Weaning from mechanical ventilation: a retrospective analysis leading to a multimodal perspective
The background of this study was the hypothesis that respiratory variability is influenced by chemoreflex regulation. In search for periodicities in the variability due to instability of the respiratory control system, spectral analysis was applied to breath-to-breath variables in 19 healthy subjects at rest. During room-air breathing, coherent oscillations in end-tidal CO2 (PET(CO2)) and mean inspiratory flow (VI/TI) were found in 15 subjects with frequencies mostly below 0.15 cycles per breath. Coherent oscillations in PET(CO2) and VI/TI were expressed by gain (0.13 to 0.34 L/second small middle dot kPa) and phase (-170 degrees to +8 degrees ). The oscillations in VI/TI were in phase with inspiratory volume (VI). A model that describes the effects of chemoreflex feedback to noise in the system could explain these gains and phases, whereas a model without chemoreflex could not. During 100% O2 breathing, only eight subjects had coherent oscillations in PET(CO2) and VI/TI. The coherent oscillations in PET(CO2) and VI/TI were interpreted as a manifestation of chemoreflex activity. We conclude that respiratory variability is not a random process but contains information on chemoreflex properties, such as the chemoreflex gain. The analysis of respiratory variability therefore provides a new tool to study the action of the chemoreflexes without application of external stimuli.