Abstract: On method of sensing a ventilation failure measures a ventilation cycle and if the pressure is not maintained throughout the whole of an inspiration period above the setting for the low datum point 10 an alarm is signaled. A normal ventilation cycle is shown in FIG. 1(a) with the pressure over the ventilation cycle indicated by curve 14, low pressure datum 10 and the high pressure datum 12. The inspiration period 16 occurs for a given length of the cycle 14. When the tube feeding air to the patient is disconnected (whether wholly or partially) the ventilation cycle is as shown in FIG. 1(b). Inspiration peak 18 is now much lower and pressure drops rapidly, once the given volume has been delivered, from peak 18 to below set low pressure datum 10 (before termination of period 16). The peak 18 and the slope of the curve as indicated at 20 will vary depending upon the circumstances experienced. Another method measures the period between excursions of the pressure above a given level above datum 10.