Remote Ischemic Preconditioning for Cardiac Surgery: Reflections on Evidence of Efficacy

Editor's Choice - Daily Remote Ischaemic Preconditioning for Intermittent Claudication: A Sham Controlled Randomised Trial

OBJECTIVE

Remote ischaemic preconditioning (RIPC) is a promising non-invasive strategy in which brief episodes of ischaemia and reperfusion can increase skeletal muscle resistance to ischaemia and improve mobility. This study aimed to determine whether 28 consecutive days of RIPC improved intermittent claudication (IC) symptoms compared with sham intervention.

METHODS

This single centre, parallel, randomised, sham controlled, double blind trial was conducted from January 2022 to April 2023 in outpatient settings. Forty two patients with stable IC Fontaine stage IIa or IIb were randomised to RIPC or sham for 28 days. The pre-specified primary outcome was a change in the maximum walking distance (MWD) after 28 days measured with a treadmill test. A > 10% change in MWD was considered clinically significant. Change in intermittent claudication distance (ICD), time to relief from claudication (TRC), and health related quality of life (HRQoL) measured with the VascuQoL-6 questionnaire were the secondary outcomes (ClinicalTrials.gov ID: NCT05084066).

RESULTS

Forty one men (RIPC = 23, sham = 18) aged 64.9 ± 7.4 years were analysed. A change of > 10% in MWD occurred in 14 patients in the RIPC group vs. eight patients in the sham group (relative risk 1.37, 95% confidence interval 0.74 - 2.25; p = .35). Changes in ICD, TRC, and HRQoL between the groups were not statistically significant.

CONCLUSION

In this trial, RIPC did not significantly improve MWD, ICD, or TRC compared with treatment with a sham device.

Nrg1β Released in Remote Ischemic Preconditioning Improves Myocardial Perfusion and Decreases Ischemia/Reperfusion Injury via ErbB2-Mediated Rescue of Endothelial Nitric Oxide Synthase and Abrogation of Trx2 Autophagy

OBJECTIVE: Remote ischemic preconditioning (RIPC) is an intervention process where the application of multiple cycles of short ischemia/reperfusion (I/R) in a remote vascular bed provides protection against I/R injury. However, the identity of the specific RIPC factor and the mechanism by which RIPC alleviates I/R injury remains unclear. Here, we have investigated the identity and the mechanism by which the RIPC factor provides protection. APPROACH AND RESULTS: Using fluorescent in situ hybridization and immunofluorescence, we found that RIPC induces Nrg1β expression in the endothelial cells, which is secreted into the serum. Whereas, RIPC protected against myocardial apoptosis and infarction, treatment with neutralizing-Nrg1 antibodies abolished the protective effect of RIPC. Further, increased superoxide anion generated in RIPC is required for Nrg1 expression. Improved myocardial perfusion and nitric oxide production were achieved by RIPC as determined by contrast echocardiography and electron spin resonance. However, treatment with neutralizing-Nrg1β antibody abrogated these effects, suggesting Nrg1β is a RIPC factor. ErbB2 (Erb-B2 receptor tyrosine kinase 2) is not expressed in the adult murine cardiomyocytes, but expressed in the endothelial cells of heart which is degraded in I/R. RIPC-induced Nrg1β interacts with endothelial ErbB2 and thereby prevents its degradation. Mitochondrial Trx2 (thioredoxin) is degraded in I/R, but rescue of ErbB2 by Nrg1β prevents Trx-2 degradation that decreased myocardial apoptosis in I/R. CONCLUSIONS: Nrg1β is a RIPC factor that interacts with endothelial ErbB2 and prevents its degradation, which in turn prevents Trx2 degradation due to phosphorylation and inactivation of ATG5 (autophagy-related 5) by ErbB2. Nrg1β also restored loss of eNOS (endothelial nitric oxide synthase) function in I/R via its interaction with Src.

