Gas exchange in a virtual respiratory system--simulation of ventilation without lung movement

The aim of this study was to supplement a previously developed virtual respiratory system (RS) with gas exchange and gas transfer in airways and circulation (GETAC); and to analyze arterial blood oxygenation during apnea when pure O2 is the ambient gas, with and without extracorporeal CO2 removal (ER), for different inspiratory O2 fractions (FiO2) before respiratory halt.

The virtual RS consists of the RS mechanics model and a model of GETAC consisting of three modules. The values of the model parameters correspond to the average values for a human being, which gave the 'standard' virtual patient. This patient was artificially ventilated in supine position with FiO2=20%, 27%, and 34%, and then the respiration was stopped, FiO2 was changed to 99%, and ER was activated or not. The percent of oxygenated hemoglobin (HbO2%) and alveolar partial pressure of O2 (PAO2) were analyzed during 40 minutes after the beginning of apnea. Both cardiac output and oxygen consumption was assumed to be independent from HbO2%.

Without ER, HbO2% decreased from 98% to 90% when FiO2=34% before apnea, while it decreased below the safety level (70%) for smaller FiO2. However, if ER was activated, HbO2% fell to 76%, 94%, and 97% from 95%, 97%, and 98%, respectively. For FiO2=27%, PAO2 was equal to 15.9, 15.1, 14.3, and 13.6 kPa after 10, 20, 30, and 40 minutes after the beginning of apnea, respectively. Thus, in the case of this particular ('standard') patient, FiO2=27% before apnea and FiO2=99% during apnea provides proper blood oxygenation with approximately normal PAO2 if ER is activated.

If ER is available, it is possible to keep both sufficient blood oxygenation and a normal level of CO2 in the blood as well as a relatively low partial pressure of toxic O2 in the lungs without cyclic mechanical action on the diseased lungs or chest.