Serum markers for early detection of patients with mesenteric ischemia after cardiac surgery

Introduction

Mesenteric ischemia (MESI) is a rare complication in patients after cardiac surgery, with an incidence of 0.1–0.5% and high mortality rates between 60% and 100% [1], [2], [3].

The pathophysiological mechanisms for MESI after cardiac surgery are multimodal. It is well known that cardiopulmonary bypass (CPB) causes systemic inflammation [4], reduction of intestinal perfusion [5], microcirculatory alterations, loss of intestinal barrier function [6], and hemolysis [7]. However, postoperatively low cardiac output, intravascular volume deficiency, and the need for vasopressors restrict mesenteric perfusion. Unfortunately, mesenteric malperfusion itself is a trigger of all the above-mentioned factors, resulting in a circulus vitiosus process.

The diagnosis of MESI after cardiac surgery is difficult in the setting of mostly sedated, intubated patients, with low cardiac output, systemic inflammatory response syndrome, need for vasopressors, and usually many differential diagnoses for acute abdomen. Early diagnosis and surgical treatment have been suggested to be the only beneficial factors for the prognosis of these patients [2, 8, 9]. Recent publications indicated that different biomarkers have some impact on the detection of MESI [10], [11], [12]. Indeed, α-glutathione-S-transferase (αGST), intestinal fatty-acid-binding protein 2 (iFABP-2), and D-lactate are the most promising biomarkers for the early and accurate detection of MESI. These biomarkers are explained in detail below.

First, the GSTs are a family of enzymes involved in the detoxification of a range of toxic and foreign compounds within the cell by conjugation to glutathione, although different isoforms exist, such as α, π, θ, and μ, which are distributed in relatively specific organs [13]. For example, αGST is a unique biomarker for hepatic and intestinal injuries.

Second, FABPs display a high affinity for long-chain fatty acids and seem to function in metabolism and intracellular transport of lipids. Three distinct FABPs are known: liver FABP, iFABP, and ileal lipid binding protein distributed proximally to distally in the intestine. The iFABP is detectable along the entire length of the small intestine and is maximally represented near the medial segment [14].

Finally, D-lactate and L-lactate result from the reduction of pyruvate by D- and L-lactate dehydrogenases, respectively. These enzymes uniquely exist in bacteria or mammals, respectively. Therefore, the quantification of D-lactate has been revealed as a marker for bacterial translocation, which follows intestinal or colonic mucosal injury caused by ischemia or other reasons [15].

Therefore, the aims of this study were (i) to quantify and monitor in a time-dependent manner these markers in patients after cardiac surgery and (ii) to evaluate their potential in identifying patients at a risk for MESI.

Methods

Results

High αGST levels are associated with laparotomy

Already 1 h after cardiac operation, nearly 10-fold higher αGST concentrations were found in the serum of patients needing a laparotomy 11±7 days later compared to patients without laparotomy. After 24 h, the αGST concentrations in the laparotomy group were >25-fold higher compared to the control group and remained significantly higher after 48 h (Table 2, Figure 1A). However, the differences in αGST serum concentration between the laparotomy patients with and without MESI were not significant within 48 h after cardiac surgery (Table 2, Figure 2A). Up to 3 days before laparotomy, the αGST levels in MESI and non-MESI patients did not differ significantly but were even lower than the initial postoperative levels of the control group (Table 3, Figure 2A).

Table 2:

Mean values±standard deviation (SD) of αGST, D-lactate, and iFABP 1, 24, and 48 h after cardiac surgery for the control group and laparotomy group with subgroup analysis for MESI and non-MESI patients.

