The value of bioactive adrenomedullin and dipeptidyl peptidase 3 to predict short-term unfavourable outcomes after cardiac surgery

A prospective cohort study

Abstract

BACKGROUND 

Adrenomedullin (ADM) is a key regulator of endothelial barrier function and vascular tone. Dipeptidyl peptidase 3 (DPP3) is a protease involved in the degradation of several cardiovascular mediators. Increased levels of bioactive ADM (bio-ADM) and circulating DPP3 (cDPP3) were found to predict short-term outcome in cardiogenic shock patients.

OBJECTIVES 

To investigate the unknown temporal profiles of bio-ADM and cDPP3 and their association with short-term outcome following cardiac surgery.

DESIGN 

Prospective observational study of 203 adult cardiac surgery patients admitted to the intensive care unit (ICU) postoperatively. Plasma bio-ADM and cDPP3 levels were measured at ICU admission (day 1) and on days 2 and 3.

MAIN OUTCOME MEASURES 

Biomarker prediction of prolonged vasopressor dependency (>3 days), acute kidney injury (AKI) and prolonged ICU length of stay (ICU-LOS) (>3 days).

RESULTS 

bio-ADM and cDPP3 levels displayed distinct temporal profiles following cardiac surgery. cDPP3 levels were highest on day 1 and strongly correlated with surgical complexity and duration but subsequently normalised on day 2 in most patients. In contrast, bio-ADM levels on day 1 were within the normal range but subsequently increased. Day 2 bio-ADM levels were strongly associated with study outcomes: the area under the receiver-operating curves (AUROC) were 0.82 (95% CI, 0.72 to 0.92) for prolonged vasopressor dependency, 0.87 (0.81 to 0.92) for AKI and 0.82 (0.75 to 0.90) for prolonged ICU-LOS (all P < 0.0001). cDPP3 levels on day 2 also predicted these outcomes, albeit to a lesser extent, with AUROCs of 0.73 (95% CI, 0.64 to 0.81) for prolonged vasopressor dependency, 0.69 (0.61 to 0.77) for AKI and 0.70 (0.62 to 0.79) for prolonged ICU-LOS (all P < 0.0001).

CONCLUSION 

Following cardiac surgery, increased bio-ADM levels are strongly associated with unfavourable short-term outcomes, whereas cDPP3 levels are mainly related to surgery complexity and duration. On the basis of these findings, ADM-modulating therapies may have beneficial effects in cardiac surgery patients whereas DPP3-targeted therapies should be reserved for patient categories with higher baseline disease severity.

KEY POINTS

• cDPP3 and bio-ADM display distinct temporal profiles following cardiac surgery.
• bio-ADM levels were strongly associated with short-term outcome.
• cDPP3 levels were mainly related to the extent and complexity of surgery.
• bio-ADM-targeted therapies might improve outcome of cardiac surgery patients.

Introduction

Cardiac surgery and the use of cardiopulmonary bypass induce a systemic inflammatory response, which is associated with several common postoperative complications.^1,2 Despite recent advances in operative techniques, including minimally invasive surgery and transcatheter approaches, up to 25% of patients develop a postsurgical vasoplegic syndrome requiring prolonged vasopressor therapy.³ Furthermore, up to 30% of cardiac surgery patients develop acute kidney injury (AKI).³ These complications result in a prolonged intensive care unit (ICU) stay and are independently associated with impaired outcome.^1,3,4 Early detection and treatment of organ dysfunction could prevent progression to organ failure. However, patients recovering from cardiac surgery present varying degrees of altered physiological and biochemical homeostasis, complicating the early identification of organ dysfunction in these patients.^2,5

Adrenomedullin (ADM) is a key hormone involved in the regulation of endothelial barrier function and vascular tone.^6,7 Dipeptidyl peptidase 3 (DPP3) is a ubiquitous cytosolic enzyme involved in the degradation of several important regulators of vascular tone, most notably angiotensin II.^7,8 Whenever cell injury occurs, intracellular DPP3 is released into the circulation (cDPP3),⁷ and the subsequent degradation of angiotensin II may contribute to haemodynamic instability. Increased levels of both bioactive ADM (bio-ADM) and cDPP3 are associated with the development of organ failure and increased mortality in cardiogenic shock and sepsis patients.^9–11 Thus, ADM and DPP3 represent two distinct molecular pathways, which could be involved in the development of circulatory shock.⁷ Furthermore, both biomarkers represent promising targets for drug therapy, as ADM-modulating and DPP3-modulating treatments have shown beneficial effects in animal models of cardiogenic and septic shock.^12–14

Whether bio-ADM and cDPP3 are also associated with short-term clinical outcomes after cardiac surgery is currently unknown. In this study, we investigated the temporal profiles of bio-ADM and cDPP3 following cardiac surgery, and their association with short-term clinical outcomes.

