The Performance of a Capnodynamic Method Compared to Transpulmonary Thermodilution in Open Abdominal Surgery

The capnodynamic method continuously calculates effective pulmonary blood flow (EPBF) with the help of a capnodynamic equation:

ELV x (FACO2(n) - FACO2(n-1)) = deltat(n) x EPBF (CvCO2 - CvCO2(n)) - VTCO2

ELV Effective lung volume [L] EPBF Effective pulmonary blood flow [L/min] n current breath n-1 previous breath FACO2 mean alveolar carbon dioxide fraction CvCO2 mixed venous carbon dioxide content [Lgas/Lblood] CcCO2n pulmonary end-capillary carbon dioxide content [Lgas/Lblood] VTCO2n volume [L] of carbon dioxide eliminated by the current, nth, breath delta t n current breath cycle time [min]

The equation above describes the mole balance between the CO2 delivered to lungs (EPBF), the volume taking part in the gas exchange (ELV) and CO2 excreted from the lungs (VTCO2). Normally there is no difference in CO2 between the actual and the preceding breath as the same amount of CO2 as delivered to the lungs as is excreted. When small changes in CO2 concentration are inserted into the equation obtained with short inspiratory or expiratory pauses in three out of nine breaths, nine different equations are obtained. The three unknown variables; ELV, EPBF and CvCO2can be solved with a linear least square optimization, a well-known numerical mathematical principle. The breathing pattern is automatically controlled by the ventilator which provides continuous calculations of EPBF where each value represents the average of the preceding nine breaths and renews with each breath as the newest replaces the oldest in the equation system.

At the day of surgery, included patients arrive 45 minutes earlier than otherwise at the surgical unit. After safe surgical checklist, vital signs are measured and epidural catheter inserted during a light sedation. Patients are anesthetized and muscle relaxed. An endotracheal tube is inserted in the trachea. Central arterial and vein catheters are inserted in sterile conditions with ultrasound guidance. Anesthesia is maintained with propofol in target controlled infusion and a short acting opioid is added as needed.