NORTH AMERICAN DRÄGER Ventilator User manual
Curves and Loops
in Mechanical Ventilation
Frank Rittner
Martin Döring
Curves and Loops
in Mechanical Ventilation
Frank Rittner
Martin Döring
5
Contents
Ventilation curve pattern
■ Pressure-time diagram 6
■ Flow-time diagram 10
■ Volume-time diagram 12
■ Interpretation of curve patterns 14
Loops – a good thing all round
■ PV loops 21
■ The static PV loop 21
■ The dynamic PV loop in ventilation 23
■ Interpretation of PV loops in ventilation 26
■ PV loops before and after the tube 34
■ Loops – other possibilities 38
■ Flow-volume loop 38
Trends reviewed
■ Documentation of a weaning process 41
■ Lung parameters based on peak and
plateau pressure
43
Capnography – locating problem areas
■ The physiological capnogram 46
■ Interpretations of capnograms 47
6
Ventilation curve pattern
All the ventilators of the Evita family offer graphic
representation of the gradual changes in ventilation
pressure and breathing gas flow. Evita 4, EvitaScreen
and the PC software EvitaView additionally show the
gradual changes in the breathing volume. Two or in
some monitors three curves can be shown on the
screen at the same time, and particularly the fact
that pressure, flow and volume can be displayed
simultaneously makes it easier to detect changes
caused by the system or the lungs. The gradual
change in pressure, flow and volume depend to an
equal extent on the properties and settings of the
ventilator, as well as on the respiratory properties of
the lung.
One respiratory cycle comprises an inspiratory and
an expiratory phase. Under normal conditions these
two periods contain a flow phase and a no flow pause
phase. No volume passes into the lung during the no
flow phase during inspiration.
Pressure-time diagram
Volume-controlled, constant flow
The pressure-time diagram shows the gradual
changes in the airway pressure. Pressure is given in
mbar (or in cmH
2
O,) and time in seconds.
At a preset volume (volume-controlled ventilation)
and constant flow the airway pressure depends on the
alveolar pressure and the total of all airway
resistances, and can be affected by resistance and
compliance values specific to the ventilator and the
lung. As the ventilator values are constant, the
pressure-time diagram allows conclusions to be drawn
about the status of the lung and changes to it.
The gradual changes in
pressure, flow and volume
depend to an equal extent
on the properties and
settings of the ventilator, as
well as on the respiratory
properties of the lung.
Ventilation curve pattern 7
Resistance = airway resistance
Compliance = compliance of the entire system
(lungs, hoses etc.)
At the beginning of inspiration the pressure between
points A and B increases dramatically on account of
the resistances in the system. The level of the pressure
at break point B is equivalent to the product of
resistance R and flow (
*).
∆p = R ∗*
This relationship, as well as the following examples, is
only valid if there is no intrinsic PEEP. The higher the
selected Flow * or overall resistance R, the greater
the pressure rise up to point B. Reduced inspiratory
flow and low resistance values lead to a low pressure
at point B.
Pressure
(mbar)
Time (s)
Resistance
Pressure
Plateau pressure
Pausephase
Gradient
Peak pressure
"Resistance
pressure"
"Compliance
pressure"
(V
T
/C)
Inspiration time
"PEEP"
A
Flowphase
B
C
D
E
F
Expiration time
const.
Pressure-time diagram for
volume controlled constant
flow ventilation.
Ventilation curve pattern8
After point B the pressure increases in a straight line,
until the peak pressure at point C is reached. The
gradient of the pressure curve is dependent on the
inspiratory flow * and the overall compliance C.
∆p/∆t = * / C
At point C the ventilator applies the set tidal volume
and no further flow is delivered (* = 0).
As a result, pressure p quickly falls to plateau
pressure. This drop in pressure is equivalent to the
rise in pressure caused by the resistance at the
beginning of inspiration. The base line between points
A and D runs parallel to the line B - C.
Further on there may be a slight decrease in
pressure (points D to E). Lung recruitment and leaks
in the system are possible reasons for this. The level of
the plateau pressure is determined by the compliance
and the tidal volume. The difference between plateau
pressure (E) and end-expiratory pressure F (PEEP) is
obtained by dividing the delivered volume V
T
(tidal
volume) by compliance C.
∆P = P
plat
- PEEP
By reversing this equation the effective compliance
can easily be calculated.
C = V
T
/∆p
The level of the plateau
pressure is determined by
the compliance and the tidal
volume.
