Simple daily technique to calculate total and interim changes of daily O2 consumption and CO2 production in premature infants

Comparison of face mask, head hood, and canopy for breath sampling in flow-through indirect calorimetry to measure oxygen consumption and carbon dioxide production of preterm infants < 1500 grams

Complete sampling of expired air is essential for accurate O2 consumption(CO2 production) [VO2(VCO2)] measurements with flow-through indirect calorimetry. In preterm infants complete sampling is critical, because only low sampling flows can be used. The accuracy of the various breath sampling systems at low flows and their patient compatibility is untested. We therefore measured 1) the accuracy of VO2(VCO2) measurements with a face mask, a head hood, and a canopy in vitro at low sampling flows; 2) the effect of breathing on measurements with the face mask; and 3) the effect of breath sampling systems on activity and body temperature of preterm infants. VO2(VCO2) were measured with a Deltatrac II. In vitro we used a methanol miniburner incorporated into a doll, which could simulate low VO2(VCO2) and tidal breathing. In vivo we studied seven preterm infants < 1500 g. With the face mask VO2(VCO2) measurements were accurate at a flow of 3 L/min (error -1 +/- 0.8%), when tidal volume was < 15 mL/breath and the distance between mask and manikin < 1 cm. With hood and canopy VO2(VCO2) were underestimated at a flow of 3 L/min (error -13 +/- 1% and -14 +/- 5%), and results were markedly influenced by body position. For accurate measurements, the hood needed a flow of 4.5 L/min, the canopy 8.3 L/min. In vivo the face mask did not increase heart rate, respiration, activity, or rectal temperature, but hood and canopy increased rectal temperature by 0.3-0.4 degree C. For VO2(VCO2) measurements in infants < 1500 g, a face mask should be used, which is accurate at low flows and does not change body temperature. Accuracy at low flows and patient compatibility of breath sampling systems should be evaluated and reported for VO2(VCO2) measurements in preterm infants.

Technical and methodologic considerations for performance of indirect calorimetry in ventilated and nonventilated preterm infants

OBJECTIVE

To evaluate and refine indirect calorimetry measurement techniques so that accurate metabolic measurements can be performed in mechanically ventilated and convalescing preterm infants who require supplemental oxygen.

DESIGN

Laboratory validation of an indirect calorimeter; clinical and laboratory assessments of technical problems in performing metabolic measurements; and clinical indirect calorimetry studies in mechanically ventilated and nonventilated preterm infants.

SETTING

Neonatal intensive care unit (ICU) in a tertiary care university hospital.

PATIENTS

Level II and level III mechanically ventilated (n = 10) and nonventilated (n = 14) neonatal ICU patients who required FIO2 levels ranging from 0.21 to 0.42.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

System calibration was assessed by combustion of 100% ethanol; the mean respiratory quotient was 0.667 +/- 0.001 (SEM). In addition, oxygen consumption (Vo2) and CO2 production (Vco2) were simulated by CO2/nitrogen infusions within the range expected for 0.5- to 7-kg infants. Mean relative errors were 0.6 +/- 0.3% and 1.8 +/- 0.3% for expected Vo2 and Vco2 values, respectively. In 27 mechanically ventilated patients with no audible endotracheal tube leak, measured endotracheal tube leak ranged from 0.0% to 7.5%. Fluctuations in FIO2 during mechanical ventilation were monitored in 30-min studies, using wall-source (n = 27) or tank-source (n = 11) supplemental oxygen. Mean FIO2 variation was 0.00075 +/- 0.00013 vs. 0.00011 +/- 0.00001 using wall-source and tank-source oxygen, respectively. Some of the difficulties of obtaining accurate measurements in supplemental hood oxygen studies were overcome by using tank-source vs. wall-source oxygen and a unique hood design.

CONCLUSIONS

Accurate indirect calorimetry studies can be performed in both ventilated and nonventilated infants weighing as little as 500 g, providing that sufficient attention is paid to technical and methodologic measurement details.

