Hypoxaemia after cardiac surgery: clinical application of a model of pulmonary gas exchange.
OBJECTIVE
To investigate the clinical application of a mathematical model of pulmonary gas exchange, which ascribes hypoxaemia to shunt and ventilation/perfusion mismatch. Ventilation/perfusion mismatch is quantified by deltaPO2, which is the drop in oxygen pressure from alveoli to lung capillaries. Shunt and deltaPO2 were used to describe changes in oxygenation after coronary artery bypass grafting.


METHODS
Fourteen patients were studied 2-4 h after surgery and on postoperative days 2, 3 and 7. On each occasion inspired oxygen fraction was changed in four to six steps to obtain arterial oxygen saturation (SaO2) in the range of 90-100%, enabling construction of FEO2/SaO2 curves. Measurements of ventilation, circulation and oxygenation were entered in a previously described mathematical model of pulmonary gas exchange.


RESULTS
We found that oxygenation was most impaired 3 days after surgery. By fitting the mathematical model to the FEO2/SaO2 curve, we found that shunt remained constant throughout the study period. However, deltaPO2 increased from 0.5 kPa (median, range 0-3.8) 2-4 h after surgery, to 3.2 kPa (range 1.2-6.4, P < 0.05) on day 2, and to 4.0 kPa (range 1.2-8.3) on day 3. On day 7, deltaPO2 decreased to 2.2 kPa (range 0-3.5, P < 0.05).


CONCLUSIONS
Ventilation/perfusion mismatch (deltaPO2), rather than shunt, explains the changes in postoperative oxygenation. The model of pulmonary gas exchange may serve as a useful and potentially non-invasive clinical tool for monitoring patients at risk of postoperative hypoxaemia.
Kjaergaard
S
S
Aalborg Hospital, Department of Anaesthesiology, Aalborg, Denmark. kjaergaard@mail.tdcadsl.dk
Rees
S E
SE
Grønlund
J
J
Nielsen
E M
EM
Lambert
P
P
Thorgaard
P
P
Toft
E
E
Andreassen
S
S
D000368:Aged; D000704:Analysis of Variance; D002196:Capillaries / Q000378:metabolism; D002245:Carbon Dioxide / Q000378:metabolism; D001026:Coronary Artery Bypass; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D000860:Hypoxia / Q000209:etiology* / Q000503:physiopathology; D008168:Lung / Q000098:blood supply; D008297:Male; D008875:Middle Aged; D008954:Models, Biological; D010100:Oxygen / Q000097:blood; D010101:Oxygen Consumption / Q000502:physiology; D011650:Pulmonary Alveoli / Q000378:metabolism; D011652:Pulmonary Circulation / Q000502:physiology; D011659:Pulmonary Gas Exchange / Q000502:physiology*; D012119:Respiration; D014692:Ventilation-Perfusion Ratio
D002245:Carbon Dioxide; D010100:Oxygen