H Odenstedt

O Stenqvist

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Alveolar Pressure/volume Curves Reflect Regional
Lung Mechanics
O. Stenqvist and H. Odenstedt
Introduction
The static pressure volume (P/V) curve has been regarded as the gold standard tool
for assessment of the mechanical properties of the lung. On this curve, a lower
inflection point (LIP) can be detected in some patients and in most patients an
upper inflection point (UIP) can be seen. The most common interpretation of the
LIP and the UIP is that LIP represents the point where alveoli collapse at the end of
expiration and reopen at the start of inspiration and that the UIP represents the
pressure above which alveoli become overdistended. It has been proposed that in
order to avoid cyclic closing and opening and overdistension of alveoli, ventilation
should be performed with pressures between the LIP and UIP, where the compliance
of the lungs is highest.
The Lower Inflection Point
The background of LIP is complex and various hypotheses have been proposed. Gat-
tinoni and co-workers have proposed that the weight of the edematous acute lung
injury/acute respiratory distress syndrome (ALI/ARDS) lung results in a superim-
posed pressure, increasing vertically, causing a collapse of the most dorsal lung parts
[1, 2]. The LIP should, according to this proposal, be the pressure that is high
enough to counteract the threshold opening pressure and the superimposed pres-
sure. This hypothesis has been further analyzed in a mathematical model by Hick-
ling, who describes a continuous recruitment process during inflation [3]. Hubmayr
has argued against this interpretation and favors a hypothesis where the LIP is
caused by a gas/fluid interface in flooded lung parts [4].
The Volume below the Lower Inflection Point
In the literature on lung mechanics, the focus has been on the pressure level of the
LIP, but very little is mentioned about the volume where the LIP is positioned. Clini-
cal data are scarce, but from published P/V curves the volume can be estimated to
lie between 50 and 150 ml [5–7]. Compliance below the LIP can be estimated to be
5–20 ml/cmH2O based on these curves. It is important in this context to realize that
compliance is closely related to the size of the lung. Thus, if you apply pressure con-
trol ventilation to a mouse or an elephant with a pressure that in a human results in
normocapnia, you will have normocapnia in both these animals, as the compliance
Fig. 1. Regional compliance (Creg), superimposed pressure (SP), and pressure/volume (P/V) curve in a
healthy lung. No lower inflection point (LIP) is detected and a decrease in compliance is noted as inflation
reaches total lung capacity.
of the elephant lung is enormous and that of the mouse is very low. Gattinoni and
Pesenti have has promoted the baby lung concept, where the ALI/ARDS patient is
supposed to have a part of the lung collapsed and the rest – a small but supposedly
healthy lung – a ‘baby lung’ that is quite normal [8]. This dichotomic view of the ALI/
ARDS lung may be questioned as, most likely, the open parts of the lung are also to
some extent affected by a lowered compliance as a result. When a P/V curve of such
a lung is obtained, the compliance below the LIP will be low, representing compliance
of the baby lung. The compliance of this baby lung will be dependent on its size, the
smaller the baby lung, the lower the compliance. As the inflation continues and more
alveoli are recruited, compliance will increase until alveoli at the very bottom of the
lung with very low compliance are recruited. The reason for elaborating on this point
is that the relationship between the size of the lung and compliance is fundamental
for understanding the P/V curve. If a healthy lung (Fig. 1) is divided into, for exam-
ple, five horizontal planes with alveoli of the same end expiratory size and compli-
ance is measured for each of these parts of the lung, it would be a fifth of the total
compliance. If an ALI/ARDS lung (Fig. 2) with a lowered total compliance of 30 ml/
cmH2O is divided in the same way, the most ventral part would have the highest
compliance and the most dependent part the lowest compliance because of the super-
imposed pressure from the edematous tissue. The compliance of these five parts
would, when added, result in a total compliance of 30 ml/cmH2O and could, if the
superimposed pressure is increasing linearly, be 10, 8, 6, 4 and 2 ml/cmH2O from top
to bottom of the lung. If the three most dependent compartments of the lung are col-
lapsed (Fig. 3), the total compliance would only be 10+8 = 18 ml/cmH2O.
Overdistension Versus Gas Compression
In ALI/ARDS, increased resistance is not a major factor, so instead of gas moving
along the path of least resistance, we see the gas moving along the path of highest
408 O. Stenqvist and H. Odenstedt
Fig. 2. Regional compliance (Creg), superimposed pressure (SP), and pressure/volume (P/V) curve in a lung
with acute respiratory failure without collapsed alveoli. SP increases along the vertical axis. Functional
residual capacity (FRC) is decreased and so is total lung capacity. The P/V curve shows no lower inflection
point (LIP), but compliance is decreased and an upper inflection zone present, indicating that ventral lung
regions are fully stretched (regional compliance ' zero) and recruitment of low compliant, dorsal alveoli at
end of inspiration still on-going.
