Normal vs Abnormal Expiratory Flow Curve

As promised some weeks back, here is a comparison of two expiratory flow curves, one normal, smooth and rounded (1st Graph) and the other abnormal, choppy, and irregular. These sleep epochs are taken from the same patient who is in stage 2 NREM sleep in the 1st graph and stage 3 NREM sleep in the 2nd graph. These are real sleep epochs taken from a 30-second period from the polysomnogram.

In the 1st graph notice how low the EPAP setting is at 3; whereas in the 2nd graph, the EPAP is now at 4.5, and the curve is no longer smooth but bumpy during this “end-expiration” period.

Now, we can all probably imagine a number of possible explanations for these changes, such as general airflow turbulence or some subtle form of leak, but in our clinical experience the pattern in the 2nd Graph is what we term “Expiratory Intolerance” or “Expiratory Instability.” We believe this description is accurate, because this pattern often emerges during a time when the patient may go on to develop a full-blown iatrogenic central apnea, that is, in response to EPAP, the patient cannot tolerate pressurized airflow coming in as they attempt to breathe out. When this “tension” reaches a tipping point, the patient simply stops breathing.

The pattern shown in the 2nd Graph is most frequently seen in patients on fixed CPAP pressure. And, we find that this expiratory pattern can be relieved by switching the patient to bilevel, through which we attempt to find the lowest possible EPAP that yields a normalized expiratory airflow curve (1st Graph) but which does not cause the jagged curves in the 2nd Graph.