An indirect calorimetry system for ventilator dependent very low birthweight infants.

With neurodevelopmental outcome of very low birthweight (VLBW) infants being adversely affected by inadequate nutrition during the first few weeks of life, there is an urgent need for more specific nutritional data on the sick VLBW ventilator dependent infant. The development of a new mass spectrometry gas analysis indirect calorimetry system which is non-invasive and can operate over several hours or days is described. Technical evaluation of each of the components of the system indicates a total random error of less than 5%. Systematic error was determined using gas infusions which simulated carbon dioxide production and oxygen consumption. The relative error in the measurement of carbon dioxide production was less than or equal to 1.5% (coefficient of variation (CV) 6.0%)) with carbon dioxide infusion rates ranging from 3.86 to 13.98 ml/min. The relative error in oxygen 'consumption' was less than or equal to 4.3% (CV 2.8%) for infusions of oxygen at rates of 7.5 to 14.80 ml/min. With nitrogen infusions simulating oxygen consumptions of 2.0 and 5.5 ml/min the relative error in the calculated nitrogen infusion was 1.5% (CV 4.1%) and 1.4% (CV 5.7%) respectively. Clinical studies on 10 infants demonstrated a mean energy expenditure of 161.7 kJ/kg/day and a respiratory quotient in excess of 1.0. The energy expenditure of ventilated VLBW infants may be less than previously indicated and the energy mix and nitrogen content of parenteral nutrition regimens recommended for these infants may be inappropriate.     

METHOD FOR DETERMINATION OF PULMONARY GAS EXCHANGE IN CONNECTION WITH BIRTH

Abstract. Tunell, R. (Department of Paediatrics, Karolinska Sjukhuset, Stockholm, Sweden). Method for determination of pulmonary gas exchange in connection to birth. Acta Paediatr Scand, 64:49, 1975.–An apparatus built on the "open" system for determination of pulmonary gas exchange in the newborn infant after birth is described. At four-minute intervals diluted expired air (5–7 1/min) was collected in bags. The oxygen and carbon dioxide fraction in the bags were analysed with a Nyons Diaferometer (working on the principle of thermoconductivity). In calibration experiments using a gas-mixing technique a high degree of linearity was found, both in the determination of the fraction of oxygen and carbon dioxide (r=0.9996). Reproducibility from duplicate readings was also good (for oxygen determination 0.9% and for carbon dioxide determination 0.8%). Duplicate determinations performed on infants with the same degree of motor activity resulted in an estimated error of the method of 5.8 % for V ₀₂ and 7.8 % for V _co2 respectively. A metabolic chamber was used to control environmental temperature. The air temperature and wall temperature in the chamber were regulated by water from a thermostatically controlled waterbath and were kept equal within 0.5^oC. As the method for determination of the fraction of oxygen and carbon dioxide is not specific, other gaseous materials exhaled by the infants influence the measurements and nitrous oxide was found to interfere with the determinations, and made V _o2 and V _Co2 determinations in these patients impossible. Experience from more than 50 investigations on newborn infants has shown that the method is well suited to this particular type of study.     

Respiratory water loss and oxygen consumption in full-term infants exposed to cold air on the first day after birth

Respiratory water loss, oxygen consumption, carbon dioxide production and skin blood flow were measured continuously in nine full-term infants on the first day after birth. After at least 18 min of measurements with the infant asleep in an incubator, with an air temperature of 33°C and a relative humidity of 50%, the temperature of the incubator air was lowered to less than 27.5°C. This resulted in a significant decrease in skin temperature and peripheral skin blood flow, while the infant's core temperature remained unchanged. At the same time, mean respiratory water loss increased from 3.7 to 6.1 mg/kg-min, which can be explained partially by the decrease in ambient humidity that accompanied the decrease in air temperature. In addition, mean oxygen consumption increased from 5.3 to 7.9 ml/kg. min and mean carbon dioxide production increased from 3.8 to 5.9 ml/kg-min. There was no concomitant increase in motor activity. Thus, when the newborn infants were exposed to cool air, they reacted with an increase in respiratory water loss, oxygen consumption and carbon dioxide production before their core temperature was affected and without increasing their motor activity.     

