Histidine–tryptophan–ketoglutarate solution versus multidose cardioplegia for myocardial protection in cardiac surgeries: a systematic review and meta-analysis

Background Surgical procedures in the heart requires protection of the heart from ischemia–reperfusion injury. Cardioplegia is the primary myocardial protective method in use. Histidine–tryptophan–ketoglutarate (HTK) solution is an intracellular cardioplegic solution that was initially used to preserve organs for transplantation. Methods A systematic electronic search was conducted in July 2021, in four databases; PubMed, Scopus, Web of Science, and Cochrane Library for eligible randomized controlled trials. The results were screened and the eligible trials were identified. Thereafter, the relevant data were extracted and pooled as mean difference or risk ratio, and 95% confidence interval in an inverse variance method using RevMan software. Results This review included 12 trials (n = 1327). HTK solution has resulted significantly in shorter intensive care unit stay (MD = − 0.09; 95% CI [− 0.15, − 0.03], p = 0.006), and shorter hospital stay (MD = − 0.51; 95% CI [− 0.71, − 0.31], p < 0.00001). Moreover, the patients who received the HTK solution had significantly lower levels of creatine kinase (after 4–7 h (MD = − 157.52; 95% CI [− 272.31, − 42.19], p = 0.007), and 24 h (MD = − 136.62; 95% CI [− 267.20, − 6.05], p = 0.04)), as well as creatine kinase muscle brain band (after 44–48 h (MD = − 3.35; 95% CI [− 5.69, − 1.02], p = 0.005)). Conclusion HTK solution had the same efficacy and safety as other cardioplegic solutions in most of the clinical parameters. Furthermore, the solution showed superiority in fastening the recovery and protecting the myocardium at the biochemical level. HTK solution provides longer myocardial protection; therefore, it limits surgical interruption. HTK solution can be used as an alternative to the currently used cardioplegic solutions.


Introduction
Myocardial damage is a major concern that accompanies cardiac surgeries. This damage is often multifactorial, but among the main attributing factors is the ischemia-reperfusion injury [1]. Myocardial damage can result in arrhythmias, myocardial infarction, or low cardiac output syndrome. As a result, major cardiac and renal morbidities, extended intensive care unit (ICU), and hospital stays, and a higher risk of mortality might occur [1,2]. To avoid these consequences, several myocardial protective methods were introduced, with the aim of minimalizing ischemia-reperfusion injury. Cardiac protection methods work by decreasing the cardiac metabolic demand, which improves its tolerance to ischemia. The main methods are chemical arrest (cardioplegia), topical hypothermia, and limiting myocardial edema.
Cardioplegia is the primary cardiac protection method in use. It is applied through the injection of a cardioplegic solution that causes heart diastolic arrest. Cardioplegic solutions can be divided into two types based on their molecular composition. Extracellular solutions, which include high amounts of sodium, calcium, potassium, magnesium, and bicarbonate, are the first type. These arrest the heart by depolarizing the myocardial membrane. In the second category, there are the intracellular solutions, sodium and calcium levels are low in this type. These induce a hyperpolarizing arrest of the myocardium [3,4].
Histidine-tryptophan-ketoglutarate (HTK) solution (Custodiol/Bretschneider) is an intracellular cardioplegic solution, introduced in the 1970s. The HTK solution was initially used to preserve organs for transplantation, thereafter, it was used in cardioplegia [5][6][7]. Added to its hyperpolarizing arrest that mimics the normal cardiac resting, histidine, tryptophan, ketoglutarate, and mannitol are all present in this solution. Each of these components adds an extra value in protecting the myocardium. Histidine buffers the ischemiainduced acidosis, therefore, improves the anaerobic glycolysis. Tryptophan is an effective cell membrane stabilizer. Ketoglutarate is a Krebs cycle intermediate, which enhances energy production and recovery following reperfusion. Moreover, mannitol minimizes cellular edema by maintaining the cellular environment osmolality, in addition to being a free radical scavenger [8,9].
A single dose of the HTK solution provides over two hours of myocardial protection. This feature allows time-saving and avoidance of surgical interruption for re-administration of the solution (as in other cardioplegic solutions, which protects for only 20 to 30 min) [10]. However, the use of the HTK solution in cardioplegia is still an off-label indication in many countries. Therefore, this systematic review and meta-analysis aims to provide updated evidence of the efficacy and safety of HTK solution in comparison with other alternative solutions in cardiac surgeries.

