Inspired CO(2) and O(2) in sleeping infants rebreathing from bedding: relevance for sudden infant death syndrome.

Some infants sleep facedown for long periods with no ill effects, whereas others become hypoxemic. Rebreathing of expired air has been determined by CO(2) measurement; however, O(2) levels under such conditions have not been determined. To evaluate this and other factors influencing inspired gas concentrations, we studied 21 healthy infants during natural sleep while facedown on soft bedding. We measured gas exchange with the environment and bedding, ventilatory response to rebreathing, and concentrations of inspired CO(2) and O(2). Two important factors influencing inspired gas concentrations were 1) a variable seal between bedding and infants' faces and 2) gas gradients in the bedding beneath the infants, with O(2)-poor and CO(2)-rich air nearest to the face, fresher air distal to the face, and larger tidal volumes being associated with fresher inspired air. Minute ventilation increased significantly while rebreathing because of an increase in tidal volume, not frequency. The measured drop in inspired O(2) was significantly greater than the accompanying rise in inspired CO(2). This appears to be due to effects of the respiratory exchange ratio and differential tissue solubilities of CO(2) and O(2) during unsteady conditions.     

Quantifying the global warming potential of CO2 emissions from wood fuels

Recent studies have introduced the metric GWP_bio, an indicator of the potential global warming impact of CO₂ emissions from biofuels. When a time horizon of 100 years was applied, the studies found the GWP_bio of bioenergy from slow‐growing forests to be significantly lower than the traditionally calculated GWP of CO₂ from fossil fuels. This result means that bioenergy is an attractive energy source from a climate mitigation perspective. The present paper provides an improved method for quantifying GWP_bio. The method is based on a model of a forest stand that includes basic dynamics and interactions of the forest's multiple carbon pools, including harvest residues, other dead organic matter, and soil carbon. Moreover, the baseline scenario (with no harvest) takes into account that a mature stand will usually continue to capture carbon if not harvested. With these methodological adjustments, the resulting GWP_bio estimates are found to be two to three times as high as the estimates of GWP_bio found in other studies, and also significantly higher than the GWP of fossil CO₂, when a 100‐year time horizon is applied. Hence, the climate impact per unit of CO₂ emitted seems to be even higher for the combustion of slow‐growing biomass than for the combustion of fossil carbon in a 100‐year time frame.     

Intracisternal DIDS enhances ventilatory response to rebreathing CO2 in rats

In 11 anesthetized rats, we tested the hypothesis that carrier-mediated anion transport in part determines the medullary chemoreceptor response to acute hypercapnia by infusing the transport inhibitor 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) in mock cerebrospinal fluid (CSF) into the cisterna magna. In five additional rats with sham CSF infusion, we found no effect of mock CSF on the response to rebreathing CO2. Dye infused into the cistern stained the putative chemoreceptor areas on the ventral surface of the medulla. DIDS, at 10 to 1,000 nM, increased the respiratory response to CO2 in a dose-related manner but had no effect on arterial pressure or heart rate. At 1,000 nM, the hypercapnic minute ventilation response was almost doubled because of both volume and rate of breathing. We conclude that the net effect of anion transport is to mitigate the stimulus to the medullary chemoreceptors during acute hypercapnia.     

Ventilatory response of the sleeping newborn to CO2 during normoxic rebreathing

The ventilatory response of the newborn to CO2 was studied using a rebreathing method that minimized changes in arterial PO2 during the test. The aim was to study the variability of the ventilatory response to CO2 and take this into account to assess the relative magnitude of the response to CO2 during rapid-eye-movement (REM) sleep and quiet sleep (QS). Five full-term babies aged 4-6 days were given 5% CO2 in air to rebreathe for 1.5-3 min. O2 was added to the rebreathing circuit to maintain arterial O2 saturation and transcutaneous PO2 (Ptco2) at prerebreathing levels. Tests were repeated four to five times in REM sleep and QS. Mean Ptco2 levels varied between individuals but were similar during REM sleep and QS tests for each subject. The mean coefficient of variability of the ventilatory response was 35% (range 15-77%) during QS and 120% (range 32-220%) during REM sleep. PtcO2 fluctuations during tests [6.0 +/- 3.0 (SD) Torr, range 1-13 Torr] were not correlated with ventilatory response. Overall the ventilatory response was significantly lower in REM sleep than in QS (12.2 +/- 3.0 vs. 38.7 +/- 3.0 ml.min-1.Torr-1.kg-1, P less than 0.001; 2-way analysis of variance) due to a small (nonsignificant) fall in the tidal volume response and a significant fall in breathing rate. In 12 REM sleep tests there was no significant ventilatory response; mean inspiratory flow increased significantly during 8 of these 12 tests. We conclude that there is a significant decrease in the ventilatory response of the newborn to CO2 rebreathing during REM sleep compared with QS.     

Diverting residual biomass to energy use: Quantifying the global warming potential of biogenic CO2 (GWPbCO2)

To calculate the global warming potential of biogenic carbon dioxide emissions (GWP_bCO2) associated with diverting residual biomass to bioenergy use, the decay of annual biogenic carbon pulses into the atmosphere over 100 years was compared between biomass use for energy and its business-as-usual decomposition in agricultural, forestry, or landfill sites. Bioenergy use increased atmospheric CO₂ load in all cases, resulting in a ¹⁰⁰GWP_bCO2 (units of g CO₂e/g biomass CO₂ released) of 0.003 for the fast-decomposing agricultural residues to 0.029 for the slow, 0.084–0.625 for forest residues, and 0.368–0.975 for landfill lignocellulosic biomass. In comparison, carbon emissions from fossil fuels have a ¹⁰⁰GWP of 1.0 g (CO₂e/g fossil CO₂). The fast decomposition rate and the corresponding low ¹⁰⁰GWP_bCO2 values of agricultural residues make them a more climate-friendly feedstock for bioenergy production relative to forest residues and landfill lignocellulosic biomass. This study shows that CO₂ released from the combustion of bioenergy or biofuels made from residual biomass has a greenhouse gas footprint that should be considered in assessing climate impacts.     

