Effect of nitric oxide on postoperative acute kidney injury in patients who underwent cardiopulmonary bypass: a systematic review and meta-analysis with trial sequential analysis

Acute kidney injury (AKI) is a multifactorial and common complication in patients undergoing cardiopulmonary bypass (CPB), as it occurs in 30–70% of patients [1‐4]. CPB leads to a decrease in the bioavailability of vascular nitric oxide (NO), both because of NO scavenging (via deoxygenation reaction in the presence of intravascular hemolysis [5, 6]) and through a reduction in NO synthesis (in the presence of ischemia/reperfusion injury, acute inflammatory reaction and endothelial dysfunction [7]). In particular, hemolysis generated during the CPB has been demonstrated to be a pivotal contributor to the increased risk of perioperative AKI in patients undergoing CPB [6].

NO regulates vascular tone and distal blood perfusion while also acting as an anti-inflammatory and anti-thrombotic mediator [8]. NO gas is traditionally used for the treatment of acute exacerbation of pulmonary hypertension [9] and pediatric hypoxemic respiratory failure [10]. At present, NO has been tested in several randomized trials for its protective role in pulmonary hypertension and myocardial injury in patients undergoing CPB [11‐13]. More recently, NO has been shown to protect against AKI and chronic renal disease, either by reprogramming metabolism [14] or via homeostatic regulation of renal hemodynamics and α1-adrenoreceptor sensitization [15]. Administration of NO gas was associated with benefit of lowering plasma NO consumption in the presence of hemolysis [16, 17]. Lei et al. conducted a single-center randomized controlled trial (RCT) and found that NO delivered from the beginning of CPB could reduce the risk of AKI and lower NO consumption in plasma [18], a finding which was confirmed by a recently completed randomized trial [19]. On the contrary, in a meta-analysis, Ruan et al. showed that NO therapy was associated with renal dysfunction, especially in critically ill patients with acute respiratory distress syndrome (ARDS) [20].

We hypothesized that the effect of NO on kidney function might be disease specific, as two of the RCTs in the meta-analysis of Ruan et al. [20] showed that NO administration started from the end of CPB in cardiac surgery patients [21, 22] had no adverse effects on renal function. Thus, a careful analysis of the effect of NO gas on renal function in cardiac surgery is warranted.

We performed a meta-analysis aiming to ascertain the effect of NO therapy on renal function in patients undergoing CPB and to further investigate whether the effect of NO varies between different initiation time points of such therapy.

This systematic review and meta-analysis was registered on PROSPERO (NO. CRD42019125948) based on the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines [23].

In this study, we investigated the effect of NO therapy on the risk of postoperative AKI in patients undergoing CPB. Overall, NO reduced the incidence of AKI in patients undergoing CPB. Although visual inspection of the funnel plot suggested publication bias, the trim-and-fill test still confirmed a beneficial effect of NO administration on the development of AKI. Sensitivity analyses also revealed consistent effect estimates for the primary outcome. However, TSA analysis suggests that further studies are required to achieve a firm conclusion. NO supplementation neither reduced the length of the hospital or ICU stay nor increased the risk of postoperative hemorrhage. NO therapy slightly increased the level of MetHb, which was clinically negligible and always below safety thresholds.

The nephrotoxicity of NO therapy has emerged since the aforementioned meta-analysis was published [20], which suggested that NO impaired renal function in critical illness settings. To interpret our results in the contest of the present literature, we asked two questions.

Our first question is: why has NO therapy shown differing effects on renal function in patients undergoing CPB as compared to patients with ARDS [20]? Several mechanisms may explain the contradictory scenario. CPB contributes to the development of AKI through multiple mechanisms including [1] hypoperfusion (microcirculatory), [2] ischemia/reperfusion injury, [3] hemodilution, [4] pro-inflammatory response, and most importantly, and [5] intravascular hemolysis [33]. Hemolysis strongly correlates to increased plasma NO depletion leading to a decrease of NO bioavailability [6, 34]. Nitric oxide is a potent endogenous vasodilator released by endothelial cells and its depletion leads to vasoconstriction, and ultimately to reduce organ perfusion [17, 35, 36]. The renal protective effects of NO gas might be twofold. On one hand, the administration of NO may act as the replenishment of NO storage in the presence of NO depletion due to hemolysis. On the other hand, NO gas might generate plasma NO metabolites that are protective against ischemia–reperfusion injury [37]. In this context, administration of therapeutic NO has shown promising properties by lowering vascular NO depletion [16], which could explain why breathing 40 parts per million (ppm) of NO markedly increased renal blood flow, glomerular filtration rate, and urine flow in a swine model of phenylephrine-induced hypertension [38].

In comparison, there is no obvious NO deficiency in ARDS. Instead, intrapulmonary NO generation due to inducible NO synthase was found in an experimental model of endotoxemia-induced ARDS and finally proved to be involved in the development of ARDS [39]. Moreover, Ruan et al. found that the duration of NO administration was longer in ARDS studies (> 7 days) [20]. Thus, prolonged NO therapy could induce plasma NO redundancy and in turn [1] induce tubular apoptosis [40], [2] produce reactive nitrogen species, such as nitrogen dioxide (NO₂) [41], and create a pro-inflammatory response leading to renal vasoconstriction and injury [20]. Based on present literature, we suggested that the effect of NO on renal function might be disease specific.

