A Comparison of Acute Kidney Injury in On-Pump Versus Off-Pump Coronary Artery Bypass Grafting

By Farina Amjad, Muhammad Zameer Rajput, Saad Ahmed Naveed

Affiliations

1. Department of Anaesthesiology, Shifa International Hospital, Islamabad, Pakistan

doi: 10.29271/jcpsppg.2025.01.145

ABSTRACT
Objective: To determine the frequency of acute kidney injury (AKI) in patients undergoing coronary artery bypass grafting (CABG) and to compare the incidence of postoperative AKI between on-pump and off-pump CABG patients.
Study Design: An observational cross-sectional study.
Place and Duration of the Study: Department of Anaesthesiology, Shifa International Hospital, Islamabad, Pakistan, from October 2024 to July 2025.
Methodology: The study included 120 patients aged 40–80 years undergoing on-pump or off-pump CABG, using a non-probability consecutive sampling. Patients with elevated baseline renal markers or those undergoing additional cardiac procedures were excluded. Serum creatinine and urea levels were measured preoperatively and at 24 and 48 hours postoperatively. AKI was defined and staged according to the Kidney Disease: Improving Global Outcomes (KDIGO) criteria. An independent t-test was applied to compare the mean serum creatinine and urea levels between the two groups at each of the specified time point (preoperative, 24 hours, and 48 hours). Data were analysed using the Statistical Package for Social Sciences (SPSS) version 23.
Results: The mean age of the patients was 59.55 ± 9.01 years, with the majority being male. Overall, AKI occurred in 15% of CABG patients, with rates of 18.8% in the on-pump CABG group and 10.7% in the off-pump CABG group; however, this difference was not statistically significant. Patients aged over 65 years, those with obesity, and those with hypertension had significantly higher odds of developing AKI. Multivariate analysis confirmed these factors as independent predictors of AKI, whereas gender and diabetes mellitus were not significantly associated.
Conclusion: AKI is a significant complication after CABG, particularly in elderly, hypertensive, and obese patients. Early identification and targeted perioperative management can reduce its incidence and improve outcomes.

Key Words: Acute kidney injury, Coronary artery bypass grafting, On-pump, Off-pump, Diabetes, Hypertension.

INTRODUCTION

Acute kidney injury (AKI) is an acute decline in kidney function characterised by retention of urea, nitrogenous waste products, and dysregulation of extracellular volume and electrolytes.¹ AKI is defined according to certain criteria in clinical and epidemiological studies. The definition and staging system of Kidney Disease: Improving Global Outcomes (KDIGO) is the latest and most widely used.² AKI after coronary artery bypass grafting (CABG) is related to a number of unfavourable consequences, including an increase in the long-term risk of end- stage renal disease (ESRD), increased duration of hospital stay, additional healthcare expenditures, and increased morbidity and mortality.³

Among the leading causes of AKI in the intensive care unit (ICU) patients, cardiac surgery is the second most common.^4,5 The best course of action for individuals with significant impairment of left ventricular function and triple-vessel coronary artery disease is surgical intervention, which remains superior to medical treatment.⁶ After CABG surgery, up to 30% of patients experience AKI of varying severity, and about 2% need short- term dialysis.⁷

CABG can be categorised into two types: On-pump CABG, which utilises cardiopulmonary bypass (CPB), and off-pump or beating heart CABG, which does not. Because CPB is linked to systemic inflammatory response, non-pulsatile blood flow, haemodilution, renal hypoperfusion, atheroembolism, and free haemoglobin, it is widely acknowledged that CPB is a risk factor for AKI following CABG.⁸ Ortega-Loubon et al. reported the prevalence of AKI as 12.4% in CABG surgery.⁹ Although the results of on-pump and off-pump approaches have been evaluated in several studies, it is under debate whether the off-pump strategy might enhance CABG outcomes. Overall, the on-pump group had a higher percentage of AKI than the beating heart CABG group, although there were no considerable variations in terms of acute renal failure.¹⁰ In contrast, a lower frequency of AKI (19.1%) was found in patients who underwent beating heart CABG compared to those who had on-pump CABG (22.2%).¹¹

This study aimed to investigate the incidence of AKI in patients undergoing first-time isolated CABG using on-pump or off-pump techniques in Pakistan, thereby addressing a current gap in the local literature and helping to identify the preferable surgical approach for improved patient outcomes.