Measurement of carbon dioxide production rate in sick ventilated premature infants

A new method is described for measuring the rate of carbon dioxide production, and hence for estimating energy expenditure, in preterm infants receiving assisted ventilation. In a validation study, the mean error in carbon dioxide measurement was 1.9%. Measurements were made, over a 45-min period, on 11 sick, ventilated subjects and carbon dioxide production rate was 5.2 +/- 0.7 (SD) ml/min X kg body weight. We suggest that continuous monitoring of carbon dioxide output will contribute to the clinical assessment of the effects of different ventilator settings on pulmonary gas exchange and that estimated values for energy expenditure will be of value in nutritional studies on sick ventilated infants.     

Supplemental oxygen delivery to the nonventilated neonate

Oxygen is one of the most commonly administered drugs in the neonatal intensive care unit. A variety of techniques exists to deliver oxygen to infants who are not on a ventilator or who are not receiving positive pressure. These techniques include oxygen hood, nasal cannula, face mask delivery or "free flow," and "flooding" the incubator with oxygen. Because each technique has unique advantages and disadvantages, the choice is frequently dependent on style. Recent advances in incubator technology have improved the delivery of supplcmental oxygen by incubator flooding. This technique is generally underutilized, but it has some advantages over thc other modalities in certain clinical scenarios. The neonatal nurse needs to be aware of the characteristics of the various oxygen delivery technologies and the tools needed to optimally care for infants who are dependent on supplemental oxygen.     

Respiratory water loss and oxygen consumption in newborn infants during phototherapy

Respiratory water loss was measured together with oxygen consumption (VO2) and carbon dioxide production (VCO2) in 11 full-term and eight preterm infants (mean gestational age 34 weeks, range 31-36 weeks) before and during 1 h of phototherapy. The method for determination of respiratory water loss, VO2 and VCO2 was based on an open flow-through system with a mass spectrometer for measurement of gas concentrations. All infants were studied naked in an incubator with an ambient relative humidity of 50% and with a controlled environment with respect to temperature and air velocity. The infants were calm during the measurements. Before phototherapy, in term infants respiratory water loss was 4.4 (SD 0.7) mg/kg min and VO2 5.9 (0.9) ml/kg min and in preterm infants respiratory water loss was 4.7 (0.8) mg/kg min and VO2 6.1 (0.8) ml/kg min. No significant difference was found between values obtained during or after 1 h of phototherapy and those obtained before.     

The accuracy and precision of an open-circuit system to measure oxygen consumption and carbon dioxide production in neonates

We measured the oxygen consumption, carbon dioxide production, and respiratory quotient during the combustion of a known mass of anhydrous ethanol and methanol to assess the accuracy of an open-circuit flow-through system. Continuous measurements were made of the mass of alcohol burned, the velocity of gas flow through the apparatus, and simultaneous measurements of the fractional concentration of oxygen, carbon dioxide and nitrogen of the inlet and outlet gas using paramagnetic oxygen analyzer, infrared carbon dioxide meter, and mass spectrometer. Standard respiratory and stoichiometric equations were used to calculate the oxygen consumption, carbon dioxide production and RQ for the mass of absolute alcohol combustion per unit time. In a series of 12 consecutive laboratory experiments (on 7 days), the measured values of gas exchange (similar to the rate of respiratory gas exchange by an infant of 1-4 kg) were in agreement within 5% of the true values for ethanol and methanol combustion, confirming the validity of the open-circuit method. The paramagnetic oxygen analyzer and the mass spectrometer gave similar oxygen consumption results and differed very little when the rate of absolute alcohol combustion was used to quantify the accuracy of the complete measurement system. A positive measurement error was observed for the carbon dioxide production results from both the IR meter and mass spectrometer, with the result that the respiratory quotient measurements were 3.4-4.7% higher than the true value. The mass spectrometer gave more precise oxygen consumption results, whereas smaller variance of carbon dioxide production measurements was observed using the infrared CO2 meter.(ABSTRACT TRUNCATED AT 250 WORDS)     