Our second question is: does the effect of NO on renal function vary by the timing of initiation? To answer to this question, one should consider the differences in renal dysfunction definition adopted in the five studies included in our meta-analysis. The first two studies in which NO was administrated at the end of CPB, renal dysfunction was defined as severe AKI (i.e., urine output < 0.3 ml/kg/h [21] or need for renal replacement therapy [22]). In the following three studies [18, 19, 31] in which NO gas was delivered at the beginning of CPB, renal dysfunction followed KDIGO criteria. Due to the dissimilar definitions of renal injury, we are unable to make a definitive conclusion on the renal protective properties of NO delivery when started at the end of CPB. Indeed, based on biochemical [6, 33, 42‐44] and hemodynamic studies [45‐50] discussed below, it is plausible that late delivery of NO might not protect the kidney function.

Red blood cells (RBCs) are damaged when passing through the CPB circuit [33] and thus releasing free hemoglobin (Hb) into the circulation [42]. High levels and prolonged duration of hemolysis can scavenge the NO produced by the endothelial cells, with deleterious effects on vascular endothelium and renal tubular cells [33, 43]. Vermeulen et al. showed that patients with postoperative AKI had higher levels of plasma Hb at the end of CPB, as compared to patients without AKI [6]. Meanwhile, circulating Hb also promotes oxidative stress and degrades to release free heme and heme iron, triggering the activation of the immune response through the innate pathway (e.g., TLR-4 pathway) [43]. Based on current knowledge, drugs that could prevent endothelial dysfunction by scavenging free Hb and reactive oxygen species could be a future therapeutic option. If NO is delivered at the beginning of CPB, it can oxidize heme iron to the ferric state and transform hemoglobin to MetHb [44] before the blood is reinfused in the patient. As a result, plasma availability of NO is preserved; blood flow to the organs is maintained, thereby lowering the risk of perioperative AKI. If NO is delivered at the end of CPB, especially when prolonged, hemolysis-associated consequences have already started and subsequently induce AKI via systemic vascular injury, inflammatory cascade, and oxidative stress.

Moreover, hemolysis-induced NO consumption increased pulmonary vascular resistance [34], thereby leading to right heart dysfunction and subsequent postoperative AKI [51]. However, administration of NO at the end of CPB [21, 22, 45‐48] or even later in the intensive care unit [49, 50] showed no effect on pulmonary hypertension. Among the five studies included in our meta-analysis, Fernandes et al. demonstrated that patients receiving inhaled NO had decreased pulmonary vascular resistance but a similar pulmonary artery systolic pressure, compared to the control group [21]. Potapov et al. found that inhaled NO did not affect right heart function in terms of risk of right heart disease, pulmonary vascular resistance index, and incidence of central venous pressure more than 16 cmH₂O [22]. Furthermore, in other clinical trials assessing effect of NO on pulmonary hypertension or heart function among patients requiring CPB, administration of NO at the end of CPB [45‐48] or in the intensive care unit [49, 50] did not affect the mean pulmonary artery pressure, and a subsequent meta-analysis also confirmed the aforementioned phenomenon [52]. Further hemodynamic studies should determine whether NO delivered at the beginning of CPB could improve right heart function and pulmonary hypertension, to better interpret the potential mechanism of NO in renal protection.

Although NO appears to improve renal function in patients requiring CPB, possible adverse effects need to be monitored during gas delivery. Hemorrhage is one of the common concerns of NO treatment in cardiac surgery patients undergoing CPB. NO ha been shown to inhibit platelet activation in in vitro studies [53] and potentially prolong bleeding time. However, no clinical trials showed an increased risk of bleeding when NO was delivered in cardiac surgery. We confirmed those findings with the present meta-analysis.

The monitoring of MetHb levels in the blood is warranted during NO delivery. MetHb (Fe³⁺) has a lower capacity to bind oxygen compared to oxy-hemoglobin (Fe²⁺), which may lead to decreased delivery of oxygen to the peripheral tissues and, consequent, tissue hypoxia. It is generally accepted that blood MetHb levels in healthy individuals is less than 2% of the hemoglobin [54]. Cyanosis is present when the MetHb levels approach 15–20% [54]. Our meta-analysis showed a slight increase in blood MetHb level in the NO group and never exceeded 10% in any patient according to the included RCTs [18, 19]. Meanwhile, Potapov et al. reported that levels of MetHb were not higher in NO group compared with the control group [22] and Kamenshchikov et al. found out that all the patients in their study had levels of MetHb less than 0.5% during CPB [31].

This study has some limitations. First, the small number of studies included in our meta-analysis may have reduced the statistical power of the analysis. To balance our interpretation, we performed trial sequential analysis, which accounts for the type I, and type II errors, using widely accepted, methods for adjusting thresholds for significance in randomized clinical trials when the required sample size has not been reached [30].

Second, although the level of statistical heterogeneity was very low in our analyses, the heterogeneity in other independent variables including AKI diagnosis criteria, duration and dosage of NO therapy, and time of CPB should not be overlooked. Over the past decades, the diagnostic criteria for AKI and AKI stage definitions have changed, i.e., decades ago common medical terminology referred to acute renal failure to describe kidney injury in acute settings, subsequently, the RIFLE criteria introduced the term AKI [55], the AKIN perfected the definition of AKI [56] and more recently those definitions have been updated in KDIGO classification [57]. Thus, it is not surprising that many meta-analyses and epidemiological studies reported in the literature include heterogeneous AKI definitions [20, 58].

Finally, some competing endpoints, such as 28- or 90-day mortality, the impact of different dose and duration of NO on renal function, and the right heart function were not reported in the present study.

In our meta-analysis, we found that NO appeared to reduce the risk of postoperative AKI in patients undergoing CPB, suggesting that the effect of NO on renal function might be disease specific. Future trials involving NO therapy are required to consolidate our findings and investigate potential mechanisms of renal protection properties.