Objectives:To compare and to analyze whether the values of rapid shallow breathing index (RSBI) determined by a ventilator display and a digital ventilometer were correlated. Methods: Twenty-two adult patients (17 males and 5 females) in the postoperative period of cardiac surgery and in mechanical ventilation were studied. Prior to the data collection, each patient was evaluated, received physical therapy, in order to promote bronchial hygiene and pulmonary reexpansion, and was positioned in elevated dorsal recumbent at 45°. After these procedures, minute ventilation (MV) and respiratory rate (RR) obtained from mechanical ventilator display and digital ventilometer were recorded. The Palavras-chave: desmame ventilatório; ventilação mecânica; índice de respiração rápida e superficial; índice de Tobin; cirurgia cardíaca.