Measurement of functional residual capacity through the transient phase of He dilution in newborns.

Helium dilution maneuver is used to determine the functional residual capacity (FRC) 14 newborns ages 1-5 mo. The model equation describes the changing alveolar fractions of He and the ventilation promoted by a rebreathing procedure that does not exceed 40 s. The model does not involve the volume of the rebreathing bag usually needed when applying rebreathing technique and which is a source of error. The equation is discretized and solved for recorded data obtained with equipment adapted to newborns. Results show a strong relationship between FRC and the biometrical indexes, and confirm those found in the literature featuring that the measurement duration of FRC can be considerably shortened.

The hyperoxic test in infants reinvestigated.

The hyperoxic test (HT) examines peripheral chemoreceptor function (PCF) by measuring the decrease in ventilation (V E) after 100% O(2) inhalation. A 30-s HT has been previously used in infants with calculation of the ventilatory response (VR) as the mean percentage change in V E during HT as compared with normoxia. However, it has been shown that during hyperoxia V E rises secondarily after the initial drop because of loss of PCF. We hypothesized that the mean V E change over a 30-s HT may underestimate the strength of PCF and may be poorly reproducible. We performed breath-by-breath analysis during 30-s HTs, calculating VR at the response time (RT) defined as the time from HT onset to the first significant HT-related change in V E. Eighteen infants (postnatal age, 21 +/- 4 d) underwent two HTs (quiet sleep, face mask attached to a pneumotachograph, and inspired and expired O(2) and CO(2) fractions measured using mass spectrometry). V E, VT, and VT/TI decreases at the RT were significantly greater than the corresponding means (-21 +/- 7 versus -15 +/- 7%, -21 +/- 8 versus -13 +/- 8%, and -22 +/- 11 versus -17 +/- 11%, respectively). Intra-individual coefficients of variation of V E, VT and VT/TI were significantly smaller when RT values were considered rather than means. We conclude that calculation of the VR to HT at RT improves assessment of PCF and enhances HT reproducibility in infants.

Mannequin-assessed dry-heat exchanges in the incubator-nursed newborn.

For manufacturers of warming devices for newborn infants, knowledge of the partition of the various channels of heat transfer between the neonate and the environment is necessary to regulate them adequately. The goal of this study was to determine the contributions of the different components of dry heat exchange using a thermal mannequin that replicated a full-term neonate. Each mannequin segment could be controlled separately at a selected surface temperature. The mannequin was placed in a reclining position on a mattress in a single-walled incubator (BioMS C2750). Conductive (K), convective (C), and radiative (R) heat exchanges were measured at incubator temperatures (T alpha) of 29 degrees, 32 degrees, and 34 degrees C and at air velocities (Va of 0 to 0.7 m/sec. Conductive heat exchanges varied from 4.5% to 7.9% of total dry heat loss (H). The conductive heat transfer coefficient was 0.21 W/degree C, and the mannequin surface area in contact with the mattress was 10.4% of the total surface area (A). Under natural convection, the convective and radiative heat transfer coefficients were 4.94 and 4.77 W/m2/degree C, respectively. The radiating surface area was 78% of total surface area. Convective heat exchange decreased from 36% to 17%, and radiative heat exchange increased from 60% to 79% of total dry heat loss as incubator temperature increased from 29 degrees to 34 degrees C. When air velocity was raised, convective heat exchange increased, whereas radiative heat exchange decreased. Whatever the incubator temperature, a fivefold increase in convective heat exchange was observed when air velocity increased, whereas radiative heat exchange was unchanged. At an incubator temperature of 34 degrees C and for air velocities between 0.1 and 0.4 m/sec radiative heat exchange is the dominant mode of heat loss. The results suggest that this thermal mannequin is a good tool for assessing dry heat exchange in incubators.

Determination of diffusing capacity by modeling the dynamics of C18O uptake in newborns.

Dynamic modeling of lung C18O diffusion is used to measure the C18O transfer factor (TLCO) of 14 newborns aged 1-4 mo. The model equation is based on the alveolar fractions of C18O and on changing alveolar ventilation induced by the rebreathing conditions. The model does not involve the volume of the rebreathing bag which is usually needed when applying rebreathing technique and which is a source of error. The equation is discretized and solved for recorded data obtained with equipment adapted to use in newborns. A least-square parameter calculation technique is applied to estimate TLCO. Results show a strong relationship between this index and the biometrical ones and confirm those found in the literature featuring that the measurement duration can be considerably shortened.

Reproducibility of the alternating breath test of fractional inspired O2 in infants.

