials were based on isolated valve or combined CABG/valve surgery [9,13,17,20,21,38]. The majority of the studies in our analysis were small, and two trials included more than 1000 patients in total [11,12]. Four trials only stated that they compared warm cardioplegia with cold cardioplegia, but did not state the exact temperature of one or both arms [7,14,27,33]. Five trials reported the data of lukewarm cardioplegia group (28–30 °C) [15,18,31,37,44], and the rest of the studies compared warm cardioplegia (32–37 °C) with cold cardioplegia (4–15 °C). A total of 19 trials have reported either complete or partial use of internal mammary artery (IMA) grafting [8,10,11,14–16,19,26,29,32–37,39,41,44,46]. In 15 studies, proximal vein grafts were implanted with side-biting clamp [11,12,22–24,26,27,29,35,36,39,42–44,46]. Publication year of included trials ranged from 1992 to 2005, and the route and timing of delivery varied among those studies.
3.2 Trial quality
Methods of randomisation and allocation concealment were unclear or inadequate for most trials, and the blinding of patients as well as observers was not always mentioned. Follow-up was completed in all the trials. Few studies reported withdrawals and drop-outs. Most of the trials reported the efficacy of randomisation by pretreatment variables in tabular form. The total Jadad scores ranged from 4 to 7.
3.3 Outcomes
In-hospital death was reported in 18 trials including 4619 patients. In the warm cardioplegia group (2307 patients), 38 patients died compared with 49 patients in the cold cardioplegia group (2312 patients). Our analysis showed that there was no significant difference in all-cause mortality between patients receiving warm cardioplegia and cold cardioplegia (RR = 0.75, 95% CI 0.49–1.15, p = 0.19) (Fig. 1 ).
Fig. 1
Meta-analysis of randomised clinical trials comparing warm cardioplegia to cold cardioplegia: in-hospital death.
As many as 24 studies with 5029 patients in total reported the results of MI. The incidence of MI was similar in patients receiving warm blood cardioplegia as compared with patients receiving cold cardioplegia (RR = 0.92, 95% CI 0.74–1.16, p = 0.49) (Fig. 2 ).
Fig. 2
Meta-analysis of randomised clinical trials comparing warm cardioplegia to cold cardioplegia: MI.
LOS was reported in 10 trials involving 2389 patients. Pooled analysis of these trials showed that there was no statistical difference in the incidence of LOS between the two groups (RR = 0.83, 95% CI: 0.52–1.32, p = 0.42) (Fig. 3 ). Because the definition of LOS varied among the trials included in our analysis, we conducted another analysis based on the data of postoperative IABP support, which included 14 trials with 4418 patients. We found that the incidences of IABP usage were also comparable in these two groups (RR = 1.19, 95% CI: 0.79–1.78, p = 0.40) (Fig. 4 ).
Fig. 3
Meta-analysis of randomised clinical trials comparing warm cardioplegia to cold cardioplegia: LOS.
Fig. 4
Meta-analysis of randomised clinical trials comparing warm cardioplegia to cold cardioplegia: IABP.
There was also no significant difference in the incidence of stroke between warm cardioplegia and cold cardioplegia (RR = 1.45, 95% CI: 0.90–2.33, p = 0.12) (Fig. 5 ). However, our subgroup analysis showed that more strokes occurred in patients receiving warm blood cardioplegia than those receiving cold crystalloid cardioplegia (RR = 2.44, 95% CI 1.07–5.59, p = 0.03), but the risk of stroke was similar when comparing warm blood cardioplegia with cold blood cardioplegia (RR = 1.13, 95% CI 0.63–2.01, p = 0.68). Compared with cold cardioplegia, warm cardioplegia perfusion did not increase the incidence of postoperative atrial fibrillation (RR = 1.07, 95% CI: 0.84–1.36, p = 0.57) (Fig. 6 ).
Fig. 5
Meta-analysis of randomised clinical trials comparing warm cardioplegia to cold cardioplegia: stroke.
Fig. 6
Meta-analysis of randomised clinical trials comparing warm cardioplegia to cold cardioplegia: AF.
Warm cardioplegia was associated with significantly improved postoperative cardiac index (WMD = 0.28, 95% CI: 0.26–0.31, p ≪ 0.00001). The cardiac troponin concentrations at day 0, day 1 and peak value after heart surgery were lower in warm cardioplegia than in cold cardioplegia (WMD = −0.61, 95% CI: −1.04 to −0.17, p = 0.006; WMD = −0.57, 95% CI: −1.26 to −0.12, p = 0.11; WMD = −1.45, 95% CI: −2.47 to −0.42, p = 0.006, respectively). The peak value of CK-MB release after surgery was also significantly reduced with warm cardioplegia (WMD = −8.03, 95% CI: −13.08 to −2.97, p = 0.002).
