Non-steady state monitoring by respiratory gas exchange

Traditionally, the study of CO2 and O-2 kinetics in the body has been mostl y confined to equilibrium conditions. However, the peri-anesthesia period a nd the critical care arena often involve conditions of non-steady state. Th e detection and explanation of CO2 kinetics during non-steady state pathoph ysiology have required the development of new methodologies, including the CO2 expirogram, average alveolar expired PCO2, and CO2 volume exhaled per b reath. Several clinically relevant examples of non-steady state CO2 kinetic s perturbations are examined, including abrupt decrease in cardiac output, application of positive end-expiratory pressure during mechanical ventilati on, and occurrence of pulmonary embolism. The lesser known area of non-stea dy state O-2 kinetics is introduced, including the measurement of pulmonary O-2 uptake per breath. Future directions include the study of the respirat ory quotient per breath, where the anaerobic threshold during anesthesia is identified by increasing respiratory quotient.