Physiological dead space increases during initial hours of chronic hypoxemia with or without hypocapnia.
A whole body plethysmograph was used to determine the minute ventilation-to-CO2 production ratio (VE/VCO2) of intact unrestrained unanesthetized adult male Sprague-Dawley rats during 7 days of hypoxemia (arterial PO2 approximately 50 Torr). In one set of rats, normocapnia (arterial PCO2 approximately 40 Torr) was maintained. Arterial blood gases and acid-base status were determined, and arterial PCO2 was used to calculate alveolar ventilation-to-VCO2 ratio (VA/VCO2) in all situations when inhaled CO2 was not elevated. In normoxia VE/VCO2 = 25 +/- 1 (mean +/- 95% confidence limits); after 12 h of hypoxemia, VE/VCO2 was maximal, 61 +/- 5 in hypoxemic hypocapnia and 200 +/- 55 in hypoxemic normocapnia. Between 2 and 7 days of hypoxemia, VE/VCO2 had plateaued, 42 +/- 3 in hypoxemic hypocapnia and 95 +/- 19 in hypoxemic normocapnia. Dead space-to-tidal volume ratio (VD/VT) = (VE/VCO2 - VA/VCO2)/(VE/VCO2), and in normoxia VD/VT = 0.17 +/- 0.04. In hypoxemic hypocapnia, VD/VT measured between 1 and 5 h was 0.38 +/- 0.04. It remained elevated at 0.29 +/- 0.04 after 24 h, but after 4-7 days in hypoxemic hypocapnia, VD/VT had recovered to 0.15 +/- 0.03. It is postulated that the disproportionate increase in VE/VCO2 observed during the first 24 h of exposure to hypoxemic normocapnia (compared with elevated steady-state plateau levels maintained from 2 to 7 days sojourn) reflects an immediate transient increase of physiological dead space on exposure to hypoxemia.
Olson
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John Rankin Laboratory of Pulmonary Medicine, Department of Preventive Medicine, University of Wisconsin School of Medicine, Madison, 53705.
D000064:Acclimatization / Q000502:physiology; D000136:Acid-Base Equilibrium / Q000502:physiology; D000818:Animals; D002245:Carbon Dioxide / Q000097:blood; D016857:Hypocapnia / Q000378:metabolism / Q000503:physiopathology*; D000860:Hypoxia / Q000378:metabolism / Q000503:physiopathology*; D008297:Male; D010101:Oxygen Consumption / Q000502:physiology; D010991:Plethysmography; D011650:Pulmonary Alveoli / Q000503:physiopathology; D051381:Rats; D017207:Rats, Sprague-Dawley; D012126:Respiratory Dead Space / Q000502:physiology*; D012129:Respiratory Function Tests; D013997:Time Factors
D002245:Carbon Dioxide