Normally, the heart and lungs maintain a peripheral oxygen saturation (SpO2) level that should ideally be above 95%. In patients with Tetralogy of Fallot, this saturation typically ranges around 85%. During periods of increased demand, such as crying or physical exertion, these levels can further decrease, potentially dropping to approximately 60%. While cyanosis (bluish discoloration of the skin) initially manifests primarily after exertion or crying, it progressively worsens with time, eventually becoming evident even at rest.