Trends_in_conventional_mechanical_ventilation_and.28.pdf

Chinese Medical Journal 2010;123(22):3319-3325 3319
Medical progress
Trends in conventional mechanical ventilation and pulmonary
graphics in the newborn
Kris C. Sekar
Keywords: pulmonary graphics; mechanical ventilation; newborn
he optimal treatment for respiratory distress
syndrome (RDS) in extremely low birth weight
newborn infants now consists of surfactant therapy,
ventilator support and aggressive nutritional support.1,2
Introduction of surfactant therapy has significantly
reduced both the mortality and morbidity in premature
infants. However, despite all the preventive efforts the
prematurity rate has increased in the United States. As a
result of this trend the majority of the infants requiring
mechanical ventilation in the current neonatal intensive
care units are less than 1000 g. This has created new
challenges in managing these infants respiratory distress
to reduce mortality, morbidity and improve neurological
outcome. Advances in optimal resuscitation, maintenance
of thermal environment, early surfactant therapy, gentle
ventilation, aggressive nutritional support, early treatment
of patent ductus arteriosus, control of infection etc. have
been adopted to reduce mortality and morbidity. However,
despite all these advancements in neonatal care the
incidence of bronchopulmonary dysplasia (BPD) has not
decreased.3,4
BPD develops in extremely premature infants who
undergo mechanical ventilation early in life. Although the
development of BPD is dependent on many factors, it has
been shown that the decision to intubate and start
mechanical ventilation is associated with a higher
incidence of BPD.5
Studies have shown that pressure
damage (barotrauma), high tidal volume (volutrauma)
and generation of inflammation (biotrauma) and exposure
to high oxygen concentration are among the main
etiologies in the development of BPD. Recent studies
have further shown that volutrauma may be more
important than barotraumas in the genesis of BPD.6-8
Therefore, one of the strategies to prevent BPD has
focused on preventing this ventilator associated lung
injury (VALI) with less invasive gentle ventilation.9
Over the last two decades the neonatal mechanical
ventilation has undergone significant changes mainly
driven by the development of the microprocessor
technology. Several new ventilators are now available
with various modes for assisted ventilation. None of these
modes have been proven to be superior in published
comparison trials. The modes of ventilator support
currently available for premature babies are as follows: 1.
Nasal continuous positive airway pressure (N-CPAP), 2.
Conventional mechanical ventilation (CMV), 3. High
frequency ventilation consisting of high frequency
oscillatory ventilation and jet ventilation (HFV), 4. Nasal
intermittent positive pressure ventilation (NIPPV), and 5.
High flow nasal cannula. Each one of these ventilator
supports has advantages and disadvantages and there are
extensive reviews published on these modes.1,10-12
None
of these modes has been shown to reduce the incidence of
BPD. Therefore CMV still remains the main primary
mode of ventilator support for premature babies. This two
part review will first focus on the newer modes that are
available in CMV and discuss the evidence in favor of
volume support rather than pressure support from
published studies. In the second part of the manuscript the
usefulness of bedside pulmonary graphics in CMV will
be discussed.
CONVENTIONAL MECHANICAL VENTILATION
CMV mainly consisted of time cycled pressure limited
(TCPL) ventilators for a long time. In this mode a preset
pressure is delivered to the lungs at a preset time over
positive end expiratory pressure (PEEP). The tidal
volume (TV) therefore varies from breath to breath and is
the dependent variable. In the traditional volume
controlled ventilation a preset tidal volume is delivered at
a preset time. The pressure here then becomes the
dependent variable. In either one of these modes the
spontaneous breaths are not synchronized with the baby’s
breaths. Therefore, significant asynchrony may develop
in these modes when the baby is exhaling while the
ventilator is giving a preset inspiratory breath. Over the
last decade synchronization of the ventilator breaths with
the baby’s breaths has become possible using various
technologies and methods. Among these synchronization
methods flow triggering at the airway opening (at the
endotracheal tube) appears to be by far the most optima.13
The various terminologies that are used in these
ventilators are described below: Synchronized
intermittent mandatory ventilation (SIMV): Here the
ventilator provides a certain number of mandatory breaths
T
DOI: 10.3760/cma.j.issn.0366-6999.2010.22.028
Department of Pediatrics, Oklahoma University Health Sciences
Center, Children’s Hospital, 1200 Everett Drive, 7th Floor North
Pavilion, Oklahoma City, OK 73104, USA (Sekar KC) (Tel:
405-271-5215. Fax: 405-271-1236. Email: Kris-sekar @ouhsc.edu)
There is no conflict of interest in this article.
