AASM criteria for scoring respiratory events: interaction between apnea sensor and hypopnea definition

Abstract

Study objectives: To examine the impact of using a nasal pressure sensor only vs the American Academy of Sleep Medicine (AASM) recommended combination of thermal and nasal pressure sensors on (1) the apnea index (AI), (2) the apnea-hypopnea index (AHI), where the AHI is calculated using both AASM definitions of hypopnea, and (3) the accuracy of a diagnosis of obstructive sleep apnea (OSA).

Design: Retrospective review of previously scored in-laboratory polysomnography.

Setting: A tertiary-hospital clinical sleep laboratory.

Patients or participants: One hundred sixty-four consecutive adult patients with a potential diagnosis of OSA, who were examined during a 3-month period.

Interventions: N/A.

Measurements and results: Studies were scored with and without the use of the oronasal thermal sensor. AIs and AHIs, using the nasal pressure sensor alone (AI(np) and AHI(np)), were compared with those using both a thermal sensor for the detection of apnea and a nasal pressure transducer for the detection of hypopnea (AI(th) and AHI(th)). Comparisons were repeated using the AASM recommended (AASM(rec)) and alternative (AASM(alt)) hypopnea definitions. AI was significantly different when measured from the different sensors, with AI(np) being 51% higher on average. Using the AASM(rec) hypopnea definition, the mean AHI(np) was 15% larger than the AHI(th); with large interindividual differences and an estimated 9.8% of patients having a false-positive OSA diagnosis at a cutpoint of 15 events and 4.3% at 30 events per hour. Using AASM(alt) hypopnea definition, the mean AHI(np) was 3% larger than the AHI(th), with estimated false-positive rates of 4.6% and 2.4%, respectively. The false-negative rate was negligible at 0.1% for both hypopnea definitions.

Conclusions: This study demonstrates that using only a nasal pressure sensor for the detection of apnea resulted in higher values of AI and AHI than when the AASM recommended thermal sensor was added to detect apnea. When the AASM(alt) hypopnea definition was used, the differences in AHI and subsequent OSA diagnosis were small and less than when the AASM(rec) hypopnea definition was used. In situations in which a thermal sensor cannot be used, for example, in limited-channel diagnostic devices, the AHI obtained with a nasal pressure sensor alone differs less from the AHI obtained from a polysomnogram that includes a thermal sensor when the AASM(alt) definition rather than the AASM(rec) definition of hypopnea is used. Thus, diagnostic accuracy is impacted both by the absence of the thermal sensor and by the rules used to analyze the polysomnography. Furthermore, where the thermal sensor is unreliable for sections of a study, it is likely that use of the nasal pressure signal to detect apnea will have modest impact.

Keywords: AASM guidelines; apnea; diagnosis; hypopnea; methodology; nasal pressure sensor; obstructive sleep apnea; polysomnography; scoring; sleep disordered breathing; thermal sensor.

Figure 1

Flow chart of process for deriving various apnea-hypopnea indexes (AHI)

Figure 2

Scatter plots comparing: (A) AI_np (the apnea index, i.e., the total number of apneic events during the sleep time divided by the number of hours of sleep, obtained using the nasal pressure sensor) with the AI_th, (B) AHI_np to AHI_th using the AASM recommended definition and (C) AHI_np to AHI_th using the AASM alternative definition. The line is line of identity. All correlations are significant at the 0.01 level (one tailed).

Figure 3

Bland-Altman plots illustrating agreement between measurements: (A) AI_np and AI_th, (B) AHI_np and AHI_th using the AASM recommended hypopnea definition and (C) AHI_np and AHI_th using the AASM alternative hypopnea definition. Solid line represents median difference. Dashed lines represent 5^th and 95^th percentiles. All differences in median values are significantly different to zero (P < 0.001)

Figure 4

Change in false positive rate using AHI_np instead of AHI_th for OSA diagnosis at various cut points. Closed symbols are using the AASM_rec hypopnea definition and open symbols the AASM_alt hypopnea definition. Least squares best fit exponential trend lines asymptotically approaching zero are shown for each definition.