CPAP/BiPAP Settings Calculator

Neck circumference is a proxy for the amount of soft tissue around the upper airway -- a thicker neck generally means more fatty and soft tissue loading on the pharynx, which increases the tendency for the airway to collapse during sleep regardless of overall body weight. It is included alongside BMI and AHI in the Miljeteig-Hoffstein formula because studies have found it adds predictive value for the pressure needed beyond BMI alone, and it is also a well-established independent risk factor for obstructive sleep apnea severity.

Comparative studies against gold-standard in-lab titration have generally found the Miljeteig-Hoffstein-style formula explains a majority but not all of the variation in the pressure patients actually need, with reported mean differences on the order of a few cmH2O and a tendency to underestimate. That is close enough to be a reasonable interim setting or an auto-titrating device's starting point, but not close enough to replace a formal titration study when one is available.

A 4-6 cmH2O pressure support difference between inspiratory and expiratory pressure is a commonly used starting convention in BiPAP prescribing because it is usually enough to reduce the work of breathing without being so large that it feels uncomfortable or causes air swallowing. This calculator uses 4 cmH2O as a reasonable default gap, but actual pressure support is routinely titrated upward for patients who also need help with hypoventilation, not just airway patency.

Yes, in the sense that AHI has a positive coefficient in the formula -- a higher Apnea-Hypopnea Index, holding BMI and neck circumference fixed, increases the estimated pressure -- but the effect is smaller per unit than BMI or neck circumference, so a patient with severe AHI but an average build and neck size will not necessarily be pushed toward the highest end of the 4-20 cmH2O range on AHI alone.