Hypoxic responses in infants

Altitude and Variable Effects on Infant Mortality in the United States

Levine, Robert S., Jason L. Salemi, Maria C. Mejia de Grubb, Sarah K. Wood, Lisa Gittner, Hafiz Khan, Michael A. Langston, Baqar A. Husaini, George Rust, and Charles H. Hennekens. Altitude and variable effects on infant mortality in the United States. High Alt Med Biol. 19:265-271, 2018.

AIMS

To explore whether altitude has different effects on infant mortality from newborn respiratory distress, nontraumatic intracranial hemorrhage, and necrotizing enterocolitis.

RESULTS

Infants born in the US Mountain Census Division (AR, CO, ID, NV, NM, UT, and WY) had lower mortality from newborn respiratory distress (p < 0.001, mortality rate ratios [MRR] = 0.5 for non-Hispanic blacks and non-Hispanic whites and 0.6 for Hispanic whites) relative to infants born elsewhere in the United States, while Mountain Division non-Hispanic white infants had significantly higher mortality from nontraumatic intracranial hemorrhage (MRR = 1.3 [1.1, 1.6] p < 0.001). After adjustment for state average birth weight, gestational age, and income inequality, a statistically significant, inverse association remained between state average altitude and non-Hispanic white infant mortality from newborn respiratory distress. County altitude (3058 counties in 9 categories from ≤0 to ≥7000 feet) was negatively correlated with newborn respiratory distress (r = -0.91, p < 0.001) and necrotizing enterocolitis (r = -0.81, p = 0.006) at ≤0 to ≥7000 feet and positively correlated with nontraumatic intracranial hemorrhage at ≤0 to 6000-6999 feet (r = 0.78, p = 0.02).

CONCLUSIONS

These data show variable cause-specific effects of altitude on infant mortality. Analytic epidemiologic research is needed to confirm or refute the hypotheses generated by these descriptive data.

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.