Perioperative Cardioprotection: General Mechanisms and Pharmacological Approaches

Cardioprotection encompasses a variety of strategies protecting the heart against myocardial injury that occurs during and after inadequate blood supply to the heart during myocardial infarction. While restoring reperfusion is crucial for salvaging myocardium from further damage, paradoxically, it itself accounts for additional cell death-a phenomenon named ischemia/reperfusion injury. Therefore, therapeutic strategies are necessary to render the heart protected against myocardial infarction. Ischemic pre- and postconditioning, by short periods of sublethal cardiac ischemia and reperfusion, are still the strongest mechanisms to achieve cardioprotection. However, it is highly impractical and far too invasive for clinical use. Fortunately, it can be mimicked pharmacologically, for example, by volatile anesthetics, noble gases, opioids, propofol, dexmedetomidine, and phosphodiesterase inhibitors. These substances are all routinely used in the clinical setting and seem promising candidates for successful translation of cardioprotection from experimental protocols to clinical trials. This review presents the fundamental mechanisms of conditioning strategies and provides an overview of the most recent and relevant findings on different concepts achieving cardioprotection in the experimental setting, specifically emphasizing pharmacological approaches in the perioperative context.

Membrane TLR9 Positive Neutrophil Mediated MPLA Protects Against Fatal Bacterial Sepsis

Sepsis is a major cause of patient mortality and morbidity from bacterial infections. Although neutrophils are known to be important in the development of sepsis, how distinctive neutrophil subtypes regulate inflammatory processes involved in septicemia remains unclear. Preconditioning protects organisms against subsequent higher-dose exposures to the same, or even different, stimuli. Several studies have reported various effects of preconditioning on immune cells. However, the detailed mechanisms underlying neutrophil-mediated protection through preconditioning in sepsis remain unknown.

Methods: Flow cytometry was conducted to sort the mice peritoneal lavage cells and the blood samples from patients with sepsis. Western blotting and ELISA were carried out to elucidate the expression of TLR9 signal transduction pathway proteins. Histological analysis was used to assess the effect of InP on intestine and liver structure in tlr9^-/- and cav-1^-/- mice. Fluorescence microscopy, Co-IP, and FRET were carried out to determine the association of TLR9 with Cav-1.

Results: We show that membrane toll-like receptor-9 positive (mTLR9⁺) neutrophils exert a protective effect against fatal bacterial infections through the process of inflammatory preconditioning (InP). InP, which occurs in the setting of a low-dose bacterial challenge, active ingredient is Monophosphoryl lipid A (MPLA), triggers the membrane translocation of TLR9 from the neutrophil cytosol, where it binds to Cav-1. Our findings showed that InP enables TLR9 to facilitate MyD88-mediated TRAF3 and IRF3 signal transduction. Depletion of either TLR9 or Cav-1 largely eliminates the neutrophil-mediated InP effect in sepsis models in vitro and in vivo. Further, examination of clinical samples from patients with sepsis showed that clinical outcomes and likelihood of recovery are closely correlated with mTLR9 and Cav-1 expression in circulating neutrophils.

Conclusion: These results demonstrate that the TLR9-Cav-1 axis is a critical signaling pathway involved in the regulation of neutrophil-dependent MPLA mediated InP, and the presence of mTLR9⁺ neutrophils could be an attractive indicator of clinical outcomes in bacterial sepsis that could be further explored as a potential therapeutic target.

Autonomic imbalance in cardiac surgery: A potential determinant of the failure in remote ischemic preconditioning

Remote ischemic preconditioning (RIPC) is a cardioprotective strategy against myocardial damage by ischemia-reperfusion. Many in-vivo and ex-vivo animal researches have demonstrated that RIPC decreases significantly the ischemia-reperfusion myocardial damage, by up to 58% in isolated rat heart. Cardiac artery bypass graft surgery (CABG) is a clinical model of myocardial ischemia-reperfusion and a clinical potential application to RIPC. However, although RIPC has shown successful results in experimental studies, clinical trials on CABG have failed to demonstrate a benefit of RIPC in humans. Strikingly, the main proposed factors associated with this translational failure also impair the balance of the autonomic nervous system (ANS), which has shown to play a key role in RIPC cardioprotection in animal models. Comorbidities, chronic pharmacological treatment and anesthesic drugs - common conditions in CABG patients - cause an ANS imbalance through parasympathetic activity decrement. On the other hand, ANS and specially the parasympathetic branch are essentials to get cardioprotection by RIPC in animal models. Consequently, we propose that ANS imbalance in CABG patients would explain the failure of RIPC clinical trials. Whether our hypothesis is true, many patients could be benefited by RIPC: a cheap, simple and virtually broad-available cardioprotective maneuver. In this paper we discuss the evidence that support this hypothesis and its clinical implications.

