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Chapter 1: What is IAP? 5
abdomen behaved as a hydraulic system and the pressures
within were hydrostatic in nature.
These findings were later challenged when it was found that
IAP measured at four different sites in the abdomen were not
homogeneous and that this pressure difference disappeared
when the abdomen was filled with 2 litres of normal saline.
Subsequent studies concluded that there were three factors
affecting IAP: gravity, uniform compression and shear
deformation. Uniform compression, such as abdominal
contraction, diaphragmatic contraction, mechanical
ventilation, rib cage excursions and abdominal binding, result
in spatially homogeneous changes in pressure that can be
superimposed on the gravitational gradients.
Figure 1.2 presents the relationship between IAV, C-abd and
IAP. The direction of the movement associated with the sole
Diaphragm
C-abd = ∞
action
IAV
Figure 1.2
Rib cage action
C-abd = 0
Abdominal
contraction
IAP
6 Section 1: Understanding IAH: what to worry about?
action of the rib cage inspiratory muscles, abdominal expiratory
muscles and the diaphragm are shown.
The abdominal wall
The abdomen can be considered as a closed box with some
parts that are rigid (spine, pelvis and costal arch) and some
flexible (abdominal wall and diaphragm). The degree of
flexibility of the abdominal wall (affected by multiple factors
such as obesity, tissue oedema and muscle relaxants – all
explained in Chapter 22) and the specific intrinsic weight of the
abdominal contents (owing to solid organs such as the liver, the
presence of fluid such as ascites, and the various part of the
bowel) will determine the pressure at a given point.
The position of the body (prone, supine, Trendelenburg, etc.)
will have an impact on IAP.
The abdominal wall forms the outer margins, extending from
the thoracic cage to the pelvis. It is made of at least seven layers:
the skin, subcutaneous fat, deep fascia, abdominal muscles,
transverse fascia, extraperitoneal fat and the parietal
peritoneum. Most of it is muscle, as shown in Figure 1.3.
Basics of fluid physics – all about pressure
Pressure is the force per unit area, and is expressed in N/m2in
the SI system. Fluid pressure is the pressure at some point
within a fluid, and can occur in an open or closed environment.
Pressure in open conditions can usually be approximated as
the pressure in ‘static’ or non-moving conditions. The pressure
Chapter 1: What is IAP? 7
Rectus sheath
Rectus abdominis
Linea alba
Aponeurosis of transverse
abdominal muscle
Parietal peritoneum
Transversalis fascia
Transverse abdominal muscle
Internal oblique
External oblique
Subcutaneous tissue
Aponeurosis of external oblique
Aponeurosis of internal oblique
Skin
Figure 1.3
at any given point of a non-moving (static) fluid is called the
hydrostatic pressure and conforms to the principles of fluid
statics described by Blaise Pascal (1623–1662). Another name
for the unit of pressure is the Pascal (Pa): 1 Pa is 1 N/m
2
.
Pascal’s Law, or the principle of transmission of
fluid-pressure, states: ‘the pressure exerted anywhere in a
confined incompressible fluid is transmitted equally in
all directions throughout the fluid such that the pressure ratio
(initial difference) remains the same’.
This means that pressure will remain the same even if
additional pressure is applied on the fluid at some point. The
best example of this is shown in Figure 1.4, illustrating the force
exerted by a piston.
Therefore Pascal’s Law can be interpreted as stating that any
change in pressure applied at any given point of the fluid is
transmitted undiminished throughout, as shown in Figure 1.4.
The difference of pressure between two points at different
heights (h1 and h2) is given by the formula shown in Figure 1.5.
The intuitive explanation of this formula is that the change in
8 Section 1: Understanding IAH: what to worry about?
Force 1
F1
Area 1
A1
P = F1/A1 =F2/A2
so F2 is A2/A1 time greater than F1
Force 2
F2
Area 2
A2
Figure 1.4
The sea level acceleration
Pressure
due to earth’s gravity
ΔP = ρ g (Δh)
Pascal’s law
Density of the fluid
Figure 1.5
pressure between two elevations is caused by the weight of the
fluid between the elevations. This means that in different body
positions the hydrostatic pressure, force of gravity, is responsible
for the change in pressure measured in those positions.
The IAV will exert a certain IAP on the compartment walls
that will be mainly determined by C-abd. The abdominal
Elevations
Chapter 1: What is IAP? 9
contents are primarily fluid in character. Closed bodies of fluid
are either ‘static’, when the fluid is not moving, or ‘dynamic’,
when the fluid is moving. The pressure in closed conditions
conforms to the principles of fluid statics by Blaise Pascal.
The abdomen at times behaves as a hydraulic system when
the viscera are not subjected to shearing forces. Shearing forces
are the strain in the structure of a substance, with layers
laterally shifting in relation to each other, and this occurs in the
abdomen. This is dependent on the shape and stability of the
tissues and the degree of deformation. It is associated with
spatially diverse pressure gradients. It is the relative importance
of these individual factors that will ultimately determine if the
abdomen behaves as a liquid-filled container.
