Viral Myocarditis: This is a common cause of heart failure and arrhythmias in infants, especially after a recent viral illness like cough and cold. The presentation of supraventricular tachycardia (SVT) with heart failure in the context of a recent upper respiratory infection makes viral myocarditis a leading consideration.

Anomalous Coronary Artery: Although less common, an anomalous coronary artery can cause myocardial ischemia leading to arrhythmias and heart failure, especially during periods of increased respiratory rate or stress, such as with a cough and cold.
Congenital Heart Disease: Undiagnosed congenital heart defects can present with heart failure and arrhythmias, especially if there's an issue with cardiac structure that affects function, such as atrioventricular canal defects or total anomalous pulmonary venous connection.
Metabolic Disorders: Certain metabolic disorders can affect the heart, leading to arrhythmias and heart failure. The recent treatment with hypertonic saline might hint at a possible electrolyte imbalance, which could be secondary to a metabolic disorder.

Hyperkalemia or Electrolyte Imbalance: The use of hypertonic saline and the development of SVT could be related to electrolyte imbalances, particularly hyperkalemia, which is life-threatening and requires immediate intervention.
Cardiac Channelopathies: Although rare in infants, conditions like Wolff-Parkinson-White syndrome can cause SVT and are crucial to diagnose due to their potential for sudden cardiac death.
Sepsis: Sepsis can cause myocardial dysfunction and arrhythmias. Given the recent history of cough and cold, and the potential for bacterial superinfection, sepsis is a critical diagnosis not to miss.

Kawasaki Disease: This is an inflammatory disease that can affect the coronary arteries and lead to heart failure and arrhythmias. It's rare but important to consider in infants with unexplained heart failure and arrhythmias.
Tumor or Mass Effect: Cardiac tumors or extracardiac masses compressing the heart can cause arrhythmias and heart failure. These are rare in infants but should be considered if other diagnoses are ruled out.
Genetic Arrhythmia Syndromes: Conditions like Long QT Syndrome can present with arrhythmias but are less likely to cause heart failure directly. However, they are important to diagnose due to their implications for family screening and management.