Respiratory water loss in relation to activity in fullterm infants on their first day after birth

Respiratory water loss (RWL), oxygen consumption and carbon dioxide production were measured in ten fullterm infants on their first day after birth, using an open flow-through system with a mass spectrometer, specially equipped with a water channel, for analysis of gas concentrations. Measurements were made both with the infant asleep and during different levels of motor activity. The ambient temperature was maintained at approximately 32.5 degrees C and the ambient relative humidity at 50%. RWL increased from 4.2 +/- 0.7 (SD) mg/kg min when the infant was asleep to 6.3 +/- 1.0 mg/kg min when he or she was awake but calm; with increasing activity there was a further increase in RWL. The oxygen consumption increased from 5.4 +/- 0.9 (SD) ml/kg min during sleep to 6.9 +/- 0.8 (SD) ml/kg min when awake, and also increased further with increasing activity.     

Day-to-day energy expenditure variability in low birth weight neonates

We estimated the metabolic rate of 13 low birth weight infants over a 9-day period, using indirect calorimetry in conjunction with serial measurements of oxygen consumption, carbon dioxide production, and total urinary nitrogen excretion. The mean percent error for oxygen consumption and carbon dioxide production measurements (determined by alcohol combustion experiments) assignable to the open-circuit system was 0.4 and 3.8%, respectively. Error in the total urinary nitrogen excretion measurement was less than 1% by the Kjeldahl technique. In the clinical setting, however, the range of deviation of measured oxygen consumption, carbon dioxide production and total urinary nitrogen excretion was +/- 12, 12, and 15% of the mean value respectively for an individual patient under standardized controlled conditions. The variability of metabolic rate between infants may be as much as 76%. Factors that had a small effect on metabolic rate were difficult to detect because of the variability inherent in the short-term measurement of metabolic rate. It was virtually impossible to control the sources of variation in the resting metabolism of low birth weight neonates over extended experimental periods. Day-to-day variations in resting energy expenditure may explain, in part, the widely different growth rates of premature infants receiving similar caloric intakes.     

Respiratory water loss in fullterm infants on their first day after birth

Measurements of water loss from the airways, oxygen consumption and carbon dioxide production were made in 12 fullterm, newborn infants on their first day after birth, using an open flow-through system. The system includes a mass spectrometer, specially equipped with a water channel, for analysis of gas concentrations. To avoid condensation of water vapour, the tubing in the flow-through system is heated. Respiratory water loss was 4.9 +/- 1.2 (SD) mg/kg min, which meant an insensible water loss from the respiratory tract of 25.4 +/- 6.9 (SD) g per infant and 24 h at rest, at an ambient temperature of 32.2 degrees C and with an ambient humidity of 50%. Oxygen consumption was 6.0 +/- 0.8 (SD) ml/kg min. An inverse relationship was found between respiratory water loss and ambient humidity, with higher losses at a low than at a high humidity. Oxygen consumption did not change very much with ambient humidity.     

Effects of intravenous glucose loading on oxygen consumption, carbon dioxide production, and resting energy expenditure in infants with bronchopulmonary dysplasia

