Cardiac output monitoring is mandatory in most surgical and critical care patients. Many techniques to assess cardiac output are invasive and/or have many limitations, contraindications and drawbacks. The measurement of cardiac output throughout the CO2 signal is an interesting option due to the simplicity and non-invasive nature of its calculation.

The present study was designed to compare the cardiac output measurement using a capnodynamic formula versus the reference thermodilution method.

This is a prospective and observational study designed to test the accuracy and precision of the non-invasive capnodynamic cardiac output measurement in mechanically ventilated patients. We will study 30 patients undergoing cardiovascular by pass in which an invasive trans-thoracic thermodilution method (PICCOScience) is indicated - the reference method.

A pair of thermodilutions and capnodynamic cardiac output measurement will be performed along the surgery. Analysis of precision will be done at the begining of the protocol and during stable hemodynamic conditions. Accuracy will then assessed using the Bland-Altmann plot and regression analysis.

Inclusion Criteria:

• Written informed consent
• Programmed cardiac surgeries
• Surgeries with cardiopulmonary bypass

Exclusion Criteria:

• Emergency surgeries.
• Acute pulmonary diseases
• Contraindication of femoral catheters.
• Need of counter-pulsation baloon

Cardiac surgical ASA II-III patients.