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Chin Med J 2010;123(22):3319-3325
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that are synchronized with the baby’s breaths. Assist
control (AC): Here every spontaneous breath is supported
by a ventilator breath. A minimum back up rate is set in
case there is apnea. Pressure support ventilation (PS):
Here the ventilator supports each breath just like AC, but
terminates each breath when inspiratory flow declines to
a preset threshold usually 10%–20% of peak flow.
Pressure regulated volume control ventilation (PRVC):
This is a pressure limited time cycled mode that adjusts
the inspiratory pressure to a set targeted tidal volume (TV)
based on the pressure achieved to reach the TV of four
test breaths. Subsequent adjustments are made based on
the previous breath (Servo-I, Maquet. Inc., Bridgewater,
NJ, USA). Volume assured pressure support (VAPS): This
is a hybrid mode designed to assure that the targeted TV
is reached. Each breath starts with a pressure limited
breath, but if the TV is not reached the devise converts to
a flow cycled mode by prolonging the inspiratory time
(Bird VIP, Viasys Medical Systems, Conhohocken, PA,
USA). Volume guarantee ventilation (VG): Here a set TV
and a set pressure limit are chosen up to which the
ventilator opening pressure may be adjusted (Draeger
Babylog 8000 plus, Draeger Medical Inc., Telford, PA,
USA). Volume limited ventilation (VL): Here when the
targeted TV is reached the devise terminates inspiration
thus avoiding excess TV delivery (Bear Cub 750 PSV,
Viasys Medical Systems). Targeted tidal volume (TTV):
Here the devise increases the rise time of the pressure
wave form to improve the TV limit to the desired target
(SLE-500, Specialized Laboratory Equipment Ltd., South
Croydon, UK).
In addition to these, there are newer novel methods of
ventilation in development as follows: proportional assist
ventilation (PAV): here the ventilator develops inspiratory
pressure in proportion to patient effort giving positive
feed back. This concept assumes a mature respiratory
system which is not the case with premature babies.13
Neurally adjusted ventilator assist (NAVA): Here the
ventilator uses the patient’s own respiratory drive from a
bipolar electrode mounted on a nasogastric tube
positioned in the esophagus at the level of the diaphragm.
The ventilator adjusts the level of support based on the
inspiratory effort.13
Despite all these available modes the unique nature of the
newborn lung mechanics with a very compliant chest wall
surrounding a very stiff lung, use of uncuffed
endotracheal tubes contributing to air leaks and the very
small tidal volume that needs precise measurement and
delivery into the lung has made conventional ventilation
still a challenging problem in the daily management of
these babies.13
In addition, the sudden changes in
compliance that occurs after surfactant replacement
requires close monitoring needing immediate adjustments
in ventilator support. Therefore it is very essential to
become familiar with the available modes and how to
effectively use them in the NICU depending on the
ventilator that is in use.