Methods
This systematic review and meta-analysis was performed in accordance with Cochrane Handbook for Systematic Reviews of Interventions [11]. Thereafter, the report was written following the preferred reporting items for systematic reviews and meta-analyses (PRISMA) statement [12].

Eligibility criteria and studies selection
This review included the randomized controlled trials (RCTs) that compared HTK solution to another cardioplegic solution in any cardiac surgery. Conference abstracts, thesis, and non-English studies were excluded from this review.
The duplicates were deleted from the search results, and a double-step screening was performed. Initially, the titles and abstracts of the retrieved articles were screened. Full-text screening was then performed for final eligibility.

Quality assessment (risk of bias)
The included trials were assessed for potential risk of bias using the Cochrane tool of Cochrane handbook for systematic reviews of interventions [13]. This tool assesses the risk of bias in six domains: (1) Random sequence generation (selection bias); (2) Allocation concealment (selection bias); (3) Blinding of participants (performance bias); (4) Blinding of assessors (detection bias); (5) Incomplete data (attrition bias); (6) selective reporting (reporting bias), in addition to any other potential source bias.

Data extraction
A summary of the trials key features, baseline data of the enrolled patients, and the treatment outcomes of efficacy and safety were extracted from the included trials. The assessed outcome included: cardiopulmonary bypass (CBP) time, aortic cross-clamping time, cardiac arrest beginning time, number of grafts, postoperative inotropic support, ejection fraction (EF) change, electrocardiogram (ECG) changes, postsurgical atrial fibrillation, hospital and ICU stay, in addition to creatine kinase (CK), creatine kinase muscle brain band (CK-MB), and troponin-I (Tn-I) levels.

Data synthesis and analysis
The statistical analysis of this review was conducted using the RevMan software (version 5.2; Cochrane Collaboration, Oxford, UK). Continuous data were pooled as mean difference (MD) and 95% confidence interval (CI), whereas dichotomous data were pooled as risk ratio (RR). Heterogeneity among the included trials was evaluated by visually inspecting the forest plot. Additionally, the I-squared (I 2 ) and chi-squared statistics were used. An I 2 value of ≥ 50% indicates statistical heterogeneity, in this case, a random-effect model is used instead of the fixedeffect model [14,15].

Literature search and characteristics of the included trials
Our systematic electronic databases search retrieved 841 articles. After removing the duplicates, 554 articles were screened. By title and abstract screening, 511 articles were excluded. Another 31 articles were excluded by full-text screening. Finally, 12 trials [6,10,[16][17][18][19][20][21][22][23][24][25] were included in our qualitative and quantitative synthesis The included trials compared HTK solution to other cardioplegic solutions-which require multiple doses (multiple doses cardioplegia (MDC))-in various cardiac surgeries. A summary of the included trial key feature, and baseline characteristics of the enrolled patients are presented in Tables 1, and 2 respectively.

Quality assessment (risk of bias)
Generally, the included trials had a low risk of reporting and attrition bias, and a low to moderate risk of selection bias. However, a potential source of performance bias was the inability to blind the participants and personnel. In most of the studies, the lack of blinding the outcomes assessors might have induced some detection bias. Having no registered protocol available was a potential source of bias as well in most of the studies. The risk of bias graph and summary are shown in Figs. 2, and 3 respectively.

Number of grafts
The analysis of this outcome was conducted upon four trials, with 220 patients enrolled (112 for HTK, and 108 for MDC). The meta-analysis showed no significant difference in the number of grafts between the two interventions (MD = − 0.04; 95% CI [− 0.25, 0.17], p = 0.7), and the results were homogenous (P = 0.13, I 2 = 47%) (Fig. 7).

