The use of a virtual patient to follow changes in arterial blood gases associated with therapeutic thoracentesis

Purposes: Some controversies exist on the effect of therapeutic thoracentesis (TT) on arterial blood oxygen tension. The aim of this study was to evaluate this issue using a previously developed virtual patient. Methods: The analysis was based and supported by clinical data collected during 36 TT. Pleural pressure and transcutaneous oxygen and carbon dioxide pressures (PtcO 2 and PtcCO 2 ) were measured during pleural fluid withdrawal. Arterial blood oxygen tension and arterial CO 2 tension (PaO 2 and PaCO 2 ) were analysed in simulations that mimicked TT. Minute ventilation was adjusted to maintain arterial CO 2 tension at a constant level unless arterial blood oxygen tension fell below 8 kPa. Specifically, the influence of hypoxic pulmonary vasoconstriction efficiency was tested. Results: In patients, PtcCO 2 remained at an approximately constant level (average amplitude: 0.63 ± 0.29 kPa), while some fluctuations of PtcO2 were observed (amplitude: (1.65 ± 1.18 kPa) were observed. In 42% of patients, TT was associated with decrease in PtcCO 2 . Simulations showed the following: (a) there were similar PaO 2 fluctuations in the virtual patient; (b) the lower the hypoxic pulmonary vasoconstriction efficiency, the more pronounced the PaO 2 fall during fluid withdrawal; and (c) the lower the atelectatic lung areas recruitment rate, the slower the PaO 2 normalization. The decrease in PaO 2 was caused by an increase of pulmonary shunt. Conclusion: Therapeutic thoracentesis may cause both an increase and a decrease in PaO 2 during the procedure. Pleural pressure decrease, caused by pleural fluid withdrawal, improves the perfusion of atelectatic lung areas. If the rate of recruitment of these areas is low, a lack of ventilation causes the arterial blood oxygen tension to fall. Effective hypoxic pulmonary vasoconstriction may protect against the pulmonary shunt.