When you watch an oscillatory flow waveform dance across the screen, it is easy to be dazzled by the amplitude. Rapid-fire oscillations that look like a vigorous tug-of-war between inspiratory flow and expiratory flow. But here is the question that separates a pretty waveform from a clinically meaningful one: Did the flow actually cross zero?
That distinction matters when we are thinking about secretion movement. Secretions do not respond to the appearance of a wave; they respond to the push created by airflow over time. A stronger pulse on the same side of zero is just a change in strength which may modulate the push in that same direction, but it does not automatically create an opposing pull.
Flow on one side of that line moves in one direction; flow on the other side moves in the opposite direction. Positive and negative conventions can vary by device or display, so the practical bedside question is simple: does the waveform cross zero?
If it does, the oscillation includes a true reversal. Otherwise, the trace may still look dynamic, but the flow remains directionally consistent. This is why “higher versus lower” flow should not be read as “expiratory versus inspiratory” flow unless the waveform changes sides.
For secretion clearance, the more useful mental model is a tug of war around zero. Expiratory and inspiratory components compete, but the winner is not decided by peak flow alone. It is decided by how high the flow rises on each side of zero and how long it stays there. A brief high spike may matter less than a lower flow that persists to keep pushing secretions in one direction.
This framing avoids overemphasizing baseline or bias flow while still preserving the essential physiology. A baseline can shift where oscillations sit relative to zero, but the secretion-relevant question is simpler: over the observed interval, which side kept pushing longer and stronger?
The takeaway is concise: do not equate a changing oscillatory waveform with changing direction. First ask whether the trace crosses zero. Then ask which side of zero carries more sustained flow. In the movement of secretions, the tug of war is won by the side that combines enough height with enough time.
Find zero, then ask whether the waveform crosses it. If it does not, the oscillation is changing the strength of the flow, not its direction. If it does, the reversals themselves are not the finding. Use tools such as cumulative position and tug-of-war views to answer the question that matters for secretion movement: which side did flow stay on longer and stronger? Where those two answers disagree, as they do here, only adding them up across the whole recording will tell you which one won.
¹ Pulmonary Research Institute for Systems & Metrics (PRISM), Founding Members
Author Affiliations & Disclosures
Leah Noaeill, MBA, and Vinay Joshi, B.Tech, MBA, are Founding Members of the Pulmonary Research Institute for Systems & Metrics (PRISM), a voluntary, precompetitive forum focused on respiratory device performance measurement. Leah Noaeill is Vice President, Marketing & Clinical Affairs at ABM Respiratory Care. Vinay Joshi is Founder & Chief Technology Officer at ABM Respiratory Care. ABM Respiratory Care is a manufacturer of respiratory therapy systems. The authors participated in this work in their individual capacities as PRISM members.
PRISM is an independent voluntary technical forum. Its publications are intended to support technical discussion and do not replace applicable standards, regulatory requirements, clinical judgment, or manufacturer responsibilities.
Read the article in the Fall 2026 issue of Respiratory Therapy: Page 65