Methods

Study design and population

We performed a single-centre prospective cohort study including all cardiac surgery patients admitted to the ICU of the Radboud university medical centre from September 2018 until December 2019. Patients underwent coronary artery bypass grafting (CABG) surgery, valve replacement or repair, combined CABG and valvular procedures, atrial/ventricular septal defect repair, intra-cardiac tumour resection surgery, or ascending/descending aortic arch procedures. To investigate the effect of surgical severity on biomarker temporal profiles, a separate subgroup analysis was performed. For this analysis, patients were stratified into three groups: elective surgery (cardiopulmonary bypass performed in a planned surgical setting); emergency surgery (cardiopulmonary bypass performed in an emergency setting because of clinical deterioration, for example, cardiogenic shock as a consequence of myocardial infarction); and minimally invasive surgery [including transcatheter aortic valve replacement (TAVI) approaches with no cardiopulmonary bypass]. The minimally invasive surgery subgroup was included only for biomarker temporal profile analyses and not considered for biomarker predictive performance analyses.

The study was carried out in accordance with the applicable rules concerning the review of research ethics committees and informed consent in the Netherlands. Ethical approval for this study (CMO 2018-4672) was provided by the research ethics committee of the Radboud University Nijmegen Medical Centre, the Netherlands on 21 August 2018. All patients or legal representatives were informed about the study details and could decline to participate. The study was conducted in accordance with the declaration of Helsinki, including current revisions, and Good Clinical Practice guidelines.

Outcomes

Study endpoints were the associations of bio-ADM and cDPP3 with prolonged need for vasopressor support, development of AKI during ICU admission and ICU and hospital length of stay (LOS). For all reported outcomes, day 1 was defined as the first day of ICU admission, directly after cardiac surgery was performed, regardless of admission time. For analyses requiring binary outcomes, prolonged vasopressor requirement was defined as the need for any vasopressor therapy to maintain blood pressure, despite adequate fluid loading, for more than 3 days after ICU admission.³ AKI was defined and staged according to the Kidney Disease: Improving Global Outcomes (KDIGO) criteria.¹⁵ For KDIGO calculations, pre-operative creatinine levels were used whenever available. Whenever pre-operative creatinine levels were not available (e.g. emergency surgery), the first available creatinine measurement following hospitalisation was used. Hospital admission days were counted as follows: day 1 was the day of admission to ICU after the cardiac surgery, irrespective of the time of admission; for a patient discharged at any time during the following day (day 2), this was recorded as a 2-day ICU admission; a prolonged ICU stay was defined as a stay in ICU of more than 3 days;^3,16 a prolonged hospital stay was defined as a hospital LOS of more than 10 days, including the days in the ICU.^3,16

Treatment protocol

Following cardiac surgery, patients were treated in the ICU according to our standardised institutional protocols. Mean arterial pressure (MAP) was maintained at at least 70 mmHg. If patients developed hypotension (MAP < 70 mmHg), fluid loading was performed if central venous pressure (CVP) was less than 14 mmHg. If CVP was greater than 14 mmHg and cardiac index was greater than 2.5 l min⁻¹ m⁻², norepinephrine was started. Inotropic medication was initiated if the cardiac index was less than 2.5 l min⁻¹ m⁻², with simultaneous hypotension and a CVP greater than 14 mmHg. Both dobutamine and milrinone were considered first-line inotropic medications, with the decision for specific therapy at the clinician's discretion.

