Source: Annual Congress 2002 - Bronchial responsiveness in asthma and COPD
Disease area: Airway diseases, Respiratory critical care

Abstract

Little is known about the effects of reduced gas tensions on the distribution of ventilation in humans. We explored the effects of hypoxia and hypocapnia in normal subjects, by using the Multiple Breath Nitrogen Washout (MBNW) technique (Crawford, A.B.H. et al. J Appl Physiol 1985; 59:838-846). The slope of the alveolar plateau of individual expiratory breaths, normalized by mean expired nitrogen concentration (Sn), is sensitive to diffusion dependent inhomogeneity (DDI) in the first five breaths of a washout and to convection dependent inhomogeneity (CDI) during the ensuing washout. The Bohr dead space can also be derived. We used MBNW in eight normal, young male and female volunteers, after they equilibrated on one of four gas concentrations: isocapnia and normoxia (N), isocapnia and hypoxia (IH), hypocapnia and normoxia (HN) and hypocapnia and hypoxia (HH). Inspired oxygen was reduced to 13% - 15% for hypoxia. Hypocapnia (PeCO2 =30 mmHg) was produced by hyperventilation. The ventilatory pattern necessary to produce hypocapnia was used for the other three experimental conditions. CO2 was bled into the inspiratory circuit to maintain isocapnic conditions. A metronome and visual feedback via an oscilloscope were used to aid the subjects in maintaining reproducible breathing patterns. Sn during the first 5 breaths was not affected by different gas tensions, indicating no effect on DDI. However, from breaths 6-30 Sn was significantly lower during HH compared to N (p<0.001); Sn for IH and HN had intermediate values, indicating improvement in CDI. The changes in Bohr dead space paralleled those in Sn, but were small. These results indicate that hypoxia and hypocapnia modify intrapulmonary ventilation distribution.
Supported by DCIEM, DND Canada.

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Citations should be made in the following way:
L. Forkert, T. Fisher (Kingston, Canada). The effect of hypocapnia and hypoxia on the homogeneity of ventilation distribution in normal humans. Eur Respir J 2002; 20: Suppl. 38, 1202

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