Correlations between SKNA and arousal index, AHI or ODI. The correlation between overall SKNA burst area and arousal index (A), AHI (B) or ODI (C) among the 18 patients in the OSA group was evaluated by Spearman’s correlation. AHI, apnea/hypopnea index; ODI, oxygen desaturation index; OSA, obstructive sleep apnea; SKNA, skin sympathetic nerve activity recorded by ECG patch electrodes.