The pressure exerted on the tracheal wall by two endotracheal tube cuffs: An
observational bench-top, clinical and radiological study
Alex Doyle1, Ramai Santhirapala1, Martin Crowe2, Mark Blunt3 & Peter Young3
1 Critical Care Trainee, 2 Consultant in Radiology, 3 Consultant in Anaesthesia and
Critical Care
Department of Anaesthesia and Critical Care, Queen Elizabeth Hospital, Kings Lynn,
Norfolk, UK, PE30 4ET.
Email addresses:
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
1
Abstract
Background: The LoTrach is a novel endotracheal tube (ETT) tube, which has a unique
low-volume, low-pressure (LVLP) cuff. We aimed to compare the pressure exerted on
the tracheal wall by a conventional ETT cuff and the LVLP cuff in a bench-top study, a
clinical study and a radiological study.
Bench-top study: A model trachea was intubated with a conventional ETT cuff and a
LVLP cuff. We tested the ability of the cuffs to support a column of water using a
standard protocol. There was leakage of water passed the conventional ETT cuff until the
column emptied. In contrast, there was only leakage of water passed the LVLP cuff until
the water column reached a height of 25 cmH2O.
Clinical study: 102 patients were intubated with a conventional ETT cuff or a LVLP cuff.
We tested the ability of the cuffs to prevent air leak during a recruitment manoeuvre. The
mean pressure at which air leak occurred was 32.4 cmH2O (SD 3.0) using the
conventional ETT cuff and 30.0 cmH2O (SD 3.8) using the LVLP cuff.
Radiological study: We examined the CT images of sequential patients who were
intubated with a conventional ETT cuff (n = 7) or a LVLP cuff (n = 6). The degree of
anatomical distortion in the antero-posterior and transverse tracheal diameter was 4.4 cm
(SD 1.4) and 2.6 cm (SD 1.6) for the conventional ETT cuff and 2.9 cm (SD 2.1) and 1.8
cm (1.4) for the LVLP cuff.
Statistical analysis: In the bench top study, it was not possible to test for non-inferiority
because of the persistent leak passed the conventional ETT cuff. In the clinical study, the
2
95% confidence intervals for each cuff were enclosed within the limits of our noninferiority margin and so deemed non-inferior.
Conclusions: The bench-top study and the clinical study demonstrated that the LVLP cuff
exerted approximately 30 cmH2O of pressure on the tracheal wall, which was equivalent
to a conventional ETT cuff. The radiological study revealed that different ETT cuffs
distort the normal tracheal anatomy by varying amounts. This may be important in the
pathogenesis of tracheal injury during mechanical ventilation.
[Word count: 350]
3
Background
It is important to ensure an appropriate endotracheal tube (ETT) cuff pressure for two
opposing reasons. Firstly, a low cuff pressure is desirable in order to minimise any
pressure injury to the trachea, especially during prolonged intubation [1, 2, 3]. This may
result in ischaemia to the tracheal mucosa, distortion of the normal tracheal anatomy and
subsequently tracheal rupture and/or tracheal stenosis. Secondly, a high cuff pressure is
desirable so that an effective airway seal is formed, which reduces both air leak and
pulmonary aspiration [4]. Indeed, the most common cause of ventilator associated
pneumonia is pulmonary aspiration of microorganisms from the oropharyngeal space [5].
Consequently, it has been recommended that the ETT cuff pressure, and therefore the
pressure exerted on the tracheal wall by the cuff, should normally be kept between 20 and
30 cmH2O [1]. This is thought to represent a balance between protecting the trachea from
injury and preventing pulmonary aspiration. However, it would appear that ETT cuffs are
routinely overinflated and therefore excess pressure on the tracheal wall is commonplace
[6]. Previously, studies in the critically ill have reported mean ETT cuff pressures of 35,
43, 62 cmH2O [7, 8, 9]. In a recent study, the ETT cuff pressure was greater than 30
cmH2O in two thirds of patients (range 10 to 180) and greater than 40 cmH2O in half of
the patients investigated [10]. Indeed, even when ETT cuff pressure was accurately
controlled to 25 cmH2O, only 18% of patients’ cuffs remained between 20-30 cmH2O
over the subsequent 8 hour period [6].
