Total parenteral nutrition in the newborn infant: energy substrates and respiratory gas exchange

Intravenous lipids for preterm infants: a review

Extremely low birth weight infants (ELBW) are born at a time when the fetus is undergoing rapid intrauterine brain and body growth. Continuation of this growth in the first several weeks postnatally during the time these infants are on ventilator support and receiving critical care is often a challenge. These infants are usually highly stressed and at risk for catabolism. Parenteral nutrition is needed in these infants because most cannot meet the majority of their nutritional needs using the enteral route. Despite adoption of a more aggressive approach with amino acid infusions, there still appears to be a reluctance to use early intravenous lipids. This is based on several dogmas that suggest that lipid infusions may be associated with the development or exacerbation of lung disease, displace bilirubin from albumin, exacerbate sepsis, and cause CNS injury and thrombocytopena. Several recent reviews have focused on intravenous nutrition for premature neonate, but very little exists that provides a comprehensive review of intravenous lipid for very low birth and other critically ill neonates. Here, we would like to provide a brief basic overview, of lipid biochemistry and metabolism of lipids, especially as they pertain to the preterm infant, discuss the origin of some of the current clinical practices, and provide a review of the literature, that can be used as a basis for revising clinical care, and provide some clarity in this controversial area, where clinical care is often based more on tradition and dogma than science.

Special circumstances: trophic feeds, necrotizing enterocolitis and bronchopulmonary dysplasia

There are many unresolved issues regarding how to feed the extremely-low-birth-weight (ELBW) infant. Trophic feedings of small volumes of breast milk or formula do not appear to increase the incidence of necrotizing enterocolitis (NEC). For prevention of NEC, breast milk, antenatal steroids and fluid restriction each confers a benefit. Because the incidence of NEC is relatively low, to determine if a particular prevention strategy is effective, large numbers of infants would need to be enrolled in a prospective, randomized controlled trial, and such trials are rare. Candidate therapies for NEC prevention that warrant further study include oral immunoglobulins, probiotics, long-chain polyunsaturated fatty acids and arginine. Suboptimal nutrition in ELBW infants is common in the early postnatal period. This is also the most critical time for the development of bronchopulmonary dysplasia, when even brief periods of malnutrition have significant effects on lung development and growth.

Assessing the effect of disease on nutrition of the preterm infant

OBJECTIVE

To review existing data on nutritional requirements of extremely low birth weight (ELBW) and very low birth weight (VLBW) preterm infants (those who weigh < 1000 g and 1000-1500 g at birth, respectively), and the effects of diseases on these nutritional requirements.

DATA SOURCES

A literature search was conducted on applicable articles related to nutritional requirements of preterm ELBW and VLBW infants and the effects of diseases in these infants on their nutritional and metabolic requirements.

DATA SYNTHESIS

The literature was analyzed to determine nutritional requirements of preterm ELBW and VLBW infants, to select the most common diseases that have significant and important effects on nutrition and metabolism in these infants, and to make recommendations about diagnostic and therapeutic approaches to nutritional problems as affected by diseases in ELBW and VLBW infants.

CONCLUSIONS

Many diseases unique to preterm infants, either directly or by enhancing the effects of stress on the metabolism of such infants, provide important changes in the nutrient requirements. The overriding observation from all studies, however, is that ELBW and VLBW preterm infants are underfed during the early postnatal period and that this condition, combined with additional stresses from various diseases, increases the risk of long-term neurological sequelae. The value of achieving a specific body composition and growth weight is less certain. There remains a critical need for determining the right quality as well as quantity of nutrients for these infants.

