A 4-French gauge Opticath catheter, inserted into a neonate's right atrium through the right internal jugular vein, can be used to continuously monitor venous blood oxygen saturation in the right atrium. This method instantly detects subtle changes in cardiopulmonary function in small infants and neonates and enables the appropriate treatment to be given in cases of critical infantile and neonatal cardiopulmonary deterioration. This report discusses four neonatal and infantile patients who underwent continuous monitoring of venous blood oxygen saturation in the right atrium. Continuous monitoring allowed appropriate treatment to be given earlier compared to conventional monitoring methods of blood pressure and heart rate.