Clinical parameters

Upon admission to the ICU (day 1), patient characteristics (age, sex, BMI), organ dysfunction scores (acute physiologic assessment and chronic health evaluation II, APACHE II¹⁷ and sequential organ failure assessment, SOFA¹⁸), medical history and cardiac surgery characteristics were collected. After patient enrolment, the following data were collected daily until ICU discharge: need for vasopressor treatment, need for renal replacement therapy, cumulative fluid balance and need for invasive mechanical ventilation. The following routine laboratory parameters were collected: serum creatinine, troponin-T, creatinine-kinase and lactate.

Biomarker measurements

Blood samples for the determination of bio-ADM and cDPP3 concentrations were obtained within 2 h after ICU admission (day 1 samples) and follow-up samples were acquired during subsequent morning rounds (day 2, day 3, etc.). Blood samples were immediately added to ethylenediaminetetraacetic acid (EDTA) anticoagulant and were centrifuged at 2000g at 4 °C for 10 min, after which the plasma was stored at −80 °C until blinded analysis occurred using luminescence immunoassays for bio-ADM (SphingoTec GmbH) and cDPP3 (4TEEN4 Pharmaceuticals GmbH). The details and design principles of these assays are provided elsewhere.^19,20 The upper limits of normal (based on levels obtained in healthy volunteers) for bio-ADM and cDPP3 with the assays used are 43 pg ml⁻¹ and 40 ng ml⁻¹, respectively.^9,21

Statistical analyses

Continuous variables are presented as median [IQR], and categorical variables as count (%). Group comparisons of continuous variables were performed using Mann–Whitney U tests or Kruskal–Wallis tests, depending on the number of groups. If Kruskal–Wallis yielded overall significance, Dunn's post hoc test was performed. Categorical data were compared using Fisher's exact tests. Correlations between continuous variables were calculated using Spearman's rank correlation coefficients.

Areas under the receiver-operating characteristic curves (AUROC) with 95% confidence intervals (CIs) are reported as x (y to z) as an effect measure of different predictor variables. For illustration, Kaplan–Meier curves using the third quartile as a cut-off were generated. Third quartile cutoffs were chosen for this specific analyses, as median cutoff levels of cDPP3 and bio-ADM were still within the normal range found in earlier studies of healthy volunteers.^9,21 To assess whether combining biomarker measurements with other known predictor variables improved the predictive value, the added value of cDPP3 and bio-ADM was evaluated using logistic regression modeling. Different items with possible predictive value were entered as independent variables, with binary outcomes as dependent variables. The predictive value of generated models was assessed by the model likelihood ratio χ² statistic. The concordance index (c-index) with its 95% CI is given as an effect measure of the multivariable models. To test for added predictive value, we used the likelihood ratio χ² test for nested models to assess whether a biomarker added predictive value to a clinical model or a risk score.

A two-sided P value of less than 0.05 was considered statistically significant. All analyses were performed using Statistical Package for the Social Sciences (SPSS) version 25.0 (SPSS Inc., Chicago, Illinois, USA) and GraphPad Prism version 8.0 (GraphPad Software, La Jolla, California, USA).

Results

Study population

Two-hundred and eight patients were evaluated for study inclusion. Five of these were excluded as no biomarker measurements were available within the specified timeframe, leaving 203 patients for analysis. Baseline characteristics of patients are summarised in Table 1. Biomarker measurements on ICU admission (day 1) were available for all but one patient, whereas day 2 biomarker data were unavailable for six patients (3%). Day 3 measurements were unavailable for 121 patients (60%) because of discharge from the ICU. Therefore, day 3 biomarker levels were only used to determine temporal biomarker profiles in the total cohort and not for subgroup or association analyses.

