Systems and methods are described for application of a transitory corrective modification to a hot-wire anemometer flow voltage and/or calculated flow rate to compensate for transient thermal response of the anemometer during a change in mixture of a mixed gas being measured. According to one embodiment a method of applying the transitory corrective modification is provided. An output signal of an exhalation flow sensor of a medical ventilator is received. The flow sensor includes a hot-wire anemometer. The output signal is indicative of a rate of flow of expired gas by a patient. Transient thermal response of the hot-wire anemometer is compensated for by applying a corrective modification to the output signal or a value based thereon. The corrective modification is based at least in part on a fraction of inspired oxygen (FiO2) being supplied by the medical ventilator to the patient.