4
Endotracheal tubes commonly have high-volume, low-pressure (HVLP) cuffs. These
have been shown to cause less injury to the trachea than traditional low-volume, highpressure (LVHP) cuffs [11]. This may be due to the different inflation profiles of the two
devices. A traditional LVHP cuff has a steep compliance curve. Consequently, when
small aliquots of air are inflated into the cuff, there is a greater increase in cuff pressure,
and so pressure on the tracheal wall, than that seen with a HVLP cuff, which has a
shallow compliance curve. However, even at supra-normal cuff pressure, HVLP cuffs
have been shown to allow pulmonary aspiration [4, 12]. This leakage occurs along
longitudinal folds, which develop in the HVLP cuff itself and produce channels.
Accordingly, there is a need to develop engineered solutions, which reduce the risk of
pressure injury to the trachea and prevent pulmonary aspiration. The LoTrach (AKA
PneuX PY System) is a novel ETT tube, which has been designed to use for mechanical
ventilation in the critically ill. It has a unique low-volume, low-pressure (LVLP) cuff,
which, despite having an intracuff pressure of 80 cmH2O, is postulated to transmit
approximately 30 cmH2O of pressure on the tracheal wall [13]. The LoTrach system also
incorporates a cuff pressure controller in order to maintain an optimal cuff pressure over
time [14]. It has already been shown to prevent pulmonary aspiration in a pig model, in
anaesthetised patients and in the critically ill [15].
The aims of this study were to compare the pressure exerted on the tracheal wall by the
HVLP cuff and the LVLP cuff in a bench-top study and a clinical study, and measure the
5
degree of anatomical distortion caused by the HVLP cuff and the LVLP cuff in a
radiological study.
6
Method
This study was conducted at the Queen Elizabeth Hospital Intensive Care Unit, Norfolk,
UK with approval from the Local Research and Ethics Committee. We compared the
SoftSeal system [HVLP, Polyvinylchloride, internal diameter 8 mm, Portex, UK] and the
LoTrach system [LVLP, Silicone, internal diameter 8 mm, Venner Medical, Singapore].
All equipment was used according to the manufacturers’ instructions.
Bench-top study: We used a model trachea with an internal diameter of 2.2 cm and a
constant pressure device [Tracoe cuff pressure controller, Tracoe, Frankfurt, Germany] to
inflate the HVLP cuff. The HVLP cuff was overpressured to a pressure of 50 cmH2O
while water was instilled above the cuff to a height of 40 cm. Next, a stopwatch was
started as the cuff pressure was reduced to the test pressure of 30 cmH2O. The rate of fall
of the water column was measured from the bottom of the ETT cuff until the column of
water reached a plateau. Theoretically, the water column would fall until it reached a
height of 30 cmH2O and then stop. At this height, the pressure exerted by the water
column would match the pressure exerted on the tracheal wall by the cuff. The
experiment was repeated 10 times so that any variation was averaged out. The same
protocol was used with the LVLP cuff. A single inflated cuff from each group was then
left for a period of 28 days.
7
Clinical study: All patients who required intubation over a 6 month period were included
in our analysis. One hundred and two patients were intubated with either a HVLP cuff (n
= 48) or the LVLP cuff (n = 54). Both ETT cuffs were inflated to the recommended
working pressure (SoftSeal at 30 cmH2O and LoTrach to 30 cmH2O above the resting
factory calibrated cuff pressure) [16]. Each patient underwent a routine staged
recruitment manoeuvre while the intracuff pressure was maintained using a constant
pressure device [Tracoe cuff pressure control, Tracoe, Frankfurt, Germany]. The positive
end expiratory pressure (PEEP) was set to 15 cmH2O and then increased in 5 cmH2O
increments every 5 seconds until 40 cmH2O was achieved. A second observer auscultated
the anterior neck and the pressure at which air leak was heard was recorded.
