What is the postoperative nutrition intake in children with congenital heart disease? A single-center analysis in China

Congenital heart disease (CHD) is the most common birth defect [1]. In this population, inadequate nutritional intake is a general problem for a variety of reasons [2, 3]. It has been found that early postoperative nutritional support can increase the energy and protein intake of children, improve their nutritional status, so that it can also help to reduce infection, duration of mechanical ventilation and hospital stay [4, 5]. Therefore, adequate nutritional intake in the early postoperative period is crucial for children with CHD. Studies have confirmed that many problems arise in the actual process of nutritional therapy, such as underestimation of energy requirements [6], feeding intolerance [7], delayed initiation of enteral nutrition (EN) or interruption of feeding [8‐10], etc., which may affect the nutritional supply of patients after surgery and exacerbate their malnutrition. However, the current status of the above-mentioned problems in children after CHD surgery is unclear.

The European Society of Pediatrics and Neonatal Intensive Care (ESPNIC) recommends use of the Schofield equation to estimate REE in critically ill children in limited conditions [11]. The equation is used to estimate REE from height and weight, and has different reference standards for different ages and genders [12]. On the other hand, the Fick method was proposed in 1987 and it is based on obtaining oxygen consumption (VO₂) from the patient's thermodilution pulmonary artery catheters, multiplying the VO₂ by the caloric value for oxygen and thus obtaining the resting energy expenditure (REE) [13].

Therefore, we reviewed data from the past six months, calculated the postoperative REE of patients with the Fick method, and tried to understand the gap between nutritional supply and actual demand in the cardiac intensive care unit (CICU) as well as factors that impede nutritional supply, in order to provide a reference for further optimization of nutritional therapy in children after CHD surgery.

ESPNIC recommends the use of the indirect calorimetry (IC) method to measure REE for guiding nutritional support in critically ill infants [11]. The IC method is based on the oxygen and carbon dioxide in the inhaled and exhaled gases per unit of time, calculates the VO₂ and carbon dioxide production (VCO₂), and then calculates the energy expenditure by Weir's formula [16]. However, the IC method is not widely used in the CICU due to its high cost, the need for specialized training of the operator, and some high requirements in practice [17]. The Fick method correlates well with the IC method (r = 0.9, P < 0.0001) and can be used to measure the REE in critically ill patients [13]. Because of the invasive operation, this study used a retrospective analysis to include patients who used a PiCCO catheter, a common alternative to pulmonary artery catheters placed in pediatric patients [18].

In a study by Zhang et al. [19], the REE of patients with a mean age of 3 months measured by indirect calorimeter was 55–57 kcal/kg/d on days 1–4 after cardiopulmonary bypass. Li et al. [20] used modified Weir’s equation to calculate the REE on days 1–3 after the Norwood procedure, and the REE was 39–41 kcal/kg/d in patients aged 4–92 days. In our study, the mean REEs on postoperative days 1–7 was 40–49 kcal/kg/d. But the patients in our study were 22 months old and had an Aristotle basic complexity (ABC) score of 8, which was different from the above study population and less comparable. We found that children had a higher REE on the first postoperative day, which was associated with an increased stress response in the early postoperative period. In addition, the REE was also higher after the fifth postoperative day, which, in addition to the reason for the stress response from the re-intervention, was also explained by the decrease in the use of sedative drugs as the tracheal tube was removed. However, there was no statistical difference in REE on different days postoperatively.

Our study found that energy supplied through EN did not account for a high proportion of patients' REE. Nicholson et al. [21] also found that the actual caloric intake of neonates who underwent modified systemic-to pulmonary artery shunt was much lower than the recommended intake. And the ratio of EN energy supply to REE started to decrease on day four and reached its lowest on day five. In addition to inadequate energy, protein supply in postoperative CHD patients is even more inadequate. For critically ill children requiring EN, the daily enteral protein intake is at least 1.5 g/kg [11, 22]. Even one study found that the protein requirement of children with CHD in the early post-cardiopulmonary bypass period is about 4 g/kg/d. High protein feeding can reverse the negative nitrogen balance earlier and improve nutritional outcomes [19]. However, in our study, the highest protein intake on postoperative days 1–7 was 1.01 g/kg/d, with an average of 0.7 g/kg, which was much lower than the actual requirement.

There are several possible reasons for such a low nutritional supply in children with CHD. The first is fluid restrictions. Children with CHD have strict fluid restriction after surgery to prevent cardiac overload [23]. In addition, while conservative feeding practices are beneficial in reducing intolerance, they may, on the other hand, further reduce nutritional intake. Therefore, faced with the conflict between fluid restriction and insufficient intake, high energy formula can provide patients with more nutrition [24, 25]. It is also possible to take a cue from another study and add pure whey protein isolate to formula [19].

The second point is fasting before and after surgical intervention or extubation. It should be noted that in our CICU, it is usually those patients who require close hemodynamic monitoring who are on PiCCO. This study retrospectively included children who had been on PiCCO, so these individuals tended to be sicker and had higher rates of re-intervention. The median time of re-intervention was 4 days, a critical time point, which on the one hand would be preceded by fasting, and on the other hand, would be followed by a higher infusion of blood products, and a reduction in EN intake was often necessary to ensure that total fluid intake was not excessive. In addition, the median duration of mechanical ventilation is almost 4 days, and fasting also occurs before and after removal of the tracheal tube. Despite guideline recommendations, there is still a large gap between theory and practice, which requires evidence-based clinical improvements to facilitate the translation of recommended preoperative fasting guidelines into clinical practice [26].

The third point is the initiation time of EN. The initiation time of EN was slightly longer than 24 h. This time, of course, includes the time from postoperative admission to the CICU until the patient receives the EN, the time of preparation and delivery by the nutrition department, and the time of delay in distributing the EN within the CICU. So, for the delay between the time the EN is ordered and the time the EN is started, we may be able to do some quality improvement projects to facilitate the early start of the EN.

Furthermore, the amount of unfinished milk was 12.2 ± 7.7 mL/kg/d. Lee et al. [9] classified the causes of nutritional interruptions into four categories, i.e., procedures, intolerance, potentially avoidable, and unknown. As this study was retrospective, the specific causes of these nutritional interruptions were not recorded. Thus, future prospective investigations are recommended to comprehensively investigate the factors influencing the nutritional supply deficiencies to provide a reference for intervention.

The Schofield equation is a method recommended by ESPNIC to estimate REE, but it only relates to height, weight, sex, and age, which cannot dynamically estimate the REE of critically ill patients. Some studies have suggested that the equation estimates of REE are highly biased [6, 27]. We found a moderate correlation coefficient of 0.467 between the REE calculated by the Fick method and the REE estimated by the equation. Therefore, the accuracy of the equation in children with CHD needs to be further verified.

There are some limitations to this study. The interpretation of the study findings is somewhat limited by the fact that it is a single-center, small-sample study. In addition, the retrospective design allows for limited data collection, which affects the interpretation of the influencing factors. Therefore, a more comprehensive exploration of the factors affecting nutrition supply is needed in the future.

In our study, the energy and protein supply in the early postoperative period in children with CHD was inadequate, especially the protein supply. Strict fluid restriction, as well as fasting, are possible causes. In addition, we found a moderate correlation between the REE calculated by the Fick method and the REE estimated by the Schofield equation.