Table 1 - Baseline patient characteristics

	Total (n = 203)	Minimal surgery (n = 22)	Elective surgery (n = 166)	Emergency surgery (n = 15)	P
Patient characteristics
Sex, male	132 (65.0)	12 (54.5)	113 (68.1)	7 (46.7)	0.138
Age (years)	68 [61 to 73]	75 [72 to 81]	67 [61 to 72]∗	62 [51 to 70]∗	<0.001
BMI (kg m−2)	27.4 [24.5 to 30.0]	28.3 [24.1 to 31.6]	27.1 [24.6 to 30.0]	27.5 [24.0 to 28.4]	0.609
Medical history
COPD	25 (12.3)	7 (31.8)	17 (10.2)∗	1 (6.7)	0.012
CKD	6 (3.0)	2 (9.1)	4 (2.4)	0 (0.0)	0.172
Diabetes	49 (24.1)	7 (31.8)	41 (24.7)	1 (6.7)	0.198
Hypertension	101 (49.8)	16 (72.7)	80 (48.2)∗	5 (33.5)∗	0.040
PVD	81 (39.9)	12 (54.5)	66 (39.8)	3 (20.0)	0.108
Characteristics during hospitalisation
Creatinineday1 (μmol l−1)	83 [69 to 105]	88 [69 to 103]	82 [68 to 102]	110 [90 to 123]	0.105
Lactateday1 (mmol l−1)	1.2 [0.9 to 1.7]	1.1 [1.0 to 1.4]	1.2 [0.9 to 1.6]	1.9 [1.5 to 2.9]∗ , #	0.001
Troponin-Tday1 (ng l−1)	341 [187 to 642]	148 [94 to 285]	343 [196 to 610]∗	940 [465 to 1665] ∗ , #	<0.001
ICU LOS (days)	3 [2 to 4]	2 [2 to 2]	3 [2 to 3]∗	8 [3 to 15]∗ , #	<0.001
Hospital LOS (days)	9 [7 to 14]	8 [6 to 14]	9 [8 to 14]	21 [10 to 29]∗ , #	0.001
AKI	13 (6.4)	3 (13.6)	6 (3.6)	4 (26.7)#	0.001
RRT	4 (2.0)	0 (0.0)	2 (1.2)	2 (12.6)	0.999
Mechanical ventilation	180 (88.7)	4 (18.2)	166 (100.0)	15 (100.0)	<0.001
Severity scores
SOFA	8 [6 to 9]	6 [3 to 7]	8 [6 to 9]∗	10 [9 to 12]∗ , #	<0.001
APACHE-II	15 [13 to 17]	14 [12 to 17]	15 [13 to 17]	17 [13 to 23]	0.484
Pre-operative cardiac status
NYHA 1–2	102 (50.2)	12 (54.5)&	90 (61.2)&	0 (0)	<0.001
NYHA 3–4	82 (40.4)	10 (45.5)&	57 (38.8)&	15 (100)	<0.001
LVEF (pre-op) >55%	126 (62.1)	15 (68.2)	114 (70.8)	9 (64.3)	0.860
LVEF (pre-op) 45–55%	28 (13.8)	7 (31.8)	26 (16.1)	2 (14.3)	0.185
LVEF (pre-op) 30–45%	16 (7.9)	0 (0)	14 (8.7)	2 (14.3)	0.256
LVEF (pre-op) <30%	8 (3.9)	0 (0)	7 (4.3)	1 (7.1)	0.520
Surgery characteristics
ECC time (min)	121 [89 to 188]	–	117 [85 to 169]	266 [175 to 310]#	<0.001
CABG	55 (27.1)	0 (0.0)	54 (32.5)∗	1 (6.7)#	0.001
Valve replacement	93 (45.8)	17 (77.3)	71 (42.8)∗	5 (33.3)∗	0.006
Combined CABG and valve replacement	23 (11.3)	0 (0.0)	22 (13.3)	1 (6.7)	0.154
Other	32 (15.8)	5 (22.7)	19 (11.4)	8 (53.3)#	<0.001

Patient and clinical characteristics according to surgical severity. Data are presented as median [IQR] or number (%). SOFA and APACHE-II scores were calculated during the first 24 h after ICU arrival. Creatinine, lactate and troponin-T were determined within 2 h after ICU arrival. Combined procedures include combined coronary artery bypass grafting and valvular procedures. The ‘other’ surgery category includes patients who underwent atrial/ventricular septal defect repair, intracardiac tumour resection surgery, or ascending/descending aortic arch procedures. AKI, acute kidney injury; APACHE-II, acute physiology and chronic health evaluation-II; BMI, body mass index; CABG, coronary artery bypass grafting; CKD, chronic kidney disease; COPD, chronic obstructive pulmonary disease; ECC, extracorporeal circulation; LOS, length of stay; LVEF (pre-op), pre-operative left ventricular ejection fraction; NYHA, New York Heart Association classification; PVD, peripheral vascular disease; RRT, renal replacement therapy; SOFA, sequential organ failure assessment.