METHODOLOGY

This observational cross-sectional study was conducted in the Department of Anaesthesiology, Shifa International Hospital, Islamabad, Pakistan, from October 2024 to July 2025. Ethical approval was obtained from the Institutional Review Board of Shifa International Hospital, Islamabad, Pakistan (IRB No. 397-24). Written informed consent was obtained from all patients before enrolment. A total of 120 patients were included using a non-probability consecutive sampling strategy. The WHO sample size calculator was used to calculate the sample size. Based on a previous study by Ortega-Loubon et al.⁹ which reported a 12.4% prevalence of AKI in patients undergoing CABG, a sample size of 116 patients was required to estimate the expected incidence with a 6% precision level.

Patients aged between 40 and 80 years, of either gender, who were admitted to the ICU after undergoing CABG for multi-vessel coronary artery disease (either on-pump or off-pump), were recruited for the study. Patients were excluded if they had preoperative serum creatinine greater than 1 mg/dL, serum urea greater than 20 mg/dL, or were undergoing additional surgical procedures such as valve surgery. Patients who underwent concomitant cardiac surgeries, reoperations, urgent or emergent procedures, or minimally invasive operations were also excluded. Patients were evaluated for comorbidities. Those with an average diastolic blood pressure above 90 mmHg or average systolic pressure above 140 mmHg on at least two separate visits, or who had been on antihypertensive treatment for at least one month, were labelled as hypertensives. Patients were labelled as diabetic if their HbA1c level exceeded 6.7% or if they had been on anti-diabetic treatment for at least one month.

Preoperative serum creatinine and urea levels were retrieved from laboratory records within 48 hours before the surgery. Similarly, serum creatinine and urea levels were obtained 24 hours and 48 hours after the surgery, while the patients were in the ICU. All surgeries were performed in the same hospital by surgeons and anaesthesiologists with over five years of experience. Data were collected using a structured proforma.

The diagnosis and classification of AKI were done using the KDIGO criteria.¹² Renal function assessment was made at baseline, 24 hours, and at 48 hours postoperatively. The staging criteria were as follows: Stage 1: increase in serum creatinine ≥0.3 mg/dL (≥26.5 µmol/L) within 48 hours, or a 1.5–1.9-fold rise from baseline within the preceding seven days; or urine output <0.5 mL/kg/h for 6–12 hours. Stage 2: increase in serum creatinine to 2.0–2.9 times the baseline value, or urine output <0.5 mL/kg/h for ≥12 hours. Stage 3: increase in serum creatinine to ≥3.0 times the baseline value, or urine output <0.5 mL/kg/h for ≥24 hours. The presence of AKI at any time point was determined within seven days based on the serum creatinine criteria.

The Statistical Package for Social Sciences (SPSS) version 23 (SPSS Inc., Chicago, IL) was used to analyse the data. Categorical variables such as gender, comorbidities, type of CABG (on-pump or off-pump), occurrence of AKI, and AKI stage were reported in terms of percentages. Normality of all continuous variables was assessed using the Shapiro-Wilk test. Mean and standard deviation were reported for age, height, weight, body mass index (BMI), hospital stay, and serum creatinine levels. An independent samples t-test was used to compare mean serum creatinine and urea levels between groups at each time point. The frequency of AKI between the two groups was compared using the Chi-square test or Fisher’s exact test as appropriate. Multivariate Logistic regression was used to identify factors associated with AKI. Using the enter model, all variables were included in the multivariate analysis based on clinical relevance, regardless of their significance in univariate analysis. Odds ratios with 95% confidence intervals were computed for all factors, and a p-value of <0.05 was considered statistically significant.

