Tidal breathing analysis is a lung function technique suggested for infants and children who are unable to cooperate with forced spirometry. This technique aims to quantify lower airway obstruction from average changes in the shape or the breath-to-breath variations of the tidal breathing flow-volume loop (TBFV) profiles. If tidal airflow is recorded with a mouth pneumotachograph (PNT), tidal breathing analysis finds the same limitations as other alternatives to spirometry. These are typically the need for sedation and the assessment of lung function only for sort times at the hospital. Recent improvements in impedance pneumography (IP) enable for the first time the continuous non-invasive monitoring of respiratory airflow overnight. This can improve the analysis of tidal breathing by capturing circadian and nocturnal worsening in lower airway obstruction. However, due to the lack of previous methods recording nocturnal airflow, little is known about how the interaction of sleep physiology and lower airway obstruction is reflected in the shape and variability of tidal breathing.
This thesis reviews the literature regarding shape and variability analysis of tidal breathing during lower airway obstruction, sleep, or maturation. The thesis also extends this knowledge by presenting four original publications. The first publication describes a technical improvement in the IP method. The other three study the nocturnal TBFV’s shape in wheezing infants and children, and the nocturnal TBFV’s variability in healthy children. Both the literature and the results agree that for the TBFVs’ shape, increasing lower air- way obstruction advances the peak of expired flow and turns the middle part from convex to concave. However, these changes occur at a different degree of obstruction for differ- ent subjects depending on the compensation strategy that they have chosen. In infants, changes putatively occur at a higher degree of obstruction because most of the expiration is controlled by the respiratory musculature. During rapid eye movement (REM) sleep, changes putatively occur at a lower degree of obstruction because muscle atony limits the compensation strategies. For the variability of TBFVs, increasing lower airway obstruction decreases the variability in the early part of expiration in the long term (the whole night).
However, the short-term variability is dominated by the stage-dependent variations in the respiratory drive.
The thesis concludes that, at the present, tidal breathing analysis can estimate lower airway obstruction but cannot quantify its degree with accuracy. However, nocturnal IP recordings are easy to conduct and can serve as a first-line diagnosis or for the monitoring of disease progression. Nonetheless, future improvements in signal processing and the understanding of the tidal airflow signal can easily increase the accuracy and find new applications
