Introduction
In patients with acute myocardial infarction (MI) or angina pectoris treated with percutaneous coronary intervention (PCI) or coronary artery bypass grafting (CABG), ischemia/reperfusion injury contributes to periprocedural myocardial injury (PMI). It has been estimated that PMI occurs in up to 30–40% of the patients treated with PCI [
1‐
3] and CABG [
4], and is associated with increased risk of cardiovascular events and mortality [
2‐
7]. The extent of PMI can be estimated by the elevation of the levels of cardiac enzymes such as creatine kinase-MB (CKMB), though troponin (Tn) is nowadays preferable as it is more sensitive and specific [
2‐
4].
In several small animal models of cardiac toxicity, vitamin C administration attenuated the rise of troponin and CKMB [
8‐
10]. In addition, vitamin C reduced infarct size in animal studies when administered alone [
11], in its oxidized form, as dehydroascorbic acid [
12], and when combined with vitamin E [
13]. These pre-clinical findings suggest that in cardiac stress vitamin C might also have protective effects on the myocardium in humans. Vitamin C plays an important role in preserving endothelial function [
14]. As an indication of clinical relevance of such biochemical effects, vitamin C improved endothelial function in atherosclerotic and heart failure patients [
15]. Moreover, vitamin C improved the perfusion of the myocardium in patients undergoing PCI and people exposed to hyperoxia [
16‐
18]. Recent meta-analyses found that vitamin C improved left ventricular ejection fraction (LVEF) in cardiac and non-cardiac patients [
19], and prevented post-operative atrial fibrillation (POAF) in high risk patients in trials carried out outside of the USA [
20].
One of the factors that contributes to ischemia/reperfusion injury is the increase in reactive oxygen species (ROS) [
21], mostly arising directly after reperfusion [
22]. Oxidative stress may cause reversible or irreversible injury to proteins, lipids and DNA [
23]. Decreased plasma vitamin C concentrations are often reported after MI and cardiac surgery, accompanying the increase in oxidative stress [
24‐
26], which indicates that the consumption of vitamin C is increased. As a result, the decreased vitamin C levels might impair its pleiotropic effects [
27] and may further be harmful as the protection against the remaining oxidative stress can become inadequate. The potential antioxidant effects on PMI should be investigated further [
28].
The aim of this meta-analysis was to investigate whether vitamin C administration may have an effect on Tn and CKMB levels in patients undergoing PCI or CABG.
Discussion
In this meta-analysis we found that on average vitamin C reduced post-operative Tn plasma levels by 43% and CKMB plasma levels by 14% in patients who underwent elective PCI or CABG. The significant reduction in cardiac enzyme levels indicate a parallel reduction in PMI.
There are multiple ways in which vitamin C may exert cardioprotective effects [
27]. Vitamin C can directly scavenge harmful oxidative stress, restore other antioxidants and antioxidant enzymes, and decrease ROS production [
47]. Vitamin C significantly lowered biomarkers of oxidative stress in all included trials that reported them [
16,
31,
33,
34,
45], but further research is needed on oxidative stress biomarkers and PMI [
28]. Vitamin C is consumed when there is overwhelming oxidative stress. Furthermore, when recycling of vitamin C is insufficient, its level may decrease systemically or locally, thereby impairing the pleotropic effects of vitamin C [
27]. Vitamin C may further exert a cardioprotective effect by preserving endothelial function as has been found in multiple clinical contexts [
14,
15,
48‐
50]. There is also evidence suggesting that vitamin C can improve myocardial perfusion [
16‐
18].
Some reports in the older literature suggested that vitamin C deficiency, scurvy, may be associated with chest pain and abnormal ECG changes, which were reversed after vitamin C administration [
51‐
53]. These early observations also indicate that vitamin C has effects on heart functions. In a further early study, vitamin C was effective in reducing creatine kinase levels in patients undergoing CABG with > 50 min ischemic time, whereas the vitamin had no effect on patients with < 50 min ischemic time [
54].
In type II diabetic patients with cardiovascular risk and in surgical patients with limb ischemia/reperfusion injury vitamin C administration significantly reduced Tn levels compared to control [
55,
56]. A recent trial, however, found no benefit of vitamin C and N-acetylcysteine versus placebo on post-operative myocardial injury in patients undergoing major non-cardiac surgery [
57]. Importantly, median Tn level was low in the placebo group. Thus, the lack of effect of vitamin C in this non-cardiac trial might be explained simply by the very low level of cardiac stress. In our study, we investigated the effects of vitamin C in the setting of PCI and CABG, two conditions that uniformly cause cardiac stress and substantial increase cardiac enzyme levels.