Ventilation curve patterns 9
During the plateau time no volume is supplied to the
lung, and inspiratory flow is zero. As already
mentioned, there is a displacement of volume on
account of different time constants, and this results in
pressure compensation between different
compartments of the lung.
Expiration begins at point E. Expiration is a passive
process, whereby the elastic recoil forces of the thorax
force the air against atmospheric pressure out of the
lung. The change in pressure is obtained by
multiplying exhalation resistance R of the ventilator by
expiratory flow *
exp
.
∆p = R ∗*
exp.
Once expiration is completely finished, pressure once
again reaches the end-expiratory level F (PEEP).
Pressure-oriented
In pressure-oriented ventilation (e.g. PCV/BIPAP) the
pressure curve is quite different.
PCV
T
insp
T
exp.
P
insp.
PEEP
BIPAP
Pressure-time diagramm
for pressure controlled
ventilation.
Ventilation curve pattern10
Pressure increases rapidly from the lower pressure
level (ambient pressure or PEEP) until it reaches the
upper pressure value P
Insp.
and then remains constant
for the inspiration time T
insp.
set on the ventilator.
The drop in pressure during the expiratory phase
follows the same curve as in volume-oriented
ventilation, as expiration is under normal conditions a
passive process, as mentioned above. Until the next
breath pressure remains at the lower pressure level
PEEP.
As pressure is preset and regulated in the case of
pressure-oriented ventilation modes such as BIPAP,
pressure-time diagrams show either no changes, or
changes which are hard to detect, as a consequence of
changes in resistance and compliance of the entire
system.
As a general rule it can be said that the pressure
curves displayed reflect the development of pressure
measured in the ventilator. Real pressures in the lung
can only be calculated and assessed if all influential
factors are taken into account.
Flow-time diagram
The flow-time diagram shows the gradual changes in
the inspiratory and expiratory flows *
insp
and *
exsp
respectively. Flow is given in L/min and time in
seconds. The transferred volume is calculated as the
integration of the flow * over time, and is thus
equivalent to the area underneath the flow curve.
During inspiration the course of the flow curve is
dependent on or at least strongly influenced by the
ventilation mode set on the ventilator. Only the
course of the flow in the expiratory phase permits
conclusions to be drawn as to overall resistance and
compliance of the lung and the system.
The course of the flow in the
expiratory phase permits
conclusions to be drawn as
to overall resistance and
compliance of the lung and
the system.
Ventilation curve pattern 11
In normal clinical practice constant flow and
decelerating flow have become established as the
standard forms for ventilator control.
As yet there has been no evidence to suggest that
particular therapeutic success could be achieved
using other flow forms.
In the case of constant flow the volume flow rate
during inspiration remains constant throughout the
entire flow phase. When inspiration starts the flow
value very quickly rises to the value set on the
ventilator and then remains constant until the tidal
volume V
T
, likewise set on the ventilator, has been
delivered (this is the square area under the curve.) At
the beginning of the pause time (plateau time) the
flow rapidly returns to zero. At the end of the pause
time expiratory flow begins, the course of which
depends only on resistances in the ventilation system
and on parameters of the lung and airways. Constant
flow is a typical feature of a classic volume-oriented
mode of ventilation.
Flow
Time
T
plat
T
insp
Flow
Time
decelerating Flowconstant Flow
Flow-time diagram
Ventilation curve pattern12
In decelerating flow the flow falls constantly after
having reached an initially high value. Under normal
conditions the flow returns to zero during the course
of inspiration. Decelerating flow is a typical feature of
a pressure-oriented ventilation mode.
The difference in pressure between the pressure in
the lung (alveoli) and the pressure in the breathing
system, maintained by the ventilator at a constant
level, provides the driving force for the flow.
As the filling volume in the lung increases the
pressure in the lung also rises. In other words,
the pressure difference and thus the flow drop
continuously during inspiration. At the end of
inspiration the pressure in the lung is equal to the
pressure in the breathing system, so there is no
further flow.
If at the end of inspiration and at the end of
expiration flow =0, compliance can also be calculated
in a pressure-oriented ventilation mode using the V
T
measured by the ventilator.
C = V
T
/ ∆P
where ∆P = P
insp.
- PEEP
Volume-time diagram
The volume-time diagram shows the gradual changes
in the volume transferred during inspiration and
expiration. Volume is usually given in ml and time in
seconds.
During the inspiratory flow phase the volume
increases continuously. During the flow pause
(plateau time) it remains constant as there is no
further volume entering the lung. This maximum
volume value is an index of the transferred tidal
At the end of inspiration the
pressure in the lung is equal
to the pressure in the
breathing system, so there is
no further flow.
