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Chapter 4: Systems available to measure IAP 45
60 mL Syringe
To Monito
Normal
nasogastric
tube
3-way stopcock
Pressure
Transducer
Figure 4.6
A pressure transducer is connected to the lumen of the
nasogastric tube, and, after instillation of 100–200 mL of
saline into the stomach and zeroing the pressure transducer, the
IAP can be read from the monitor. The stomach should
be completely empty (including air) before this method is used.
Transgastric: gastric balloon method
An oesophageal balloon catheter is inserted into the oesophagus
and connected to a pressure transducer. Using a syringe, 1 mL
of air is introduced into the balloon and, after zeroing to
atmospheric pressure, the IAP can be read from the monitor.
The position of the balloon can be checked using an oscillation
test or by observing the pressure readings from the catheter.
The air from the balloon is reabsorbed after a few hours
so recalibration is necessary. This is illustrated in the lower
panel of Figure 4.7. The reabsorption of air is recorded as a loss
of IAP signal.
r
46 Section 1: Understanding IAH: what to worry about?
120
m
m
H
g
0
30
m
m
H
g
0
Figure 4.7
APP: Abdominal Perfusion Pressure
−20 −16 −12 −8 −4
IAP: Intra-abdominal Pressure
10:00
14:00
18:00
22:00
02:00
0 h
06:00
Figure 4.8
Figure 4.8 shows an IMD Allegiance balloon-tipped catheter;
Figure 4.9 the Ackrad balloon-tipped catheter; Figure 4.10 the
Datex tonometer catheter; and Figure 4.11 the NutriVent
™
catheter with two balloons.
Figure 4.9
Chapter 4: Systems available to measure IAP 47
Figure 4.10
Continuous IAP measurement
Continuous transvesicular IAP measurement
IAP can be measured continuously using a 3-way Foley
catheter, with one lumen connected to a pressure transducer
48 Section 1: Understanding IAH: what to worry about?
Figure 4.11
Figure 4.12
(see Figure 4.12). A continuous flow of saline (e.g. from an
infusion pump) is required at a low speed (2–4 mL/h) in the
dedicated lumen for reliable IAP measurement.
The issue of draining urine and measuring IAP at the same
time needs to be solved; this is analogous to the brain where
Chapter 4: Systems available to measure IAP 49
one cannot measure the intracranial pressure (ICP) and drain
the cerebrospinal fluid at the same time.
Once a urine column is formed from the bladder to the
collection bag, a negative suction force can be created, causing
underestimation of the real IAP. Air bubbles on the other hand
can cause an overestimation of the real IAP.
Continuous IAP monitoring – CiMON (Pulsion Medical Systems)
CiMON provides continuous IAP monitoring via a special
nasogastric probe combining feeding, decompression and
IAP measurement function (Figure 4.13). The IAP is measured
via a small air-filled balloon located at the distal tip of the
CiMON probe.
The catheter is connected to a specificmonitorthat
allows continuous IAP and APP measurements and
monitoring. In the future, new probes are likely to be
available in the form of nasojejunal 3-lumen postpyloric
feeding tubes or even 4-lumen probes equipped with an
oesophageal and gastric balloon for transdiaphragmatic
pressure measurement.
Details about the CiMON device can be found at http://
www.pulsion.com.
The IAP-Catheter and IAP-Monitor (Spiegelberg)
The IAP-Catheter is introduced like a nasogastric tube. It is
equipped with an air-pouch at the tip. The catheter has one
50 Section 1: Understanding IAH: what to worry about?
Figure 4.13
lumen that connects the air-pouch with an IAP-Monitor and
one lumen that takes the guidewire.
Details about the IAP-Catheter can be found at http://www.
spiegelberg.de/products.
Key points
IAP measurement techniques are safe, reproducible and
accurate.
IAP measurement techniques have greatly improved over
the years, with multiple assembled kits commercially
available.
Chapter 4: Systems available to measure IAP 51
Continuous IAP measurement is simple and offers added
values in specific situations.
FURTHER READING
Balogh Z, De Waele JJ, Malbrain ML. Continuous
intra-abdominal pressure monitoring. Acta Clinica Belgica
Supplement 2007; 62(1): 26–32.
De Waele JJ, De Laet I, Malbrain ML. Rational
intraabdominal pressure monitoring: how to do it? Acta
Clinica Belgica 2007; 62(1): 16–25.
Chapter 5
Pitfalls of IAP monitoring
Introduction
Accurate and reproducible IAP measurement is essential in
the management of patients with IAH and ACS. IAH is defined
as an IAP between 12 and 20 mmHg. It is therefore important
to identify an accurate value to define a potential problem.
Trends in IAP may be more important in certain conditions
and therefore repeated reliable IAP measurements are essential.
Clinicians involved in the care of the patient with IAH
should be aware of pitfalls in the measurement of IAP. Most can
easily be avoided if guidelines and protocols are properly set.
The pitfalls
Pitfalls in IAP measurement can be related to the patient, the
measurement technique or the interpretation of the value
obtained.
Pitfalls related to the patient
Positioning of the patient
The patient should be put in the supine position for IAP
measurement. Measuring IAP with the head of the bed elevated
Chapter 5: Pitfalls of IAP monitoring 53
leads to higher values, and measurements in one position
should not be compared with the values obtained in another.
In some patients, it may be contraindicated to change the
body position and it is accepted that IAP can be measured
with the head of the bed elevated as long as the classical
thresholds for IAH and ACS are not applied.
The awake patient
In awake patients, spontaneous breathing and abdominal
muscle contractions (notably after abdominal surgical
procedures) can influence the IAP value. This can lead to falsely
elevated values in case of forced expiration or increased
abdominal muscle tension.
Deep inspiration changes the classical IAP curve observed in
mechanically ventilated patients, and may confuse the observer
and the monitor software.
Intra-abdominal space-occupying lesions
The presence of mass in the abdomen or pelvis (e.g.
haematomas in patients with pelvic fractures or spontaneous
haemorrhages in the rectus sheath or muscles) may compress
the bladder and lead to falsely elevated IAP values.
Obesity
Patients with BMI higher than normal have elevated IAP. Other
thresholds for IAH and ACS may be applicable.
54 Section 1: Understanding IAH: what to worry about?
Children
Small children tend to have lower blood pressure values. IAP
values will also be lower. Other thresholds for IAH and ACS may
be applicable.
Pitfalls related to the measurement technique
Zero reference level
Most of the techniques to monitor IAP use fluid-filled
systems for pressure transduction. Correct calibration of the
transducer is of paramount importance for reliable IAP
measurement. It is important to use the correct zero reference
level, as described in Chapter 2. It is advisable to mark this level
on the patient, and the transducer should be zeroed before
each IAP measurement.
Gastric route
When measuring IAP by the oesogastric route, it is important to
consider rhythmic changes caused by peristaltic waves. Gastric
dilatation can also affect readings.
Intrathoracic positioning of the balloon can be excluded from
the analysis of the waveform: the pressure elevation during
inspiration is mostly dampened when correctly positioned; and
the oscillation test – which consists of gently tapping on the
abdomen and should result in an oscillating curve on the
monitor – may be helpful. Gastric pressure IAP tracings often
include cardiac rhythm artefacts.