ECG change
Three trials contributed with analyzable data to this analysis, with 480 patients enrolled (240 in each group). In terms of effectiveness, there was no significant difference between the two methods in ECG changes (RR = 0.81; 95% CI [0.61, 1.09], p = 0.17), and the results were homogenous (P = 0.29, I 2 = 20%) (Fig. 10).

Postsurgical atrial fibrillation
The analysis of this outcome was based upon five trials, with 376 patients enrolled (188 in each group). The comparative meta-analysis revealed no significant difference between the two interventions in the risk of postsurgical atrial fibrillation (RR = 0.82; 95% CI [0.61, 1.10], p = 0.18), and the results were homogenous (P = 0.22, I 2 = 30%) (Fig. 11).

ICU stay (days)
This analysis was conducted upon five trials, with 466 patients enrolled (325 for HTK, and 319 for MDC). HTK solution has significantly resulted in shorter ICU stay

CK level (IU/L)
The analysis of this outcome was based upon three trials, with 173 patients enrolled (88 for HTK, and 85 for MDC).  (Fig. 14).

CK-MB level (ng/ml)
Three trials were included in this analysis, with 204 patients enrolled (100 for HTK, and 104 for MDC).

Tn-I level (ng/ml)
Three trials had participated with analyzable data for this outcome, with 282 patients enrolled (139 for HTK, and 143 for MDC).

Discussion
This systematic review and meta-analysis provides an update to the current evidence by summarizing the findings of 12 RCTs that compared HTK solution to other cardioplegic solutions in various cardia surgeries. Data from 1,327 cardiac patients were summarized, among them, 666 patients had received the HTK solution. HTK solution has resulted significantly in shorter hospital (p < 0.00001) and ICU (p = 0.006) stay. Moreover, in comparison with other cardioplegic solutions, the HTK solution has significantly decreased the release of CK (after 4-7 h (p = 0.007), and 24 h (p = 0.04)), as well as CK-MB (after 44-48 h (p = 0.005)). These findings indicate superiority in myocardial protection at the biochemical level.
This article updates the previous meta-analysis Reynolds et al. 2020 (26), with four added RCTs [16, 17, 23, 25]. Our findings were consistent with the previous ones to a large extent. However, our update revealed the significant role of the HTK solution in reducing the release of CK-MB; an outcome that was insignificant in the previous meta-analysis. Furthermore, this updated review has investigated more outcomes than those reported previously. Among these outcomes were the hospital and ICU stay duration, which favored the HTK solution. Other newly investigated outcomes were cardiac arrest beginning time, number of grafts, EF change, and ECG change.
HTK solution was found to be as effective as other inuse-cardioplegic solutions. Moreover, it provides longer protection for the myocardium. This long protection makes it easier to administer, with minimal interruption of the surgical site. Furthermore, the analysis showed a superiority of the solution in shortening the recovery period, given the shorter ICU stay duration. Hospital stay days were reduced as well, which prevents the acquisition of nosocomial infection, and deterioration of physical and psychological health. Added to that, HTK solution protects the heart more, given the lower level of cardiac enzymes detected in the serum.
Among the included studies, Huet et al. [6] and Cvetković et al. [17] have compared HTK with St Thomas cardioplegia and concluded that no difference between both solutions in terms of safety, efficacy, and hemodynamics.
This review was strengthened by including only experimental controlled trials with adequate randomization. The selected studies design provides the highest power of evidence. However, this review was limited by the variation between the included trials data in some outcomes. This heterogeneity could not be resolved on some occasions. The inability to blind the study participants and personnel, as well as the outcome assessors in the majority of the trials was a probable source of bias. Most of the trials had no registered protocol. Due to the difference between cardioplegic solutions compared to HTK, a subgroup analysis could not performed according to the comparator. Therefore, further studies are recommended to compare HTK to the most frequently used solutions, such as St Thomas and BuckBerg, in order to determine the best option for each case and surgery.
The study concluded that, HTK solution had the same efficacy and safety as the in-use-cardioplegic solutions in most of the measured parameters. Furthermore, HTK solution showed superiority in reducing ICU and hospital stay, as well as CK and CK-MB release. Given its high efficacy and simple administration, the HTK solution constitutes an important alternative for MDC.