What is trace alternans in polysomnography (PSG)?

T-Wave Alternans in Polysomnography

T-wave alternans in polysomnography refers to beat-to-beat variations in T-wave amplitude that can be detected during sleep studies and may serve as a marker for cardiac electrical instability and potential vulnerability to ventricular arrhythmias.

Definition and Characteristics

• T-wave alternans (TWA) is defined as T-wave amplitude variations that alternate every second beat, representing latent instability of cardiac repolarization that may predict malignant arrhythmias 1
• In polysomnography (PSG), TWA appears as microvolt-level variation (microvolt T-wave alternans) and sometimes as more pronounced variations in alternating complexes 1
• TWA is typically quantified using various modifications of moving-average analysis or as the variance of specific frequency components in spectral analysis 1

Clinical Significance in Sleep Studies

• TWA during sleep indicates potential cardiac electrical instability and may identify patients at high risk for serious arrhythmic events 1
• TWA patterns vary based on sleep stages and can be influenced by respiratory events that occur during sleep 2
• TWA is generally not present at resting state even in high-risk patients but may be detected during sleep studies when cardiac stress occurs 1

Relationship with Sleep Disorders

• In obstructive sleep apnea (OSA) patients, TWA patterns differ from those in healthy subjects, with OSA patients unexpectedly showing lower TWA that does not increase with heart rate 2
• The marked swings in intrathoracic pressure that occur during obstructive events in OSA may influence cardiac electrical activity and potentially affect TWA patterns 3
• TWA assessment during polysomnography may provide additional diagnostic information in patients with suspected cardiac arrhythmias and sleep disorders 1

Technical Aspects of Detection

• Detection of TWA requires special electrodes and processing to record the microvolt-level T-wave alternans with high fidelity 1
• Modern polysomnography equipment can detect subtle electrical alternans that would not be detectable by visual inspection of the electrocardiogram 4
• TWA primarily involves the ST segment and T wave (the phase of repolarization) 4

Clinical Applications

• TWA has been shown to be a strong predictor of arrhythmic events in patients with ischemic heart failure (relative risk 2.42) and nonischemic heart failure (relative risk 3.67) 1
• TWA assessment during polysomnography may help identify patients who require further cardiac evaluation 1
• The absence of TWA during sleep studies may indicate lower risk for sudden cardiac death in certain patient populations 1

Limitations and Considerations

• TWA activity level tends to increase with heart rate in healthy populations, with an unexpected nadir around 60-70 beats per minute 2
• The role of TWA regarding its clinical utility has not been fully defined, though it holds substantial potential in identifying patients at high risk of serious arrhythmic events 1
• Interpreting isolated T-wave abnormalities, including TWA, can be difficult and requires careful analysis to avoid inappropriate diagnoses 1

Relationship with Other PSG Parameters

• TWA should be evaluated in the context of other polysomnography parameters, including apnea-hypopnea index (AHI), oxygen desaturation index (ODI), and sleep architecture 5
• Proper interpretation of TWA requires standardized criteria according to American Academy of Sleep Medicine (AASM) guidelines 5