General findings
We previously reported that blood mediators of pro-inflammation, hypoperfusion and ischaemia reperfusion injury can predict AKI in cardiac surgery (9) and major orthopaedic surgery (26). Reasoning that the anti-inflammatory response in blood and urine would protect against direct and indirect renal injurious effects of proinflammation, hypoperfusion and IR, we investigated the hypotheses that in cardiac surgery those patients who develop CS-AKI would, in comparison with normal renal function patients, have lower ratios of biomarkers of A) anti-inflammation in blood and urine /pro-inflammation in blood and urine (hypothesis I-c, II-c, II-d); B) anti-inflammation in urine/hypoperfusion markers in blood (as detected by HFABP and VEGF in blood) (III-b) and C) anti-inflammation in blood and urine/IRI (as detected by MK in blood) (hypothesis IV-c and IV-d).
In CS-AKI patients, although the blood pro-inflammatory markers were higher than non CS-AKI patients (I-a), the corresponding blood anti-inflammatory response was even greater (I-b and I-c), such that the ratio of anti-inflammation/pro-inflammation in blood was higher in CS-AKI than non CSAKI (I-c). Similarly, the renal injurious effects of the greater blood based IRI responses in the CS-AKI patients (IV-a) were not effectively countermanded by the even greater blood anti-inflammatory responses which accompanied them (IV-c). It seems that whereas the blood anti-inflammatory response is commensurate with and even more than the blood pro-inflammatory response which drives it (I-c), this blood anti-inflammatory response appears to lack adequate renal protective efficacy. This may be because the blood anti-inflammatory biomarkers are larger (>20kDa) than many of the smaller blood pro-inflammatory biomarkers whose effects they limit and thus are unable to be filtered to the same extent. This means that whereas much of the body is shielded from effects of excessive pro-inflammation in blood by an accompanying blood anti-inflammatory response, the glomerular filtrate and tubular cells do not have this same level of protection (13). Accordingly, the ratios of blood anti-inflammation/blood pro-inflammation and blood anti-inflammation/sMK are elevated in CS-AKI compared with non-CS-AKI, to the extent that they are predictive of CS-AKI. The elevated ratios reflect a greater renal injurious underlying pro-inflammatory response. Although this pro-inflammatory response drives the compensatory blood anti-inflammatory response, its renal injurious effects are not constrained by the latter due to these smaller pro-inflammatory biomarkers being more readily able to escape from the blood to the glomerular filtrate. Our results suggest that the blood anti-inflammatory response doesn’t adequately protect against the renal injurious effects of pro-inflammation and IRI. Since ischemia reperfusion is normally preceded by a period of hypoperfusion and blood antiinflammation/sMK was observed to be higher in CS-AKI, then we suggest that that the blood anti-inflammatory response is also unlikely to protect against hyoperfusion associated CS-AKI.
In contrast to blood, urinary ratios of biomarkers of 1) anti-inflammation/pro-inflammation, 2) anti-inflammation/hypoperfusion and 3) anti-inflammation/IR were lower in urine in the CS-AKI compared with non-CS-AKI. This suggests, that in contrast to the blood anti-inflammatory response which does not appear to confer adequate renal protection, the urinary anti-inflammatory response is effective against mechanisms of renal injury arising from proinflammation, hypoperfusion and IRI. The renal anti-inflammatory response, as detected by increased perioperative uTNFsr1 and 2 and uIL1Ra, is believed to be largely an intrarenal response, since it develops in the urine before significant increases of these anti-inflammatory biomarkers in the blood (10), and at much higher concentrations (10). Presence of an adequate urinary anti-inflammatory response seems to mitigate against the renal injurious effects of proinflammation, hypoperfusion and IRI. This may have implications for understanding CS-AKI pathogenesis and its management both preventative and reactive.
Detailed remarks:
Result (I-a), confirms existing evidence of the perioperative pro-inflammatory response being injurious to kidneys (10). However, our demonstration of significantly elevated concentrations of pre-operative blood pro-inflammatory mediators in those who later developed CS-AKI suggests that pre-operative drivers of pro-inflammation are important in subsequent pathogenesis of CS-AKI.
Similarly, consistent with elevated pro-inflammation preoperatively in blood of patients who developed CS-AKI, there was also elevated urinary pro-inflammatory biomarkers (e.g. pre- and postoperative uIL-12p40; postoperative uNGAL, uIP-10).
