Estimation of nitrogen balance based on a six-hour urine collection in infants

Comparison of Urine Urea Nitrogen Collection Times in Critically Ill Patients

Twenty-four-hour urine urea nitrogen (UUN) collections are used to assess nitrogen loss in critically ill patients but are often difficult to obtain accurately. This prospective study compared 6- and 12-hour UUN collections with 24-hour UUN collections in critically ill patients receiving continuous nutrition support. ICU patients admitted from September 1999 through January 2003 who had UUN collections as part of routine care were recruited into the study. Patients were not receiving oral diets, were receiving continuous parenteral or enteral nutrition, and had indwelling urinary catheters. We excluded patients with hepatic or renal failure. Urine samples were collected every 6 hours starting at 6:00 am and kept refrigerated until the 24-hour collection was complete. Samples were analyzed using an automated urease enzymatic reaction. Samples were multiplied by a factor of 4 (6-hour samples) or 2 (12-hour samples) to estimate 24-hour totals and then compared with actual 24-hour totals. Twenty-four patients (18 men) completed the study; 21 patients had 6-hour samples (84 samples), and 24 patients had 12-hour samples (24 samples). Estimated 24-hour UUN from 6-hour (14.7-15.7 g/d) and 12-hour (15.2 g/d) samples did not differ significantly (p > .5) from actual 24-hour totals (15.1 g/d). Shortened UUN collection times may be used to estimate 24-hour nitrogen losses in critically ill patients receiving continuous nutrition support.

Achieving positive protein balance in the immediate postoperative period in neonates undergoing abdominal surgery

OBJECTIVES

To determine whether neonates undergoing major abdominal surgical procedures in the first day of life could achieve a positive protein balance without protein toxicity in the immediate perioperative period by using parenteral amino acids and fentanyl analgesia.

STUDY DESIGN

Newborns undergoing major surgery for gastroschisis in the first 24 hours of life (n = 13) were alternately allocated to immediate postoperative parenteral administration of 1.5 g/kg(-1)/day(-1) versus 2.5 g/kg(-1)/day(-1) amino acids. Protein balance was determined at 1.6 to 2.6 days postoperatively with both nitrogen balance and leucine stable isotope methodology. Statistical analyses were conducted with the unpaired t test and linear regression.

RESULTS

Protein balance was significantly different in the 2 groups with both nitrogen balance and leucine stable isotope methodology. There was no evidence of protein toxicity as determined with blood urea nitrogen, creatinine, and ammonia concentrations.

CONCLUSIONS

Neonates undergoing the metabolic stress of abdominal surgery shortly after birth are able to achieve a net positive protein balance with parenteral amino acid administration without evidence of protein intolerance.

Amino acid administration to premature infants directly after birth

OBJECTIVES

To test the hypothesis that the administration of 2.4 g amino acids (AA)/(kg.d) to very low birth weight infants is safe and results in a positive nitrogen balance.

STUDY DESIGN

We conducted a randomized, clinical trial. Preterm infants with birth weights <1500 g received either glucose and 2.4 g AA/(kg.d) from birth onward (n=66) or solely glucose during the first day with a stepwise increase in AA intake to 2.4 g AA/(kg.d) on day 3 (n=69). Blood gas analysis was performed daily during the first 6 postnatal days; blood urea nitrogen levels were determined on days 2, 4, and 6; AA plasma concentrations and nitrogen balances were determined on days 2 and 4. Student t tests, Mann-Whitney tests, and chi2 tests were performed to compare groups.

RESULTS

Infants supplemented with AA had no major adverse side effects. Their blood urea nitrogen levels were higher, nitrogen balance turned positive upon AA administration, and more AA concentrations were within reference ranges.

CONCLUSIONS

High-dose AA administration to very low birth weight infants can be introduced safely from birth onward and results in an anabolic state.

Aggressive early total parental nutrition in low-birth-weight infants

OBJECTIVE

This study aimed to compare nitrogen balance and biochemical tolerance of early aggressive versus late total parenteral nutrition in very-low-birth-weight (VLBW) infants over the first week of life.

STUDY DESIGN

In all, 32 ventilator-dependent preterm infants were prospectively randomized into two groups. The Early Total Parenteral Nutrition (ETPN) group received 3.5 g/kilo-day amino acids (AA), and 3 g/kilo-day of 20% Intralipid (IL), starting within 1 hour after birth. The Late Total Parenteral Nutrition group (LTPN), started on a solution containing glucose during the first 48 hours of life, followed by 2 g/kilo-day of AA and 0.5 g/kilo-day of IL. For the LTPN group AA and IL were each increased by 0.5 g/kilo-day to a maximum of 3.5 and 3 g/kilo-day, respectively.