No statistical heterogeneity was evident among the studies, and we recorded no evidence of publication bias by the Egger test.
3.4 Sensitivity analysis and subgroup analysis
A sensitivity analysis was made, only including the studies which clearly reported the temperatures of cardioplegia. The four trials which did not state the exact temperature of cardioplegia were excluded. We found the outcome of our analysis did not change significantly with these studies excluded.
We conducted another sensitivity analysis only including the studies using warm cardioplegia, excluding the five studies which included patients receiving lukewarm cardioplegia. The outcomes of our analysis were not significantly altered by exclusion of those studies either.
Although all the warm groups used blood cardioplegia, two different cardioplegia solutions were used by the cold cardioplegia group: cold blood cardioplegia and cold crystalloid cardioplegia. We made a subgroup analysis according to these two different solutions used in the control group, and we found no relevant differences when cold blood trials and cold crystalloid trials were independently analysed, except in analysis for postoperative stroke, which showed significantly higher incidence of stroke in warm group as compared with the cold crystalloid-only subgroup, but no difference existed between warm blood cardioplegia and cold blood cardioplegia, as described above.
3.5 Meta-regression
Meta-regression analysis did not disclose any significant interactions between several variables of interest (publication year, Jadad score, type of surgical procedure, IMA grafting, proximal vein grafts implantation with side-biting clamp, the route (antegrade/retrograde) and timing (continuous/intermittent) of delivery of cardioplegia) and the main outcomes of our analyses.
4 Discussion
The aim of myocardial protection during heart surgery was to preserve myocardial function while providing a bloodless and motionless operating field. In the early stage, myocardial protection was obtained by decreasing myocardial oxygen demand as a consequence of hypothermia. Although intermittent cold cardioplegia perfusion is associated with excellent clinical outcomes in cardiac surgery, this standard technique results in myocardial hypothermia, ischaemia and a delay in the recovery of postoperative myocardial metabolism and function [47]. In addition, it was demonstrated that with electromechanical arrest alone, one could reduce the oxygen requirements of the heart by nearly 90%, with only a slight further decrease attributable to lowering myocardial temperature to 11 °C [48]. Based on these considerations, warm blood cardioplegia perfusion during heart surgery procedure was clinically applied since 1980s. It was once believed that the use of warm blood cardioplegia could improve metabolic and functional recovery, and this technique has been adopted by many cardiac surgeons since the early 1990s [49,50]. However, after being used for 30 years, there is still much controversy regarding whether warm cardioplegia is superior to cold.
In this meta-analysis, we identified 41 RCTs that compared warm cardioplegia with cold cardioplegia for myocardial protection in patients undergoing heart surgery. The risk of in-hospital death was similar in both groups. Although several large retrospective clinical trials reported that patients receiving warm blood cardioplegia sustained significantly fewer MIs after surgery [51,52], our pooled analysis of RCTs demonstrated that the incidence of MI was comparable between these two groups. LOS caused by cardiac damage from inadequate myocardial preservation is a strong predictor of both perioperative and late death, and it could also prolong hospital stay and cost. The Warm Heart Trial reported that fewer incidences of LOS occurred in the warm cardioplegia group [12]; however, our pooled results showed that there was no statistical difference between these two group in LOS. Previous meta-analysis comparing blood with crystalloid cardioplegia indicated that LOS was significantly reduced in patients who received blood cardioplegia [53]. Interestingly, our subgroup analysis comparing warm blood cardioplegia with cold crystalloid cardioplegia indicated that the incidence of LOS in these two groups was similar. To avoid the bias which can be caused by the different definitions of LOS, we pooled the data of postoperative IABP usage which involved almost 4500 patients, and we still found there was no difference in the use of IABP. Cerebral damage is one of the most serious complications following cardiac surgery. The Emory Trial confirmed similar efficacy of warm and cold cardioplegia for myocardial protection but extremely high rate of postoperative neurological events in the warm group [11]. However, it may relate to significantly higher blood glucose level in the warm group, which was caused by the blood cardioplegic solution they used. Besides, normothermic bypass, which was applied in their trial, was not necessary to administer warm blood cardioplegia, and the use of a partially occluding clamp technique may also exacerbate brain injury. In this meta-analysis, warm cardioplegia perfusion did not result in higher rate of stroke, except in the subgroup analysis comparing warm blood with crystalloid cardioplegia which favored the latter. However, only three trials were included for this subgroup analysis, and the result may be also greatly affected by the Emory Trial, as mentioned above. The incidence of AF was comparable in these two groups. The pooled results showed that warm cardioplegia was associated with improved postoperative cardiac index.
