Desaturating. Fixed pressure CPAP. Strategies?

[StuartC]

Okay, I get what you are saying with your analogy and I think I have been lazy or ineffective with my explanation and wording.  Let me try to explain better.  If nothing else, it will help you understand where my thinking is wrong and to educate me better.

The term flow limit implies (to me at least) an obstruction.  In industry, doing process control work, I would partially close a valve or introduce some other obstruction when I wanted to limit a flow rate.  For humans the equivalent might be strangulation, choking, the tongue dropping back or the airway collapsing.  In this case, the flow limit is what causes the reduction / cessation of air flow, which causes desaturation.

But I can stop or reduce flow with my mouth and airway open (for a period of time) but nothing obstructing flow in any way.  I do this by ceasing / reducing movement my diaphragm, i.e by holding my breath as you said.  As you point out, the reduced flow will lead to desaturation but it is the diaphragm (which I incorrectly labelled the desaturation) that is the cause rather than a physical obstructive flow limitation.

Now, the machine only knows that flow has reduced (expected flow < actual flow, with the numerical difference being used to determine the flow limit value), perhaps that the pressure has started to increase and that it needs to reduce the turbine speed to maintain the pressure set point.  I am guessing that the machine calculates and records this flow limit, regardless of cause, and then takes steps to determine if the limit can be classified as an apnea, and which type, a hypopnea or something else. Statistically, of course, it is likely to be obstructive.  But I am not seeing OAs - and I am so sick and tired of statistical medicine that ... No, rather don't let me derail the discussion by going there. 

My point is simply that if I see flow limits during desaturations but not at other times, I think the cause is not obstructive but rather is related to my diaphragm.  As I said, I am new to this, so opinions are welcome.  Suggestions of what I can do to improve matters before my sleep study are sought.



Apart from the discussion above, you have asked for 3 minute charts.  Here are two from last night with an overview of the whole night for context.  I have picked last night because a chart from the returned DS1 in May is of little interest to me using an AS10 six months later.  I have picked two consecutive 3 minute charts where there was a reasonable flow limit value that can be seen on the chart for the whole night.

Thanks for your help.

[Sleeprider]

I'll step in long enough to say there are many reasons for flow limitation. Consider your lungs as an overfilled vessel that restricts further inflation or a lung that has lost elasticity as in COPD (pulmonary flow limitation), or the thoracic restriction you describe which may be neurological or positional. The more common source of flow limitation is upper airway inspiratory flow limits caused by everything from a restricted or inflamed nasal passage or throat (esophageal flow limitation), to a physical narrowing of the airway caused by body position, especially chin-tucking. In your process control analogy this is similar to the limitation of flow at pressure, but respiration is considerably more dynamic than pipeline flow. We also see what appears as flow limits when large leaks occur. Less common is expiratory flow limitation (EFL) more common in COPD which limits functional residual capacity and is not strictly a sleep disorder.

ResMed determines and graphs inspiratory flow limitation on a breath by breath basis. From the S9 onward, flow limitation is calculated using a combination of flatness index, breath shape index, ventilation change, and breath duty cycle. Ventilation change is the ratio of the current breath ventilation to recent 3-minute ventilation. Breath duty cycle is the ratio of current breath time of inspiration to total breath time of recent 5 minutes. If a breath is severely flow limited, the flow limitation index will be closer to one and when the breath is normal or round, the flow limitation index will be zero.

[StuartC]

Thanks

This is definitely an area of learning for me,so that is very helpful.