Background
Acute kidney injury (AKI) is a serious complication of major surgery and severe AKI requires treatment with acute dialysis [
1,
2]. Annually 230 million patients undergo major surgery worldwide and around one million major surgeries are complicated by AKI treated with dialysis (AKI-D) [
1‐
4]. The development of AKI is associated with increased morbidity, mortality, and health care expenditures [
5]. A patient’s risk for developing postoperative AKI is influenced by a variety of clinical factors including pre-existing comorbidities (e.g. chronic kidney disease (CKD), congestive heart failure, coronary artery disease, diabetes, hypertension), clinical factors around the time of surgery (e.g. volume depletion, blood loss, hypotension, administration of vasoconstrictors), type of surgery (e.g. cardiac, vascular), and certain medications used in the perioperative period (e.g. antihypertensive medications) [
2,
3,
6‐
10]. Medication use prior to surgery is of particular interest given the possibility of modification prior to planned elective surgeries.
Angiotensin converting enzyme inhibitor (ACEi) and angiotensin receptor blocker (ARB) are frequently prescribed medications [
11,
12]. Despite their demonstrated benefits in outpatient settings, their continued use in the perioperative period remains controversial as preoperative ACEi/ARB use may lead to the development of perioperative hypotension and subsequent AKI [
8,
13‐
16]. However, previous observational studies on the association between preoperative ACEi/ARB use and AKI have had conflicting results, and have focused on milder forms of AKI rather than the most serious renal outcome of AKI-D. We conducted this large retrospective cohort study to test the hypothesis that preoperative ACEi/ARB use compared to non-use associates with a different risk of AKI-D following major elective surgery.
Discussion
Contrary to the results of other studies, in this analysis preoperative ACEi/ARB use versus non-use was associated with 17% lower risk of AKI-D and 9% lower risk of all-cause mortality, when we adjusted for a set of prespecified potential confounders. We found similar results in the propensity score matched analyses. The association of preoperative ACEi/ARB use versus non-use on AKI-D was only evident in patients with CKD.
With respect to mechanism, it is possible that potential benefits of ACEi/ARB use in surgical settings relates to angiotensin II inhibition [
36,
40‐
42]. Angiotensin II is a potent vasoconstrictor, which can cause postoperative AKI (i.e., true tubular injury) by increasing oxidative stress, endothelial dysfunction, inflammatory response, and renal vascular resistance, and by reducing renal blood flow [
36,
42]. By inhibiting angiotensin II, ACEi/ARB use may increase the risk of functional AKI (i.e., a drop in GFR) but may paradoxically reduce the risk of “true AKI” (true tubular injury) [
36,
40‐
42]. In this regard, ACEi and ARB may improve renal blood flow and oxygenation to the renal tubules by vasodilatation of efferent arterioles and may prevent tubular cell necrosis during ischemic insults around surgery [
13]. To wit, a recent analysis from a large multi-center cohort of patients undergoing cardiac surgery (TRIBE-AKI cohort) showed that while AKI defined by changes in serum creatinine, was higher in those that had ACEi or ARB continued pre-operatively, there was not a concomitant increase in several biomarkers of kidney injury [
43]. While that study may also suffer confounding by indication, most impressive is that despite greater comorbidities in patients that continued ACEi/ARB, the kidney injury biomarkers still were not more elevated, suggesting some protective benefit of these agents [
43].
Our results are also similar to two small observational studies by Benedetto
et al. and Barodka
et al. [
40,
41] Benedetto
et al. studied 536 patients who underwent cardiac surgery and observed a lower risk of AKI with preoperative ACEi use compared to non-use (adjusted OR: 0.48; 95% CI: 0.23 to 0.77) [
41]. The authors presumed the pathophysiologic benefit of ACEi use stemmed from the preservation of renal blood flow during surgery [
41]. Barodka
et al. found similar benefits with preoperative ACEi/ARB use compared to non-use in 346 patients who underwent cardiac surgery (adjusted OR: 0.19; 95% CI: 0.04 to 0.84) [
40].
Other observational studies by Rady
et al. [
44], Ouzounian
et al.[
37], and Yoo
et al.[
45], studied 11330, 1647, and 472 cardiac surgery patients, respectively, and demonstrated no significant association between preoperative ACEi/ARB use and AKI. However, all three studies observed a non-significant trend towards a benefit with preoperative ACEi/ARB use raising the possibility of insufficient statistical power to detect the association [
37,
44,
45].
Miceli
et al. performed a propensity score matched analysis in 9,274 patients who underwent cardiac surgery and noted a 1.36 fold higher risk of AKI (adjusted OR: 1.36; 95% CI: 1.10 to 1.67) with preoperative ACEi/ARB use compared to non-use and a two-fold higher risk of mortality (adjusted OR: 2.00; 95% CI: 1.17 to 3.42) [
8]. The authors speculated that the AKI occurred as a result of a decrease in renal perfusion, mainly due to reduction in mean arterial pressure along with increased use of vasoconstrictors [
8]. Both Cittanova
et al. and Arora
et al. also observed an increased risk of AKI with preoperative ACEi and ARB use, although the study samples were small (249 aortic surgery patients and 1358 cardiac surgery patients, respectively) [
11,
46]. Railton
et al. studied the outcome of AKI-D and observed a higher incidence of AKI-D with ACEi/ARB use compared to non-use in patients who underwent abdominal aortic aneurysm repair (4.6% vs. 0.8%;
P value = 0.01) [
47]. However, the number of patients (n = 883) and number of events (n = 24) were small [
47].
