Comparison of propofol-ketamine and propofol-fentanyl combinations for sedation in patients undergoing gastrointestinal endoscopy: a randomized clinical trial oleh Milena Vasconcelos Falcão, João Nogueira Neto, Ed Carlos Rey Moura, Caio Márcio Barros Oliveira, Rodrigo Pinto Diniz, Almir Vieira Dibai-Filho, Plínio da Cunha Leal