Because of the high sensitivity and specificity in revealing cardiac injury following ischaemic insult, postoperative cTn and CK-MB have been considered to be ideal markers to evaluate the efficiency of the technique for myocardial protection during heart surgery. In our analysis, the concentrations of both cTn and CK-MB were significantly reduced in warm group after surgery, as compared with the cold group. Our results demonstrated that warm cardioplegia was associated with lower postoperative enzyme release, which might indicate less cardiocyte injury.
Our sensitivity analysis demonstrated that pooling the data on death, MI, LOS, stroke, AF, cardiac index, cTn and CK-MB did not alter the results of our analysis significantly, after excluding the trials which did not report the temperature of cardioplegia, or included lukewarm cardioplegia. Subgroup analysis according to the two different solutions used in the control group (cold blood/cold crystalloid) showed no significant difference between these two subgroups in the majority of outcomes, except stroke. Evidence from meta-regression suggested that the outcomes were unaltered by differences in publication year, Jadad score, type of surgical procedure, IMA grafting, proximal vein grafts implantation with side-biting clamp and the routine and timing of delivery among the trials.
This meta-analysis has limitations. First, only a few of the included trials clearly stated the methods of randomisation, and blinding was not always specifically reported. Second, just like the previous meta-analysis which compared blood cardioplegia with crystalloid cardioplegia [53], there were many potentially confounding factors that might impact the analysis, such as intermittent versus continuous, antegrade versus retrograde, composition of cardioplegia, volume of delivery, systemic temperatures, type of surgical procedure and so on. The presence of these issues made it difficult to analyse the primary intervention of our interest. Although we found no statistical heterogeneity among the studies included, and our subgroup analysis, sensitivity analysis and meta-regressions demonstrated that there was no significant interaction between several factors and the outcomes, it was hard to sufficiently resolve all these issues. Third, the results of this analysis were obtained by pooling data from a number of clinical trials, and the definitions of MI and LOS varied between the included trials. MI was diagnosed by ECG or enzyme markers. In the Warm Heart Trial, completely different outcomes were achieved by applying these two criteria. There might be limitations in the diagnostic accuracy by either of the two criteria alone, and use of combination of these methods may be more appropriate. Different definitions might lead to bias; in fact, the observed discrepancy in our analysis between lower myocardial enzyme release and comparable incidence of MI might be also caused by the different definitions of MI. However, the large number of patients included and the use of the random effect model might be helpful to overcome this limitation. Furthermore, the analysis based on data of postoperative IABP support also confirmed the result of our analysis for LOS. Fourth, because of the low incidence of events and lack of follow-up, this study was unable to evaluate the long-term effects of warm and cold cardioplegia. It has been repeatedly shown that the amount of postoperatively released enzymes was a strong predictor of long-term survival; however, the lack of follow-up data in our analysis might prevent to demonstrate an impact of warm blood cardioplegia on late clinical outcomes. Fifth, the trials included in our analysis spanned 13 years, a period in which both surgical and anaesthetic techniques changed dramatically. It is worth noting that surgeons who use cold cardioplegia these days may modify it to include warm blood as well. Unfortunately, we found only a few RCTs that reported their results based on combined use of cold and warm cardioplegia (such as warm induction, hot-shot or both), and their strategies of myocardial preservation varied. Therefore we could not assess their effects for myocardial protections in the present analysis, and those RCTs were also not included in our analysis in order to avoid bias.
In summary, our analysis demonstrated that there was no statistical difference in the risk of in-hospital death, MI, LOS, IABP and AF between these two groups. Warm cardioplegia resulted in significantly improved cardiac index and lower enzyme release after surgery. Despite the limitations, our results indicated that both warm and cold cardioplegia were safe and effective for myocardial protection, and warm cardioplegia was associated with even better postoperative haemodynamic performance and less cardiocyte injury.
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© 2009 European Association for Cardio-Thoracic Surgery. Published by Elsevier B.V. All rights reserved.
European Association for Cardio-Thoracic Surgery
© 2009 European Association for Cardio-Thoracic Surgery. Published by Elsevier B.V. All rights reserved.
Source: https://academic.oup.com/ejcts/article/37/4/912/406682
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