Theoretically, there should be no air leak up to a PEEP of 30 cmH2O. Above this, the
PEEP should exceed the pressure exerted on the tracheal wall by the cuff and an air leak
heard.
Statistical analysis: We tested for non-inferiority in both studies using a two sided 95%
confidence interval. We hypothesised that the mean difference in the pressure exerted on
the tracheal wall by the HVLP cuff and the LVLP cuff was within a clinically relevant
magnitude. A figure 20% either side of 30 cmH2O was chosen to delineate the limits of a
clinically relevant magnitude [17].
Radiological study: All patients who required chest CT examinations as part of their
ongoing medical care over a 6 month period were included in our analysis. An
independent radiologist examined the images of 7 sequential patients who were intubated
8
using a HVLP cuff and 6 sequential patients who were intubated using the LVLP cuff.
The tracheal antero-posterior (AP) and transverse diameters were measured at the midcuff level, and 2 cm above the cuff and 2 cm below the cuff where there was no contact
with the tracheal wall. The degree of anatomical distortion of the trachea was recorded.
Anatomical distortion was defined as the difference in diameter between the mid-cuff and
the mean of the above and below cuff diameters.
9
Results
Bench top study: Once the HVLP cuff pressure was reduced to the test pressure of 30
cmH2O there was leakage of water passed the cuff (figure 1). Leakage continued after the
water column had reached a height of 30 cmH2O. Leakage passed the cuff stopped at a
height of 4 cm because this represented the top of the cuff and so measurement was no
longer possible. In contrast, once the LVLP cuff pressure was reduced to the test pressure
of 30 cmH2O, there was only leakage of water passed the cuff until the water column
reached a height of 25 cmH2O. There was no leakage of water passed the cuff once the
water column reached a height of 25 cmH2O. It was not possible to test for noninferiority (in terms of the pressure exerted on the tracheal wall by the cuffs) because of
the persistent leak passed the HVLP cuff. In terms of preventing leakage, the LVLP cuff
was superior – within 300 seconds the HVLP cuff retained no water above it, however
the LVLP cuff continued to retain the 25 cm column of water for 28 days (with the cuff
pressure controller attached) when the study was terminated.
Clinical study: The mean tracheal wall pressure at which air leak occurred using the
HVLP cuff was 32.4 cmH2O (SD 3.0). The mean tracheal wall pressure at which air leak
occurred using the LVLP cuff was 30.0 cmH2O (SD 3.8). The 95% confidence interval at
which air leak occurred using the HVLP cuff was 32.4 +/- 0.7 cmH2O. The 95%
confidence interval at which air leak occurred using the LVLP cuff was 30.0 +/- 0.8
cmH2O. The limits of our clinically relevant magnitude were 24 cm and 36 cm. The
10
confidence intervals for each cuff were enclosed within these limits and so each cuff was
deemed non-inferior.
Radiological study: The mean mid-cuff AP and transverse tracheal diameter using the
HVLP cuff was 2.26 cm (SD 3.6) and 2.01 cm (SD 2.6) (table 1). The mean AP and
transverse tracheal diameter above and below the HVLP cuff was 1.82 cm (SD 3.1) and
1.81 cm (SD 2.3). The mean mid-cuff AP and transverse tracheal diameter using the
LVLP cuff was 1.96 cm (SD 3.2) and 1.85 cm (SD 1.9) (table 2). The mean AP and
transverse tracheal diameter above and below the LVLP cuff was 1.68 cm (SD 2.0) and
1.7 cm (SD 2.0).
The 95% confidence intervals for the mean mid-cuff AP and transverse tracheal diameter
using the HVLP cuff were 2.26 +/- 3.3 cm and 2.01 +/- 2.4 cm. The 95% confidence
interval for the mean AP and transverse tracheal diameter above and below the HVLP
cuff was 1.82 +/- 2.9 cm and 1.81 +/- 2.1 cm. The 95% confidence intervals for the mean
mid-cuff AP and transverse tracheal diameter using the LVLP cuff were 1.96 +/- 3.4 cm
and 1.85 +/- 2.0 cm. The 95% confidence interval for the mean AP and transverse
tracheal diameter above and below the LVLP cuff was 1.68 +/- 2.1 cm and 1.7 +/- 2.1
cm.