^∗P less than 0.05 vs. minimal surgery.

^#P less than 0.05 vs. elective surgery.

^&P less than 0.05 vs. emergency surgery. P values were calculated using χ² tests followed by Fisher's exact post hoc testing.

The median [IQR] age of the patients was 68 [61 to 72] years, 65% of patients were men, median APACHE-II score was 15 [13 to 17], median SOFA score was 8 [6 to 9]. One hundred and sixty-six patients (82%) underwent elective surgery, 22 patients (11%) minimally invasive surgery and 15 patients (7%) emergency surgery. The majority of the cohort (171, 84%) was admitted after CABG, valve replacement, or a combination of these two surgical procedures. Cardiopulmonary bypass was performed in 181 (89%) patients, with nonbypass surgery only being performed in the minimally invasive surgery subgroup (22).

Patient characteristics of the minimally invasive, elective and emergency surgery subgroups are listed in Table 1. In short, patients in the minimally invasive surgery subgroup were significantly older than patients in the elective and emergency surgery subgroups, whereas patients in the emergency surgery subgroup were more severely ill on admission than patients in the other two subgroups, as reflected by the significantly higher admission SOFA scores as well as higher admission lactate and troponin-T levels. Furthermore, compared with elective surgery, patients in the emergency surgery subgroup were exposed to more prolonged extracorporeal circulation (ECC) during surgery.

Temporal profiles of bio-adrenomedullin and circulating DPP3 following cardiac surgery

Bio-ADM and cDPP3 displayed markedly different temporal profiles following cardiac surgery.

At day 1, bio-ADM levels were comparatively low and within the normal range (<43 pg ml⁻¹) in most patients (87.6%) (Fig. 1a). Median bio-ADM levels significantly increased on day 2, with 32.5% of patients having values above the normal range (Fig. 1a). A further increase was observed on day 3 (57.3% of patients above normal range, Fig. 1a). No clear association between surgical complexity and duration and day 1 bio-ADM levels was present (Fig. 2a). On day 2 of ICU admission, higher bio-ADM levels were present in the emergency surgery subgroup compared with elective surgery patients (P = 0.004, Fig. 2a).

Fig. 1: Temporal profiles of bioactive adrenomedullin (panel a) and circulating DPP3 (panel b) following ICU admission after cardiac surgery.

Fig. 2: Temporal profiles of bioactive adrenomedullin (panel a) and circulating DPP3 (panel b), with differences in levels between the different surgical severity subgroups.

cDPP3 levels were highest on ICU admission (day 1), subsequently decreased on day 2 and remained stable on day 3 (Fig. 1b). Importantly, whereas most patients (76.7%) displayed cDPP3 levels above the normal range (>40 ng ml⁻¹) on day 1, the majority of patients displayed normalised cDPP3 levels on days 2 and 3 (72.1 and 57.3%, respectively) (Fig. 1b). In contrast to bio-ADM, a clear association of higher day 1 cDPP3 levels in patients with increasing surgical complexity and duration was observed (Fig. 2b).

Prolonged vasopressor dependency

Twenty-eight patients (15.6%) still required vasopressor therapy 3 days after ICU admission. The number of vasopressor-dependent days was strongly associated with ICU length of stay (r = 0.66, P < 0.001), underlining vasoplegia as a significant factor contributing to prolonged ICU stay after cardiac surgery. In univariate analysis, day 2 bio-ADM levels (bio-ADM_day2) proved to be the best predictor of prolonged vasopressor dependency, with an AUROC of 0.82 (0.72 to 0.92, P < 0.001), compared with an AUROC of 0.70 (0.57 to 0.82, P = 0.001) for bio-ADM_day1 (Fig. 3a). cDPP3 concentration on day 1 (cDPP3_day1) showed limited prediction of prolonged vasopressor dependency, with an AUROC of 0.64 (0.55 to 0.74, P = 0.021). cDPP3_day2 performed better with an AUROC of 0.73 (0.65 to 0.81, P < 0.001, Fig. 3b).