Ventilation curve patterns 13
volume and does not represent the entire volume in
the lung. The functional residual capacity (FRC) is
not taken into account. During expiration the
transferred volume decreases as a result of passive
exhalation.
The relationships between pressure, flow and
volume are particularly obvious when these
parameters are all displayed at the same time.
Pressure, flow and volume
diagram of volume-oriented
and pressure-oriented
ventilation
Pressure
Flow
Volume Pressure oriented
Flow-phase
Pausephase
Inspiration Expiration
Flow-phase
phase
Time
Time
Time
Pause-
Pressure
Time
Flow
Time
Volume Volume oriented
Time
Inspiration Expiration
Flow-phase
Pausephase
Pausephase
Flow-phase
Ventilation curve pattern14
Changes in compliance
When compliance changes the plateau and peak
pressures change by the same amount of the pressure
difference ∆p.
increasing compliance → plateau and peak pressures
fall
decreasing compliance → plateau and peak pressures
rise
Paw
Pressure
Time
Interpretation of curve
patterns
Ventilation curve pattern 15
Changes in inspiratory airway resistance
When the inspiratory airway resistance changes the
peak pressure changes and the plateau pressure
remains the same.
increasing resistance → peak pressure rises
decreasing resistance → peak pressure falls
The expiratory lung resistance cannot be seen on
the pressure curve as the alveolar pressure would
need to be known. Conclusions can be drawn however
from the expiratory f low curve (see «Flow curve at
increased expiratory resistances»).
p
peak
Paw
Time
Pressure
The expiratory lung
resistance cannot be seen
on the pressure curve as
the alveolar pressure would
need to be known.
Ventilation curve pattern16
Spontaneous breathing
During a ventilator breath the patient will try to
breathe spontaneously, and will «fight» against the
machine. Reducing the amount of time for inspiration
or, even better, changing to a mode of ventilation
where the patient is allowed to breathe spontaneously
even during a mandatory breath, is an option worth
thinking about. BIPAP or AutoFlow
®
are examples of
suitable modes.
Paw
Pressure
Time
Inspiration Expiration
Ventilation curve pattern 17
Adaptation of the flow curve
In volume-controlled modes of ventilation, AutoFlow
®
results in automatic flow adaptation with the aim of
applying the set tidal volume at the lowest possible
airway pressure. The constant flow typical of volumeoriented ventilation modes (square) becomes at the
same time a decelerating flow form, while tidal
volume remains constant even if the compliance in
the patient’s lung changes.
Pressure limitation at a constant tidal volume can
also be achieved in Dräger ventilators by using the
P
max
setting. If the compliance of the patient changes
this set value may need to be checked and reset.
automatic flow adaption in
pressure-oriented ventilation mode
PCV, BIPAP and in volume-oriented
mode with Autoflow
®
Flow
Time
Flow
Ventilation curve pattern18
The flow curve in the case of insufficient inspiration time
If the flow does not return to zero during inspiration
this means that the inspiration time is insufficient to
apply the volume which could be achieved for the set
pressure.
T
I
Flow does not return
to zero during inspiration
Time
Flow
Flow
Ventilation curve pattern 19
The flow curve in the case of insufficient expiration time
If the flow does not return to zero during expiration,
the expiration time is not sufficient for full expiration.
This indicates the presence of an intrinsic PEEP.
This results in an increase in lung pressure in the
case of volume-controlled ventilation.
In Evita ventilators it is possible to measure
intrinsic PEEP and trapped volume directly. An
intrinsic PEEP can have considerable effects on the
exchange of gases and pulmonary blood circulation.
In some applications, however, there may be
attempts to establish an intrinsic PEEP on purpose
(Inverse Ratio Ventilation IRV), due to the fact that
this will probably then only occur in certain desired
sections of the lung, while a PEEP set on the ventilator
will affect the entire lung.
T
E
Expiration flow does
not return to zero
Flow
Time
Flow
In Evita ventilators it is
possible to measure intrinsic
PEEP and trapped volume
directly.
Ventilation curve pattern20
Flow curve in the case of increased expiratory resistances
A more gentle expiratory flow curve indicates
increased expiratory resistances which may be caused
by expiratory filters which have become damp or
blocked as a result of nebulization. This may lead to a
considerable increase in expiration time and a
deviation from the set PEEP value.
Flow
Time
Flow
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