 The apneas (pauses in breathing) must last for at least 10 seconds and be associated with a decrease in blood oxygenation.Combining AHI and oxygen desaturation gives an overall sleep apnea severity score that evaluates both the number of sleep disruptions and the degree of oxygen desaturation (low oxygen level in the blood).The AHI is calculated by dividing the number of apnea events by the number of hours of sleep.