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

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.

[1]  T. Similowski,et al.  Clinically relevant diaphragmatic dysfunction after cardiac operations. , 1994, The Journal of thoracic and cardiovascular surgery.

[2]  R. Muraoka,et al.  Leukocyte and platelet depletion with a blood cell separator: effects on lung injury after cardiac surgery with cardiopulmonary bypass. , 1996, The Journal of thoracic and cardiovascular surgery.

[3]  T. K. Hunt,et al.  Tissue oxygenation, anemia, and perfusion in relation to wound healing in surgical patients. , 1991, Annals of surgery.

[4]  O. Akca,et al.  Supplemental perioperative oxygen to reduce the incidence of surgical-wound infection. , 2000, The New England journal of medicine.

[5]  Steen Andreassen,et al.  The Automatic Lung Parameter Estimator (ALPE) System: Non-Invasive Estimation of Pulmonary Gas Exchange Parameters in 10-15 Minutes , 2004, Journal of Clinical Monitoring and Computing.

[6]  J. Jones,et al.  A noninvasive method for evaluating the effect of thoracotomy on shunt and ventilation perfusion inequality , 1997, Anaesthesia.

[7]  S. Andreassen,et al.  Modelling of hypoxaemia after gynaecological laparotomy , 2001, Acta anaesthesiologica Scandinavica.

[8]  D. Taggart,et al.  Respiratory dysfunction after uncomplicated cardiopulmonary bypass. , 1993, The Annals of thoracic surgery.

[9]  P. Roe,et al.  Intra-operative gas exchange and post-operative hypoxaemia. , 1997, European journal of anaesthesiology.

[10]  C. J. Morgan,et al.  Hypoxemia after coronary bypass surgery modeled by resistance to oxygen diffusion. , 1999, Critical care medicine.

[11]  L. Aakerlund,et al.  Postoperative delirium: treatment with supplementary oxygen. , 1994, British journal of anaesthesia.

[12]  G. Hedenstierna,et al.  Atelectasis and Gas Exchange after Cardiac Surgery , 1998, Anesthesiology.

[13]  G. Hedenstierna,et al.  V/Q distribution and correlation to atelectasis in anesthetized paralyzed humans. , 1996, Journal of applied physiology.

[14]  Sapsford Dj,et al.  The PIO2 vs. SpO2 diagram: a non-invasive measure of pulmonary oxygen exchange. , 1995 .

[15]  H. Lenzen,et al.  Gas exchange impairment and pulmonary densities after cardiac surgery , 1992, Acta anaesthesiologica Scandinavica.

[16]  A. Gillinov,et al.  Platelet activating factor inhibition reduces lung injury after cardiopulmonary bypass. , 1995, The Annals of thoracic surgery.

[17]  G. Hedenstierna,et al.  Ventilation‐Perfusion Inequality in Patients Undergoing Cardiac Surgery , 1994, Anesthesiology.

[18]  G. J. Jones,et al.  The effect of surgery with cardiopulmonary bypass on alveolar-capillary barrier function in human beings. , 1985, The Annals of thoracic surgery.

[19]  H. Kehlet,et al.  Late postoperative nocturnal episodic hypoxaemia and associated sleep pattern. , 1994, British journal of anaesthesia.

[20]  S Westaby,et al.  Complement and the damaging effects of cardiopulmonary bypass. , 1983, The Journal of thoracic and cardiovascular surgery.