BIRD MARK 7 - SECTION 4 PNEUMATIC SYSTEMS

SECTION 4
PNEUMATIC SYSTEMS
8 VENTURI SYSTEMS
The original rationale for the use of a VENTURI in a
medical ventilator was two fold: 1) to act as an air
dilutor when oxygen was used as the source gas
powering the unit and 2) to increase the gas flow
capability of the unit. With the proper design, a
gated venturi can function as a “pneumatic clutch”
yielding a tapered (decelerating) wave form as
backpressure within the system begins to increase
(Fig. 1).
GATE CAGE
THROAT
ENTRAINMENT DISTAL GATE
PORT
FIGURE 1
A venturi has two flow gradients. The primary gra-
dient consists of a positive flow of gas passing
through the jet orifice. The secondary gradient is
developed by that flow of gas creating a pressure
drop (sub-ambient) at the throat of the venturi (Fig.
2). The laws of physics are such that gas pres-
sures always try to equilibrate. When a sub-
ambient pressure is created by a venturi, gas from
a higher pressure source will always move to a
lower pressure in an attempt to maintain equi-
librium between the two. The end result is the
entrainment of additional gas molecules by the
original source gas. This entrainment mechanism
(pressure gradient) allows a pneumatic device to
increase its total flow potential being delivered to
the patient.
50 Р5.1. OXYGEN
SOURCE GAS
SUBAMBIENT PRESSURE
ENTRAINMENT AIR MOLECULES
FIGURE 2
When the veniuri is functioning, the mixed gas is
delivered to the lungs under pressure (Fig. 3).
However, as the lung continues to fill, pressures
begin to equalize within the venturi and less air
molecules are entrained (Fig. 4). The result is a
higher oxygen concentration delivered to the lungs
as the venturi entrains less air due to the back-
pressure increasing within the system. When air
molecules cease to be entrained at the end of
inspiration, oxygen concentrations rise upward
toward 100%.
Due to its design, a venturi CANNOT hoid a con-
stant dilution ratio unless an oxygen blending
device and entrainment kit are utilized, especially
in the presence of rising pressures within the sys-
tem. Because of this design, oxygen concentra-
tions being delivered to a patient may range from
40%-90% when the unit is being powered by
100% oxygen.
ENTRAINMENT GRADIENT
FIGURE 3
LUNG FILLING PRESSURE
LUNG FILLING PRESSURE
AMBIENT ZERO
ENTRAINMENT GRADIENT
ZERO
FIGURE 4
The major advantage of delivering gas via a ven-
turi is achieved by the addition of a distal gate
(Fig. 1). Bird venturis utilize this gate to achieve a
“pneumatic clutch” during the inspiratory flow of
gas. As pressures increase within the lungs, the
backpressure is sensed at the venturi gate. The
increase in pressure, as the lungs fill, causes the
gate to close. The greater the pressure, the
quicker the gate closes. As the gate approaches
the end of the venturi, it reduces the size of the
delivery port thus allowing less gas to flow to the
patient (Fig. 5). Toward the end of inspiration, this
“pneumatic clutch” achieves a more laminar gas
flow to the lungs.
ip
NOILDAS
SINALSAS
ILVIANINA