Fig. 3: Areas under the receiver-operating characteristic curve of bioactive adrenomedullin (panels a, c, e, g) and circulating DPP3 (panels b, d, f, h) levels on days 1 and 2 of ICU admission.

To assess the biomarker's values independent of ECC time and lactate, two known predictor variables of prolonged vasopressor dependency, day 2 biomarker performance was subsequently assessed in multivariable models. Bio-ADM_day2 had additive value on top of ECC time to predict vasoplegia, with a c-index of 0.83 (0.75 to 0.91) for ECC time combined with bio-ADM_day2 compared with 0.75 (0.66 to 0.85) for ECC time alone, P = 0.008 (Supplementary Figure 1, https://links.lww.com/EJA/A679). Correspondingly, bio-ADM_day2 also improved predictive capacity of lactate, with a c-index of 0.83 (0.75 to 0.92) for lactate_day2 combined with bio-ADM_day2 compared with 0.68 (0.57 to 0.78) for lactate_day2 alone, P = 0.001 (Supplementary Figure 1, https://links.lww.com/EJA/A679). In contrast, cDPP3_day2 did not have additive predictive value on top of either ECC-time or lactate_day2 (Supplementary Figure 1, https://links.lww.com/EJA/A679).

Kaplan–Meier analysis using third quartile cutoff of bio-ADM_day2 (>50.6 pg ml⁻¹) further illustrates the association between bio-ADM_day2 and prolonged vasopressor dependency, with a corresponding hazard ratio of 2.1 (95% CI, 1.5 to 2.9, P < 0.0001, Fig. 4a).

Fig. 4: Kaplan–Meier analysis of duration of vasopressor dependency (a), development of acute kidney injury (b), duration of ICU stay (c) and duration of hospital stay (d) in cardiac surgery patients on day 2 after ICU admission.

Acute kidney injury

Fifty-two patients (28.7%) developed postoperative AKI. The majority of patients (36, 20%) developed KDIGO stage 1 AKI, with stage 2 and 3 encountered in 8 (4%) and 3 patients (2%), respectively. Both bio-ADM and cDPP3 concentrations were associated with development of AKI. Although the predictive performance of bio-ADM_day1 was poor with an AUROC of 0.67 (95% CI, 0.57 to 0.76, P = 0.001), the performance of bio-ADM_day2 was good with an AUROC of 0.87 (0.81 to 0.92, P < 0.0001, Fig. 3c). Kaplan–Meier analysis using third quartile cutoff of bio-ADM_day2 (>50.6 pg ml⁻¹) resulted in a corresponding hazard ratio for development of AKI of 4.7 (95% CI, 2.4 to 9.0, P < 0.0001 (Fig. 4b). Overall predictive performance of cDPP3 for AKI was poor; with AUROCs of 0.61 (0.52 to 0.70, P = 0.018) and 0.69 (0.61 to 0.77, P < 0.001) for cDPP3_day1 and cDPP3_day2, respectively (Fig. 3d).

ICU and hospital length of stay

Fifty patients (27.6%) required a prolonged ICU stay (more than 3 days). In univariate analysis, both bio-ADM and cDPP3 levels were associated with prolonged ICU stays, with day 2 measurements providing the best prediction of outcome for both biomarkers and bio-ADM performing better than cDPP3 (Fig. 3e and f). Associations with prolonged hospital stays were also present for both biomarkers, albeit less pronounced (Fig. 3g and h).

In multivariable analysis, bio-ADM_day2 improved the ability of APACHE II to predict prolonged ICU stays: the c-index increased from 0.74 (0.65 to 0.83) for APACHE II alone to 0.78 (0.70 to 0.86) for APACHE-II combined with bio-ADM_day2 (P = 0.001). Likewise, the SOFA score c-index increased from 0.73 (0.64 to 0.81) for the SOFA score alone to 0.79 (0.71 to 0.87) for the SOFA score combined with bio-ADM_day2 (P < 0.001) (Supplementary Figure 1, https://links.lww.com/EJA/A679). cDPP3_day2 improved the predictive ability of APACHE-II, with the c-index increasing to 0.76 (0.68 to 0.85) for APACHE-II combined with cDPP3_day2, P = 0.023. However, cDPP3_day2 did not improve predictive ability of the admission SOFA score (Supplementary Figure 1, https://links.lww.com/EJA/A679).