compliance. Following the path of highest compliance in an ALI lung without col-
lapse, as in Figure 2, when inflation starts, the initial gas will naturally flow towards
the non-dependent lung with highest compliance. Continuously during the inflation,
when pressure increases, the alveoli of the most non-dependent lung will be
expanded until not yielding any more (regional compliance = zero). Already before
that, as pressure rises, gas will flow to more dorsal parts of the lung where compli-
ance is low but, at this time point, higher than in the most ventral parts of the lung.
The P/V curve of this lung will show a continuously decreasing compliance as infla-
tion proceeds. When the pressure is high enough to inflate the most dorsal parts of
the lung, the alveoli of the most ventral parts are already stretched to their struc-
tural limits and will not expand any further, i.e., compliance is decreasing towards
zero. This will result in the final part of the P/V curve deflecting as a sign of low
compliance in the dependent part of the lung rather than a sign of overdistension of
the ventral part of the lung. The term overdistension is thus misleading as the
deflection of the P/V curve indicates that pressure rises more than volume, i.e., gas
is compressed.
Let us consider the behavior of another example of a five-compartment lung,
where the three most dependent compartments are collapsed during the start of an
inflation (Fig. 3). In this case the initial compliance will be 18 ml/cmH2O and when
the airway pressure is high enough to overcome the superimposed pressure and the
threshold opening pressure of the mid compartment alveoli (the most non-depen-
dent compartment of the three collapsed compartments), compliance will increase
by 8 ml/cmH2O, which is the compliance of that very compartment. In contrast to
the situation where all compartments are open already from the start of inspiration,
this sudden increase in compliance will result in a LIP of the P/V curve. As the air-
way pressure increases enough to open or recruit the two most dependent compart-
ments, compliance will further increase by 6 and 4 ml/cmH2O. When inflation pro-
Alveolar Pressure/volume Curves Reflect Regional Lung Mechanics 409
Fig. 3. Regional compliance (Creg), superimposed pressure (SP), and regional and global pressure/volume
(P/V) curves in a lung with ALI/ARDS with collapsed alveoli. SP increases along the vertical axis. Functional
residual capacity (FRC) is decreased and so is total lung capacity. The P/V curve shows a lower inflection
point (LIP) as ventral, open parts represent a small lung volume and thus have low compliance, which
increases when more dorsal parts of the lung are recruited. An upper inflection point (UIP) is present, indi-
cating recruitment of low compliant, dorsal alveoli at end-inspiration when ventral lung regions are already
stretched to their structural limits (compliance ' zero).
ceeds, the P/V curve will show a continuous decrease in compliance, as compliance
in the most ventral parts of the lung decreases towards zero (as in the previous
example, where all five compartments were open from the start of inflation). Com-
pliance of the last part of the P/V curve will reflect the compliance of the dependent
parts of the lung.
Volume Dependent Compliance
The changing of compliance along the P/V curve, i.e., volume-dependent compli-
ance, reflects that different parts of the lung have different properties. Normally the
regional differences are arranged along a vertical axis, so that the highest compli-
ance of the P/V curve represents the most ventral parts of the lung and the lowest
compliance the most dependent, dorsal part of the lung (Figs. 4, 5) [9]. However, in
some cases the regional differences in mechanical properties of the lung occur more
randomly. In any case, whether regional differences in compliance are arranged ver-
tically or randomly, the volume-dependent compliance of the P/V curve is a mea-
surement of these differences.
410 O. Stenqvist and H. Odenstedt
Fig. 4. Electric impedance tomogra-
phy tracings from left lung of a
patient with pneumonia. Relative
regional tidal volume on the Y-axis
and airway pressure on the X-axis.
The pressure/volume (P/V) curves
are obtained during a low flow
inflation, which gives time for par-
tial equilibration of visco-elastic
forces. Note that the lower inflec-
tion point (LIP) is positioned at a
much higher pressure in the dorsal
parts of the lung. No gas enters the
mid region until a pressure of
' 6 cmH2O is reached and aeration/
ventilation of the most dependent region requires a pressure of about 15 cmH2O. When compliance of the
ventral lung regions are close to zero (P/V curve parallel to x-axis) compliance in the most dorsal lung
regions still increases. Modified from [9] with permission
Fig. 5. Impedance changes (corresponding to lung volume changes) in the ventral, mid and dorsal region
of the lung, in a pig during one tidal breath, before (a) and after saline lavage (b). In the healthy lung (a),
gas enters all three regions from the start of inspiration. Compliance is equal in the ventral and dorsal
regions, and higher in the mid region. Saline lavage (b) changes regional compliance and, thereby, the rel-
ative tidal volume distribution. At the start of the breath, gas will enter the ventral region with the highest
compliance, now representing a proportionally larger part of the open lung. The mid region initially fills