The mean degree of anatomical distortion of the trachea in AP and transverse tracheal
diameter using the HVLP cuff was 4.4 cm (SD 1.4) and 2.6 cm (SD 1.6). The mean
degree of anatomical distortion of the trachea in AP and transverse tracheal diameter
11
using the LVLP cuff was 2.9 cm (SD 2.1) and 1.8 cm (1.4). It was not possible to test for
non-inferiority in terms of anatomical distortion in our radiological study because of the
small number of patients in each group. However, it did give a snapshot of the degree of
anatomical distortion caused by different ETT cuffs in normal clinical practice.
12
Discussion
Previously, it has been recommended that ETT cuffs are operated at an intracuff pressure
of 30 cmH2O [1]. This intracuff pressure corresponds to a tracheal wall pressure of 30
cmH2O. This is thought to represent a balance between protecting the trachea from injury
and preventing pulmonary aspiration. However, even at this recommended cuff pressure,
HVLP cuffs have been shown to permit aspiration [15].
Although, the LoTrach has been shown to prevent pulmonary aspiration in a pig model,
in anaesthetised patients and in the critically ill, this is the first study to investigate the the
pressure exerted on the tracheal wall by the LoTrach system in vivo and in vitro [15]. The
LVLP cuff is calibrated so that the sum of the elastic forces within the cuff remains
constant throughout the inflation profile [13]. Despite having an intracuff pressure of 80
cmH2O, the constant elastic forces in the cuff wall only allow approximately 30 cmH2O
of pressure to be transmitted to the tracheal wall no matter what the tracheal diameter is
across the human range.
Our bench top study demonstrated that the LVLP cuff transmitted approximately 30
cmH2O of pressure on the tracheal wall. This is demonstrated by the fact that there was
only leakage of water passed the cuff until the water column reached a height of 25
cmH2O. This differs from 30 cmH2O because there is a range of acceptable values, which
are allowed as part of the quality control during cuff manufacture. All LVLP cuffs
13
operate within the range of 25-35 cmH2O. In contrast to the HVLP cuff, the LVLP cuff
provided an effective seal at this pressure. The LoTrach system has a unique LVLP cuff,
which does not develop longitudinal folds in cuff wall and therefore prevents subglottic
secretions from leaking around the cuff [13, 15]. Our clinical study also demonstrated
that the LVLP cuff exerted approximately 30 cmH2O of pressure on the tracheal wall,
which was equivalent to that provided by a HVLP cuff.
Concerns regarding tracheal injury and pulmonary aspiration are especially important
during mechanical ventilation of the critically ill given the length of time an ETT may
remain in place. During this time, there are likely to be episodes of low cuff pressure and
high cuff pressure [6]. Nseir et al reported that 54% of patients’ cuffs were underinflated,
73% of patients’ cuffs were overinflated and 44% of patients’ cuffs were either
underinflated or overinflated at some point in time. Episodes of low cuff pressure may
allow pulmonary aspiration to occur, whereas any episode of high cuff pressure may
damage the tracheal wall [18, 19]. Crucially, the LoTrach system also incorporates a cuff
pressure controller to maintain an optimal cuff pressure over time [14]. Our bench top
study demonstrated that the LoTrach system prevented episodes of low cuff pressure over
a 28 period (with the cuff pressure controller attached) until the study was terminated.
In our radiological study, the degree of anatomical distortion of the trachea was greater
using the HVLP cuff in both the AP and transverse diameters on radiological imaging.
However, this was observational data and so we could guarantee that the HVLP cuffs
were not overinflated. Nonetheless, it did give a snapshot of the degree of anatomical
14
distortion caused by different ETT cuffs in normal clinical practice. Our findings suggest
that different ETT cuffs distort the normal tracheal anatomy by varying amounts. This
may be important in the pathogenesis of tracheal injury during mechanical ventilation of
the critically ill. Ideally, we would have liked to have a group of patients with HVLP
cuffs in place, which were measured to have a cuff pressure of 30 cmH2O, but as a purely
observational study of practice these were not available to us. Furthermore, as only
patients who required chest CT examinations as part of their ongoing medical care were
included in our analysis, there were a limited number of patients in our radiological
study. These represented all of the available patients at the time of our analysis. Further
studies in a larger patient group would be welcomed to confirm our findings.