Kaplan–Meier analyses using the third quartile cutoffs of bio-ADM_day2 (>50.6 pg ml⁻¹) for ICU-and hospital length of stay demonstrated similar results to those obtained for prolonged vasopressor dependency. Patients with high bio-ADM_day2 remained in the ICU for longer, 2 [2 to 3] vs. 4 [3 to 8] days: hazard ratio 2.2 (1.6 to 3.0, P < 0.0001). A similar effect was also observed for hospital length of stay, increasing from 9 [7 to 12] to 18 [12 to 30] days: hazard ratio 2.8 (2.1 to 3.8, P < 0.0001) (Fig. 4c and d).

Combined biomarker approaches

To further investigate the predictive performance of different study variables, a combined biomarker approach (either combining two different time-points of the same biomarker, or two different biomarkers) was also considered. An overview of these analyses is presented in Supplementary Table 1, https://links.lww.com/EJA/A680. Results should be interpreted with caution, as further subgroup stratification severely limits statistical power given the study cohort size.

Discussion

The main finding of this prospective cohort study is that bio-ADM and cDPP3 show distinctively different temporal profiles and predictive characteristics in postoperative cardiac surgery patients. High cDPP3 levels were observed at ICU admission but these levels subsequently normalised the next day in the majority of patients. In contrast, admission bio-ADM levels were within the normal range in the majority of patients but significantly increased in the days following ICU admission. Furthermore, high admission cDPP3 levels were mainly related to the extent and complexity of the surgical insult, likely reflecting direct tissue injury caused by the procedure. Increases in bio-ADM concentrations were more strongly related to subsequent organ dysfunction, such as vasopressor dependency and development of AKI, resulting in prolonged ICU stay after cardiac surgery. Bio-ADM concentrations obtained on day 2 after ICU admission effectively differentiated between high-risk and low-risk patients.

The observed differences in the kinetics and outcome associations in cardiac surgery patients might be explained by the molecular pathways involved in the release of these biomarkers. During systemic inflammation, release of bio-ADM is a physiological response aimed at attenuating endothelial barrier dysfunction.²² If this compensatory response falls short, endothelial barrier compromise occurs, substantially contributing to the development of shock.²² Correspondingly, we found significant time-dependent increases in bio-ADM after cardiac surgery, with the most pronounced bio-ADM increase occurring in patients who subsequently developed prolonged circulatory failure and AKI. DPP3, on the other hand, is a mainly intracellular enzyme that only reaches high circulating levels in case of extensive cell death.¹⁰ This explains the observed higher cDPP3 levels at ICU admission following procedures with increasing surgical severity, and thus tissue damage. This confirms the findings of a recently performed pilot-study in abdominal aneurysm repair surgery patients.²³ As cDPP3 has a half-life of only 20 to 70 min,^13,19 high levels induced by surgery should quickly normalise, provided tissue perfusion is adequate and no additional cellular necrosis occurs. By analogy, if cDPP3 levels remain high despite treatment, this signals that tissue perfusion is persistently impaired through factors not directly related to surgery-induced tissue injury, indicating an increased risk for the development of organ failure and impaired outcomes.⁷ In line with the above, the vast majority of patients in our cohort showed normalised cDPP3 levels (below the upper normal range of 40 ng ml⁻¹) on day 2 of ICU admission, with only emergency surgery patients being the notable exception. Earlier studies in cardiogenic shock patients reported sustained cDPP3 levels well above the upper normal range in most patients.^10,13 Of note, these cardiogenic shock patients were more severely ill compared with our cardiac surgery cohort, as reflected by their higher admission SOFA scores as well as a substantial short-term mortality, which was absent in our cohort. Therefore, sustained pathological levels of cDPP3 appear to be dependent on a high disease severity and ongoing cell death. As sustained pathological cDPP3 levels were only rarely observed in our less severely ill cohort, this could explain the more limited predictive value of day 2 cDPP3 measurements in cardiac surgery patients.

