You ask interesting questions. I think you may have a "collapse of noncompliant upper airway" going on.
My understanding is that "flow limitation" means approximately the same thing as "flow rate limitation".
The flow or flow rate of a particular breath is limited if it doesn't have the normal rounded-peak shape.
Why is that a bad thing? I think
for the same reasons that apneas and hypopneas are bad. Flow limitations mean that there is airway obstruction, which equals abnormal pressures in our chest cavity, and can cause arousals.
Some charts and graphs from a good article on flow limitations:
The role of flow limitation as an important diagnostic tool and clinical finding in mild sleep-disordered breathing from Sleep Science Volume 8, Issue 3, November 2015, Pages 134–142
Quote:Assessing for inspiratory flow limitation requires recognition of both flow and intrathoracic pressure changes . A decrease in flow normally is accompanied by a compensatory increase in intrathoracic pressure. This is illustrated in Fig. 1 in the setting of differentiating a central and obstructive event via airflow and esophageal pressure monitoring . Inspiratory flow limitation, which does not meet criteria for an obstructive hypopnea is illustrated in Fig. 2.
Airflow and esophageal pressure monitoring on PSG. (A) A central event with reduction in airflow without any change in esophageal pressure signal during the event. (B) A reduction in airflow with an increase in esophageal pressure for the duration of the event, indicative of an obstructive hypopnea. Flattening of the airflow signal can also be seen during the obstructive event suggestive of flow limitation.
Polysomnographic recording of a subject with snoring and inspiratory flow limitation during slow-wave sleep. There is no fluctuation in oxygen saturation on pulse oximetry however flattening of the nasal waveform, coinciding with snoring, is present.
Result in one patient of lowering CPAP pressure by 1 cm H20.
(with permission) from: Condos et al. (1994) : Continuous tracing of flow in one patient during transition to lower CPAP. At the arrow, CPAP was lowered by 1 cm H20. Note the increase in the esophageal pressure swings that occurs over five breaths and results in more than a twofold increase in estimated resistance. Simultaneously, the inspiratory flow contour loses its rounded shape and develops the characteristic plateau of flow limitation. )
Quote from the linked article:
Quote:The upper airway is submitted to at least three forces during inspiration: phasic activity of the dilator muscles (activated at or prior to the onset of inspiration), negative airway pressure (maximal at mid-inspiration), and tracheal traction support (maximal at end-inspiration). The investigators reasoned that the inspiratory flow shape could provide information on upper airway behavior throughout inspiration
Explanation for how data was collected for Table 1 & Figure 5:
Quote:Postmenopausal women and male patients with established OSAS treated with uvulopalatopharyngoplasty were selected as patient groups. A control group was also selected. Inspiratory flow signals were evaluated on a breath-by-breath basis, and each inspiratory flow shape was extracted from an automated classifier. Recorded breaths were sorted into 7 classes of inspiratory flow shapes with their significance as it relates to anatomic flow abnormality (Table 1, Fig. 5). Based on the results seen, significant differences were seen in control subjects and patient group, but also between males and females .
(with permission) from: Aittokallio et al. (2001) : The distribution of flow shape classes in experimental and control subjects.
I stopped here. It's a very long article.
(06-16-2016 12:35 AM)chill Wrote: I don't really understand the flat top explanations. For a flow limitation, I would expect a rapid return to closer to zero. These seem likes a high level of inhalation maintained evenly over time. I don't know what that would be.