In contrast with the secondary hypothesis (II-b), which was based on the demonstration of a significantly reduced urinary anti-inflammatory response in CS-AKI patients (7), patients who developed CS-AKI in the present study had higher pre- and postoperative urinary concentrations of anti-inflammatory biomarkers (pre- and postoperative uTNFsr1, uTNFsr2; postoperative uIL-1RA).
Since our previous study (7), clinical practice has changed in the ICU at the Royal Victoria Hospital Belfast where routine use of noradrenaline has increased providing consistently higher perioperative blood pressures particularly during CPB. In the 2013 study (7) only 10.6% of all patients were on noradrenaline perioperatively compared to 60% in the current study. In neither study was there a difference in noradrenaline use between CS-AKI and non-CS-AKI groups. It could be argued that the increased noradrenaline use, may increase perioperative blood pressure and reduce perioperative hypotensive episodes sufficiently to lead to somewhat enhanced filtration of the relatively difficult to filter, large molecular weight blood anti-inflammatory biomarkers (TNFsr1, TNFsr2, IL-1RA). However, less noradrenaline usage (7), may have resulted in lower mean arterial pressures perioperatively. This would cause reduced filtration of the perioperatively increased blood anti-inflammatory biomarkers, resulting in changes in the magnitude of urinary anti-inflammatory biomarkers, since the latter are largely intrarenally generated, without a significant top up from filtered blood biomarkers.
In the present study, blood anti-inflammatory biomarkers are of higher concentration in CS-AKI than non-CS-AKI patients. Any improvement in filtration of these blood cytokines due to greater noradrenaline usage, could lead to an augmentation of the intrarenally-generated anti-inflammatory response. Since blood anti-inflammatory biomarkers are higher in CS-AKI than non-CS-AKI, this could explain the increased urinary concentrations of these biomarkers in the CS-AKI (finding II-b). Furthermore, any increase in urinary anti-inflammation is likely to be accompanied by an even greater increase in filtration of blood pro-inflammatory biomarkers, although this is difficult to measure directly. This may explain the increased urinary anti-inflammatory biomarkers in CS-AKI, and yet despite this, such increases seemed inadequate to confer renal protective advantage. The proportion of filtered anti-inflammatory biomarkers was not commensurate with the filtered pro-inflammatory biomarkers as indirectly measured by the latter in the urinee. However, in patients who can develop an adequate intrarenally-generated anti-inflammatory response, renal protection against pro-inflammatory biomarkers is conferred.
This leads to the key finding in the present study, namely confirmation of the secondary hypotheses II-c and II-d, that patients who developed CS-AKI had lower ratios of urinary anti-/pro-inflammatory biomarkers (II-c) and urinary anti-/blood pro-inflammatory biomarkers (II-d).
Findings II-a and II-b suggest that although the urinary pro- (II-a) and anti-inflammatory (II-b) responses were greater in CS-AKI than non-CS-AKI patients, in contrast to blood, urinary anti-/pro-inflammatory ratios (II-c) and urinary anti-/blood pro- inflammatory ratios (II-d) were consistently lower (II-c and II-d) in CS-AKI than non-CS-AKI patients. This suggests that in CS-AKI patients, while there is a measure of compensatory anti-inflammatory activity in urine (II-b), in contrast to blood (I-b), this intrarenal anti-inflammatory response is of inadequate magnitude (II-c and II-d) to protect against inflammatory-mediated CS-AKI. This work suggests, for the first time, that a compromised intrarenal anti-inflammatory response constitutes an important and until now, undescribed mechanism involved in the pathogenesis of CS-AKI.
We hypothesized that an anti-inflammatory response in blood and urine adequately protected against a hypoperfusion insult indicated by increased blood VEGF and H-FABP. Blood VEGF preoperatively and blood H-FABP pre- and postoperatively were higher in CS-AKI than non-CS-AKI patients (III-a). However, this risk can be reduced where there are compensatory increases in pre- and postoperative uIL-1RA and postoperative uTNFsr1 and uTNFsr2.