	Control, mean±SD (n)	Laparotomy, mean±SD (n)	p-Value	MESI, mean±SD (n)	Non-MESI, mean±SD (n)	p-Value
αGST (μg/L)
1 h	82±126 (492)	727±1382 (18)	<0.01	280±245 (9)	1173±1884 (9)	0.35
24 h	73±167 (484)	4549±9889 (18)	<0.01	3741±8542 (9)	1459±3013 (9)	0.63
48 h	83±153 (295)	2300±5895 (18)	<0.01	1010±1710 (9)	3204 ±8282 (9)	0.63
D-lactate (nmol/mL)
1 h	25.1±20.1 (492)	37.1±17.9 (18)	<0.01	36.3±20.8 (9)	37.8±15.9 (9)	0.51
24 h	23.2± 21.1 (484)	52.2±40.5 (18)	<0.01	58.7±40.9 (9)	45.7±41.4 (9)	0.23
48 h	24.8±23.6 (295)	58.7±54.3 (18)	<0.01	56.3±44.6 (9)	61.1±65.2 (9)	0.69
iFABP (ng/mL)
1 h	4.05±4.04 (492)	1.04±0.91 (18)	<0.01	0.78±0.71 (9)	1.30±1.05 (9)	0.29
24 h	3.42±4.95 (484)	2.03±1.86 (18)	0.38	1.15±1.30 (9)	2.90±1.99 (9)	0.04
48 h	4.28±6.12 (295)	2.88±2.66 (18)	0.95	1.89±1.78 (9)	3.87±3.11 (9)	0.12

Figure 1:

Already 1 h after cardiac surgery and 24 and 48 h thereafter, the serum concentrations of D-lactate and αGST in patients undergoing laparotomy were increased compared to the control group.

Timeline of αGST (A), D-lactate (B), and iFABP (C) mean serum concentrations and 95% confidence interval (CI) at 1, 24, and 48 h after cardiac surgery for the control group and the laparotomy group.

Figure 2:

Comparing laparotomy patients with and without MESI up to 3 days before laparotomy, no significant differences between these two groups were found for αGST and D-lactate, but the iFABP-2 levels were significant lower in the MESI group.

Timeline of αGST (A), D-lactate (B), and iFABP (C) mean serum concentrations and 95% CI at 1, 24, and 48 h after cardiac surgery for the control, MESI, and non-MESI group at 1, 24, and 48 h after cardiac surgery and 72, 48, 24, and 12 h before laparotomy.

Table 3:

Mean serum concentration±SD of αGST, D-lactate, and iFABP 72, 48, 24, and 12 h before laparotomy for MESI and non-MESI patients. One patient underwent laparotomy 48 h after cardiac surgery.

	MESI, mean±SD (n)	Non-MESI, mean±SD (n)	p-Value
αGST (μg/L)
−72 h	35.8±82.4 (8)	22.6±29.1 (8)	0.40
−48 h	12.3±20.6 (8)	52.1±79.8 (8)	0.29
−24 h	28.6±30.9 (9)	70.6±100.5 (9)	0.55
−12 h	16.3±21.6 (9)	55.4±96.7 (9)	0.77
D-lactate (nmol/mL)
−72 h	24.1±9.5 (8)	24.9±9.6 (8)	0.83
−48 h	23.3±4.7 (8)	38.1±30.9 (8)	0.34
−24 h	25.9±9.8 (9)	36.4±19.9 (9)	0.09
−12 h	40.6±41.8 (9)	35.4±17.1 (9)	0.63
iFABP (ng/mL)
−72 h	0.56±0.72 (8)	2.51±1.96 (8)	0.01
−48 h	0.58±1.02 (8)	2.79±1.57 (8)	0.01
−24 h	0.76±0.78 (9)	3.57±1.77 (9)	<0.01
−12 h	0.78±1.04 (9)	3.50±2.17 (9)	0.01

Discussion

The incidence of MESI after cardiac surgery in the literature varies. In a review with 35 included papers and a total of 151,652 patients, Rodriguez et al. calculated an incidence of 0.16% with 58% mortality [1]. Pang et al. described an even higher incidence (0.31%) and mortality (71%) [2], and retrospective autopsy studies found MESI in 0.49% of the patients, responsible for 11% of all postoperative deaths [3]. Interestingly, 96% of these had a non-occlusive MESI (NOMI), which is interesting regarding the definition and therapy of MESI, which will be addressed later. Our high laparotomy and MESI rate of 1.6% might be the result of a bias during the study period; however, it reflects the severely ill patient population with a mean EuroSCORE I of 11±14%. According to the GICS model introduced by Nilsson et al. [16], the expected MESI rate in our patient population is >1.9%, emphasizing the accuracy of the suggested score and the frailty of the observed patient cohort.

