VII.
T H E O R Y O F O P E R A T I O N - C O N T .
39
C- SIMV (Synchronized Intermittent Mandatory Ventilation) MODE
In the SIMV mode, the Crossvent-2i+ delivers two main types of breaths: ASSISTED and SPONTANEOUS. Both are
initiated by the patient’s inspiratory effort, which is sensed by the ventilator. The assisted breaths are the same as assisted
breaths in the Assist Control mode. The rate of administration of assisted breaths is set using the SIMV RATE key and
entering the desired value.
In-between the assisted breaths, patient inspiratory efforts initiate spontaneous breaths. When the ventilator detects an effort
to breathe, it initiates inspiration and delivers gas to the patient at a rate determined by the FLOW control and for a period of
time equal to the inspiratory time of a normal assisted breath.
The difference between a spontaneous and assisted breath is that during a spontaneous breath, PIP is not applied, only the
PEEP pressure signal is applied to the exhalation valve, thus delivering gas to the patient at zero or PEEP pressure, if PEEP
is on.
The full tidal volume is made available at the proximal airway. Whatever gas is not inspired by the patient is vented to
atmosphere. At the end of the inspiratory time, should the patient desire a greater tidal volume than that which is set, he
need only continue to inhale. As long as pressure below baseline (zero or PEEP) is detected, the ventilator will continue to
deliver boluses of gas as determined by the TV setting. The patient is free to trigger as many spontaneous breaths as needed
between assisted breaths. At the end of the time interval established by the SIMV RATE control, the ventilator administers
an assisted breath, synchronized to the patient’s breathing effort.
Pressure Support: When pressure support is turned on, the patient circuit pressure is allowed to increase above baseline (zero
or PEEP) by the number of cmH
2
O as set by PRESSURE SUPPORT. This occurs only during spontaneous breaths. When
the airway pressure reaches the pressure support level, the baseline pressure is restored, allowing the exhalation valve to open,
but the patient gas continues to flow for the duration of a normal assisted breath as set by the RATE and TIDAL VOLUME
controls.
WARNINGS: IT IS EXTREMELY IMPORTANT THAT THE FLOW TRIGGER
CONTROL BE CAREFULLY ADJUSTED TO ASSURE PROPER OPERATION IN THE
SIMV MODE. ALSO, UNDER CERTAIN CONDITIONS IN SIMV WITH PEEP, EVEN
THOUGH THE LOW PEAK PRESSURE ALARM IS SET CORRECTLY FOR ASSISTED
BREATHS, THERE MAY BE NO LOW PEAK PRESSURE ALARM FOLLOWING A
PATIENT DISCONNECT UNTIL THE NEXT ASSISTED BREATH. (THIS PERIOD
MAY BE UP TO 2 MINUTES.) AS AN ADDED PRECAUTION, SET THE LOW
PEEP/CPAP AND LOW EXHALED TIDAL VOLUME ALARMS SO THAT THEY ARE
OPERATIVE.