Measurement of oxygen uptake in newborn infants during assisted and spontaneous ventilation

Measurements of oxygen uptake (VO2) and CO2 output (VCO2) are useful in critically ill patients. However, VO2 is not routinely measured in intensive care during mechanical ventilation (MV) especially in premature newborns. The present study describes a new method of measuring VO2 and VCO2 using a double open circuit which accounts for gas leaks around the uncuffed tracheal tube. The accuracy of the method was assessed with N2 and CO2 infusion. In case of leaks, VO2 and VCO2 measurement was significantly underestimated by the simple circuit method. This underestimation was not present with double circuit method. Five preterm newborns were studied. VO2 and VCO2 using the double open circuit were compared with the classic simple circuit. During MV, the mean underestimation assessed by the difference between simple and double circuit measurement was -12% (range from 0 to -29%) for VO2 and -14% (range 0 to -26%) for VCO2.

Metabolic rate and energy balance in infants with bronchopulmonary dysplasia

To determine energy use and growth of infants with bronchopulmonary dysplasia (BPD), we studied metabolic rate and energy balance in five infants with stage III-IV BPD (birth weight 1309 +/- 530 gm, gestational age 32 +/- 3 weeks, postnatal age 59.8 +/- 14.2 days) and in five control infants (birth weight 1540 +/- 213 gm, gestational age 33 +/- 2 weeks, postnatal age 42.0 +/- 4.2 days). Infants with BPD had significantly lower energy intake but higher energy expenditure than did control infants. Weight gain and energy cost of growth were significantly less in BPD infants than in control infants, as were urine output and output/intake ratio. We conclude that infants with BPD (1) absorbed caloric intake as well as did normal control infants, (2) had low energy intake and high energy expenditure, resulting in poor weight gain, and (3) had low energy cost of growth, suggesting an alteration in composition of tissue gain, with relatively high water content.

Errors in measurements of CO2 with the use of drying agents

To assess the errors in measurements of CO2 concentration that arise from the widespread use of drying agents in clinical and physiological studies, continuous measurements of CO2 concentration were made with an infra-red analyser before and after standard gas mixtures were passed through a tube containing one of several drying agents: calcium chloride; calcium sulphate (Drierite); alumina; silica gel; and magnesium perchlorate. The response time of the infra-red analyser was independent of the concentration of CO2 used (0-1%), but it was related to flow rate, dead space and the amount and drying agent used. At a low rate of 1 litre/min, alumina (145 g anhydrous weight) trapped virtually all the CO2 (0.8% in air) passing over it; silica gel (145 g anhydrous weight, mesh 6-20) adsorbed approximately 2 mmol CO2 (38% of total) over a period of 15 min; calcium sulphate (Drierite) absorbed a variable amount, depending on the bath; and calcium chloride adsorbed virtually no CO2. Increasing hydration of the drying agents reduced their capacity to adsorb CO2. Nitrogen was found to elute CO2 from the drying agents at about the same rate as it had been adsorbed. It is concluded that awareness of these adsorption/elution phenomena by drying agents should prevent errors from being made in the calibration of CO2 analysers, in the analysis of CO2, and in the measurement of specific activity or isotopic enrichment of CO2.

Anemia of prematurity: determinants of the erythropoietin response

This study was undertaken to determine the factors that are important in determining the erythropoietin response in low-birth-weight infants during the period of so-called anemia of prematurity. In the first weeks of life oxygen consumption in a group of 21 infants gradually increased as hemoglobin level fell. The magnitude of the erythropoietin response inversely varied with the central venous oxygen tension (P-vO2) (r = -0.55, P less than 0.001). When the P-vO2 declined to less than 30 torr, erythropoietin values were uniformly increased above the "normal" range (defined as the values associated with P-vO2 greater than 38 torr). Erythropoietin values varied inversely with hemoglobin but in general did not exceed the values observed for normal adult men. The erythropoietin values in the infants were remarkably lower at any given hemoglobin level when compared with those of older children with anemia resulting from bone marrow failure. In general, elevations of erythropoietin were seen when the hemoglobin concentration declined to less than 10.0 gm/dl. Change in heart rate did not appear to be a reliable indicator of the presence of anemia; rather, it correlated best with oxygen consumption.