To keep it simple, we assume that the impact of shear
deformation on the measurement of IAP is probably not
significant in the fully sedated mechanically ventilated patient
with sepsis, capillary leak and a positive fluid balance, with or
without neuromuscular blocking agents. It might not be the
same in the athlete swimming through a cold swimming pool.
The relation between IAP and IAV – compliance and elastance
The relationship between pressure and volume can be
expressed by the analysis of pressure–volume (P/V) curves. The
same is done in respiratory physiology. The relation between
IAP and IAV is the abdominal compliance (C-abd) and is
calculated by the change in volume over the change in pressure
(Figure 1.6).
10 Section 1: Understanding IAH: what to worry about?
Abdominal
compliance
Figure 1.6
60
50
40
30
20
IAP (mmHg)
10
0
0
Figure 1.7
Difference in
intra-abdominal volume
C-abd = ΔIAV/ΔIAP
Difference in
intra-abdominal pressure
Patient with
low abdominal compliance
high abdominal compliance
246810
Abdominal Volume (L)
Patient with
The relation between IAV and IAP is curvilinear, with an
initial linear part followed by an exponential increase once a
critical volume is reached, as shown in Figure 1.7.
The linear part of the abdominal pressure–volume
relationship is the elastance (E-abd; Figure 1.8). In humans it
was found that body weight, body mass index (BMI) and the use
Chapter 1: What is IAP? 11
Abdominal
elastance
Abdominal compliance
E-abd = 1/C-abd
Figure 1.8
of pharmacological muscle relaxation influences the baseline
pressure (called Pv
abdominal surgery affects the elastance (or the slope of the
initial portion of the abdominal P/V loop). This initial part of the
curvilinear relationship between IAV and IAP has been studied
in patients undergoing laparoscopic surgery and the elastance
measured at 3 mmHg/1000 mL when Pv
around 5 mmHg.
It has been shown that the higher the initial IAP the greater
the variation in IAP will be for the same added volume or
pressure, as shown in Figure 1.9.
It has been known for a long time that effects produced in the
thorax by respiration are inverse to those present in the
abdomen, as shown in Figure 1.10.
The C-abd can be estimated by looking at the changes in IAP
during mechanical ventilation: a low C-abd is characterized by
large respiratory swings and this could help to identify patients
at risk of the detrimental effects associated with elevated IAP.
The observed respiratory variations are dependent on the
respiratory setting and the tidal volume excursions.
) whereas age, pregnancy and previous
0
was measured at
0
12 Section 1: Understanding IAH: what to worry about?
20
15
10
5
0
Figure 1.9
20
15
10
All Patients
IAH
Relation between baseline IAP and Δ IAP
Intra-abdominal pressure
at end of inspiration
IAPei
IAP
Δ IAP
Normal IAP
Variation in IAP
ΔIAP
5
0
Figure 1.10
IAPee
Intra-abdominal pressure
at end of expiration
respiratory cycles during I minute
Chapter 1: What is IAP? 13
Key points
The abdomen is a closed anatomical space.
The abdominal contents are primarily fluid in character.
Pascal’s Law states that any change in pressure applied at any
given point of the fluid is transmitted undiminished
throughout the fluid. This means that IAP can be measured
by way of different (in)direct routes.
IAV will exert a certain IAP on the compartment walls that will
be mainly determined by C-abd.
The relationship between IAV and IAP is curvilinear, with
an initial linear part followed by an exponential increase once
a critical volume is reached.
FURTHER READING
De Keulenaer BL, De Waele JJ, Powell B, Malbrain ML. What is
normal intra-abdominal pressure and how is it affected by
positioning, body mass and positive end-expiratory
pressure? Intensive Care Medicine 2009; 35(6): 969–76.
Emerson H. Intra-abdominal pressures. Archives of Internal
Medicine 1911; 7: 754–84.
van Ramshorst GH, Salih M, Hop WC et al. Noninvasive
assessment of intra-abdominal pressure by measurement of
abdominal wall tension. The Journal of Surgical Research
2011; 171(1): 240–4.
Chapter 2
Definitions
Introduction
For any pathophysiological entity, a common nomenclature
and definitions are essential for effective clinical
communication and appraisal of the scientific literature.
Consensus definitions have been proposed for
intra-abdominal hypertension (IAH) and abdominal
compartment syndrome (ACS). These definitions are now
widely accepted around the world.
Use of these definitions will further improve communication
and future research in this area. They will need to be revised
when new evidence emerges.
Background
IAH and ACS have been increasingly recognized as causes of
significant morbidity and mortality over the past decade.
Initially, there was little agreement regarding the definition of
IAP, IAH and ACS. Comparing the results of clinical trials was
difficult.
The World Society of the Abdominal Compartment
Syndrome (WSACS) developed a set of consensus definitions