To determine the effects of intravenous glucose loading on basal oxygen consumption, resting energy expenditure, and basal carbon dioxide production in infants with bronchopulmonary dysplasia who were still oxygen dependent, we administered intravenous glucose loads of 4 and 12 mg/kg-1/min-1 on 2 consecutive days, under identical experimental conditions, to six infants with bronchopulmonary dysplasia and six healthy control subjects. Infants were not fed for 9 hours before and during the 4- to 6-hour study periods; the intravenous glucose infusion, along with an amino acid mixture (2 gm.kg-1.24 hr-1), was started at the beginning of the fasting period. Oxygen consumption and carbon dioxide production and resting energy expenditure were measured by a flow-through indirect calorimetry technique under basal conditions. Infants with oxygen-dependent bronchopulmonary dysplasia had significantly higher basal oxygen consumption and resting energy expenditure than did control infants and significantly higher basal carbon dioxide production during the high glucose infusion. With glucose loading, infants with bronchopulmonary dysplasia had a significant rise in basal oxygen consumption (7.91 +/- 0.91 ml.kg-1.min-1 to 9.65 +/- 1.35 ml.kg-1.min-1, p less than 0.05), basal carbon dioxide production (5.93 +/- 0.72 ml.kg-1.min-1 to 7.10 +/- 1.04 ml.kg-1.min-1), and resting energy expenditure (53.8 +/- 5.75 kcal.kg-1.24 hr-1 to 65.3 +/- 7.0 kcal.kg-1.24 hr-1, all p values less than 0.05). Control infants had no significant changes with intravenous glucose loading. We conclude that intravenous glucose loading in infants with bronchopulmonary dysplasia resulted in a net increase in resting energy expenditure, which should be taken into account in assessing their energy intake during nutritional management. The risk of pulmonary stress caused by an increase in basal oxygen consumption and carbon dioxide production resulting from glucose load should also be considered.     

Transcutaneous estimation of arterial carbon dioxide in intensive care. Which electrode temperature?

Transcutaneous and arterial carbon dioxide were measured simultaneously in 57 children (age range 10 days to 14.3 years) undergoing intensive care. All were haemodynamically stable at the time of study. Mean calibration time with 5 and 10% carbon dioxide was 43 (range 38-58) minutes and mean arterialisation time was 10.5 (range 3-30) minutes. Duplicate hourly arterial samples over a four hour period showed that transcutaneous: arterial carbon dioxide correlation was independent of electrode temperatures over the range 42-44 degrees C and was independent of child age. One and a half hours after electrode placement transcutaneous carbon dioxide (kPa) = arterial carbon dioxide X 1.41 + 0.02. Use of a simple ratio (arterial carbon dioxide = transcutaneous carbon dioxide/1.40) resulted in a mean estimate of arterial carbon dioxide within 3% of the actual value with coefficients of variation of 11 and 15% at 1.5 and 4.5 hours, respectively. For prediction of arterial carbon dioxide 95% confidence limits around the regression mean rose from +/- 1.04 kPa (7.8 mm Hg) at 1.5 hours to +/- 1.56 (11.7 mm Hg) at 4.5 hours. Baseline drift was more than +/- 0.67 kPa (5 mm Hg) during 22% of the studies. For a drift of less than 0.67 kPa 95% confidence limits were 0.87 and 1.12 kPa, respectively. Arterial carbon dioxide can be estimated with clinically acceptable tolerances from surface electrodes operating at temperatures between 42-44 degrees C. Improved electrode stability and speed of calibration should make this a valuable monitoring technique.     

Energy metabolism and substrate utilization in low birth weight neonates under radiant warmers

We evaluated the metabolic response to the thermal demands of an open radiant warmer device, as distinct from convection incubator, in 13 healthy premature infants (1.395 +/- 169 g, 28 +/- 12 days of age, mean +/- SD). Metabolic rate was 10% higher for infants under the radiant warmer than in the incubator (2.60 +/- 0.4 v 2.36 +/- 0.3 kcal/kg/h; P less than .05). The radiant warmer also induced a small (4%), but significant, increase in nonprotein respiratory quotient (0.94 +/- 0.1 v 0.90 +/- 0.1; P less than .05) and a 13% increase in carbon dioxide production (8.26 +/- 1.1 v 7.31 +/- 1.1 mL/kg/min; P less than .05). Subcutaneous fat accumulation (estimated from 60-second skin-fold thickness measurements) was greater under the radiant warmer than in the incubator (0.08 +/- 0.05 v 0.04 +/- 0.04 mm/d; P less than .05). Under the warmer, the infant's mean skin temperatures and core temperatures were normal and similar to those found in the incubator, but the foot temperature was on average 0.6 degrees C cooler. The average rate of weight gain (18 g/kg/d) was the same in the radiant environment. The pattern of the elevated metabolic rate, shift of respiratory quotient coupled with the accumulation of subcutaneous fat, and cool extremities of infants under the radiant warmer may represent a physiologic adaptive response to thermal stress. However, the reasons for the elevated metabolic rate are unclear, because activation of the sympathetic nervous system with the release of catecholamines is not apparently involved.(ABSTRACT TRUNCATED AT 250 WORDS)     

Arterialisation of transcutaneous oxygen and carbon dioxide.

