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Hypothesis: During alternating ventilation (AV) (i.e. differential ventilation (DV) of both lungs with a phase difference of half a ventilatory cycle) volume expansion of the inflated lung will occur partly by compression of the opposite lung.Objective: We studied whether and how large an undulating flow would occur out of and into the non-ventilated lung during unilateral ventilation.Design: In 20 anaesthetized, paralysed piglets (11.0 +/- 1.0 kg) DV was applied at a rate of 10 breaths per minute (bpm). In 6 animals, 15 and 20 bpm were also applied with the tidal volume adapted no normocapnia. As the measure of interaction (cross-talk) served the volume change in the non-ventilated lung, found by integration of the low signal, in percentage of the tidal volume to the other lung.Results: In all pigs, tidal volume to the left lung. (VT,l = 7.33 +/- 1.06 ml kg-1) caused a volume change in the right lung of about 21% of VT,l at 10 bpm. The right-to-left cross-talk was significantly lower and about 15% of VT,r (9.07 +/- 1.21 ml kg-1). At higher ventilatory rates, the l-to-r and the r-to-l cross-talk did not change.Conclusion: During unilateral ventilation, volume expansion of the inflated lung occurs partly by compression of the opposite lung. The lower mean lung volume during AV compared to synchronous differential ventilation can be explained by such compression. The mechanism of compression is similar at different ventilatory rates. [ABSTRACT FROM AUTHOR]