In the setting of cardiac and aortic surgery, some studies focused on αGST, D-lactate, and iFABP, and their elevation during operations with and without CPB [17], [18], [19]. On-pump coronary artery bypass grafting (CABG) patients showed significantly higher serum αGST levels at the end of surgery (10 μg/L) compared to off-pump CABG patients (4 μg/mL), normalizing 24 h after surgery [17]. Although the reported mean values are low compared to our total control group, they are similar to the mean values of an uncomplicated CABG subgroup (6 μg/mL), demonstrating the general impact of CPB on mesenteric perfusion and intestinal barrier function. In another study focusing on intestinal barrier function, Sun et al. found the highest D-lactate concentration (11 nmol/mL) 2 h postoperatively and the highest iFABP concentration (1.4 ng/mL) 6 h postoperatively in aortic valve repair/replacement patients. Both findings and values are conclusive with the results of our control group, again emphasizing damage and loss of intestinal barrier function during any operation using CPB. Also, in patients operated for abdominal aortic aneurysm (AAA), elevated D-lactate levels (33 vs. 11 nmol/mL [20]) were found in patients with histologically diagnosed ischemic colitis 24 and 48 h after open aortic reconstruction. Patients with a fatal intestinal necrosis after AAA repair had increased iFABP levels (0.6 vs. 4 ng/mL) early postoperatively and decreased concentrations on postoperative days 3 and 4 [21].

D-lactate and αGST showed no differences between patients with and without MESI, neither 72, 48, 24, or 12 h before laparotomy nor early after cardiac surgery. However, both markers were significantly increased in the laparotomy group within the first 2 days after cardiac surgery, suggesting a pathologic gastrointestinal process 11±7 days before clinical signs or imaging indicated laparotomy.

Interestingly, in our study, iFABP-2 was decreased in the laparotomy group compared to the control group early after cardiac surgery and increased again within the next 48 h, although the iFABP-2 concentration remained significantly lower in the MESI subgroup compared to the non-MESI subgroup. Before laparotomy, MESI patients had significantly lower iFABP-2 levels compared to non-MESI patients. These findings might be explained by our definition of MESI, which was a necrotic intestine diagnosed by laparotomy. In a human model, Schellekens et al. demonstrated the reduction of tissue and serum iFABP-2 within 120 min reperfusion after up to 60 min ischemia by subtotal destruction of the intestinal enterocytes and villus structures [22]. In histology of the resected intestinal segments in our study, transmural necrosis was found in 5/9 patients, and submucosal necrosis in 4/9 patients. In all specimens, no mucosa that could have released iFABP-2 was found.

The definition of MESI as intestinal ischemia treated by laparotomy and resection is unambiguous and used in most retrospective studies. However, it does not reflect the dynamics of this disease and its pathomechanism, which is non-occlusive (NOMI) in 95% [3] with an incidence after cardiac surgery of up to 10% [23]. We assume an even higher rate of mild and non-apparent MESI after most cardiac operations, demonstrated by elevated markers after CPB in all studies including ours. αGST and D-lactate seem to be suitable markers for early detection of gastrointestinal complications after cardiac surgery, and iFABP-2 could help differentiate patients with ischemia. Especially in high-risk patients with CPB times >150 min, pre- and postoperative need for inotropic support, atrial fibrillation, and high GICS and EuroSCORE, these markers might be used for differentiation. Nevertheless, resection of necrotic tissue is only damage control, and the benefit of a very early resection is questionable. Accepting the pathologic mechanism and the dynamic character, new therapeutic and protective approaches are required. Substances like glycine [24, 25] and pyruvate [26] demonstrated protective effects in MESI models in rats. The evaluated markers could not only be useful for early detection of patients with MESI, but also represent a new routine for evaluation of protective effects of different substances or conditioning maneuvers in further clinical studies.