An aspect of computer or microprocessor control is the loop system. Most ventilators that are not microprocessor controlled are called open loop systems.

 

The operator sets a control (e.g., tidal volume), and the ventilator delivers that volume to the patient circuit. This is called an unintelligent system because the ventilator cannot be programmed to respond to changing conditions. If the gas leaks out of the patient circuit (and therefore does not reach the patient), an open loop ventilator cannot adjust its function to correct for the leakage. It simply puts out, or outputs, a set volume and does not measure or change it.

 

Closed loop systems are often described as intelligent systems because they compare the set control variable to the measured control variable. For example, some closed loop systems are programmed to compare the tidal volume setting to the measured tidal volume exhaled by the patient. If the two differ, the control system may alter the volume delivery.

 

An example of a closed loop system is the mode of ventilation called mandatory minute ventilation. The operator selects a minimum minute ventilation setting that is lower than the patient's spontaneous minute ventilation. The ventilator monitors the patient's spontaneous minute ventilation, and if it falls below the operator's set value, the ventilator increases its output to meet the minimum set minute ventilation.