The LoTrach system also has a flexible shaft, an atraumatic tip and three supra-cuff ports,
which allow subglottic secretion drainage. Furthermore, large volume saline
irrigation/decontamination of the subglottic space and the entire upper airway can be
performed because of the complete airway seal afforded by the LoTrach system [14].
This can be used to clear potentially damaging bile and acid bathing the glottis [20].
Collectively, these features are designed to limit airway injury. Importantly cuff pressure
controllers can also carry serious risks. The connection of the cuff pressure controller to
the ETT is by a luer connector. Accidental connection of the cuff pressure controller male
luer to an intravenous line would result in a rapid fatality due to massive air embolism.
The LoTrach system incorporates a unique connector which cannot be misconnected to a
luer lock [21]. Other potential advantages of the LoTrach system are that it has a securing
flange to prevent accidental extubation. There is also a nonstick lining, which is designed
15
to reduce biological materials and biofilm from adhering to the inner lumen of the ETT
and perpetuating VAP [22]. In addition, the LVLP cuff may have another advantage over
a HVLP cuff. A HVLP cuff contacts a larger surface area of the trachea and previously
has been shown to cause damage to a larger surface area of the trachea than a low volume
cuff [23].
16
Conclusions
In conclusion, there is a need to develop engineered solutions, which reduce both the risk
of injury to the trachea and the risk of pulmonary aspiration. Previously, the LoTrach
system has been shown to completely prevent pulmonary aspiration. We conclude that
the LoTrach system exerts an acceptable level of a pressure on the tracheal wall and
incorporates a number of design features to limit tracheal injury.
17
Competing interests
Dr Peter Young is the inventor of the LoTrach/PneuX PY system endotracheal tube. In
the past he has received funding and consultancy fees from Venner Medical. This work
was not funded or financed by Venner Medical. Dr Peter Young is a minor shareholder in
the intellectual property ownership of the LoTrach endotracheal tube and tracheal seal
monitor (cuff pressure controller).
The remaining authors do not declare any conflict of interest or competing interest.
18
Author contributions
AD carried out the bench-top and radiological study, drafted the write up for the benchtop study and is the lead author of the submitted manuscript.
RS carried out the clinical study, drafted the write up for this section and gave final
approval for the version to be published.
MC interpreted the CT scans in the radiology study, drafted the write up for this section
and gave final approval for the version to be published.
MB devised the bench-top study, critically revised the manuscript and gave final
approval for the version to be published.
PY devised the clinical and radiological studies, critically revised the manuscript and
gave final approval for the version to be published.
19
Acknowledgments
None declared.
20
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23
Figures
Figure 1. Line graph displaying the leak of water passed a HVLP cuff (line) and the
LVLP cuff (dotted line) for a period of 300 seconds. y axis = height of water column
(cm), x axis = time (seconds). Leak passed the HVLP cuff stopped at a height of 4 cm
because this represented the top of the cuff and so measurement was no longer possible.
24
Tables
A
B
AP
Transverse
AP
Transverse
1
22
20
17.5
15.5
2
27
22
21
23
3
25
24
21
20
4
17
16
15
15
5
21
20
15
16
6
26
21
22
22
7
20
18
16
15
Table 1. The mean mid-cuff AP and transverse tracheal diameter (A) and the mean AP
and transverse tracheal diameter above and below (B) the HVLP cuff.
25
A
B
AP
Transverse
AP
Transverse
1
16
16
14
15
2
21
20
17.5
19
3
19
17
18
18
4
17
18
16.5
16.5
5
20
19
14.5
14.5
6
25
21
19
19
Table 2. The mean mid-cuff AP and transverse tracheal diameter (A) and the mean AP
and transverse tracheal diameter above and below (B) the LVLP cuff.
26
Figure 1