CURRENT TRENDS IN CONVENTIONAL
VENTILATOR MANAGEMENT
There is no consensus regarding the superiority of one
ventilator mode over the others. Each ventilator mode has
certain unique features which are not available in others
making comparison difficult. There are no large
randomized controlled studies looking at the long term
outcome with these newer ventilator modes. However,
one concept that is emerging in the ventilator
management of neonates is volume ventilation as it is
now well established that volutrauma not pressure that
contributes to the development of BPD.14-17
Studies
published comparing volume vs. pressure ventilation
have only looked at short term outcome with fewer
numbers of patients lacking the power to predict long
term benefits. In addition, the various study designs have
also made the comparisons difficult. Currently
synchronized ventilation is the accepted normal mode of
ventilation in the majority of the NICUs in the USA
irrespective of the ventilator mode that is chosen.
Addition of SIMV and AC will depend on the operator
preference. Likewise, the preference of pressure or
volume ventilation is dependent on the operator and how
effective the ventilator is able to deliver the preferred TV.
Although several studies have been published comparing
volume ventilation to pressure ventilation a recent
meta-analysis of Cochrane review included only four
studies that met their criteria for comparison.18
The
ventilators used in these four studies were all different. A
total of 178 infants were included in this pooled analysis.
The results of the analysis showed no difference in
mortality between the groups which was the primary
outcome of the analysis. However, no study reported the
combined outcome of death or supplemental oxygen
requirement (BPD). In addition, the volume targeted
group showed a significant reduction in duration of
ventilation (weighted mean difference –2.9 (–4.28,
–1.57)), incidence of pneumothorax (RR 0.23 (0.07, 0.76))
and the incidence of severe intracranial hemorrhage (RR
0.32 (0.11, 0.90)) when compared to the pressure targeted
group. There was no difference in the incidence of BPD
defined as requirement for oxygen at 36 weeks corrected
age.19-22
The conclusion of the analysis affirmed a sound
theoretical basis for the use of volume targeted ventilation.
The review did not identify any adverse events with
volume ventilation when compared to TCPL ventilation.
Finally, the analysis failed to show any long term benefit
in the outcome of death or neurodevelopmental
impairment.
Among the studies that looked at PRVC vs. TCPL
ventilation, Piotrowski et al19
compared PRVC with
intermittent mandatory ventilation in 60 infants <2500 g
birth weight. There was no difference in the duration of
ventilation or BPD. A subgroup analysis showed a
reduction in duration of ventilation in the PRVC group in
infants <1000 g. D’Angio et al23
compared PRVC to
SIMV in 213 infants with birth weight of 500 g –1249 g.
There was no difference in the incidence of BPD or
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duration of ventilation.
Among the studies that looked at volume controlled
ventilation, Sinha et al21
studied 50 infants weighing 1200
g or more and randomized them to either VC or TCPL
ventilation. The VC group reached success criteria (time
to achieve AaDO2 <100 mmHg, mean airway pressure <8
cmH2O) faster and a shorter mean duration of ventilation.
There was a trend towards a reduced incidence of
intraventricular hemorrhage and BPD in the VC group.
Singh et al24
in 109 infants between 600 g and 1500 g
randomized in similar fashion between VC and TCPL and
showed no difference in the time to reach success criteria
or total duration of ventilation. A sub group analysis
showed faster weaning in the VC group in babies
weighing <1000 g.
Among the studies that looked at volume guarantee
ventilation, Cheema et al25
studied volume guided
ventilation compared to SIMV in a group of 40 infants
(GA 27.9 weeks, BW 1064 g) using a randomized,
crossover trials using the volume guarantee mode (VG).
They showed that VG is feasible in neonates and can
achieve equivalent gas exchange with significantly lower
peak airway pressures. Keszler and Abubakar22,26,27
have
published several studies showing breath to breath TV
variability was significantly reduced in VG mode
compared with AC mode, VG mode reduced hypocarbia
and excessively large TV when AC was compared with
AC plus VG and a higher variability of TV and increased
work of breathing noted with AC plus VG when
compared with SIMV plus VG. Lista et al28
showed
decreased inflammatory cytokines in infants with RDS
when PS plus VG was compared with PS alone (target
TV 5 ml/kg). They speculated that VG may reduce
ventilator associated lung injury reducing volutrauma.
