flow–volume loop
The flow–volume loop is the picture spirometry draws when you both blow all the way out and then suck all the way back in: airflow on the vertical axis, lung volume on the horizontal, tracing a closed loop. Its shape tells a story at a glance, the way a heartbeat tracing does for cardiologists.
A healthy expiratory curve shoots up to a sharp peak and then descends in a straight line. In obstruction the descending limb sags inward, scooped out like a ski slope, because flow collapses as the lungs empty. In restriction the whole loop is small but keeps its tall, narrow shape, like a normal loop shrunk down.
The loop is uniquely good at flagging blockage of the central airways — the windpipe or voice box. A flattened top of the expiratory limb, the inspiratory limb, or both points to a fixed or variable large-airway obstruction such as a tracheal stricture, a tumour or vocal-cord dysfunction, patterns that the bare numbers alone would miss.
A young woman with episodic stridor mislabelled as asthma shows a flattened inspiratory limb on her flow–volume loop, leading to a diagnosis of vocal-cord dysfunction rather than airway disease.
The shape of the loop can reveal large-airway problems the numbers miss.
Always inspect the loop's shape, not just the numbers: a flattened inspiratory limb can be the only clue to an extrathoracic large-airway lesion that spirometric values fail to reveal.