We compared previously calculated global correction factors for oxygen and carbon dioxide arterial/transcutaneous ratios with individual in vivo calibrations from the first arterial sample. In infants beyond the neonatal period and older children in vivo calibration confers little benefit over the use of a global calibration correction factor for transcutaneous carbon dioxide, and may reduce the precision with which arterial oxygen can be estimated from transcutaneous oxygen.     

The Effect of a Warm Environment on Respiratory Water Loss in Fullterm Newborn Infants on Their First Day after Birth

ABSTRACT. Continuous measurements of respiratory water loss (RWL), oxygen consumption and carbon dioxide production were made in 21 fullterm infants on their first day after birth. The infants were first studied in incubators with a temperature of 32.5°C and an ambient humidity of 50%. After an interval with stable conditions the incubator temperature was raised to 36.5°C while the water vapour pressure was kept constant. When the rectal temperature had increased to 37.8°C or when the infant had started to sweat, the relative humidity in the incubator was increased to 50%. At the start of the measurements mean RWL was 4.9 mg/kg min. On the average, RWL increased to a maximal value of 7.0 mg/kg min in the warm environment. Mean oxygen consumption only increased from 5.3 to 5.8 ml/kg min. This meant that when nursed in this warm environment the infants were able to increase their respiratory water loss by nearly 50% without a significant change in oxygen consumption.     

Thermal balance in term and preterm newborn infants nursed in an incubator equipped with a radiant heat source

A radiant hood warmer, a device that heats the incubator roof independently of the incubator's main heat source, was used to study the thermal balance of 11 full term and 13 preterm (gestational age 25–34 weeks) infants exposed to an isolated elevation of incubator roof temperature at stable ambient air temperature and humidity. After initial measurements without active heating of the incubator roof, the hood warmer was set to 33°C, 36°C and finally (preterm infants only) to 39°C. At least 18 min of measurements with the infant asleep were made at each hood warmer setting. In the term infants an increase in roof temperature from 30.5°C to 35.6°C resulted in an increase in skin temperature from 35.4 to 35.9°C, and a decrease in radiative heat loss from 32.8 to 20.7 W/m² exposed skin. In the preterm infants an increase in roof temperature from 31.0 to 38.4°C led to an increase in skin temperature from 35.7 to 36.3°C and a decrease in radiative heat loss from 34.1 to 13.0 W/m² exposed skin. The increased inner roof surface temperature did not affect evaporative or convective heat loss, skin blood flow, respiratory water loss, oxygen consumption or transepidermal water loss in either group. Thus, at stable ambient air temperature and humidity, the increase in incubator roof temperature resulted in an increase in skin temperature and a decrease in radiative heat loss in both term and preterm infants.     

The effect of a warm environment on respiratory water loss in fullterm newborn infants on their first day after birth

Continuous measurements of respiratory water loss (RWL), oxygen consumption and carbon dioxide production were made in 21 fullterm infants on their first day after birth. The infants were first studied in incubators with a temperature of 32.5 degrees C and an ambient humidity of 50%. After an interval with stable conditions the incubator temperature was raised to 36.5 degrees C while the water vapour pressure was kept constant. When the rectal temperature had increased to 37.8 degrees C or when the infant had started to sweat, the relative humidity in the incubator was increased to 50%. At the start of the measurements mean RWL was 4.9 mg/kg min. On the average, RWL increased to a maximal value of 7.0 mg/kg min in the warm environment. Mean oxygen consumption only increased from 5.3 to 5.8 ml/kg min. This meant that when nursed in this warm environment the infants were able to increase their respiratory water loss by nearly 50% without a significant change in oxygen consumption.     

A new transport incubator for primary care of low birth weight babies.