Low birthweight infants and total parenteral nutrition immediately after birth. III. Randomised study of energy substrate utilisation, nitrogen balance, and carbon dioxide production

This study aimed to investigate energy substrate utilisation and nitrogen balance in low birthweight infants receiving total parenteral nutrition during the first days of life, and in particular, to determine the effect of two different glucose intakes on carbon dioxide production. Twenty infants (mean (SE) birthweight 1314 (65) g, mean (SE) gestation 30.9 (0.4) weeks) were recruited to the study. Immediately after birth they were randomised to a carbohydrate intake of 8 g/kg/day (5.5 mg/kg/minute) or 12 g/kg/day (8.3 mg/kg/minute). After 24 hours they were changed to the alternative regimen which was continued for a further 24 hours. Fat and protein intakes were kept constant throughout the study. Indirect calorimetry was performed during each of the regimens, urine was collected for urinary nitrogen, and substrate utilisation calculated for 12 infants. The carbohydrate utilisation rate was increased during the higher carbohydrate intake. Lipid utilisation rates were significantly different, with net lipid synthesis occurring during high carbohydrate intake. Protein utilisation rates were not influenced by the different carbohydrate intakes. The mean plasma glucose concentration was higher during the high carbohydrate intake but the mean highest and lowest values were not significantly different during the two study periods. A plasma glucose below 2.6 mmol/l was recorded more frequently during the low glucose intake (9/20 v 5/20). Capillary PCO2 values measured during high and low glucose intakes were similar (5.9 (0.2) v 6.2 (0.3) kPa. Carbon dioxide production rates were increased during the higher carbohydrate intake but the differences were not significant. Similarly, there was no significant difference in the respiratory quotients (RQ), oxygen consumption, or energy expenditure during the two study periods.

Influence of energy substrates on respiratory gas exchange during conventional mechanical ventilation of preterm infants

OBJECTIVE

The purpose of this study was to determine the optimal parenteral feeding regimen for infants with compromised respiratory function.

METHODS

We studied the influence of varying the source of energy on respiratory gas exchange in 10 infants who were supported by mechanical ventilation and who received intravenous feedings. Two isoenergetic parenteral regimens were infused consecutively; the level of fat intake was varied inversely with that of glucose. Under similar ventilator settings, transcutaneous partial pressures of oxygen and carbon dioxide, as well as indirect calorimetry were measured during each regimen.

RESULTS

Despite the higher carbon dioxide production during the glucose-rich regimen (8.9 +/- 0.7 vs 7.9 +/- 0.4 ml/kg per minute, p < 0.05 by analysis of variance), transcutaneous partial pressure of carbon dioxide remained unaffected, suggesting ventilatory compensation as documented by the increased (p < 0.002) alveolar ventilation. This was not associated with a detectable rise in oxygen consumption, but with a significant change in partial pressure of oxygen (77 +/- 5 vs 66 +/- 3 mm Hg, p < 0.05).

CONCLUSIONS

Ventilator-dependent infants with early and mild bronchopulmonary dysplasia, who receive intravenous feedings of a moderate load of glucose-based energy, can compensate for enhanced carbon dioxide production by increasing their respiratory drive, with a beneficial effect on oxygenation compared with that observed when energy is derived from lipid-based solutions.

Diet and body composition of preterm infants

There have been few systematic studies of the effects of energy and protein intake on the body composition of preterm infants. Analysis of published studies suggests a roughly inverse relation between energy stored per gram of weight gain (a measure of the fatness of new tissues) and the ratio of protein to energy in the preterm infant's diet. At least within a certain range of energy and protein intakes, a higher protein diet promotes leaner body composition. Studies of the effects of varying the dietary ratio of carbohydrate to fat in preterm infants have shown reduced rates of carbon dioxide production with high-fat diets, fed by either the parenteral or enteral route. The little information available suggests no clear effect of varying the carbohydrate-to-fat ratio on body composition. The mineral content of the body can be influenced by diet. Insufficient intakes of calcium and phosphorus reduce the bone mineral content and thus the whole-body content of these minerals.