RESULTS

Nitrogen retention was significantly greater in all infants in the ETPN group throughout the 7-day study period. All infants in the LTPN group were in negative nitrogen balance during the first 48 hours of life, while those in the ETPN group were in positive nitrogen balance throughout. The mean (+/-SD) nitrogen retention in the ETPN was 384.5 mg/kilo-day (+/-20.2), compared to 203.4 mg/kilo-day (+/-20.9) in the LTPN group (p <0.001). In each of the first 5 days of life, energy intake was significantly greater in the ETPN group compared to the LTPN group (p <0.001). Mean fluid intake during the study period was similar between, the ETPN and the LTPN groups (162 and 165 cm3/kilo-day, respectively). The mean weight gain was similar in the ETPN and LTPN groups. Plasma levels of cholesterol, triglycerides, bicarbonate, blood urea nitrogen, creatinine, and pH were similar in both groups during the study period. Mean (+/-SD) serum glucose in the LTPN group was higher, but remained in normal range (101.1+/-5.2 and 80.8+/-5.4 mg/kilo-day, respectively). The mean peak serum bilirubin was significantly higher in the ETPN group, compared to The LTPN group (7.7 and 6.2 mg/dl).

CONCLUSION

This study shows that aggressive intake of AA and IL can be tolerated immediately after birth by VLBW infants. Also, ETPN significantly increased positive nitrogen balance and caloric intake, without increasing the risk of metabolic acidosis, hypercholesterolemia, or hypertriglyceridemia.

Effect of low versus high intravenous amino acid intake on very low birth weight infants in the early neonatal period

Greater protein intakes are required than have been commonly used to achieve fetal in utero protein accretion rates in preterm neonates. To study the efficacy and safety of more aggressive amino acid intake, we performed a prospective randomized study in 28 infants [mean wt, 946 +/- 40 g (SEM)] of 1 (low amino acid intake, LAA) versus 3 g.kg(-1).d(-1) (high amino acid intake, HAA) at 52.0 +/- 3.0 h of life. After a minimum of 12 h of parenteral nutrition, efficacy was determined by protein balance and was significantly lower in the LAA versus HAA groups by both nitrogen balance (-0.26 +/- 0.11 versus 1.16 +/- 0.15 g.kg(-1).d(-1), p < 0.00005) and leucine stable isotope (0.184 +/- 0.17 versus 1.63 +/- 0.20 g.kg(-1).d(-1), p < 0.0005) methods. Leucine flux and oxidation and nonoxidative leucine disposal rates were all significantly higher in the HAA versus LAA groups (249 +/- 13 versus 164 +/- 8, 69 +/- 5 versus 32 +/- 3, and 180 +/- 10 versus 132 +/- 8 micro mol.kg(-1).h(-1), respectively, p < 0.005), but leucine appearance from protein breakdown was not (140 +/- 15 in HAA versus 128 +/- 8 micro mol.kg(-1).h(-1)). In terms of possible toxicity with HAA, there were no significant differences between groups in the amount of sodium bicarbonate administered, degree of acidosis as determined by base deficit, or blood urea nitrogen concentration. Parenteral HAA versus LAA intake resulted in increased protein accretion, primarily by increasing protein synthesis versus suppressing protein breakdown, and appeared to be well tolerated by very preterm infants in the first days of life.

Assessing the validity of adjusted urinary urea nitrogen as an estimate of total urinary nitrogen in three pediatric populations

Nitrogen excretion is a useful measurement for determining efficiency of protein utilization. Knowledge of nitrogen losses is especially important in the treatment of stressed, postsurgical, or catabolic patients, in whom optimizing the amount of nitrogen intake in the diet may spare visceral and somatic proteins and encourage anabolism. Many methods have been used to estimate total urinary nitrogen (TUN) in different patient populations. Urinary urea nitrogen (UUN) values are routinely adjusted and used by investigators who are not able to measure TUN directly by either Kjeldahl or pyrochemoluminescent methods. The rationale for the use of adjusted UUN concentrations to predict TUN is based on adult experiences. No similar experience in pediatrics has been published. We have compared TUN with adjusted UUN in a study of 250 urine samples from pediatric patients (n = 34) and normal pediatric volunteers (n = 109). Our findings suggest that adjusted UUN (determined by previously established formulas) may be of limited use in estimating TUN in neonates, infants, and critically ill pediatric patients; however, adjusted UUN may be useful in approximating TUN in healthy school-aged children. Good correlations were found between UUN and TUN for critically ill children and postsurgical neonates and infants, suggesting that these newly described regression equations (once validated) may be useful in predicting TUN from a measured UUN.