Several other similar studies have been published all
favoring the beneficial effect of volume ventilation.29
But
it is not known whether the short term benefits seen
translate to long term effects in reducing mortality, BPD
and improved neuro developmental outcome. There is
definitely increasing evidence that volume ventilation is
better than pressure ventilation in neonates. Some of the
newer ventilators even have algorithms built in to
compensate for potential excessive or lower TV delivery
and thus deliver the targeted TV (volume guarantee). The
tidal volume needed to maintain adequate ventilation
appears to be 5 ml/kg on day 1 advancing to 6 ml/kg by
the end of the third week.13
PULMONARY GRAPHICS MONITORING IN
NEWBORN MECHANICAL VENTILATION
In the seventies, mechanical ventilation of the neonate
primarily consisted of devises using continuous flow,
pressure limited and time cycled modes, without patient
synchronization. The neonatal ventilators provided very
basic information such as positive end expiratory
pressures (PEEP), peak inspiratory pressures (PIP),
ventilator rates, inspiratory time and oxygen
concentration. The clinician was left to adjust ventilator
modes by clinical observation, chest excursions, chest X
ray findings and blood gas measurements.30
Adjusting
ventilator parameters were based on best clinical
judgment as no physiological measurement was possible
in real time. Later, the introduction of pulse oximetry
enabled clinicians to dynamically titrate oxygen
requirement in real time without the need for frequent
blood gas measurements. In the late eighties, pulmonary
function measurement was available at the bedside. The
main component of this portable equipment was called a
pneumotachograph. This was a bulky devise requiring
cleaning between patients and also required the babies to
be taken off the ventilator to insert the devise. There was
thus the potential for the endotracheal tube to be
dislodged and ventilation lost during the set up. In
addition, it also added dead space to the circuit which
increased the work of breathing. The values obtained
from this devise were tidal volume, compliance and
resistance. While these measurements were useful it only
gave a “snap shot” of events at the time of measurement
and was not helpful in ventilator management with
constant changes in clinical status.31
The various newer models of ventilation such as PRVC,
PS, VG etc. have been described previously. All these are
now possible due to the introduction of proximal airway
sensors that are positioned between the endotracheal tube
and the ventilator circuit. These devises are extremely
light, stay in line and add very minimal dead space. They
are disposable and therefore very easy to use. These
sensors are either thermal or differential in type. The
sensor detects either flow or pressure and converts it into
a useful analog value. The flow is processed by the
software in the machine and a continuous display of tidal
volume measurements both during inspiration and
expiration, pulmonary compliance, resistance and work of
breathing is displayed. In addition, these sensors also
detect patient’s breaths or “triggers” and distinguish them
from ventilator breaths and patient breaths. This helps
pressure support ventilation for spontaneous breaths. In
association with these developments, they also display
real time pulmonary graphics on the ventilator screen.32,33
The variables measured are now available as a continuous
display rather than “snap shots” enabling the operator to
monitor pulmonary function in “real time” at the bed side.
A brief description of the common wave forms and
examples are described below.
Pulmonary graphics display
This continuous display consists of graphs and numerical
values of the various parameters measured, such as mean
airway pressure (MAP), peak inspiratory pressure (PIP),
positive end expiratory pressure (PEEP), inspiratory and
expiratory tidal volumes (TV), compliance and work of
breathing (WOB). Clinicians can use these values
displayed in real time and optimize the ventilatory
assistance close to normal respiratory physiology. These
measurements can be significantly variable because of
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constantly changing dynamics in babies. However, it does
help the clinician to assess the trend and make changes in
the ventilator support as needed for a given clinical
situation. Assessment of real time pulmonary graphics is
now the accepted standard of care in most neonatal
intensive care units in the USA.34
Basic wave forms
The three basic pulmonary wave forms displayed are
pressure, volume and flow. The typical pressure wave
form has upward (inspiration) and downward (expiration)
scalars. The peak of the upward scalars represents the PIP
and the area under this curve represents the MAP. The
inspiratory time (IT), flow and frequency can be
determined from this tracing (Figure 1). Oxygenation is a
function of MAP. MAP can be increased by increasing
the PEEP, PIP, IT, flow and/or frequency (Figure 2).
