Effect of maternal smoking on ventilatory responses to changes in inspired oxygen levels in infants.

Maternal smoking is a major independent risk factor for sudden infant death syndrome (SIDS). Respiratory control deficits have been implicated in SIDS. The aim of this study was to test the hypothesis that infants born to smoking mothers have reduced ventilatory responses to changes in inspired oxygen. Smoking and nonsmoking women were recruited in midpregnancy and smoking status confirmed by measurement of urinary cotinine. To control for confounding factors of smoking each mother-infant pair in the smoking group was matched to a pair in the nonsmoking group for social class, maternal age and parity, feeding, birthweight, gestational age, and infant sex. Infants were seen overnight at approximately 10 wk of age for tests of respiratory control, using the alternating breath test. Ventilation was measured by respiratory inductance plethysmography and inspired and end-tidal oxygen levels were determined by mass spectrometry. Data were obtained from 40 infants (17 in the smoking group). Responses were similar in both groups for 10 respiratory parameters including respiratory drive and timing, and there were no significant differences. The mean end-tidal oxygen level when 40% O(2) was delivered was an average of 1.13% higher in the smoking group (p = 0.0067), although the inspired oxygen levels were not different. In conclusion, we did not find an independent effect of maternal smoking on respiratory control. The differences in end-tidal oxygen levels during the alternating breath test may represent differences in alveolar ventilation.

Respiratory inductance plethysmography in healthy infants: a comparison of three calibration methods.

Respiratory inductance plethysmography (RIP) measures respiration from body surface movements. Various techniques have been proposed for calibration in order that RIP may be used quantitatively. These include calculation of the proportionality constant of ribcage to abdominal volume change (K). The aims of this study were to 1) establish whether a fixed value of K could be used for calibration, and 2) compare this technique with multiple linear regression (MLR) and qualitative diagnostic calibration (QDC) in normal healthy infants. Recordings of pneumotachograph (PNT) flow and RIP were made during quiet (QS) and active sleep (AS) in 12 infants. The first 5 min in a sleep state were used to calculate calibration factors, which were applied to subsequent validation data. The absolute percentage error between RIP and PNT tidal volumes was calculated. The percentage error was similar over a wide range of K during QS. However, K became more critical when breathing was out of phase. A standard for K of 0.5 was chosen. There was good agreement between calibration methods during QS and AS. In the first minute following calibration during QS, the mean absolute errors were 3.5, 4.1 and 5.3% for MLR, QDC and fixed K respectively. The equivalent errors in AS were 11.5, 13.1 and 13.7% respectively. The simple fixed ratio method can be used to measure tidal volume with similar accuracy to multiple linear regression and qualitative diagnostic calibration in healthy unsedated sleeping infants, although it remains to be validated in other groups of infants, such as those with respiratory disease.

The effect of hand immersion on body temperature when wearing impermeable clothing.

The effects of hand immersion on body temperature have been investigated in men wearing impermeable NBC clothing. Six men worked continuously at a rate of approximately 490 J. sec-1 in an environmental temperature of 30 degrees C. Each subject was permitted to rest for a period of 20 minutes when their aural temperature reached 37.5 degrees C, and again on reaching 38 degrees C, and for a third time on reaching 38.5 degrees C (three rest periods in total). Each subject completed three experimental conditions whereby, during the rest periods they either: a. Did not immerse their hands (control). b. Immersed both hands in a water bath set at 25 degrees C. c. Immersed both hands in water at 10 degrees C. Physiological measures of core temperature, skin temperature and heart rate were recorded at intervals throughout the experiment. Measures of mean aural temperature and mean skin temperature were significantly (P less than 0.05) reduced if hands were immersed during these rest periods, compared to non immersion. As a result, the total work time of subjects was extended when in the immersed conditions by some 10-20 minutes within the confines of the protocol. It is concluded that this technique of simple hand immersion may be effective in reducing heat stress where normal routes to heat loss are compromised.