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Hypoxaemia after cardiac surgery: clinical application of a model of pulmonary gas exchange

Published online by Cambridge University Press:  23 December 2004

S. Kjærgaard ,
S. E. Rees ,
J. Grønlund ,
E. M. Nielsen ,
P. Lambert ,
P. Thorgaard ,
E. Toft  and
S. Andreassen
S. Kjærgaard
Affiliation:
Aalborg Hospital, Department of Anaesthesiology, Aalborg, Denmark
S. E. Rees
Affiliation:
Aalborg University, Centre for Model-Based Medical Decision Support, Aalborg, Denmark
J. Grønlund
Affiliation:
Aalborg Hospital, Department of Thoracic Surgery, Aalborg, Denmark
E. M. Nielsen
Affiliation:
Aalborg Hospital, Department of Anaesthesiology, Aalborg, Denmark
P. Lambert
Affiliation:
Aalborg Hospital, Department of Anaesthesiology, Aalborg, Denmark
P. Thorgaard
Affiliation:
Aalborg Hospital, Department of Anaesthesiology, Aalborg, Denmark
E. Toft
Affiliation:
Aalborg Hospital, Department of Cardiology, Aalborg, Denmark
S. Andreassen
Affiliation:
Aalborg University, Centre for Model-Based Medical Decision Support, Aalborg, Denmark
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Extract

Summary

Background and 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 ΔPO2, which is the drop in oxygen pressure from alveoli to lung capillaries. Shunt and ΔPO2 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, ΔPO2 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, ΔPO2 decreased to 2.2 kPa (range 0–3.5, P < 0.05).

Conclusions: Ventilation/perfusion mismatch (ΔPO2), 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.

Keywords

CARDIAC SURGICAL PROCEDURESMODELS, BIOLOGICAL, models, cardiovascularMONITORING, PHYSIOLOGICALPOSTOPERATIVE COMPLICATIONSRESPIRATION, respiratory transport, pulmonary gas exchange, ventilation/perfusion ratio
Type
Original Article
Information
European Journal of Anaesthesiology , Volume 21 , Issue 4 , April 2004 , pp. 296 - 301
Copyright
2004 European Society of Anaesthesiology

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