Interestingly, we found strong associations of day 2 bio-ADM levels with short-term postoperative outcomes, while associations with day 1 bio-ADM levels were less pronounced. By contrast, in sepsis, both day 1 bio-ADM levels, as well the level of reduction of bio-ADM levels following the first 24 h of ICU treatment proved to be the best outcome predictors.^21,24 These differences in predictive performance might be explained by differences in the onset of illness between these patient categories. In sepsis, patients are admitted to the ICU already displaying marked organ dysfunction/failure and haemodynamic compromise has often been developing for hours before initial ICU presentation: meaning bio-ADM-associated pathways are already markedly engaged when patients are admitted to the ICU. Thus, in sepsis, admission bio-ADM levels already reflect the level of haemodynamic compromise and consequently, illness severity. Following cardiac surgery, haemodynamic compromise often develops in the first 24 to 48 h after the initial surgical insult, as a consequence of systemic inflammatory responses.^1,2 As this postoperative systemic inflammatory response is only just developing at ICU admission, compensatory bio-ADM responses aimed at preventing haemodynamic compromise are not yet engaged. This molecular rationale could explain why admission bio-ADM levels had limited predictive capacity for negative outcome in our study, while levels 24 h after surgery were strongly associated with haemodynamic compromise and organ dysfunction.

Our results have several clinically relevant implications. First, surgery acts as an additional factor leading to an increase of cDPP3, which is not directly related to cell death induced by tissue hypoperfusion or progressive organ dysfunction. This may complicate the interpretation of high cDPP3 levels in patients directly after surgery. Second, sustained high cDPP3 levels are not encountered often following elective cardiac surgery, and may therefore, be used to prompt clinicians to re-assess patients for causes of reduced tissue perfusion, such as cardiac dysfunction, hypovolaemia, or microcirculatory dysfunction. Third, high bio-ADM levels on day 2 of ICU admission may identify patients with a high risk of developing prolonged vasopressor dependency and AKI, improving on the risk stratification provided by conventional clinical risk scores like APACHE-II and SOFA score.

In contrast to many recently investigated prognostic biomarkers of cardiovascular insufficiency, both cDPP3 and ADM-targeted therapies have already shown promise in preclinical models of cardiogenic and septic shock.^12–14 The availability of specific therapeutic options able to correct dysregulated pathways associated with high levels of these biomarkers greatly increase their potential clinical relevance.⁷ Adrecizumab, an antibody directed against bio-ADM, is currently undergoing phase-2 investigation for the treatment of septic shock.²⁵ On the basis of our results, Adrecizumab may also hold promise as a therapeutic option aimed at preventing prolonged vasopressor dependency and subsequent AKI following cardiac surgery. In this regard, bio-ADM measurements might also serve as a strategy for targeting treatment, by identifying high-risk cardiac surgery patients with dysregulated bio-ADM responses, with possibly higher chances of beneficial effects from Adrecizumab therapy. In contrast, the absence of sustained pathologically high cDPP3 levels after elective cardiac surgery in the vast majority of cardiac surgery patients makes it unlikely that cDPP3-targeted therapies would be of clinical benefit in this patient category. Rather, research into the potential benefits of cDPP3 inhibition should probably focus on conditions with a higher disease severity, like cardiogenic or septic shock.

Our study also has several limitations. First, as the prognostic properties of both cDPP3 and bio-ADM in cardiac surgery have never been examined beyond the current study, optimal cut-off values need to be defined and externally validated in future investigations. Second, the observational nature of this study means no formal causal relations between predictor variables and outcomes can be inferred. Third, the unavailability of pre-operative and interoperative biomarker levels reduces the understanding of the temporal profiles and the timing of the true peak levels of both cDPP3 and bio-ADM after cardiac surgery. Lastly, it is currently unknown whether factors, such as kidney and/or liver function disorders influence cDPP3 metabolism: this will need further exploration in patient groups with varying levels of preexisting kidney and liver dysfunction.

Conclusion

Increased bio-ADM levels following cardiac surgery, especially when sustained, are strongly associated with vasopressor dependency, development of AKI and prolonged ICU stay. In contrast, despite high peak levels related to the extent of surgery, cDPP3 levels decrease swiftly and are associated with these clinical outcomes to only a limited extent. Therefore, compared with cDPP3, bio-ADM-modulating therapies might be of more benefit in cardiac surgery patients by improving haemodynamic stability and renal function.