Sleep Heart Health Study

6.6.2.7 Scoring respiratory events

SHHS will identify the following categories of discrete breathing events: obstructive apneas, central apneas, and hypopneas. Additionally, periodic breathing and periodic large breaths will be identified. No attempt will be made to distinguish mixed apneas from obstructive apneas. This decision was based on previous data that indicated mixed events cannot be reliably identified. Central hypopneas and increased upper airway resistance (RERAs) will not be identified because of controversies in the defining these events and the probable need to use invasive monitoring to identify these accurately (ResFigures 1a, 1b, 1c, and 1d).

Obstructive Apneas are identified when the amplitude (peak to trough) of the airflow signal decreases to a flat or almost flat signal (showing a 75% reduction of the amplitude of “baseline” breathing) if this change lasts for ≥ 10 s. Baseline breathing is defined as a period of regular breathing with stable oxygen levels (ResFigures 2a, 2b, 2c, and 2d).

Hypopneas are identified if the amplitude of any respiratory signal is reduced by 30% of the amplitude of “baseline” (identified during a period of regular breathing with stable oxygen levels), if this change lasts for ≥ 10 s and for >2 breaths (ResFigures 3a and 3b). Sometimes more subtle changes in breathing are observed (not clearly reduced by 30% or more from baseline). These require at least a 2% desaturation (ResFigure 3c).

• Distinguishing Between Hypopneas and Apneas. This distinction only can be made for events in which airflow is interpretable. (If airflow is uninterpretable, the event-based on inductance data is considered by default to be a hypopnea.) (ResFigure 4a). Apneas are marked if > 50% of the event shows absent or nearly absent airflow on the thermister channel (and this reduction is 75% the amplitude of the surrounding breaths). (ResFigures 4b and 4c)
• Variation in signal amplitude on airflow, thoracic and abdominal channels: In some studies, information appears qualitatively different from different channels. Scoring will be done from the channel that correlates the best with the changes in the oxygen saturation (ResFigure 4d). If there are no changes in O2 saturation, scoring will be done from the channel that shows the clearest amplitude variation (ResFigure 4e). In cases where the thermister varies from the inductance channels, and the inductance channels appear mostly artifact free, the inductance channels will be used for event identification and classification (ResFigure 3b, 4f). For example, sometimes the airflow channels will suggest prolonged periods of “no flow”, but changes in inductance and oxygen saturation suggest that breathing is occurring. When in doubt, use data from the inductance and saturation channels to identify/classify events.

A Central apnea event is scored if NO displacement is noted on both chest and the abdominal inductance channels (ResFigures 5a and 5b). Otherwise, events are noted as “obstructive.”

• Distinguishing Between Central and Obstructive Events. Only events in which there is clear data from both the abdominal and chest signals can be distinguished as “central” or “obstructive”. (Events where one or both of these channels are missing or contain artifact are considered “obstructive.”) (ResFigure 5c).

• Often determining whether an event is central or obstructive is influenced by where the event is noted to begin and end. Sometimes, small efforts are seen following a completely flat area, followed by a large (”breaking”) breath. If a single small breath is seen at the beginning or the end of the period of flat signal, the event will be marked as “central.” (This recognizes that shortening the event slightly would make it a central event) (ResFigure 6a). However, if 2 or more consecutive small breaths are seen in the period in question, the event is marked as “obstructive.” Many of these events would be noted as “mixed” by non-SHHS scorers (ResFigures 6b and 6c).

• Determining whether an event is central or obstructive in areas of periodic breathing can be difficult because of uncertainties in deciding when to start and end such events. Often, these areas contain breaths that gradually increase and decrease, sometimes decreasing to an imperceptible level. Marking “longer” events in these areas would result in identifying “obstructive” events; “shorter” events are more likely to appear “central.” When it is unclear as to when to start an event, look for evidence of paradoxical breathing. Change in phase angle between thoracic and abdomen is an indicator of upper airway obstruction (such events will be designated as obstructive) (ResFigure 6d). When still unclear, the event duration will be marked using the airflow channel. Identify the areas where airflow stops and starts, then assess whether the period is also associated with effort on either channel/band. Then, the inductance channels will be visualized to decide whether during this period, any effort occurred. If any effort was visualized, the event will be considered “obstructive”, otherwise, “central.” (ResFigures 6e, 6f, and 6g).

Duration criteria: The beginning of an Apnea/Hypopnea is marked at the end of the last “normal” breath; the end of the event is identified as the beginning of the first breath that exceeds the amplitude of the first reduced breath used to mark the beginning of the event. Duration is based on a “trough to trough” marking (ResFigures 6h, 6i, and 6j).