The effect of hypo-, normo-, and hypercapnia induced by mechanical ventilation on intrapulmonary shunt.
The effects of hypo-, normo- and hypercapnia on the variations in arterial oxygenation and their indices in critical patients with acute respiratory failure (ARF) receiving mechanical ventilation are studied. It is a prospective and randomized study carried out in multidisciplinary ICU. Fifteen ARF patients, intubated and mechanically ventilated, were studied within the first 48 h of evolution. Three stages were delimited: I) 30 min after the beginning of anaesthesia; II) 30 min after adding 30 cm of dead space (VD); III) 30 min after replacing the previous VD with VD of 60 cm. Ventilation parameters and FiO2 were kept stable. Stage I was characterized by respiratory alkalosis and stage II by normal acid-base balance with an increase in PaO2 (p < 0.01) and a decrease in intrapulmonary shunt (Qsp/Qt) (p < 0.001); the indices alveolar to arterial oxygen tension gradient [P(A-a)O2], respiratory index (R.I.) and estimated shunt (Est Shunt) also decreased significantly, whereas arterial to alveolar oxygen tension ratio (PaO2/PAO2) and arterial oxygen tension to inspired oxygen fraction ration (PaO2/FiO2) increased significantly. In stage III there was pure hypercapnic acidosis, with decreases in PAO2 (p < 0.001), P(A-a)O2 (p < 0.01) and R.I. (p < 0.05), while PaO2, Qsp/Qt, Est Shunt, PaO2/PAO2 and PaO2/FiO2 remained stable with respect to the previous situation. The observed PaO2 differs (p < 0.05) from the expected PaO2 in stage III. It is suggested that local or regional modifications of pulmonary perfusion are responsible for the observed variations. The P(A-a)O2 and R.I. indices do not make it possible to differentiate the causes of arterial hypoxemia in the presence of hypercapnia.
Boix
J H
JH
UCI, Hospital, Castellón, Spain.
Alvarez
F
F
Tejeda
M
M
Peydro
E
E
Olivares
D
D
Arnau
A
A
D000208:Acute Disease; D000328:Adult; D000368:Aged; D000471:Alkalosis / Q000097:blood / Q000209:etiology / Q000503:physiopathology*; D002245:Carbon Dioxide / Q000097:blood*; D005260:Female; D006801:Humans; D006935:Hypercapnia / Q000209:etiology / Q000503:physiopathology*; D016857:Hypocapnia / Q000209:etiology / Q000503:physiopathology*; D008297:Male; D008875:Middle Aged; D010100:Oxygen / Q000097:blood; D010313:Partial Pressure; D011446:Prospective Studies; D012121:Respiration, Artificial / Q000009:adverse effects*; D012126:Respiratory Dead Space; D012129:Respiratory Function Tests; D012131:Respiratory Insufficiency / Q000097:blood / Q000503:physiopathology*
D002245:Carbon Dioxide; D010100:Oxygen