Operating Principles of Ascending Bellows Ventilators
Contemporary examples of ascending bellows, double-circuit, electronic
ventilators include the North American Dräger AV-E and the Datex-Ohmeda 7000,
7800, and 7900 series. A generic ascending bellows ventilator is shown in Figure
9-24
. It may be viewed as a breathing bag (i.e., bellows) located within
a clear plastic box. The bellows physically separate the driving-gas circuit from
the patient's gas circuit. The driving-gas circuit is located outside the bellows,
and the patient's gas circuit is inside the bellows. During the inspiratory phase
(see Fig. 9-24A
), the driving
gas enters the bellows chamber, causing the pressure within it to increase. This
increase in pressure is responsible for two events. First, the ventilator's relief
valve closes, preventing anesthetic gas from escaping into the scavenging system.
Second, the bellows are compressed, and the anesthetic gas within the bellows is
delivered to the patient's lungs. This compression action is analogous to the hand
of the anesthesiologist squeezing the breathing bag.[51]
During the expiratory phase (see Fig.
9-24B
), the driving gas exits the bellows' chamber. The pressure within
the bellows' chamber and the pilot line decline to zero, causing the mushroom portion
of the ventilator's relief valve to open. Gas exhaled by the patient fills the bellows
before any scavenging occurs. This happens because a weighted ball similar to those
used in ball-type positive end-expiratory pressure (PEEP) valves is incorporated
into the base of the ventilator's relief valve. The ball produces 2 to 3 cm H2
O
of backpressure; therefore scavenging occurs only after the bellows fill completely
and the pressure inside the bellows exceeds this pressure threshold. This design
causes all ascending bellows ventilators to produce 2 to 3 cm H2
O of PEEP
within the breathing circuit. Scavenging occurs only during the expiratory phase
because the ventilator relief valve is open only during expiration.[51]
Gas flow from the anesthesia machine into the breathing circuit
is continuous and independent of ventilator activity. During the inspiratory phase
of mechanical
ventilation, the ventilator's relief valve is closed, and the breathing system's
APL valve (pop-off valve) is closed or out of circuit. The patient receives a volume
from the bellows and flow meters during the inspiratory phase. Factors that influence
the correlation between set tidal volume and exhaled tidal volume include the flow
meter's settings, the inspiratory time, the compliance of the breathing circuit,
external leakage, and the location of the tidal volume sensor.[15]
[16]
[139]
[140]
Usually, the volume gained from the flow meters during inspiration is counteracted
by the volume lost to the breathing circuit compliance. The set tidal volume generally
approximates the exhaled tidal volume. However, oxygen flushing during the inspiratory
phase can result in barotrauma because excess volume cannot be vented.[51]
