Tafonius
Junior
– A User Guide
Pressure Gauges
Junior w/gas also has 2 pressure guages on the front panel. One for Breathing System
Pressure the range of which is -10 – +60 cmH
2
O and one for Oxygen Supply pressure with a
full scale of 100psi (700kPa).
Junior
and Spontaneous Breathing
Thinking of the Piston as a “Virtual bag
When the ventilator is idle and a patient is connected, the system behaves like a ‘perfect’
bag. By a perfect bag we mean that when the patient breathes out there is no expiratory
resistance and when the patient breathes in there is no inspiratory resistance. This action is
possible because the airway pressure is sensed at the centre of the Y-piece. As soon as a
pressure deviation in excess of 0.5 cmH
2
O from the ambient pressure is detected the piston
is moved so as to negate this pressure change. If the patient breathes out then the piston
withdraws, effectively filling. If the patient inspires then the piston moves down, effectively
emptying. Because a motor drives the piston it is the motor that overcomes the resistances
of any piping, valving or soda lime. To the patient it feels like breathing to room air with no
resistance.
Dictating the size of the “Virtual Bag”
The size bag you use on your anaesthesia machine depends on the size of the patient. The
breathing bag must be large enough to hold the largest size breath the patient may take.
But it should not be overly large because the excess volume increases the time constant of
the breathing system making it take longer for changes in vaporizer settings or the FiO
2
setting equilibrate and take effect.
The tidal volume and the concept of a buffer volume are used to control the size of the
“Virtual Bag”, the volume under the piston, in
Junior
. The piston is not allowed to rise
above the combined volumes of TV + BV. Whenever the piston volume reaches this upper
limit, gas is allowed to flow out to the scavenging system.
In this manor
Junior
is changing the Virtual bag size every time you change the TV.
Add-a-Litre feature
In addition to continually managing the maximum volume is the system
Junior
also works
to keep a the minimum volume in the system at all times.
Junior
considers the piston to
be in the low volume zone whenever it dips below the 1.5 litre point. When this happens
gas flows into the system during current and subsequent the inspiratory phase(s) until a liter
has been added and the
LOW SYSTEM RESERVE VOLUME
alarm will sound.
Example 1:
A 300kg patient breathing spontaneously. Rule of thumb would suggest setting the TV
control to 3.0L in preparation for IPPV. The BV is set to its default of 5.0L. The upper limit
of the piston volume is then 3.0 + 5.0 L = 8.0L, the patient is effectively breathing from an
8L bag.
Say the patient has just been induced, initial gas flow rates may be of the order of 5-10L per
minute. This will tend to fill the system rather quickly.
When the piston volume reaches the 8.0L limit the piston stops rising and the excess gas
will be discharged from the system through the electronically controlled Dump Valve. You
will hear the dump valve opening and closing as a clunk... clunk. After induction you would
turn the fresh-gas-flow down but lets say you turn it down a little to far. The piston volume
falls a little breath by breath, the clunking sound will stop, until finally the piston dips below
the 1.5L level towards the end of a cycle. At this point an extra 1.0L of gas will be added to
Printed: 26/Oct/18
Page 15 of 20
DOCA4962B
