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.
John Rankin Laboratory of Pulmonary Medicine, Department of Preventive Medicine, University of Wisconsin School of Medicine, Madison, 53705.
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Olson
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