RESULTS

The study included 120 participants who had CABG, either on-pump or off-pump. The patients in the study had a mean age of 59.55 ± 9.01 years, varying from 40 to 80 years. Male gender was predominant, accounting for 83.3% (n = 100) of the sample, compared to 16.7% females (n = 20). Hypertension (57.5% n = 69) and diabetes mellitus (55.8% n = 67) were the most commonly encountered associated conditions. The detailed demographic features are presented in Table I.

The prevalence of AKI in patients undergoing CABG was 15% (n = 18/120 cases), as illustrated in Figure 1. At baseline, 24 hours, and 48 hours, individuals with AKI had substantially higher mean blood creatinine levels than those without AKI. Additionally, the mean urine output was remarkably lower in patients who developed AKI than in those without AKI, as presented in Table II.

Table  I:  Demographic  characteristics  (n  =  120).

Variables	Mean ± SD/ number of patients (n)	Min-max (%)
Age (years)	59.55 ± 9.01	40-80
Weight (kg)	76.42 ± 12.82	48-129
Height (cm)	166.83 ± 8.12	145-190
BMI (kg/m2)	27.64 ± 4.20	19.9-39.8
Genders
Male	100	83.3%
Female	20	16.7%
Comorbidities
Hypertension	69	57.5%
Diabetes mellitus	67	55.8%
Asthma	6	5%
Smoker	9	7.5%
COPD	4	3.3%
Stroke	4	3.3%
Others	9	7.5%

Table II: Comparison of the mean serum creatinine and urine output according to time points.

Variables	AKI present (n = 18)	AKI absent (n = 102)	p-values
Serum creatinine
Baseline (mg/dL)	1.23 ± 0.39	1.02 ± 0.25	0.005
24-hour post-CABG (mg/dL)	1.59 ± 0.55	0.98 ± 0.25	0.001
48-hour post-CABG (mg/dL)	1.65 ± 0.92	0.94 ± 0.24	0.001
Urine output
Baseline (mg/dL)	66.11 ± 19.74	85.05 ± 18.05	0.001
24-hour post-CABG (mg/dL)	49.17 ± 26.91	94.48 ± 29.06	0.001
48-hour post-CABG (mg/dL)	71.67 ± 24.62	101.18 ± 31.69	0.001
An independent sample t-test was applied for mean comparison between the groups.

Table III: Comparison of characteristics of patients with and without AKI.

Variables	n	Postoperative AKI		p-values	Odd ratios
		Presence	Absent
CABG
On-pump	64	12 (18.8%)	52 (81.3%)	0.219	Reference
Off-pump	56	6 (10.7%)	50 (89.3%)		0.52 [0.18-1.43]
Age groups
≤55 years	40	5 (12.5%)	35 (87.5%)	0.045*	Reference
56-65 years	48	4 (8.3%)	44 (91.7%)		0.63 [0.15-2.55]
>65 years	32	9 (28.1%)	23 (71.9%)		2.74 [1.14-9.22]
Genders
Male	100	16 (16%)	84 (84%)	0.734	1.71 [0.63-8.12]
Female	20	2 (10%)	18 (90%)		Reference
BMI (kg/m2)
<30	88	9 (10.2%)	79 (89.8%)	0.022*	Reference
≥30	32	9 (28.1%)	23 (71.9%)		3.43 [1.22-9.62]
Hypertension
Yes	69	15 (21.7%)	54 (78.3%)	0.016*	4.44 [1.21-16.29]
No	51	5 (5.9%)	48 (94.1%)		Reference
Diabetes mellitus
Yes	67	10 (14.9%)	57 (85.1%)	0.979	0.98 [0.36-2.71]
No	53	8 (15.1%)	45 (84.9%)		Reference
Chi-square tests were used to assess the associations and calculate the p-values. Odds ratios were calculated independently from contingency tables for each predictor.