MK is a biomarker of IRI associated with CS-AKI (9). Since pro-inflammatory cytokine generation is an important aspect of IRI (20) we evaluated if the ratio of urinary anti-inflammatory cytokine/serum MK was lower in CS-AKI patients. In general, the magnitude of change in concentrations of blood and urinary anti-inflammatory biomarkers is greater than corresponding changes in blood and urinary pro-inflammatory biomarkers which are usually at lower concentrations. For example, concentrations of blood TNFa and IL-1b and changes in their level are small in comparison with serum TNFsr1, TNFsr2 and IL-1RA. Moreover, filtered TNFa and IL-1b are normally undetectable in the urine (as they are absorbed and destroyed by the tubules) (16). Therefore, surrogates for urinary TNFa were used, namely uIP-10 and uIL-12p40 whose presence in urine may reflect TNFa activity more proximally in the filtrate. However, if it is assumed that the sTNFsr1 and sTNFsr2 constitute an adequate compensatory response to transient, small increases in blood TNFa, and serum IL-1RA represents the response to blood IL-1b, then sTNFsr1 and sTNFsr2 could be taken as surrogates for filtered TNFa, whereas IL-1RA is a surrogate for filtered IL-1b. Therefore, the ratios of sTNFsr1 or sTNFsr2 or sIL-1RA/ uTNFsr1 or uTNFsr2 or uIL-1RA were investigated as an extrapolation of II-d. This is effectively a ratio of filtered blood pro-inflammatory biomarker/urinary anti-inflammatory biomarker, if blood TNFsr1 and TNFsr2 are surrogates for filtered blood TNFa and blood IL-1RA a surrogate for filtered IL-1b. Postoperative ratios sTNFsr1/uTNFsr2, sTNFsr2/uTNFsr2, sIL-1RA/uTNFsr2, sTNFsr1/uTNFsr1, sTNFsr2/uTNFsr1, sIL-1RA/uTNFsr1 were all significantly higher in Day 5 CS-AKI patients. The data suggests that the urinary anti-inflammatory response is protective against filtered pro-inflammatory biomarkers (Figure 2).
Clinical application
When the above blood and urinary ratios were subjected to Backward Wald and Forced Entry logistic regression, preoperative Any Day ratio uIL-1RA/pIL-6 had the highest predictive ability to identify postoperative CS-AKI with AUROC 0.616. However, postoperatively the combination of Any Day ratios uIL-1RA/sIL-12p40 + uTNFsr2/sH-FABP + uIL-1RA/sMK had the highest predictive ability to identify patients at risk of developing CS-AKI with AUROC 0.824 (Table 5, Figure 1a and 1b). The results illustrate the imbalance between postoperative urinary anti-inflammation and three factors: (i) blood pro-inflammation (uIL-1RA/sIL-12p40), (ii) hypoperfusion (uTNFsr2/sH-FABP) and (iii) IRI (uIL-1RA/sMK). This model reflects the underlying concept of urinary anti-inflammatory response protecting against the three insults of perioperative pro-inflammation, hypoperfusion and IRI. This information can be used to inform clinical management in the areas of treatment choices and planning intraoperative management.
Treatment choices and planning
Increasing use of minimally invasive techniques (e.g. mini-sternotomy, robotic surgery), allow a reduced perioperative pro-inflammatory response as compared with traditional cardiac surgery. These options may be more attractive in patients where preoperatively an inadequate urinary anti-inflammatory response has been identified. Furthermore, since this response renders the patient more vulnerable to hypoperfusion, a treatment option involving temporary permissive hypoperfusion such as off pump coronary artery bypass surgery would not be advisable.
Intraoperative management
More studies are needed to confirm if managing patients perioperatively on higher than normal perfusion pressures improves the urinary anti-inflammatory response. However, it could be argued that preoperative identification of patients exhibiting inadequate urinary anti-inflammatory levels and a resulting reduction in protection in subsequent hypoperfusion, could result in these vulnerable patients being managed at supra-normal perioperative blood pressures to avoid hypoperfusion (even though this may make the surgical field more challenging for the operator).
Post-operative care
A post-operatively identified unfavorable urinary anti-inflammatory response may be taken as a contraindication to non-steroidal anti-inflammatory analgesics.
In the long term, these findings may help identify at risk patient populations who could be enrolled in interventive trials. One concept in need of testing is the effectiveness of deploying strategies to precondition the anti-inflammatory response of patients found preoperatively to have deficient anti-inflammatory protection.