The Siemens Servo 900C is electrically and pneumatically powered, electronically (microprocessor) controlled, pressure or time triggered, time cycled with flow-controlled CMV (Vol Contr) and SIMV modes and a pressure-controlled CMV mode. CPAP and Pressure Support modes are also available. Pressure support is flow cycled when the flow falls to 25% of the peak flow.

The 900C is divided into two sections, the pneumatic section on the top and the electronic section on the bottom containing the ventilator settings.The two sections are connected by a cable and therefore could be separated if desired.

High presssure gas enters the pneumatic section from the blender. A second gas inlet connection (not shown) accepts low pressure anesthesia gas. The gas then passes the Galvanic cell oxygen analyzer and Main Flow Bacterial Filter and enters the Spring Loaded Bellows. From the bellows gas passes through the Inspiratory Flow Transducer (strain gauge in blue) and then through the Inspiratory Scissors Valve [><] before exiting the unit to

go to the patient (ventilator circuit not shown). Exhaled gas from the patient returns to the unit and passes through the Expiratory Flow Transducer (strain gauge in blue) and Expiratory Scissors Valve (><) before exiting out a one-way valve to the atmosphere.The inspiratory scissors valve is opened and closed by a stepper motor. The expiratory scissors valve is controlled by a pull magnet.

The flow servo loop controls volume delivery during inspiration in volume controlled mode. Actual gas flow is measured at the inspiratory flow transducer and compared to the electronics section containing the operator preset volume [Preset Inspiratory Min Volume and BPM] and flow [Preset Inspiratory Min Volume and IT%]. If the actual flow does not match the preset value, the stepper motor opens or closes the inspiratory scissors valve to adjust the flow delivery. Inspiratory time is always determined by the master BPM, Inspiratory Time %, and Pause Time % for all modes. The maximum inspiratory time allowed is 80%

A toggle switch in between the Preset Min Volume and the BPM knobs allow either a rectangular or descending ramp flow pattern in the volume controlled modes. The inspiratory time remains the same, so the peak flow will be higher in a descending ramp than in a rectangular flow waveform. See the Lab Review for the 900C for an explanation of how to calculate peak flow, inspiratory time, and I:E ratios.

The modes of the 900C are volume control [CMV], volume control + sighs, SIMV, SIMV + PS, pressure control [CMV], pressure support, and CPAP. There is no SIMV mode in pressure control. SIMV has a separate SIMV Breaths/min control [see above] which must be set less than the BPM setting in order to function in SIMV mode. If the SIMV Rate equals or exceeds the BPM setting, the actual SIMV rate in SIMV mode is determined by the BPM setting. A toggle switch next to the SIMV Breaths/min knob allows High Rates [4 - 40 breaths/min] or Low Rates [0.4 - 4 breaths/min]. The SIMV rate of 0.5 breaths/min is really one breath every two minutes, and 0.4 is one breath every 2.5 minutes.

In pressure control and pressure support mode, the pressure level is set by the Inspiratory Pressure above PEEP control. Set the Inspiratory Pressure Level before switching to pressure support or pressure control modes.

The Monitor knob allows the operator to view selected pressures or volumes which are displayed in the LED window shown in red. Monitored parameters are Breaths/min, FIO2, inspiratory or expiratory tidal volume, expiratory minute volume, peak, pause, or mean airway pressures. High and Low Oxygen % Alarm knobs are located above the display window. Both the Oxygen Alarms and the High and Low Minute Volume Alarms have amber indicator lights that will flash if the alarm knobs are set off scale.

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