Table IV: Multivariate analysis showing the factors associated with acute kidney injury (AKI).

Factors	aOR	[95% CI]	p-values
CABG
On-pump	Reference
Off-pump	0.55	0.17-1.83	0.333
Age groups
≤55	Reference
56-65	0.62	0.13-2.91	0.46
>65	2.27	0.57-8.98	0.244
Genders
Male	1.61	0.29-8.73	0.58
Female	Ref
BMI (kg/m2)
<30	Reference
≥30	3.63	1.15-11.37	0.027
Hypertension
Yes	4.53	1.03-19.87	0.045
No	Reference
Diabetes mellitus
Yes	0.69	0.19-2.45	0.57
No	Reference
aOR = Adjusted odds ratio. Multivariate logistic regression analysis was used to obtain p-values. An enter model was applied to include all variables due to their clinical relevance.

The prevalence of postoperative AKI in patients undergoing on-pump and off-pump CABG was 18.8% (n = 12) and 10.7% (n = 6), respectively. However, this difference was not statistically significant, indicating no substantial variation in AKI risk between the two surgical techniques.

As shown in Table III, the odds of developing AKI were 2.74 times higher in patients aged over 65 years (OR = 2.74; 95% CI: 1.14–9.22; p = 0.045). Similarly, the risk was three to four times higher in obese patients (OR = 3.43; 95% CI: 1.21–9.62; p = 0.022) and those with hypertension (OR = 4.44; 95% CI: 1.21–16.29; p = 0.016). In contrast, gender and diabetes mellitus were not found to be statistically significant predictors of AKI.

Multivariate analysis, as presented in Table IV, demons-trated that obesity and hypertension were independently associated with an elevated risk of AKI following CABG, while gender and diabetes mellitus did not show a statistically significant association.

Figure 1: Frequency distribution of AKI in patients with CABG (n = 120).

DISCUSSION

AKI is an important medical complication, especially after cardiac surgery, leading to increased illness and fatality, and the research on its pathogenesis is still in progress in recent years. CABG carries a risk of AKI.^13,14 This study assessed the incidence of AKI in patients undergoing CABG and compared outcomes between the on-pump and off-pump techniques. The study population had an average age of 59.55 ± 9.01 years, with 83% males and 17% females. Hypertension and diabetes mellitus were the common comorbidities. The overall frequency of AKI was 15%, consistent with both national and international literature showing an incidence ranging from 12 to 45% following CABG.^15-18 The incidence among patients of the on-pump CABG group was 18.8% compared to the off-pump group (10.7%). Multivariate analysis identified obesity and hypertension as significant independent risk factors for AKI. In contrast, gender and diabetes mellitus did not show a statistically significant association. Most cases in this cohort were classified as stage 1 AKI as per the KDIGO criteria, implying early-stage renal injury and its potential for favourable outcomes with timely intervention.

These findings align with those of Cheungpasitporn et al., who reported a modest protective effect of the off-pump approach with a relative risk of 0.87 (95% CI: 0.77–0.98).¹¹ Similarly, Yadav and Pradeep reported that patients undergoing on-pump CABG have a higher incidence of AKI compared to beating heart CABG. In the latter study, there were 12 (10.8) cases of AKI, out of which, 5 (8%) patients were in the off-pump group and 7 (10.8%) patients were in the on-pump group.¹⁹ In general, the off-pump CABG group had fewer cases of AKI than the on-pump CABG group. In their systematic review and meta-analysis of 31 prospective randomised controlled trials (RCTs), Spanuchart et al. noted that the off-pump CABG approach significantly reduced the incidence of AKI when compared to traditional on-pump CABG.²⁰ Their meta-analysis, however, did not demonstrate an advantage in survival for individuals receiving both forms of CABG. The findings of this study correlate with those of King et al.’s study, the biggest RCT with 4,752 patients enrolled from 79 institutions across 19 nations, with significant 5-year results, which demonstrated that the incidence of AKI did not significantly differ between the off-pump and on-pump groups.²¹