A new portable, cheap and indigenous incubator made of polystyrene has been devised for delivery of primary health care services to the newborn babies in the community. Twenty six babies with a mean weight of 1726 g (range 1388-1981g) and gestational age of 35.3 weeks (range 34-38 wks) were continuously evaluated for 2 hours observation period, in naked and clothed conditions. Rectal, abdominal skin, foot, ambient air and nursery temperatures were recorded. The baseline core temperature of the babies was 36.58 (+/- 0.21) degrees C; after incubator care it was recorded s 36.80 (+/- 0.10) degrees C in naked infants. The baseline core temperature of the clothed babies was 36.63 (+/- 0.21) while it was 37.01 (+/- 0.18) after 2 hours of incubator care. An ambient air temperature of 33-34 degrees C in the incubator (thermoneutral temperature range for these babies being 31.0-33.8 degrees C) was achieved within 30-60 minutes of incubator stay (nursery temperature being 28 +/- 0.6 degrees C). No evidence of carbon dioxide narcosis, hypoxia, acidosis, or adverse thermoregulatory behavior was observed. One baby had hypoglycemia (blood sugar less than 35 mg/dl) and another had sweating. There is a scope for providing additional facilities like administration of oxygen, phototherapy, X-rays through the incubator without disturbing the baby.     

Respiratory metabolism in preterm infants: the measurement of oxygen consumption during prolonged periods

We have developed a method for measuring oxygen consumption (Vo2) in preterm infants in their normal incubator environment over prolonged periods. The results of measurements made over 24 h in 18 infants are presented. In normally grown infants, the mean Vo2 was 9.66 +/- 1.25 liters/kg X 24 h (SD) (6.71 +/- 0.87 ml/kg X min), and in small for gestation infants it was 10.09 +/- 1.21 liters/kg X 24 h (7.00 +/- 0.84 ml/kg X min). During the 24-h measurements, the highest mean Vo2 during 3 consecutive h was 7.75 +/- 0.89 ml/kg X min and the lowest was 5.95 +/- 0.92 ml/kg X min. The difference between the highest and the lowest values was significant (p less than 0.001). There is room for considerable error if short term measurements are assumed to represent values over a whole day. "Short" measurements should be made over at least 6 h.     

Hypercapnic and hypoxic ventilatory and cardiac responses in school-aged siblings of sudden infant death syndrome victims.

Siblings of sudden infant death syndrome (SIDS) victims have been shown to have abnormal ventilatory patterns and altered responses to respiratory stimuli during infancy. To evaluate whether these abnormalities persist, we studied ventilatory responses in 20 older SIDS siblings (9.8 +/- 0.9 (mean +/- SEM) years of age) and 20 control subjects (10.2 +/- 0.9 years of age). To evaluate hypercapnic ventilatory responses, we had subjects rebreathe 5% carbon dioxide and 95% oxygen until end-tidal carbon dioxide tension reached 65 mm Hg. Instantaneous minute ventilation, mean inspiratory flow, and respiratory rate were calculated breath by breath. Hypercapnic responses did not differ between SIDS siblings (2.08 +/- 0.14 L/min per mm Hg) and control subjects (1.90 +/- 0.10 L/min per mm Hg; not significant). To assess hypoxic ventilatory responses, we asked subjects to rebreathe 13% oxygen and 7% carbon dioxide, with the balance nitrogen, at mixed-venous end-tidal carbon dioxide tension, until arterial oxygen saturation by pulse oximetry fell to 75%. No differences in hypoxic ventilatory responses were found between the SIDS siblings (-1.39 +/- 0.15 L/min/% saturation) and the control subjects (-1.22 +/- 0.17 L/min/% saturation; not significant). The mean inspiratory flow, tidal volume, respiratory rate, and heart rate responses to hypercapnia and hypoxia were also similar in the two groups. We conclude that there is no difference in hypercapnic and hypoxic ventilatory and cardiac responses, as assessed by rebreathing techniques, between school-aged SIDS siblings and control subjects. We speculate that in SIDS siblings the control of breathing is immature during infancy and that they achieve maturity of control and resolution of breathing abnormalities with time.