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

Abstract

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 (PtcO₂ and PtcCO₂) were measured during pleural fluid withdrawal. Arterial blood oxygen tension and arterial CO₂ tension (PaO₂ and PaCO₂) were analysed in simulations that mimicked TT. Minute ventilation was adjusted to maintain arterial CO₂ 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, PtcCO₂ 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 PtcCO₂. Simulations showed the following: (a) there were similar PaO₂ fluctuations in the virtual patient; (b) the lower the hypoxic pulmonary vasoconstriction efficiency, the more pronounced the PaO₂ fall during fluid withdrawal; and (c) the lower the atelectatic lung areas recruitment rate, the slower the PaO₂ normalization. The decrease in PaO₂ was caused by an increase of pulmonary shunt.

Conclusion:: Therapeutic thoracentesis may cause both an increase and a decrease in PaO₂ 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.

Keywords: Blood oxygenation; hypoxic pulmonary vasoconstriction; pleural effusion; pleural pressure; pulmonary shunt; thoracentesis.