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bag, etc. The expiratory time is set in the IMV range allowing the patient one
or more spontaneous breaths between the machine assisted breaths. It
should be noted that in the SIMV mode it is essential that the inspiratory
effort control be set so that the patient can trigger the machine at all times.
When the NORMAL/SIMV selector is set in the SIMV position it will be
observed that while the ventilator cycles with each inspiratory effort, the
pressure builds up in the patient circuit only after the end of the expiratory
time. This is achieved by pressurizing the exhalation valve only after the
patient triggered breath at the end of the expiratory time (assisted breath).
At the end of the expiratory time the machine waits for the next inspiratory
effort and therefore when the assisted breath is provided it is synchronized to
the patient's breathing effort. In the SIMV mode, in the event no patient
inspiratory effort is sensed for a period of ten seconds, a "backup timer" will
provide a "backup breath." Every time an inspiratory effort is sensed,
whether for a spontaneous or an assisted breath, the backup timer is reset.
The interval between assisted breaths may still be set to the maximum
expiratory time of at least 45 seconds.
Each time the machine is cycled it provides gas flow to the patient even
when the exhalation valve is not pressurized. If the inspiratory time and flow
rate are set in a way that provides more gas than needed by the patient, the
excess passed to the atmosphere. Should the patient require more gas
during a spontaneous breath than is provided, and if a negative pressure is
still being generated (following the termination of the inspiratory period),
another inspiratory period (and as many more as necessary) will be initiated,
thus providing as much gas as required. The patient may exhale at any time
during spontaneous breathing since the exhalation valve is not pressurized.
It is, however, desirable to set the flow rate and inspiratory time to give a tidal
volume as close to the spontaneous tidal volume as possible. With this
triggered demand-flow system, it is only necessary to trigger the unit initially.
The IC-2A then provides a bolus of gas equal to the inspiratory time
multiplied by the flow rate. Unlike other systems it is not necessary to
maintain a constant negative pressure of several cmH
2
O during each
spontaneous breath. This eliminates the oscillations observed in other
systems at low flow rates and makes possible the use of a normal bubble
type humidifier with the bubbler in place. Note that during the spontaneous
breaths, even though the exhalation valve is not pressurized, there may be a
buildup of 3-5 cmH
2
O pressure depending on flow rate set. This is due to
slight resistance of the exhalation valve at high flow rates.
3.
Positive End Expiratory Pressure (PEEP)
The PEEP/CPAP control applies a constant pressure to the exhalation valve
which may be adjusted to give a pressure in the patient circuit from 0 to 25
cmH
2
O. The PEEP/CPAP control is operative in all modes of operation. The
inspiratory effort control may be set to compensate for the PEEP/CPAP
pressure level in the patient circuit. Whenever the PEEP/CPAP level is
changed, the inspiratory effort control should be readjusted.
4.
Continuous Positive Airway Pressure (CPAP)
When the CYCLE/CPAP selector switch is placed in the CPAP-MANUAL
position the function of the IC-2A is very similar to the SIMV mode. In the
Summary of Contents for 2005IC
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