Metabolic response of preterm infants to variable degrees of respiratory illness

In older children and adults, physiologic instability associated with severe illness causes increased cellular oxygen consumption (VO2), increased serum lactate and cortisol levels, and more negative nitrogen balance. To determine the metabolic response of preterm infants to severity of respiratory illness, we analyzed VO2, nitrogen balance, urinary 3-methyl-histidine and norepinephrine concentrations, and serum levels of lactate and cortisol as a function of ventilatory index (VI). Twelve 2-day-old premature infants who were appropriate in size for gestational age (mean +/- SEM birth weight: 1460 +/- 251 gm) and who required mechanical ventilation for respiratory distress syndrome had VO2 and carbon dioxide production measured by indirect calorimetry and blood and urine samples obtained concurrently. All infants received amino acids, 1.0 gm/kg per day, and a mean energy intake of 27 +/- 3 kcal/kg per day, provided as a parenteral dextrose solution. The resting energy expenditure exceeded energy intake in all infants. The VO2 value ranged from 5.5 to 9.2 ml/kg per minute and was directly correlated with VI (r = 0.79; p = 0.002). Nitrogen balance ranged from -160 to 53 mg/kg per day (mean: -33 +/- 21 mg/kg per day) but was not dependent on VI (r = 0.04) or VO2 (r = 0.01). The serum lactate level correlated directly with VI (r = 0.82; p = 0.002) and VO2 (r = 0.60; p = 0.05), but cortisol and urinary norepinephrine levels did not. We conclude that preterm infants with respiratory distress syndrome have increased VO2 rates and serum lactate concentrations directly related to the degree of respiratory illness. They are generally in a state of mildly negative nitrogen balance, the degree of which is not related to severity of illness. Although these infants may require increased energy delivery during illness, they do not appear to require excessive amounts of amino acids.

Nitrogen balance studies in clinical nutrition

Nutrition support is recognized as an important cofactor in altering morbidity and mortality of hospitalized patients. Paramount in delivering proper nutrition support is the accurate determination of baseline metabolic and nutritional status, thus influencing necessary protein requirements. After nutritional intervention, routine laboratory monitoring is used to measure the efficacy and to reassess metabolic stress level. Accurate determination of nitrogen excretion (and nitrogen balance) remains the standard in prescribing and monitoring the protein and nutritional treatment regimen. This article examines nitrogen excretion determinations in the clinical setting, including proper collection techniques, laboratory measurements, and analyses and their effect upon nitrogen balance studies.

Estimation of nitrogen excretion based on abbreviated urinary collections in patients on continuous parenteral nutrition

The physiologic response to stress can create a net loss of nitrogen, which is indicative of a catabolic state. Nitrogen balance has been demonstrated to be a useful clinical indicator of a patient's catabolic state and the effectiveness of parenteral nutrition, but requires an estimate of total urinary nitrogen excretion. The standard method for determining total urinary nitrogen excretion is 24-hour urinary urea nitrogen excretion. Unfortunately, the 24-hour urine collection is inconvenient, cumbersome, sometimes inaccurate, and induces a lag in response time to changes in therapy. Although shorter collection times have been proposed, the validity of shorter-timed determinations remains open to question. To evaluate the estimation of 24-hour urine urea excretion from shorter-timed determinations, the urinary urea nitrogen excretion of 4-, 8-, and 12-hour durations was regressed against the 24-hour collection in 56 critically ill adult patients. The 12-hour determination provided satisfactory estimates of the 24-hour nitrogen excretion, but the 4-hour and 8-hour determinations did not. Thus, two times the 12-hour urine urea nitrogen determination can be substituted for the 24-hour determination in calculating nitrogen balance. A 12-hour determination can provide a more rapid turnaround of biochemical analysis, allow more timely nutritional intervention, decrease nursing time, and reduce the frequency of inaccurate or lost specimens.

Pediatric nutritional assessment

Nutritional assessment is an important facet of the medical care of infants and children. An accurate assessment allows for the early detection and treatment of nutrient deficiencies. The traditional nutritional assessment includes anthropometric measurements, biochemical evaluation, clinical examination, and dietary evaluation. Newer techniques include the assessment of body composition using total body electrical conductivity or bioelectrical impedance. The sophistication of nutritional support makes it necessary to identify more sensitive methods for accurate nutritional assessment.