The percentage of ETF trading relative to overall volume tends to shoot higher in headline-driven markets when asset classes are moving together on macroeconomic or political events.

That’s exactly what happened on Monday when global markets swooned on fears parliamentary elections in Italy will result in political gridlock. After months of simmering on the back burner, Europe’s debt crisis roared back into the news. [Italy ETF Swings Lower on Berlusconi, Election]

On Monday, ETFs accounted for 32% of overall dollar volume, and there have been multiple sessions in the past week when the share rose above 30%, says Chris Hempstead, director of ETF execution services at WallachBeth Capital.

“It’s rare when ETF volume goes above 30%,” he said in a telephone interview Tuesday morning. [Using ETFs to Gauge Market Sentiment]

Hempstead said he has seen the figure approach 40% on some days during the past few years.

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