Ventilation is a function of tidal volume and frequency. In
this scalar machine triggered breaths will have no
negative deflection at the start. The patient triggered
breaths will have negative deflection at the start if the
breaths are being pressure triggered. The greater the
patient’s effort to trigger the breaths the greater will be
the negative deflection. There will be no deflections seen
with flow triggering. If PEEP is added the baseline will
be above zero.
Figure 1. Typical pressure wave form showing beginning of
inspiration and expiration, PIP and distending pressure.
The volume wave form is similar in appearance to the
pressure wave form except that the peak volume is
reached earlier in pressure ventilation as opposed to the
end of inspiration in volume ventilation (Figure 3). In
contrast in volume targeted ventilation the peak volume
delivery occurs at the end of inspiration (shark’s fin
appearance).
The flow wave form has two components. There is a
positive deflection and a negative deflection above the
base line. Deflection above the baseline represents flow
into the lungs (inspiration) and deflection below the
baseline represents flow away from the lungs (expiration).
The highest point of the curve above and below the
baseline represents peak inspiratory and peak expiratory
flow. These wave forms help distinguish between
pressure targeted and volume targeted breaths,
inadvertent PEEP and resistance. The pressure and volume
Figure 2. Mean airway pressure. 1. Flow; 2. PIP; 3. IT; 4. PEEP;
5. Rate.
Figure 3. Typical volume wave form showing inspiratory tidal
volume.
targeted breaths look different on display. The volume
targeted breaths are more “square” and the pressure
targeted breaths are more “sinusoidal” in shape. There is no
evidence to support one flow pattern is superior compared
to the other. However, squire wave might distribute gas
more evenly in the lungs as the initial burst of flow at the
beginning of inspiration would pop open the alveoli and
allow for better gas exchange. If the expiratory flow does
not return to baseline before the next breath starts there will
be auto PEEP or inadvertent PEEP. If inadvertent PEEP is
detected this could be corrected by decreasing the
frequency, inspiratory time to give more time for expiration
and sometimes by adjusting the PEEP levels (Figure 4).
Pressure-volume loop
The pressure volume (PV) loop is the relationship between
pressure and the generated volume. This loop begins at
PEEP. The inflation curve ends at PIP and the lungs start
to empty. The inflation curve (upward) and the deflation
curve (downward) of the loop are different and describe
the mechanical properties of the lung (hysteresis).
Spontaneous breaths go clockwise and the positive
pressure breaths go counter clockwise. The line connecting
the beginning of inflation to the end of inflation
represents the dynamic compliance of the lung. The
compliance is mathematically determined as the change
in volume divided by the change in pressure and
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Figure 4. Typical flow wave form showing inspiratory and
expiratory flow and auto-PEEP.
displayed both as a loop and numerical value on the
screen. The distortion of the PV loop may indicate
disturbances in the lung mechanics. The PV loop helps to
optimize inflation and adequate tidal volume delivery
avoiding over inflation. Inadequate hysteresis may also be
indicative of inadequate flow. The pressure volume loop
may be used to determine the change in compliance after
surfactant therapy as the loop will become more vertical
with better inflation with improving compliance. PV loop
will also help to optimize inflation if the loop has a
“beaked” appearance. In such situations either the
pressure or the volume will need adjustment to correct the
over inflation of the lung. PV loop could also be used to
optimize PEEP (Figures 5 and 6). The loop will not meet
at the bottom with air trapping or leaks.
Figure 6. Pressure-volume loop showing change in compliance.
Figure 7. Flow volume loop showing inspiratory and expiratory
flow.
recoil. The flow “scoops” with increase in resistance.
