A client who has been mechanically ventilated for 5 days following ARDS is being evaluated for weaning readiness. The respiratory therapist reports the following parameters: Rapid shallow breathing index (RSBI) of 85, negative inspiratory force (NIF) of -28 cm H2O, spontaneous tidal volume of 6 mL/kg, and FiO2 requirement of 40% to maintain SpO2 >92%. Which parameter indicates the client may NOT be ready for weaning?