FIG 14: The cartoon in this Figure addresses these questions.  A patient may have high loop gain and hyperventilation with large shifts between overshoot and undershoot, as for example in patients with high altitude periodic breathing.  Other patients may have OSA, meaning that they have airway obstruction, whereas patients with Cheyne-Stokes respiration (CSR) are again on the high loop gain side but with almost open airways. 

But some patients, like this one, present with co-existing OSA and CSR.  As a result they have upper airway obstruction, meaning that it is necessary to open the airways, and on the other hand they have hyperventilation, with a high overshoot and undershoot of ventilation. 

Some patients have complex sleep apneas, and some patients have breathing disturbances while using CPAP.  They have upper airway obstruction because they have obstructive sleep apnea, but they are also intermediate between high- and low-loop gain. 

Patients with opioid-induced sleep apnea may have hypoventilation because opioids reduce their ventilation, but they also have increased loop gain in some phenotypes. 

Some patients present with neuromuscular diseases.  They often have open airways, but with hypoventilation so that they cannot really breathe. 

Patients with chronic obstructive pulmonary disease (COPD) have occluded upper or lower airways, and have low ventilation with a reduced respiratory drive.  If there is high loop gain with hyperventilation, then therapy is needed to address this hyperventilation; if it is a case of overshoot, then this is best addressed with ASV.  If there is a low loop gain and hypoventilation with reduced respiratory drive, then the solution should be noninvasive ventilation.  Finally if the patient has high upper airway obstruction, then this needs more CPAP or expiratory positive airway pressure (EPAP) to stabilize the upper airways.