Demographic and socioeconomic factors affecting infant mortality in Egypt

This paper analyses the relative importance of demographic and socioeconomic factors with respect to their role in reducing infant mortality in Egypt. Logit analyses of data from a nationally representative sample of Egyptian households, and for urban and rural households separately, indicate that demographic factors have more effect on infant mortality than socioeconomic factors. The results also show the need to improve housing in urban areas and sewerage systems in rural areas in order to reduce infant mortality. One of the most important policy conclusions, however, concerns the importance of providing a vigorous educational campaign to enlighten mothers and prospective mothers in both rural and urban areas on the positive effects of breast-feeding, longer birth intervals, and fewer children on the survival of infants.     

Effects of CO2 rebreathing on pulmonary mechanics in premature infants

The effects of hypercapnia produced by CO2 rebreathing on total pulmonary, supraglottic, and lower airway (larynx and lungs) resistance were determined in eight premature infants [gestational age at birth 32 +/- 3 (SE) wk, weight at study 1,950 +/- 150 g]. Nasal airflow was measured with a mask pneumotachograph, and pressures in the esophagus and oropharynx were measured with a fluid-filled or 5-Fr Millar pressure catheter. Trials of hyperoxic (40% inspired O2 fraction) CO2 rebreathing were performed during quiet sleep. Total pulmonary resistance decreased progressively as end-tidal PCO2 (PETCO2) increased from 63 +/- 23 to 23 +/- 15 cmH2O.l-1.s in inspiration and from 115 +/- 82 to 42 +/- 27 cmH2O.l-1.s in expiration between room air (PETCO2 37 Torr) and PETCO2 of 55 Torr (P less than 0.05). Lower airway resistance (larynx and lungs) also decreased from 52 +/- 22 to 18 +/- 14 cmH2O.l-1.s in inspiration and from 88 +/- 45 to 30 +/- 22 cmH2O.l-1.s in expiration between PETCO2 of 37 and 55 Torr, respectively (P less than 0.05). Resistance of the supraglottic airway also decreased during inspiration from 7.2 +/- 2.5 to 3.6 +/- 2.5 cmH2O.l-1.s and in expiration from 7.6 +/- 3.3 to 5.3 +/- 4.7 cmH2O.l-1.s at PETCO2 of 37 and 55 Torr (P less than 0.05). The decrease in resistance that occurs within the airway in response to inhaled CO2 may permit greater airflow at any level of respiratory drive, thereby improving the infant's response to CO2.     

Abdominal muscle activity during CO2 rebreathing in sleeping neonates

Comparison of the abdominal muscle response to CO2 rebreathing in rapid-eye-movement (REM) and non-REM (NREM) sleep was performed in healthy premature infants near full term. Eight subjects were studied at a postconceptional age of 40 +/- 1.6 (SD) wk (range 38-43 wk) during spontaneous sleep. Sleep stages were defined on the basis of electrophysiological and behavioral criteria, and diaphragmatic and abdominal muscle electromyographic activity was recorded by cutaneous electrodes. The responses to CO2 were measured by a modified Read rebreathing technique. The minute ventilation and diaphragmatic and abdominal muscle electromyographic activities were calculated and plotted against end-tidal CO2 partial pressure. Both the ventilatory and diaphragmatic muscle responses to CO2 decreased from NREM to REM sleep (P less than 0.05). Abdominal muscles were forcefully recruited in response to CO2 rebreathing during NREM sleep. In REM sleep, abdominal muscle response to CO2 was virtually absent or decreased compared with NREM sleep (P less than 0.05). We conclude that 1) the abdominal muscles are recruited during NREM sleep in response to CO2 rebreathing in healthy premature infants near full term and 2) the abdominal muscle recruitment is inhibited during REM sleep compared with NREM sleep, and this REM sleep-related inhibition probably contributes to the decrease in the ventilatory response to CO2 rebreathing in REM sleep.     

Arterial-alveolar CO2 equilibration in exercising dogs during prolonged rebreathing

Arterial-alveolar equilibration of CO2 during exercise was studied by normoxic CO2 rebreathing in six dogs prepared with a chronic tracheostomy and exteriorized carotid loop and trained to run on a treadmill. In 153 simultaneous measurements of PCO2 in arterial blood (PaCO2) and end-tidal gas (PE'CO2) obtained in 46 rebreathing periods at three levels of mild-to-moderate steady-state exercise, the mean PCO2 difference (PaCO2-PE'CO2) was -1.0 +/- 1.0 (SD) Torr and was not related to O2 uptake or to the level of PaCO2 (30-68 Torr). The small negative PaCO2-PE'CO2 is attributed to the lung-to-carotid artery transit time delay which must be taken into account when both PaCO2 and PE'CO2 are continuously rising during rebreathing (average rate 0.22 Torr/s). Assuming that blood-gas equilibrium for CO2 was complete, a lung-to-carotid artery circulation time of 4.6 s accounts for the observed uncorrected PaCO2-PE'CO2 of -1.0 Torr. The results are interpreted to indicate that in rebreathing equilibrium PCO2 in arterial blood and alveolar gas are essentially identical. This conclusion is at variance with previous studies in exercising humans during rebreathing but is in full agreement with our recent findings in resting dogs.