Increase in resistance is seen in conditions like meconium
aspiration syndrome, respiratory distress syndrome and
bronchopulmonary dysplasia (BPD). In addition, the
flow-volume loop may help to optimize PEEP, and to
detect air leaks and turbulent flow. During pressure
support ventilation, flow-volume loops help detect
supported breaths. The loop becomes very jagged with
water or secretion build up in the circuit. They also help
in detecting endotracheal tube leaks when the ventilator
starts auto-cycling, misinterpreting the leak as
spontaneous breaths. If there is abnormal flow volume
loops detected the cause should be identified and
interventions undertaken as needed. For example if leak
is detected all the connections need to be checked for
leaks and the flow sensor is working appropriately. If
there are still leaks either pressure or tidal volume needs
to be decreased. If there is water in the circuit this should
be drained.
Figure 5. Typical pressure volume loop showing inspiratory and
expiratory curves, peak inspiratory pressure and tidal volume.
Flow-volume loop
The flow-volume loop describes the changes in these
parameters over the inspiratory and expiratory phase of
respiration. The flow is plotted on the Y axis and the
volume on the X axis. Inspiration is above the horizontal
line and the expiration is below. In some ventilators this
is reversed. The shape of the inspiratory flow will match
what is set on the ventilator. The shape of the expiratory
flow represents passive exhalation. This curve should be
generally smooth and circular in appearance (Figure 7).
When there is resistance to flow either during inspiration
or expiration the characteristics of the loop changes and
helps both in diagnosis and treatment. The expiratory
flow is long and more drawn out in patients with less
Bed side pulmonary graphics help distinguish mechanical
breaths from patient triggered breaths. From the scalar
tracings it is easy to distinguish SIMV, IMV and AC
breaths (Figure 8).
The configuration of these wave forms as a continuous
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Figure 8. Scalar tracings showing control mode, SIMV mode, assist mode and SIMV with pressure support. Notice that the ventilator
breaths look different from spontaneous breaths.
display will help fine tune ventilator management to
prevent complications. In addition, the waveforms will
also readily distinguish when the baby starts “auto
cycling” the ventilator. This is a situation where the leak
around the endotracheal tube is perceived as a breath and
the machine starts delivering rapid breaths more than the
set breaths.
Current ventilators available differ in the way the
graphics are displayed. The options available now are
unlimited starting from basic displays to changing the
configuration of the flow patterns as the disease
progresses. Every clinician should become familiar with
the available modes and interpreting the displayed
graphics of the machine they are using and take the
necessary corrective actions as needed. Most ventilators
also have the capability to store captured information for
later down load and analysis of data.
Pulmonary graphics help the clinician with real time data
that will compliment bedside examination, blood gas
determination, chest X-ray and the state of the disease
process. Careful monitoring of pulmonary graphics will
help to monitor changes after surfactant administration,
bronchodilator and diuretics treatments, air leaks, changes
in airway resistance, air trapping and inadvertent PEEP.
Closely monitoring these changes will help clinicians to
optimize ventilatory support.
CONCLUSIONS
It is still unclear which modality is superior in preventing
morbidities associated with mechanical ventilation
despite the availability of several modalities of ventilation
and real time pulmonary graphics. Likewise, there is no
real consensus as to how to optimize ventilation based on
real time pulmonary graphics among clinicians. Each
machine is different in the way the data are displayed and
the computer algorhythm used to calculate data making
comparison very difficult among them. Because of this,
there are no large randomized studies or evidence
supporting that ventilator management based on
pulmonary graphics reduce alveolar over distension,
barotrauma or chronic lung disease.6
However, there is
evidence to support some superiority of volume
ventilation over pressure ventilation in preterm
infants.18,24,29
Bedside pulmonary graphics should be used
as an additional tool complementing clinical examination,
blood gas measurements, oximetry trends and chest X-ray
evaluation in the management of ventilator supported
neonates.
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(Received April 11, 2010)
Edited by CHEN Li-min
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