slowly due to lower compliance and the start of the inflation of the dorsal region is markedly delayed.
So far, all references to P/V curves are static P/V curves that are rarely used in clini-
cal practice. The most common lung mechanics monitoring modality is the pressure
volume loop based on pressure and flow measured in the ventilator or at the
Y-piece. These dynamic loops are to a high degree influenced by the endotracheal
tube resistance, which distorts the loop, resulting in a right shift of the inspiratory
limb and a left shift of the expiratory limb of the loop. A loop that represents the
lung mechanics more closely can be obtained by plotting the tracheal pressure ver-
sus volume instead. The tracheal pressure can either be calculated from the y-piece
airway pressure and an algorithm for the endotracheal tube resistance [10] or mea-
sured directly by insertion of a narrow pressure line through the tube [11]. From the
tracheal pressure loop an alveolar P/V curve can be obtained by multiple linear
regression analysis of the loop, which is divided into six slices, where compliance is
assumed to be constant within each slice, for volume dependent compliance calcula-
tions [12]. We have used another approach for obtaining an alveolar P/V curve from
direct tracheal pressure measurements: The Dynostatic algorithm. This algorithm is
Alveolar Pressure/volume Curves Reflect Regional Lung Mechanics 411
based on the assumption that inspiratory and expiratory resistance of the airway is
reasonably similar at the same lung volume during inspiration and expiration [13,
14].
Fast and Slow Lung Mechanics
The most prominent feature of the dynamic alveolar P/V curve, which represents the
mechanic properties of the lung during on-going therapeutic ventilation, is that
compliance is lower than compliance of the static P/V curve of the same patient.
Also, if any LIP is present in the dynamic P/V curve it is usually not very prominent
and the UIP is not an inflection point but rather a zone of decreasing compliance.
The reason for this difference in behavior of the lung during static and dynamic
conditions can probably be explained by the time factor playing a more important
role than expected. As seen in Figure 6, during a low flow inflation in an ALI patient
a prominent LIP at 8 cmH2O is seen followed by a high and constant compliance
throughout the inflation up to 1100 ml. In contrast, during tidal breathing in the
same patient where inspiration starts from a pressure level of 4 cmH2O no LIP can
be detected and there is a tendency for an upper inflection zone in the P/V curve in
spite of the volume being only half of that during the low flow inflation [15]. This is
explained by the fact that during tidal breathing there is not sufficient time for
equilibration of visco-elastic forces of the lung.
There are several studies indicating that low tidal volume ventilation causes less
damage to the alveoli than a high tidal volume [16, 17]. The level of positive end-
expiratory pressure (PEEP) seems to have less impact on ventilator-induced lung
injury (VILI) as long as it is not set at very low levels or at 0 cmH2O. However, the
UIP of the static or the dynamic P/V curve does not represent overdistension, but
rather recruitment of the most dorsal lung compartments with the lowest compli-
Fig. 6. Total respiratory system alveolar pressure/volume (P/V) curves obtained by the Dynostatic algo-
rithm, in a patient with acute lung injury (ALI). During a low flow inflation (left panel) a prominent lower
inflection point (LIP) is seen at ' 8 cmH2O followed by a high constant compliance without an apparent
upper inflection point (UIP) even though the total volume inflated reaches 1100 ml. During tidal breathing
(right panel) with inspiration starting from a pressure level of 4 cmH2O, no LIP is seen but there is a ten-
dency to an upper inflection zone in spite of the inflated volume being only half of the low flow inflation
volume. The calculated compliance of the tidal breath at the end of inspiration, between 11 and 14 cmH2O
is 52 ml/cmH2O but at the same pressure range during low flow inflation, it is as high as 85 ml/cm H2O.
Modified from [15] with permission
412 O. Stenqvist and H. Odenstedt
ance, during the last part of the inspiration. These alveoli are the same alveoli that
collapse early at the beginning of the expiration. During static measurements this
occurs at a higher lung volume than during dynamic conditions. This indicates that
in patients it is important to monitor the lung mechanics during prevailing condi-
tions to be able to set the ventilator optimally.
Conclusion
The compliance below the LIP, when present, is usually very low indicating that only
a small lung volume is open when inspiration starts. The LIP of a dynamic P/V
curve is usually not very prominent when compared to the LIP of a static P/V curve.
There is probably no LIP without partial lung collapse. The UIP is not a sign of
overdistension but rather a sign of low compliant, dorsal lung parts being recruited
at the end of the inspiration, when the most compliant, ventral parts of the lung do
not expand any further.
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414 O. Stenqvist and H. Odenstedt


Alveolar Pressure/volume Curves Reflect Regional Lung Mechanics

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Alveolar Pressure/volume Curves Reflect Regional Lung Mechanics

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