Remote ischaemic conditioning reduces infarct size in animal in vivo models of ischaemia-reperfusion injury: a systematic review and meta-analysis

Aims

The potential of remote ischaemic conditioning (RIC) to ameliorate myocardial ischaemia-reperfusion injury (IRI) remains controversial. We aimed to analyse the pre-clinical evidence base to ascertain the overall effect and variability of RIC in animal in vivo models of myocardial IRI. Furthermore, we aimed to investigate the impact of different study protocols on the protective utility of RIC in animal models and identify gaps in our understanding of this promising therapeutic strategy.

Methods and results

Our primary outcome measure was the difference in mean infarct size between RIC and control groups in in vivo models of myocardial IRI. A systematic review returned 31 reports, from which we made 22 controlled comparisons of remote ischaemic preconditioning (RIPreC) and 21 of remote ischaemic perconditioning and postconditioning (RIPerC/RIPostC) in a pooled random-effects meta-analysis. In total, our analysis includes data from 280 control animals and 373 animals subject to RIC. Overall, RIPreC reduced infarct size as a percentage of area at risk by 22.8% (95% CI 18.8–26.9%), when compared with untreated controls (P < 0.001). Similarly, RIPerC/RIPostC reduced infarct size by 22.2% (95% CI 17.1–25.3%; P < 0.001). Interestingly, we observed significant heterogeneity in effect size (T2 = 92.9% and I2 = 99.4%; P < 0.001) that could not be explained by any of the experimental variables analysed by meta-regression. However, few reports have systematically characterized RIC protocols, and few of the included in vivo studies satisfactorily met study quality requirements, particularly with respect to blinding and randomization.

Conclusions

RIC significantly reduces infarct size in in vivo models of myocardial IRI. Heterogeneity between studies could not be explained by the experimental variables tested, but studies are limited in number and lack consistency in quality and study design. There is therefore a clear need for more well-performed in vivo studies with particular emphasis on detailed characterization of RIC protocols and investigating the potential impact of gender. Finally, more studies investigating the potential benefit of RIC in larger species are required before translation to humans.

Remote Ischemic Conditioning in Cardiovascular Surgery

The existing clinical studies on remote ischemic preconditioning in patients undergoing cardiovascular surgery are critically reviewed, with a focus on infarct size reduction and clinical outcome as end points. Confounders, notably the use of propofol anesthesia are identified. The need for better designed trials with a more targeted approach is emphasized.

Time to Give Up on Cardioprotection?

The mortality from acute myocardial infarction (AMI) remains significant, and the prevalence of post-myocardial infarction heart failure is increasing. Therefore, cardioprotection beyond timely reperfusion is needed. Conditioning procedures are the most powerful cardioprotective interventions in animal experiments. However, ischemic preconditioning cannot be used to reduce infarct size in patients with AMI because its occurrence is not predictable; several studies in patients undergoing surgical coronary revascularization report reduced release of creatine kinase and troponin. Ischemic postconditioning reduces infarct size in most, but not all, studies in patients undergoing interventional reperfusion of AMI, but may require direct stenting and exclusion of patients with >6 hours of symptom onset to protect. Remote ischemic conditioning reduces infarct size in patients undergoing interventional reperfusion of AMI, elective percutaneous or surgical coronary revascularization, and other cardiovascular surgery in many, but not in all, studies. Adequate dose-finding phase II studies do not exist. There are only 2 phase III trials, both on remote ischemic conditioning in patients undergoing cardiovascular surgery, both with neutral results in terms of infarct size and clinical outcome, but also both with major problems in trial design. We discuss the difficulties in translation of cardioprotection from animal experiments and proof-of-concept trials to clinical practice. Given that most studies on ischemic postconditioning and all studies on remote ischemic preconditioning in patients with AMI reported reduced infarct size, it would be premature to give up on cardioprotection.