The increase in vasomotor tone induced by a parenteral lipid emulsion is linked to an inhibition of prostacyclin production

The aim of the study was to verify whether the infusion of a lipid emulsion causes a rise in vascular pressure related to an imbalance in the production of vasoconstricting and vasodilatating eicosanoids. Segments of umbilical veins were perfused with and without 1.5 microM indomethacin (cyclooxygenase inhibitor) in solutions differing only in their lipid content (control vs. lipid). The lipid-induced higher pressure (p < 0.05) was associated with an inhibition (p < 0.05) in the output of the vasodilatator PGI2, and an increase (p < 0.01) in the production of the vasoconstrictor PGF2 alpha. Indomethacin abolished differences in pressure, but produced a rise (p < 0.01) in vascular tone of both the control and lipid-containing solutions by inhibiting PGI2 synthesis. Prostacyclin was the only eicosanoid significantly correlated (p < 0.01) to vascular tone. The lipid emulsion was therefore linked to the inhibition of the conversion of PGH2 to PGI2. The ensuing greater PGH2 availability would result in vivo, in the increased synthesis of vasoconstricting eicosanoids. The lipid-containing solution produced vasoactive responses similar to those reported with tert-butyl hydroperoxide, suggesting that hydroperoxides contaminating commonly used lipid emulsions could be causing a prostanoid-dependent vasoconstriction.

Use of high-fat formula for premature infants with bronchopulmonary dysplasia: metabolic, pulmonary, and nutritional studies

The use of dietary fat in preference to carbohydrate offers the theoretic advantage of diminishing carbon dioxide production and thus the respiratory quotient, which may be beneficial for babies with chronic lung disease. Ten premature infants (birth weight (mean +/- SEM), 1.13 +/- 0.12 kg; postnatal age, 9 +/- 1 weeks) with bronchopulmonary dysplasia were alternately fed a high-fat and a high-carbohydrate formula each for 1 week, in randomized order. Lower rates of carbon dioxide production (6.6 +/- 0.3 versus 7.4 +/- 0.4 ml/kg per minute; p < 0.05), and consequently lower respiratory quotients (0.80 +/- 0.02 versus 0.94 +/- 0.01 ml/kg per minute; p < 0.005), were observed during the administration of the high-fat formula. There were no significant differences in results of pulmonary function tests with the use of either formula. Both formulas were equally well tolerated and able to promote adequate growth and normal biochemical profiles. However, weight gain was significantly greater with the administration of the high-carbohydrate formula, possibly because of an increase in the accretion of body fat. We conclude that the short-term use of high-fat formula for infants with bronchopulmonary dysplasia decreases carbon dioxide production while maintaining adequate growth and nutritional status.

Measured energy expenditure in mechanically ventilated very low birth weight infants

Forty-nine calorimetric studies were performed on 24 very low birth weight infants with respiratory distress syndrome on mechanical ventilation during the first seven postnatal days. Mean resting energy expenditure for the entire study was 59 +/- 21 kilocalories/kg daily, with a respiratory quotient of 0.93 +/- 0.1, reflecting the predominance of carbohydrate calories. A comparison of mean energy intake with mean resting energy expenditure showed a mean caloric deficit of 31 kcal/k daily during the first 4 postnatal days, followed by 3 days where resting energy expenditure was met by equivalent caloric intake. Wide variation was noted in resting energy expenditure.

Relative contribution of physical activity to neonatal oxygen consumption

To explore the relevance of distinguishing between resting and global energy expenditure in newborn infants, oxygen consumption (VO2) was measured during extremes of physical activity in 17 parenterally fed newborn infants with a large range of body weights (1.0-3.4 kg) and gestational ages (28-41 weeks). Under constant nutrient intakes, each infant served as his/her own control when comparing VO2 during resting conditions and spontaneous intense physical activity, called exercise. VO2 was significantly correlated with body weight at rest (r = 0.96). But during intense activity, the better predictor of exercise-induced VO2 was body weight in the smaller infants (< 2.0 kg) and gestational age in the larger infants (> 35 weeks). The difference in VO2 between both levels of activity represented the oxygen cost of exercise, which decreased (P < 0.01) with body weight. For clinical purposes, the physical activity of low-birth-weight infants does not contribute substantially to their energy balance.

