Validation of Capnodynamic Assessment of Mixed Venous Oxygen Saturation

Background: Mixed venous saturation (SvO2) is an indicator of the balance between oxygen supply and demand in the body's tissues The primary objective of the current study is thus to investigate the agreement and trending ability for capnodynamically derived SvO2 against the gold standard CO-oximetry. The secondary objective is to investigate the agreement and trending ability for capnodynamically derived CO (COEPBF) against the modified CO2-Fick method.

Methods:

The rationale of capnodynamic SvO2 (capno-SvO2) can be briefly described as follows:

The Capno-SvO2 method is based on differential Fick's principle and utilizes a combination of continuous estimation of Effective Pulmonary Blood Flow (COEPBF) and oxygen consumption (VO2) incorporated in a rearranged Fick's equation. COEPBF is estimated by applying a special breathing pattern of variations in I:E relationships (six breaths with normal I:E relations followed by three breaths vid an approximately 2 s expiratory pause). This breathing pattern causes small fluctuations in alveolar CO2 concentration and VCO2, related to the pulmonary blood flow participating in gas exchange which allows for estimation of COEPBF. VO2 can be estimated using continuous volumetric capnography measurement of VCO2 combined with the Respiratory Quotient (RQ).

Modified CO2-Fick: CO estimation using modified CO2-Fick is assessed by incorporating measured CO2 content in mixed venous and arterial blood into the equation below where VCO2 is estimated using volumetric capnography.

COFick=VCO2/(CvCO2-CcCO2)

Study protocol

All patients planned for percutaneous intervention of cardiac conditions at our department will be screened for inclusion in this study. Exclusion criteria: ongoing ECMO treatment or intracardiac shunts not subjected to intervention. All these patients are routinely anaesthetized, intubated with a cuffed endotracheal tube and kept on controlled ventilation, a prerequisite for the capnodynamic method.

After induction of anesthesia, the special breathing pattern will be applied and continuous data from the capnodynamic monitoring will be collected. After catheterization of the pulmonary artery two baseline blood gases from the PAC and arterial line will be drawn and analyzed, and the hemoglobin level (Hb) from this sample will be used for calibration of the capnodynamic method together with a tabulated RQ. After completion of the interventional procedure, a sequence of paired testing of SvO2 from blood gases drawn from the pulmonary artery and arterial line. Firstly 2 samples (3 min apart) forming a baseline. After this follows a short increase in PEEP (3 cmH2O to 8cmH2O), which temporarily decreases cardiac output and SvO2. A sample is collected and then once more after PEEP has been brought back to 3 cmH2O after 3 minutes. Next, a raised level of inhaled oxygen from 30 to 80% (FiO2) is applied for 3 minutes and then turned back to baseline. Mixed venous and arterial samples will be drawn at FiO2 30% and 80%.

Paired recordings of COEPBF and CO2-Fick as well as capnodynamic SvO2 and co-oximetry will be examined for agreement and ability to detect change using Bland-Altman and four quadrant plots.

Power analyses (power 80% and alfa of 0.05) implies a minimal of 120 paired recordings corresponding to approximately 20 patients. 30 patients will be included for upward margin.