Economists noted that Draghi had said only 20 percent of purchases would be the responsibility of the ECB. This means the bulk of any potential losses, should a euro zone government default on its debt, would fall on national central banks.
Critics say this casts doubt over the unity of the euro zone and its principle of solidarity, and countries with already high debts could find themselves in yet deeper water.
"It is counterproductive to shift the risks of monetary policy to the national central banks," said former ECB policymaker Athanasios Orphanides. "It does not promote a single monetary policy. This path toward Balkanisation of monetary policy would signal that the ECB is preparing for a break-up of the euro."