Thoraco-abdominal asynchrony (TAA) during upper airway obstruction (UAO) in small children can be documented by phase angle analysis of the Lissajous figure from the output of a noncalibrated respiratory inductance plethysmograph. Phase angle measurements have not been related to levels of inspiratory resistance, nor to the effect of breathing a 79% helium-21% oxygen mixture (heliox) during inspiratory resistance. We examined the effects of graded inspiratory loading (5-1000 cm H2O/L/sec) on TAA as measured by phase angle in 10 male, anesthetized, and intubated Rhesus monkeys, breathing room air and heliox. Phase angles increased with inspiratory loading from a baseline value of 22 +/- 3 degrees to 165 +/- 8 degrees at 1,000 cm H2O/L/sec resistance and correlated significantly with the level of inspiratory loading (r = 0.82). End-tidal carbon dioxide PETCO2 increased from 39 +/- 1 to 49 +/- 3 mm Hg at the highest load, but correlated only weakly with phase angle measurements (r = 0.60) and the level of inspiratory loading (r = 0.56). By changing to heliox breathing at the highest tolerated resistance, PETCO2 dropped significantly from 49 +/- 3 to 40.5 +/- 4 mmHg (P < 0.001) with no significant change in phase angles: 169 +/- 13 degrees and 165 +/- 8 degrees, respectively (P > 0.05). We conclude that heliox therapy for acute alveolar hypoventilation during UAO improves ventilation, but does not decrease TAA at high inspiratory resistance. Continuous monitoring of the relative changes in phase angles is useful to observe the severity of UAO in the early stages.(ABSTRACT TRUNCATED AT 250 WORDS)