The most common aetiology of heart failure in previously well children is myocarditis. A review of 62 cases whose final diagnosis was viral myocarditis showed that common features included:

• multiple visits / evaluations before the diagnosis was made
• initial misdiagnosis as respiratory disease (eg. pneumonia, asthma)
• presenting symptoms of shortness of breath, vomiting, poor feeding, lethargy, fever
• signs included hepatomegaly (50%), tachypnoea, respiratory distress
• although a normal heart rate for age was found in 41(66%) patients, ALL patients had an abnormal ECG
• Most had abnormal chest x-rays

The take home messages regarding this rare disease are that initial misdiagnosis is common, do not expect a resting tachycardia, look for hepatomegaly, CXR signs, and particularly ECG abnormalities.
Pediatric myocarditis: presenting clinical characteristics
Am J Emerg Med. 2009 Oct;27(8):942-7