An apparatus for providing back up ventilation to a patient comprising: a controllable blower device (1, 2) to generate a controlled supply of breathable gas, the supply of breathable gas providing ventilatory support for the patient, the blower device (1,2) being adapted for being coupled to a patient interface and a conduit for delivering the supply of breathable gas to the patient; and a processor coupled with the blower device (1, 2) to control the delivery of the supply of breathable gas to the patient, the processor being adapted for connection with a pressure transducer configured to detect a measure of patient respiration through the interface and to generate a signal indicative of the measure to the processor; wherein the processor includes coded instructions to control a spontaneous mode in which synchronized cyclical ventilatory support is initiated by detecting the presence of a respiratory cycle of the patient from the signal from the pressure transducer; wherein the processor further includes coded instructions to control a timed mode in which cyclical ventilatory support is delivered in accordance with a timing threshold in the absence of detecting a respiratory cycle of the patient from the signal from the pressure transducer, and characterized in that the processor further includes coded instructions to set the timing threshold as a function of normal expected respiratory timing for the patient, said function chosen to deviate the timing threshold from the normal expected respiration timing for the patient by setting it to a rate that is lower than the normal expected respiratory timing of the patient in order to promote patient initiated synchronization of the ventilatory support but permit back-up ventilation in the event of an apnea wherein the processor further includes coded instructions to automatically adjust the timing threshold in the timed mode in the absence of detecting a respiratory cycle of the patient from a timing threshold deviating from a normal expected respiration timing for the patient toward the normal expected respiration timing for the patient. a controllable blower device (1, 2) to generate a controlled supply of breathable gas, the supply of breathable gas providing ventilatory support for the patient, the blower device (1,2) being adapted for being coupled to a patient interface and a conduit for delivering the supply of breathable gas to the patient; and a processor coupled with the blower device (1, 2) to control the delivery of the supply of breathable gas to the patient, the processor being adapted for connection with a pressure transducer configured to detect a measure of patient respiration through the interface and to generate a signal indicative of the measure to the processor; wherein the processor includes coded instructions to control a spontaneous mode in which synchronized cyclical ventilatory support is initiated by detecting the presence of a respiratory cycle of the patient from the signal from the pressure transducer; wherein the processor further includes coded instructions to control a timed mode in which cyclical ventilatory support is delivered in accordance with a timing threshold in the absence of detecting a respiratory cycle of the patient from the signal from the pressure transducer, and