To evaluate the efficacy and safety of levosimendan versus conventional intensified inotropic treatment, in critical children with severe acute heart failure which persists despite of having received conventional inotropic treatment.

Inclusion Criteria:

• Patients who give written consent to participate in the study and for the transfer of material for cellular and molecular studies.
• Understand the purpose of the study and to be available to perform the procedures written in the protocol.
• Any child of 1 day old to 18 years old, admitted to the Paediatric Intensive Care Unit due to severe acute heart failure of any etiology.

Exclusion Criteria:

All patients who

• express their wish of not to participate in the protocol
• have hypertrophic or restrictive cardiomyopathy
• have aortic Stenosis
• have a known allergy to any drug used in the study
• it is not possible to prescribe the study medication because it is contraindicated according to the Summary of the Product Characteristic or according to the criteria of the physician responsible for patient
• are pregnant