Significant degrees of main-stem bronchial obstruction may not have a detectable effect on ventilation distribution at normal breathing frequencies. We determined the effect of graded left main-stem bronchial obstruction (area reduction of 50 and 70%) on the distribution of tidal volume (VT) and mean lung volume (VL) using radioactive 13NN and two-dimensional planar positron imaging in six supine anesthetized tracheotomized dogs. Measurements were made during eucapnic high-frequency oscillatory ventilation at frequencies (f) of 0.2, 1, 5, and 10 Hz. Right and left lung respiratory system complex impedance (Z) values were assessed by simultaneous measurements of dynamic regional lung volume by positron imaging and carinal pressure. The results show a progressive shift of VT away from the obstruction at f > 1 Hz, with VT left-to-right (L/R) ratios of 0.9, 0.9, 0.58, and 0.46 at f of 0.2, 1, 5, and 10 Hz, respectively, for 70% obstruction. VT shifts with f for 50% obstruction were similar but of lesser magnitude. VL L/R ratio was 0.88 and did not change with f or obstruction. The real part of Z was frequency dependent and increased at low f independent of obstruction. The real part of Z L/R ratio increased with obstruction at 5 and 10 Hz. At low f there was a difference between left and right imaginary parts of Z due to the difference in VL. There was no significant change in the imaginary part of Z as a result of obstruction. We conclude that up to a 70% unilateral bronchial obstruction is not detectable by distribution of ventilation at f < or = 1 Hz.(ABSTRACT TRUNCATED AT 250 WORDS)