1. Department of Pediatrics, Medical University Pleven, 5800 Pleven, Bulgaria
2. Dr. Georgi Stranski University Hospital, 5800 Pleven, Bulgaria
3. Department of Anatomy, Histology, Cytology and Biology, Medical University Pleven, 5800 Pleven, Bulgaria
4. Department of Anaesthesiology and Resuscitation, Medical University Pleven, 5800 Pleven, Bulgaria