In the large PHS-II trial, long-term daily supplementation of 0.5 g/day vitamin C did not reduce the risk of major cardiovascular events [
58]. However, the population consisted of well-nourished physicians without prevalent cardiovascular disease. Therefore, the context was very different from the trials included in our analysis. The findings of the PHS-II trial are not a relevant comparison for findings with PCI and CABG patients.
Previous studies have shown that PMI is associated with the incidence of adverse outcomes and the risk of death after PCI and CABG [
2‐
7,
59]. Therefore, reducing prognostically important PMI, especially in high risk patients, might improve patient outcome, e.g. by reducing Type 4–5 myocardial infarction or death [
2‐
4]. Three of the included trials also reported beneficial effects of vitamin C on cardiac function [
16,
34,
60]. In addition, a recent meta-analysis found a beneficial effect of vitamin C on LVEF in cardiac and non-cardiac patients [
19]. A meta-regression analysis in that study found a highly significant relationship between the baseline LVEF level and the size of effect by vitamin C [
19]. Vitamin C had no effect for people with high baseline LVEF level, while the effect was progressively larger with lower LVEF levels. Another meta-analysis in cardiac surgery patients found that vitamin C shortened the length of hospital stay, ICU stay and the occurrence of POAF [
20]. However, there was significant heterogeneity in the findings as the benefits were only found in non-United States trials [
20]. In our current analysis, all included trials were carried out outside of the USA, and therefore we could not carry out a similar comparison.
The effect of vitamin C may depend on dose, timing, route of administration and the total duration of treatment. In the included trials vitamin C dose ranged from 1 to 16 g at the day of surgery, but no dose-effect was evident. All trials administered vitamin C prior to, and some during, the procedure. One trial had a prolonged treatment for 6 days, however TnI was measured at 18 h [
44]. The data from the trial by Oktar et al. suggests that early administration of vitamin C, prior to the procedure, is more beneficial compared to during the procedure (Fig.
4), though this requires further study. Also the optimal duration of vitamin C therapy remains to be investigated as it appears that oxidative stress is increased for multiple days post-operatively [
26]. None of the trials administered vitamin C orally, so we could not compare oral and intravenous administration. Absorption of oral doses become saturated at about 3 g/day, whereas i.v. administration can lead to over 100 times higher plasma concentrations [
61,
62]. Nevertheless, in the context of PCI and CABG, intravenous administration is convenient as all patients have i.v. routes available because of the procedures. Finally, measuring plasma vitamin C concentrations would add valuable information about size of effect by baseline and follow-up vitamin C status [
63,
64].
In addition to differences in dosing regimens, treatment/clinical settings also differed among the included trials. In two trials patients were scheduled for PCI, whereas in five trials patients underwent cardiac surgery. Cardiac surgery may generate higher levels of ROS due to extracorporeal circulation compared to PCI. The size of the effect by vitamin C may differ between the two clinical contexts, but the included trials were too small to compare the two contexts. Oxidative stress parameters such as the static oxidation-reduction potential (sORP) can be measured quickly to quantify the amount of oxidative stress that is generated in different settings [
64,
65], and may be used in future research to explore the relation between oxidative stress and PMI [
28].
No side effects of vitamin C administration were reported in the included trials. Previous evidence also indicates that both oral and intravenous vitamin C administrations are remarkably safe [
66,
67]. The US nutritional recommendations monograph considers various proposed harms (e.g. kidney stone formation, increased oxygen demand and pro-oxidant effects) of high doses of vitamin C, but concluded that the great majority of them are unsubstantiated [
66]. Nevertheless, high dose vitamin C may be harmful for the kidneys, but only when administered for a longer period of time or at an extremely high dose. The US recommendations stated that 2 g/day is safe for ordinary people, yet encourages research of higher doses in the contexts of controlled trials [
66].
The recently published LOVIT-trial investigated the effect of 4-day vitamin C administration in septic patients [
68]. Persistent organ dysfunction and mortality were increased in the vitamin C group. However, the harm occurred after vitamin C was stopped, and not during the vitamin C administration. Therefore, the harm is explained by the rebound effect, and does not indicate harm of ongoing vitamin C administration [
69].
Limitations
The diverse dosing regimens and differences in treatment/clinical settings among the included trials impact the generalizability of the overall conclusions. In addition, the number of included trials is small. Therefore, the evidence we provide does not allow practical conclusions at clinical level. Our goal was to figure out whether the limited number of trials encourage further research and our positive conclusion is not limited by the heterogeneity in dosing regimens and clinical contexts. The blinding of treatment and outcome measures was not described in three included trials [
33,
34,
45], but we do not consider that such objective laboratory measures as Tn and CKMB could be meaningfully influenced by the knowledge about the intervention [
46]. Therefore, we judged that performance and detection bias were low also for these three trials. Finally, concerns about blinding do not influence our conclusion that this topic should be investigated further. The number of studies and participants was small; however, small size can lead to a false negative finding, but cannot lead to a false positive finding.
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