Cardiovascular Journal of Africa: Vol 32 No 6 (NOVEMBER/DECEMBER 2021)

CARDIOVASCULAR JOURNAL OF AFRICA • Volume 32, No 6, November/December 2021 312 AFRICA cardiac surgery. Its importance is emphasised in the Enhanced Recovery After Cardiac Surgery (ERACS) recommendations. 15 It is highlighted that ‘early detection of kidney stress and interventions to avoid acute kidney injury after surgery’ are necessary. 15 This single-centre study was a retrospective records review of adult patients who presented for cardiac surgery. The cases included CABG, cardiac valve repair surgery, aortic surgery, and other cardiac-related surgeries on CPB. The prevalence of CSA-AKI, defined using the KDIGO criteria, at 28% in our study, is similar to that reported in previous reports that show the frequency of CSA-AKI to be up to 30%. 3,16 Thirty-two (24%) of these patients had presented with a normal pre-operative eGFR. This suggests a peri-operative and possibly modifiable factor in the development of their AKI. The majority had early kidney stress (KDIGO 1), which could have been improved with early detection and intervention. The study highlights several modifiable factors that can be mitigated for better post-operative outcomes. Pre-operative eGFR and SCr levels, and long CPB times were three such factors associated with CSA-AKI. An elevated pre-operative SCr level was found to be an independent risk factor for the development of CSA-AKI and to contribute to mortality in one study. 4 Identifying patients with low eGFR and planning for renal rescue in the peri-operative period could lead to better outcomes, including in-hospital mortality rate. Although a previous study 17 found no relationship between low cardiac output and need for IABP, duration of CPB and cross-clamping, we postulate that these factors were reflective of the peri-operative course and complications. Early identification of patients with these factors and planning for renal rescue in the peri-operative period could lead to better outcomes, including in-hospital mortality rate. 4 Patients who developed AKI displayed a steady increase in SCr levels from baseline, notable on day one, the peak occurring on day three. The changes in SCr ranged from 1 (0.78–1.4) to 1.4 (1.18–1.74) over seven days. Patients in the non-AKI group, on the contrary, displayed a decline in SCr levels from day one, which continued over seven days. Although these changes seemed minor, Lassnigg et al . 18 in their study stated that ‘even minor degrees of post-operative AKI, as manifest by only a 0.2 to 0.3 mg/dl rise in serum creatinine from baseline, predicted a significant increase in short-term mortality’. As part of a management strategy for CSA-AKI, the following recommendations have been made: prevention, optimisation pre-operatively, modification of risk factors, and peri-operative management using the KDIGO care bundle. 19 In an attempt to detect early ‘kidney stress’, newer modalities of detection have been described and include the use of biomarkers such as neutrophil gelatinase-associated lipocalin (NGAL), interleukin-18 (IL-18), cystatin C, insulin-like growth factor binding protein 7 (IGFBP-7) and tissue inhibitor of metalloproteinases-2 (TIMP-2). 20 These biomarkers have been investigated in line with the principles of the KDIGO care bundle. 21 Early biomarker-based prediction of imminent AKI followed by implementation of the KDIGO care bundle reduced AKI severity, post-operative creatinine increase, and length of ICU and hospital stay in patients after major non-cardiac surgery. Use of renal vascular ultrasound is one other modality being investigated. 22,23 The mortality rate was 9.6% (46 out of 476 patients). Mortality rates were significantly higher in those with AKI compared to those without [28 (61%) vs 18 (39%), respectively] ( p = 0.001). The incidence was significantly worse in those with more severe kidney injury. Similarly, a previous study has shown an association of increased 30-day mortality 24 with a higher KDIGO class. In a meta-analysis of 91 observational studies with 320 086 patients, Hu et al . 25 found pooled short- and long- term mortality rates of 10.7 and 30%, respectively, with increases along with the severity of stages. The current study found pre-operative eGFR to be the sole predictor of in-hospital mortality. A Japanese study also found pre-operative eGFR to be predictive of all-cause mortality, cardiac mortality, and an increased risk of major adverse events (MACE) (HR 0.983, p = 0.026; HR 0.963, p = 0.006; HR 0.983, p = 0.002), respectively, in patients following cardiac surgery. 26 Due to the association with increased mortality rate and length of hospital stay, with mortality rates varying from 22 to 36%, and rates of 50 to 80% in severe AKI requiring renal replacement therapy, early detection of CSA-AKI is necessary to mitigate risk. 20 This is also emphasised in the ERACS recommendations. 15 Limitations Retrospective cohort studies rely on accurate record keeping, as errors related to bias and confounding factors are high, affecting the level of the evidence. Large sample sizes are required, as some statistical analysis cannot be measured. Data were obtained from a single centre, therefore, results are contextual and may not be extrapolated to other population groups. The probable presence of unknown confounding factors affects the interpretation of the results. A multicentre study would be beneficial to permit larger sample sizes. The mortality rate was low, making the regression models likely to be over-fitted. It also narrowed the scope of variables that could be included in the multivariable regression models. This may have confounded the results. Relationships were, Table 6. Cox regression analysis of predictors of in-hospital mortality Predictors of mortality (n = 46) Univariable regression analysis Multivariable regression analysis UHR (95% CI) p -value HR (95% CI) p -value Age 1.02 (0.99–1.04) 0.115 Male gender 0.86 (0.44–1.69) 0.667 African race 1.34 (0.71–2.55) 0.368 Valve surgery 0.54 (0.27–0.06) 0.073 0.60 (0.22–1.61) 0.309 CABG 2.02 (1.02–3.99) 0.043 1.16 (0.41–3.31) 0.775 Aortic surgery 22.56 (6.96–73.09) < 0 .001 0.54 (0.06–4.69 ) 0.575 Hypertension 0.89 (0.44–1.78) 0.742 Cholesterol 0.97 (0.34–2.84) 0.962 Diabetes mellitus 1.53 (0.77–3.02) 0.222 Smoker 2.89 (1.21–6.91) 0.017 0.45 (0.17–1.23) 0.121 ECMO 1.20 (0.67–2.15) 0.540 IABP 1.46 (0.74–2.89) 0.273 Pre-operative eGFR 0.983 (0.97–0.99) 0.006 0.99 (0.97–0.99) 0.019 AKI 1.52 (0.72–3.23) 0.269 RRT 0.64 (0.43–0.97) 0.034 2.64 (0.13–1.61) 0.223 UHR, unadjusted hazards ratio; HR, hazard ratio; CI, confidence interval; CABG, coronary artery bypass graft; ECMO, extracorporeal membrane oxygen- ator; IABP, intra-aortic balloon pump; eGFR, estimated glomerular filtration rate; AKI, acute kidney injury; RRT, renal replacement therapy.