Non-invasive measurement of cardiac output using an iterative, respiration-based method

Abstract

Background. Current non-invasive respiratory-based methods of measuring cardiac output (Q) make doubtful assumptions and encounter significant technical difficulties. We present a new method using an iterative approach (QIT), which overcomes limitations of previous methods. Methods. Sequential gas delivery (SGD) is used to control alveolar ventilation (VA) and CO2 elimination (VCO2) during a continuous series of iterative tests. Each test consists of four breaths where inspired CO2 (PICO2) is controlled; raising end-tidal PCO2 (PECO2′) by about 1.33 kPa (10 mm Hg) for the first breath, and then maintaining PECO2′ constant for the next three breaths. The PICO2 required to maintain PECO2′ constant is calculated using the differential Fick equation (DFE), where Q is the only unknown and is arbitrarily assumed for the first iteration. Each subsequent iteration generates measures used for calculating Q by the DFE refining the assumption of Q for the next test and converging it to the true Q when PECO2′ remains constant during the four test breaths. We compared QIT with Q measured by bolus pulmonary artery thermodilution (QTD) in seven pigs undergoing liver transplantation. Results. QIT implementation and analysis was fully automated, and QTD varied from 0.6 to 5.4 litre min-1 through the experiments. The bias (between QIT and QTD) was 0.2 litre min-1 with 95% limit of agreement from -1.1 to 0.7 litre min-1 and percentage of error of 32%. During acute changes of Q, convergence of QIT to actual Q required only three subsequent iterations. Conclusions. QIT measurement is capable of providing an automated semi-continuous non-invasive measure of Q.