An Observational Study of the Relationship Between Respired Gases, Mixed Venous Oxygen Content and Cardiac Output in Mechanically Ventilated Patients Post Cardiac Surgery

The cardiovascular system of patients undergoing major cardiac surgery may be unstable during the immediate post-operative period. The core interventions employed as part of routine cardiothoracic ICU management of such patients include the administration of fluids and blood, cardiac pacing, and inotropic support. All of these therapies can be guided by a knowledge of the patient's cardiac output and mixed venous oxygen saturation (SvO2). Pulmonary artery catheters provide an invasive approach that allows the measurement of SvO2 (by blood sampling) and cardiac output (by thermodilution).

For any particular patient, there is always a trade-off between the risks of placing a pulmonary artery catheter and the advantages that it brings when managing patients whose cardiovascular status may be unstable. The primary objective of this study is to explore whether continuous measurement of respired gas exchange, when coupled with small (clinically insignificant), transient variations in inspired oxygen and alveolar carbon dioxide, can be used to calculate mixed venous oxygenation and cardiac output without pulmonary artery catheterisation. If so, then this might provide the basis for a non-invasive approach by which estimates of these parameters can be obtained in patients for whom the benefits of a placing a pulmonary artery catheter do not outweigh the risks.

In order to know the mixed venous oxygenation and cardiac output, this study needs to be conducted in patients who are undergoing pulmonary artery catheterisation as part of their standard clinical care. Patients receiving non-invasive cardiac output monitoring will also be considered. In relation to this, in the UK there are approximately 34,000 major cardiac surgeries which take place each year. Major surgeries in this context include, but are not limited to, coronary artery bypass grafting (CABG), valve replacement or repair and proximal aortic repairs or reconstruction. Following surgery, these patients will invariably be admitted to a Cardiothoracic Intensive Care Unit (CTICU) for the purposes of close cardiorespiratory monitoring and intervention. Many of the patients admitted to CTICU in the postoperative period will require cardiac output monitoring as well as SvO2 measurement. To facilitate this pulmonary artery (PA) catheters are inserted in the perioperative period or non-invasive cardiac output monitoring is utilised postoperatively. Standard operating procedures in our CTICU involve mixed venous sampling for continuous mixed venous oximetry and modern thermodilution via heated catheter for cardiac output measurements. Arterial blood gas samples are taken approximately every hour while the patient is mechanically ventilated.

If it is possible to use measurements of respired gas exchange to estimate cardiac output and SvO2, then they have to be very accurate. The opportunity to obtain measurements with the required precision has arisen from the development of technology that uses laser absorption spectroscopy to measure gas exchange: the Optical Gas Analyser (OGA).

The predicted cardiac output and mixed venous oxygenation are obtained from the measurements of gas exchange by non-linear regression. This process involves a computational model of the lung and circulation that, given particular physiological parameter values and an overall respired gas flow, can calculate the respired gas flows for oxygen, carbon dioxide and nitrogen. The process of non-linear regression is used progressively to adjust the physiological parameter values of the model until the calculated respired gas flows from the model closely match those measured with the OGA. The parameters of the model then provide the cardiac output and SvO2.