Hypopnea: ‘rule of thumb’ or ruler and plumb?

Abstract

The need to establish clinically meaningful indices of sleep disorders are driven by the necessity for equity in access to healthcare resources and clinical treatment as well as the desire to understand the relationship between pathological markers, pervasive symptoms and comorbid consequences of disease. The controversy that surrounds the definition of hypopnea springs from a contrast between symptomatology and clinical significance that may not be fully captured by current metrics required by payers for reimbursement of treatment [1,2]. Sleep apnea clinical thresholds defining disease, treatment recommendations, and prevalence estimates are heavily influenced by hypopneas since obstructive apneas contribute only an estimated 5 to 25 % of the spectrum of sleep disordered breathing [3]. Inclusion of hypopneas in the indices of disease spectrum is intended to reflect ventilatory insufficiency by defining periods of reduced respiration. Developing more accurate measurements of inspired and expired airflow, and then providing more precise definitions for hypopneas, will be paramount for determining how mild, moderate and severe ventilation insufficiency are associated with clinically identifiable consequences such as magnitude of oxygen desaturation or duration of arousal from sleep.The recent study by Dr. BaHamman and colleagues, examines the impact of altering the hypopnea definition on the indices of sleep breathing [4] as described for previous updates to scoring consensus [5]. In the current manuscript, the authors examine how changing the requisite percentage for reduction in airflow, and threshold level for oxygen desaturation and association with arousal, alters the distribution and significance of the clinical thresholds for disease. Agreement between the AASM 2012 [1] and 2007 (recommended and alternate) [2] scoring guidelines were assessed using Bland Altman plots that display the difference between two scores plotted versus their average. Comparing criteria of a 50 % (2007) versus 30 % (2012) reduction in airflow both with 3 % desaturation or arousal, demonstrates approximately 10 events per hour less, are detected when the threshold for reduced airflow is decreased by 20 %. In this population, approximately one fifth of the sample have solely hypopneas with a greater than 50 % reduction in airflow, while the majority of the sample is evenly distributed with individuals that have hypopneas both above and below 50 % ( Fig. 1; also see the original article Figure 2-C [4]). This raises the question of whether changing percent reduction in airflow to define hypopneas obscures information important to an individuals' ventilatory capacity. On the one hand, a greater reduction in airflow may reflect a greater degree of airflow obstruction [6]; however, on the other hand may reflect increased ventilatory reserve [7] or lower arousal threshold [8].The analysis of BaHamman et. al. demonstrates systematic bias of agreement by applying comparisons of the indices of 30 % reduction in airflow for events classified wi...