Triggering sensitivity, ventilators

In this mode, the ventilator maintains a positive pressure at the airway as the patient attempts to inspire. Figure 18.6 illustrates a typical airway pressure waveform during continuous positive airway pressure (CPAP) breath delivery. The therapist sets the sensitivity level lower than PEEP. The sensitivity is the pressure level that the patient has to attain by making an effort to breathe. This, in turn, triggers the ventilator to deliver a spontaneous breath by supplying air (or a mixture of air and oxygen) to raise the pressure back to the PEEP level. Typically, the PEEP and sensitivity levels are selected such that the patient will be impelled to exert effort to breathe independently. As in the case of the mandatory mode, when the patient exhales, the ventilator shuts off the flow of gas and opens the exhalation valve to allow the patient to exhale into the atmosphere. 

Based on reflex studies, the hypoxic bradycardia exhibited by most teleost fish appears to be triggered by activation of externally oriented receptors that monitor aquatic O2 levels. These receptors are found largely on the first gill arch but have been reported on other gill arches in some species (see Refs. 13,65 for reviews). Not all fish follow this pattern, however. In the gar and the tambaqui they appear to be sensitive to both internal and external changes in Pq (79,85) while in the neotropical fish the traira they appear to only monitor the P02 of the blood (84). By contrast, the 02-sensitive receptors instrumental in producing the increases in ventilation frequency and amplitude in most teleost fish appear to monitor both the blood and the water (13,67,84,85). 

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