We conducted a reproducibility study of the alternating breath test (ABT) for assessing peripheral chemoreceptor function in infants. The ABT delivers a rapid hypoxic stimulus to the peripheral chemoreceptors with breath-by-breath alternations of the inspired O2 fraction. The reproducibility of the ABT performed on a single occasion has not been extensively studied in infants. Eight unsedated infants (postnatal age, 22+/-19 d; weight, 3.2+/-0.4 kg) were studied in standardized conditions: morning naps, supine position, room temperature 22-24 degrees C, quiet sleep, and face mask attached to a pneumotachograph connected to a two-way electric valve. Respiratory gases were analyzed by mass spectrometer. Two ABTs were performed. Each included a 2-min control run (CR) alternating between air and air, and a 2-min test run (TR) alternating between air and 0.15 O2. After data preprocessing, on average 13+/-11% of the data were rejected because of sighs, apneas, and cycles with the fraction of inspired oxygen above 0.17. Using the remaining validated breaths, the response to ABT was calculated for the CR, for all breaths in the TR (TR(T)), and for the first 50 breaths of the TR (TR50). During the ABTs oxygen saturation did not fall below 96%, and heart rate was not affected. Inspired and end-tidal CO2 fractions remained unchanged during the ABTs. FetO2 oscillated in TRs at a lower values than in CRs and differed significantly between breaths of air and hypoxic breaths of TRs. All infants responded to ABT with percentage alternation coefficients of TRs significantly greater than those of CRs for all respiratory variables. The values of the coefficients were not significantly different between both ABT, and between TR50 and TR(T). The greatest values of the coefficients were for timing variables compared with flows and volume. We conclude that the ABT is a reproducible test of peripheral chemoreceptor function under standardized conditions.

Skin derivative control of thermal environment in a closed incubator.

Defining a thermoneutral environment remains difficult because thermoneutrality depends on both physical and physiological factors. A servocontrolled skin temperature derivative (SCS) heating device has been designed to control the thermal environment in closed incubators without the necessity of setting an air or skin reference temperature. The thermal environment obtained with the SCS program is controlled only by the neonate's skin temperature changes. For each neonate, the program allows the attainment of a specific individual thermal equilibrium (Teq). Although the mean value of the thermal equilibrium level measured on 29 neonates does not differ significantly from the neutral air temperature defined from the charts of other researchers, individual values of Teq differed greatly among neonates of similar birthweight and postnatal age. When compared with on/off heating programs, the SCS system permits greater quiet sleep occurrence and seems to provide an optimal thermal environment. The results suggest that the skin temperature derivative heating program takes into account both the ambient and physiological factors affecting body temperature regulation of each neonate.

Regulation of sleep and body temperature in response to exposure to cool and warm environments in neonates.

BACKGROUND AND OBJECTIVE: Thermoregulation is impaired during desynchronized sleep in animals and in adults. This can lead to a conflict between homeothermy and sleep in nonthermoneutral conditions. This study aimed to analyze thermoregulation during sleep, especially during desynchronized sleep (active sleep, AS) and to determine whether the conflict between thermoregulation and sleep might exist in the newborn sleeping in warm or cool conditions. METHODS: Esophageal and skin (cheek and abdomen) temperatures, local sweating rate (ventilated sweat collection capsule stuck on the abdomen), metabolism (indirect respiratory calorimetry), and sleep variables were recorded in 10 newborns exposed, in an incubator, to thermoneutral, warm, and cool environments. Body movements and apneas were also considered. Exposures were performed after a first habituation condition. RESULTS: Sleep structure was not modified by the first exposure nor by the warm environment. Exposure to cool temperatures increased AS duration (+13% of total sleep time) and the quantity of body movements during AS (+11.3% of AS duration), whereas these parameters were not modified during quiet sleep. The thermoregulatory response to warm and cool environments was not impaired during AS. During exposure to mild thermal load, analyses revealed large interindividual differences in the strategy for thermoregulation during AS. Depending on the newborn, the thermoregulatory response to cool temperatures could be described by an increase either in nonshivering thermogenesis or in frequency of body movement. In warm conditions, most newborns exhibited an increased sweating rate. The interindividual differences (lack of increase sweating in three newborns) seemed to be linked to changes in the sensitivity of the sweating response. CONCLUSION: Because thermoregulation is not impaired during AS, this sleep stage seems to be a well-protected one from a thermoregulatory point of view. This difference from adults and animals may be due to the important role of AS in newborn's nervous maturation.

Maximal oxygen uptake and aerobic-anaerobic transition on treadmill and bicycle in triathletes.

The maximal aerobic capacity and the aerobic-anaerobic transition were analyzed on 14 triathletes performing an incremental work load on a bicycle ergometer and on horizontal or inclined treadmills. To compared the cardiorespiratory responses between cycling and running, the subjects were divided into 2 groups of 7 with similar aerobic capacity determined from cycle runs. The first group ran on horizontal treadmill while the second group performed similar exercise on inclined treadmill at constant grade (1.5%). Heart rate was recorded by electrocardiogram. Oxygen uptake (VO2), CO2 production (VCO2), respiratory frequency, and pulmonary ventilation were monitored at 30 s intervals through a Rudolph valve connected to a calibrated Oxycon V. Tidal volume, respiratory exchange ratio, equivalent O2 and CO2 were calculated from on-line computer. Aerobic and anaerobic thresholds were determined by a non-invasive method from pulmonary ventilation curves. The results showed that maximum oxygen uptake (VO2max) did not differ between the 2 types of ergometers. Pulmonary ventilation, heart rate and VO2 recorded at aerobic and anaerobic thresholds depended on the mode of exercise and reached the highest values on inclined treadmill. The amount of muscle mass, the type and the distribution of active motor units involved in each exercise test might be at the origin of these differences. This indicates that, when assessing a training program from anaerobic threshold values, it is necessary to take into consideration the type of ergometer used and the protocol performed.