Major considerations when comparing our results with previous studies are the heterogeneous AKI definitions, type of surgery, and the consideration of preoperative CKD in statistical analysis. Our primary outcome was AKI-D, which is the acute renal outcome most important to patients and their health-care providers [
1‐
3,
5]. The outcome of AKI-D is distinct from AKI defined solely by acute changes in serum creatinine [
48]. The latter is a surrogate outcome and may be misleading particularly in the ACEi or ARB setting (i.e. use of these medications in outpatient settings may increase serum creatinine concentration despite evidence that the drugs prevent progression to ESRD requiring chronic maintenance dialysis) [
11,
49,
50].
CKD is considered to be the most important risk factor for AKI [
7,
10]. However, the majority of previous studies did not account for CKD in their analyses [
8,
11,
37,
40,
41,
44‐
46]. In this study, we observed an association of less AKI-D with preoperative ACEi/ARB use compared to non-use only in patients with preoperative CKD . One concern interpreting this result is that CKD patients not receiving preoperative ACEi/ARB may represent an advanced disease population where ACEi/ARB might have been stopped due to the risk of early onset chronic maintenance dialysis [
51]. Another concern is that we identified CKD patients using database codes which have limited validity. These codes underestimate CKD prevalence may have impacted the study results. Moreover, this prevented us from examining CKD stages according to preferred glomerular filteration rate (eGFR) categories [
30,
52].
Study strengths and limitations
Our study has several strengths. To the best of our knowledge, this is the largest study to describe the association between preoperative ACEi/ARB use and AKI-D (over 230,000 patients from 118 hospitals). We included both cardiac and non-cardiac major surgeries. Unlike other studies, the large number of events of AKI-D (810 events) reduced concerns about statistical overfitting [
53]. Given there is <1% yearly emigration from Ontario, the loss to follow up was minimal. Finally, the information available within the large Ontario health care databases reflects routine clinical practice and may be less prone to participation biases that can arise in other types of studies [
3,
19‐
23].
There are several limitations to our study. Due to possible difference in underlying mechanism for the risk of postoperative acute dialysis in emergent surgeries compared to elective surgeries, we excluded emergent surgeries from our analyses. In an emergency situation it is also difficult to manipulate pre-operative medication use. However, exclusion of emergency surgeries may have reduced the generalizability of the study results. We could not determine medication compliance from evidence of a dispensed prescription for ACEi, ARB, or other medications in our data sources. Our study was limited by the absence of key information on perioperative ACEi/ARB use particularly whether it was held prior to surgery and if so when it was held, and when it was restarted after surgery. Important information such as preoperative and intraoperative blood pressure was also not available. These are key elements to guide the optimal and safe use of ACEi/ARB use in the perioperative period, including regimens to be tested in large randomized controlled trials (RCTs). We were also not able to adjust for variables unavailable in our data sources such as body mass index, preoperative proteinuria, non-prescription medication use and in hospital medication use. Another limitation with our data sources is the accuracy of codes for patient related health information. In attempt to limit these concerns, we did use database codes supported by validation studies whenever possible [
25‐
32].
Study implications and future directions
Our study results support the need for RCTs in this setting, to test a regimen of perioperative ACEi/ARB selected for optimal efficacy and safety. Given the low incidence of acute dialysis (about 0.45% in all types of major surgery [
1‐
3]), a very large trial (over 150,000 patients) would be needed to examine a meaningful difference with this as a primary outcome. Such a trial is unlikely to occur. However, if we consider a primary outcome of 90-day all-cause mortality (4.67% of patients in our study), the sample size is tenable at about 15,000 patients. The sample size could be further reduced if there is a rationale to consider a composite of clinically important events including peri-operative cardiac events. Enrolling a large number of patients with CKD may be prudent, as in the current study the signal of benefit for AKI-D was strongest in this group of patients. Given the large number of surgeries complicated by AKI-D worldwide each year, we propose that such RCTs should be undertaken.
Competing interests
Amit Garg’s institution received an unrestricted grant from Pfizer for research unrelated to the current project. All the authors declare no competing interests.
Authors’ contributions
MS, AJ and AG conceived of the study and developed the study protocol. MS, AJ, SB, SC, PD, MJ, JL, AM, MM, NP, CP, MP, SS, RW, MW, RW, DW and AG contributed to the study design. JL carried out the statistical analysis. MS drafted the manuscript. MS, AJ, SB, SC, PD, MJ, JL, AM, MM, NP, CP, MP, SS, RW, MW, RW, DW and AG read, edited and approved the final manuscript.