The Pacer System

In the Avery system, the pacer transmitter unit is powered by 9-V batteries. The Mark IV transmitter has a battery indicatar light, which flashes on inspiration. The battery indicator will stop flashing as the power is reduced to a critical level, approximately 24 to 36 hours in advance of loss of transmitter output. In the latest transmitter model, there is an additional circuit that maintains transmitter output as voltage decreases, so that there is no effect on ventilation however, in older models, as battery output decreases, so does the stimulus output and consequently the ventilation. Therefore, battery maintenance must occur on a regular schedule. 

The respiratory rate control may be located externally or internally—in both cases, the factory preset rate is 12 breaths per minute (bpm). This rate can easily be adjusted by the clinician through the transmitter control box. The combination of rate and tidal volume will determine the minute volume. This can be titrated to achieve adequate carbon dioxide and oxygen saturation levels as well as patient comfort. 

The remaining settings are internal and require slightly more advanced technology for adjustments. The inspiratory time is factory preset at 1.3 seconds. This represents the total time for inspiration and may be adjusted to alter the inspiratory to expiratory ratio, for patients requiring high respiratory rates or to increase patient comfort. 

The pulse interval is factory preset at 50 milliseconds and represents the distance from the beginning of each pulse to the next within the pulse train. This period may be lengthened in order to decrease the number of pulses during inspiration. Increasing the pulse interval will decrease the total number of pulses delivered within the inspiratory time. This dropping of extra pulses will decrease unnecessary electrical stimulation of the phrenic nerve and diaphragm. 

The slope is factory preset at zero, i.e., all of the pulses in the pulse train are of the same amplitude. Changes in slope will change the pattern of nerve fiber recruitment and may result in a smoother and more comfortable contraction. 

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