Respiratory gas exchange in response to fat-free parenteral nutrition: a comparison after thoracic or abdominal surgery in newborn infants

Thoracic surgery is known to cause a postoperative respiratory failure because of the mechanical problems following chest wall disruption and/or diaphragmatic dysfunction. This study was to verify whether the fat-free intravenous nutritional support of neonates who underwent thoracic surgery could lead to a CO2 production exceeding the patients' respiratory reserves. Respiratory gas exchange and alveolar ventilation were obtained by indirect calorimetry and continuous recordings of transcutaneous PO2 and PCO2. These noninvasive measurements were compared at the same age of 7 +/- 1 days between a group of 7 newborn infants (mean +/- SEM: 3.09 +/- 0.14 kg, 39 +/- 1 weeks) after thoracic surgery versus a group of 8 newborn infants (2.88 +/- 0.17 kg, 37 +/- 1 weeks) after abdominal surgery. The intravenous macronutrient support was the same between both groups: 14 g/kg/d of glucose, 2 g/kg/d of amino acids, 250 kJ/kg/d of energy. One week after surgery, the global metabolic rate (195 kJ/kg/d) was not increased, and comparable between both groups. We documented that early after thoracic surgery, the ventilatory compensation required to handle the CO2 production (6.7 +/- 0.2 mL/kg/min) associated with a positive energy balance (45 +/- 8 kJ/kg/d) was effective.

Metabolism and nutritional frontiers in pediatric surgical patients

The successful treatment of pediatric surgical disease requires an understanding of the acute metabolic stress response. Poor clinical outcome can result when the metabolic demands of acute injury exceed the ability of endogenous host mechanisms to compensate. Appropriate exogenous supplementation may provide the metabolic and nutritional support crucial to recovery. As knowledge in this area grows, more effective treatment strategies are evolving. The potential for further advances, especially in the infant critical care population, offers the hope for substantial progress in the near future.

Validation of a portable indirect calorimetry system for measurement of energy expenditure in sick preterm infants

A portable indirect calorimeter adapted from adult use was validated for use in preterm infants. Oxygen consumption (VO2) and carbon dioxide production (VCO2) were subsequently measured in 16 preterm infants breathing spontaneously in room air (canopy mode) and in nine preterm infants receiving intermittent positive pressure ventilation (ventilator mode). Validation of the system was performed using a gas injection technique with nitrogen to simulate VO2 and carbon dioxide for VCO2. Mean errors in validation of the canopy mode were 1.4% and 0.2% for VO2 and VCO2 with limits of agreement of 0.6 (+2SD) ml/min and -1.3 (-2SD) ml/min, and 0.9 (+2SD) ml/min and -2.3 (-2SD) ml/min respectively. In validation of the ventilator mode mean errors were -1.8% and -5.05% for VO2 and VCO2 with limits of agreement of 1.02 (+2SD) ml/min and -0.74 (-2SD) ml/min, and 0.93 (+2SD) ml/min and -1.45 (-2SD) ml/min respectively. Values of VO2 and VCO2 in 16 preterm infants in the canopy mode were 6.2 ml/kg/min (0.5 1SD) and 6.7 ml/kg/min (0.6 1SD) and in nine preterm infants in the ventilator mode 4.98 ml/kg/min (1.09 1SD) and 4.74 ml/min/kg (1.08 1SD) respectively. Mean energy expenditure was 45.5 kcal (191 kJ)kg/day for infants measured in the canopy mode and 35.5 kcal (149 kJ)/kg/day for ventilated infants. This metabolic system can be adapted for use in the newborn but accuracy is reduced when it is used in those weighing less than 1000 g.

Energy expenditure and severity of respiratory disease in very low birth weight infants receiving long-term ventilatory support

We attempted to determine whether the hypermetabolism of infants with bronchopulmonary dysplasia was detectable during assisted ventilation. Respiratory gas exchange variables were measured with a metabolic gas monitor in 10 infants under similar nutritional conditions. Oxygen consumption increased linearly with the need for ventilatory support (R2 = 0.75), as documented by the ventilatory index.