We evaluated whether measuring arterial lactate level in addition to ScvO2 would improve the prediction of complications after pediatric cardiac surgery among 73 patients admitted to a cardiac tertiary referral center in Aswan, Egypt. In this study, 35 patients (49.95%) developed complications. The most common complications after cardiac surgery were prolonged MV, chest infection, prolonged PICU LOS, and sepsis. In addition to age, ScvO2 measured 6 h after PICU admission, blood lactate level measured 12 h after PICU admission, and MAP measured 18 h after PICU admission were significantly associated with complications. Furthermore, all of these markers had good validity in predicting complications after cardiac surgery. However, multivariate regression analysis revealed that ScvO2 and weight were the only predictors of complications after cardiac surgery.

The demand for tools to predict complications after cardiac surgery is not new. Research has focused on lactate clearance, duration of high lactate levels, duration of cardiopulmonary bypass, inflammatory markers, and other indicators, with an emphasis on predicting low cardiac output syndrome; however, no optimal prognostic method has been established [17]. A study of the role of pro-inflammatory cytokines in inducing cardiac injury after arterial switch procedure in 63 newborns revealed that cardiac troponin T, interleukin-6, and interleukin-8 levels were higher in patients who developed low cardiac output syndrome [18].

In our study, we observed that all measured lactate levels and ScvO2 during the first 24 h after admission were significantly correlated with hospital LOS, PICU LOS, duration of MV, and duration of inotropic support. To best predict clinical outcomes, the optimal times to measure lactate levels and ScvO2 are 12 and 6 h after admission to the PICU, respectively. Maillet et al. [19] found that compared to patients whose lactate levels were < 3 mmoL/L, those with lactate levels > 3 mmoL/L on ICU had considerably longer durations of both MV and LOS of the ICU, as well as an increased mortality rate. In a large study (n = 1820), Kogan et al. [20] found that a maximum lactate level of 4.4 mmol/L during the first 10 h after admission to the ICU was associated with prolonged MV, prolonged LOS in the ICU, and an increased rate of mortality. Similarly, Pearse et al. [9] found that the rate of complications was considerably higher among patients with a ScvO2 of < 64.4% during the first 8 h after surgery than among those with a higher ScvO2. On the other hand, lactate levels in patients with and without complications were < 2 mmoL/L; the two groups did not differ significantly. Of note, ScvO2 and lactate measurements play an important role in the diagnosis of occult hypoperfusion. In individuals with normal macrohemodynamics, occult hypoperfusion is characterized as moderate to severe GTH with a mean arterial pressure of 65 mmHg, a mean central venous pressure of 8 mmHg, and mean urine output of 0.5 mL/kg/h [15]. Hu et al. [10] found that on admission to the ICU, 19 patients (32%) had occult hypoperfusion. GTH was found in 21 individuals (35%): 13 (22%) had moderate GTH and 8 (13%) had severe GTH.

In our study, arterial lactate level 12 h after PICU admission had 63.2% sensitivity and 82.0% specificity for predicting postsurgical complications, ScvO2 had 81.6% sensitivity and 82.9% specificity, VIS had 92.1% sensitivity and 40.0% specificity, and MAP measured 18 h after PICU admission had 97.1% sensitivity and 94.1% specificity. Similarly, Gaies et al. [20] reported that the maximum VIS estimated during the first 24 h after cardiac PICU admission was significantly associated with morbidity and death. A maximum VIS of ≥ 20 indicates a higher risk of a bad composite clinical outcome. Seear and his colleagues [21] found that arterial lactate and ScvO2 levels were the only postoperative measures that could predict serious adverse outcomes after cardiac operations in pediatric patients. Moreover, the ScvO2/lactate ratio appeared to have better predictive value: when the ratio was < 5, the PPV for complications was 93.8%, with 78.9% sensitivity and 90.5% specificity. Each measurement separately had high specificity but limited sensitivity. Single-measure predictive power was of only fair quality, but it could have been increased for patients at high risk of complications by tracking repeated measures over time 0.986.

Rocha et al. (2021) [22] found that, overall, the ScvO2/lactate ratio distinguished patients with and without major adverse events very effectively (AUC = 84%), outperforming either variable alone, with 48% sensitivity, 94% specificity, 60% PPV, and 91% NPV. However, when we calculated the ratio of ScvO2 6 h after PICU admission and lactate level 12 h after PICU admission, the AUC did not improve. Moreover, when we included all significant variables in the bivariable analysis, in addition to weight, the regression model showed that ScvO2 and weight were the only significant predictors of complications, with an AUC of 92%. This finding may be attributable to the correlation between ScvO2 and lactate. Bisaya and his colleagues [23] found that when ScvO2 was 65%, arterial lactate level was weakly correlated with ScvO2 (r2 = 0.0431, p < 0.001), whereas they were strongly correlated in individuals with an O2 extraction ratio of 50% (r2 = 0.93, p = 0.0019). Of note, about one-third of patients studied by Bisarya et al. had a ScvO2 of 50% or less. In our study, approximately one-third of the patients had a ScvO2 6 h after PICU admission of ≤ 50 which explains the high correlation between arterial lactate and ScvO2.

In this study, we found that one of the main predictors of postoperative complications was the weight of the patient. Similarly, an examination of demographic data, preoperative results, and surgical details of the population, patients with serious postoperative morbidity: they were considerably with younger age, weighed less, and had higher Aristotle scores [24]. This finding can be explained by the fact that these conditions are associated with poor growth and development. In contrast, Seear et al. [21] reported that both body weight and duration of circulatory arrest were weak predictors of complications, but they were not predictors at every measured time point.

To the best of our knowledge, this study is one of the first in pediatrics to determine the prognostic value of using combined markers in the prediction of complications after cardiac surgery. In this study, we obtained serial measurements of both markers and determined measurement times that were most strongly correlated with the outcomes. Additionally, the prospective nature of the study reduces the bias of missing data and allows intensive clinical and laboratory follow-up of the patients. However, this study had limitations. First, the significant heterogeneity of heart conditions and surgeries limits generalizability to other centers and populations; as a result, our findings must be interpreted with care. Actually, we included patients with different categories of congenital heart disease: some were acyanotic (VSD, ASD, coarctation of the aorta), and others were cyanotic of whom some have total correction (as arterial switch for patients with TGA and total correction of tetralogy of Fallot), and the others had only palliative repair (shunt for patients with complex congenital heart and Glenn for patients with single ventricle physiology); all these would have different baseline saturation and different oxygen delivery and consequently different ScVo2 and lactate owing to different degrees of mixing due to residual shunting, or amount of shunting is dependent on the patient’s hemodynamics as MAP and pulmonary vascular resistance PVR that are continuously changing according to different conditions and other variables (fever, sedation, over or under shunting). Another limitation was the different age categories; we had different age groups with different physiology, age, weight, and hemodynamic parameters as blood pressure that could not be accounted for, so we strongly recommend further research in every age category to avoid these limitations. Finally, this study was a single-center observational study; future multicenter studies are mandatory to provide more robust evidence.