Patients over 65 years of age showed a significantly higher likelihood of developing AKI. This is consistent with a previous study indicating diminished renal reserve and increased susceptibility to perioperative insults in the elderly individuals.²² Obesity emerged as another independent predictor, potentially due to obesity-related haemodynamic changes and activation of the renin-angiotensin system.²³ Likewise, hypertension was significantly associated with AKI risk, likely due to chronic microvascular damage and compromised renal autoregulation, even when baseline serum creatinine was normal.²⁴

Unexpectedly, diabetes mellitus did not show a significant association with AKI. This finding, although surprising, is not unique and may reflect limitations in relying solely on serum creatinine without accounting for subclinical nephropathy or microalbuminuria, as highlighted by Silva et al.²²

In terms of diagnostic methodology, different AKI definitions might result in inconsistent clinical study findings. Based on a thorough evidence-based evaluation of literature, the KDIGO's guidelines on AKI offer physicians’ much-needed clinical guidance.²⁵ This study applied the KDIGO criteria, which are currently the most widely accepted and sensitive classification system for AKI.

These findings underscore the importance of individualised risk stratification in cardiac surgical patients, especially focusing on modifiable factors such as obesity and hyper-tension. While surgical technique (on-pump vs. off-pump) remains a clinical consideration, patient-specific characte-ristics may have a major contribution in predicting AKI risk. The use of standardised KDIGO criteria enhanced the diagnostic accuracy and allowed comparability with international studies. Data collection at multiple time points postoperatively (0, 24 hours, and 48 hours) improved the sensitivity of diagnosis, while all procedures were performed by experienced surgical teams, ensuring consistency in operative technique.

This study has certain limitations. It was conducted at one centre, limiting its generalisability. The relatively small sample size, particularly in subgroup comparisons, may have underpowered the analysis for detecting statistically significant differences in AKI incidence between surgical techniques. Furthermore, urine output data were not incorporated into the AKI diagnosis, possibly leading to an underestimation of incidence. The study also excluded high-risk patients with pre-existing renal dysfunction and those undergoing emergency surgery, which may have further underestimated the true AKI burden. Lastly, short-term follow-up precluded the analysis of long-term renal outcomes and survival.

CONCLUSION

AKI remains a clinically significant complication following CABG, with an incidence of 15% in this study population. While the utilisation of CPB (on-pump CABG) was related to a higher rate of AKI compared to off-pump CABG, the difference was not statistically significant. Instead, patient-specific factors, including obesity and hypertension, emerged as independent predictors of postoperative AKI. These findings highlight the need for vigilant perioperative risk assessment and optimisation, particularly in high-risk subgroups. Early identification, preventive strategies, and targeted interventions may reduce the incidence and severity of AKI, improving postsurgical outcomes. Further large-scale, multicentre studies with long-term surveillance are essential to validate these findings and formulate evidence-based guidelines for AKI prevention in cardiac surgery.

ETHICAL APPROVAL:
Ethical approval was obtained from the Institutional Review Board of Shifa International Hospital, Islamabad, Pakistan (IRB No. 397-24).

PATIENTS’ CONSENT:
Informed consent was obtained from all participants involved in the study.

COMPETING INTEREST:
The authors declared no conflict of interest.

AUTHORS’ CONTRIBUTION:
FA: Conception, design, data analysis, interpretation, and drafting.
MZR: Conception and critical analysis of the manuscript for important intellectual content.
SAN: Analysis and interpretation of data, drafting, and accountable for all aspects of the work.
All authors approved the final version of the manuscript to be published.

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