High Frequency Ventilation Back to Basics
Case Two hfov
•
•
BG is 39 wk, 3400 g infant vaginally delivered to 27 YO
G1P0 mom with complete prenatal care.
At delivery, amniotic fluid is meconium
• stained and BG is distressed.no time for amnioinfusion of
saline,because of variable deceleration that is
recommmended in 41 gw
Case Two hfov
•
•
Direct laryngoscopy reveals meconium in airways.
BG intubated with 3.5 mm ETT and suctioned with meconium
aspirator for thick meconium.BG lavaged with Surfactant and
placed on SIMV: f = 40; PIP = 25; PEEP = 5;FIO2 = 1.0-ABG:
7.21/78/73
• Over several hours, f increased to 60;PIP increased to 40.
•
BG worsened. CXR revealed Rt pneumothorax. Post-chest tube
ABG:7.08/85/46.
•
HFO initiated. f = 5 Hz; delta P = 32;MAP = 26.-ABG: 7.19/75/45
•
What to do about PaO2?What to do about PaCO2?
• 4/5/2011-Copyright 2008 AP Jones 11
Case Two hfov
•
ABG: 7.19/75/45
•
What to do about PaO2?
•
MAP increased to 30, observing SPO2
• and CXR
•
What to do about PaCO2?
•
delta P increased to 36
•
ABG: 7.32/52/85
Case Two hfov
•
Over two days, BG improves; but small
• air leak persists.
•
FIO2 weaned to 40% with SPO2
•
ABG: 7.56/24/213
•
Next changes?
Case Two hfov
•
Over two days, BG improves; but small
• air leak persists.
•
FIO2 weaned to 40% with SPO2
•
ABG: 7.56/24/213
•
Next changes?
•
reduce MAP, using SpO2 = 94%
•
reduce delta P to 30 for PaCO2
hfjv
Case One
• 26 wk 700 g BG-Intubation and surfactant in DR
•
Initial ventilator settings TV = 12 mL,rate = 60/min, FIO2
= 60%; PEEP = 6 cm H2O - couldn't wean FIO2’More
surfactant - no changes (RDS)Over 36 H, PIP increased
from low 30s to 55 cm H2O - CXR after CMV
• Click to see CXR after 36 H on CMV
Case Two
41 wk, 3500 g BB Delivered with meconium in amnion
and in upper airways,Intubated, suctioned through ETT
Lavaged with surfactant,Placed on nCPAP = 6; FIO2 = 35%;
SpO2 = 89% then to NICU-6 H later, SpO2 decreased and RR
increased to 80/min
Case Two
Placed on volume-control
ventilator
with FIO2 = 50%; TV = 22 mL;
rate =
40/min; PEEP = 6 cm H2O; PIP =
48
cm H2O; MAP = 18 cm H2O
ABGs: PaO2 = 45 mm Hg;
SaO2 =
81%; PCO2 = 76 mm Hg; pH =
7.18
Changed to jet ventilator
Settings??
Case Two
•
•
Initial settings for jet ventilation companion FIO2 =
60%; PEEP = 8 cm H2O for MAP = 18 cm H2O;rate = 5/min
jet FIO2 = 60%; rate = 360/min;
• PIP = 46 cm H2O-it=0/02 seconds
•
ABGs: PaO2 = 42 mm Hg; PaCO2 =75 mm Hg; pH = 7.10
• Click to see a chest radiograph of RDS
• http://img.medscape.com/pi/emed/ckb/radiology/336139-
410756-9164.jpg
Case Two
•
•
•
Ventilator adjustments-companion PEEP increased to
10 cm H2O for MAP = 20 cm H2O; rate decreased to zero
jet rate decreased to 240/min
ABGs: PaO2 = 59 mm Hg; SaO2 =91%; PaCO2 = 55 mm
Hg; pH = 7.27
•
CXR - less hyperinflation
•
Note: increased PIP might decrease
• PaCO2; but decreased rate worked
• by decreasing I:E
Case Two
Over two days, CXR improved and patient stable on
FIO2 = 38%; PIP =22 cm H2O; PEEP = 8 cm H2O
PIP weaned to zero; FIO2 weaned to 30% with patient
stable
Patient extubated to nCPAP
Radiographs during the first day of life with increasing
with small amount of MAP with the danger of silent
recruitment
• For a 30-week premature
infant with RDS managed
with HFOV to achieve mean
lung volume(MVL).
• A=initial PAW=10
cmH2O,fio2=1
• B=at 12 hours of age,with
PAW=15 cmH2OFIO2=0/45
• C=at 24 hours of
age,PAW=12cmH2O,FIO2=/
28.silent recruit has occured
Radiographs of a 2-day-old
preterm ON HFOV
• With pulmonary intersticial
emphysema(PIE)
• A=,paw=16
cmH2O,FIO2=0/65.
• B=six hours later,paw=8
cmH2O,FIO2=1
• C=twelve hours later,settings
were unchanged.
• D=thirty-six hours
later,reinflation was
beginning,paw=12cmH2O,FI
O2=0/45
Reductions in MAP would
usually be made when the
FiO2 is < 40%
The Optimal Lung Volume Strategy recruits alveoli and
lung segments and once an optimal lung volume is
achieved recruitment, lung volume and oxygenation can
be maintained with a lower MAP.
The repeated stretch group received 15-sec
sustained inflations at 30 cm H2O mean
airway pressure every 20 mins, with
maintenance mean airway pressure sufficient
to keep PaO2 > 350 torr (46.7 kPa)
Chest wiggle frequency
ƒneonates from nipple line to
umbilicus
ƒadults
• In neonate from nipple line to umbilicus
Ventilation
Ventilation is primarily determined by the stroke
volume (Delta-P) or the frequency of the ventilator.
Alveolar ventilation during CMV is defined as:
F x Vt
Alveolar Ventilation during HFV is defined as:
F x Vt
2
Therefore, changes in volume delivery (as a function of
pressure-amplitude, frequency, or % inspiratory time)
have the most significant affect on CO2 elimination
HFV: Start
MAP(PEEP): 2-5-(8) mbar above
MAP of conventional
ventilation;
if necessary, increase MAP until
pO2 ()
after 30 min: X-ray: 8-9 rib level
IMV rate: 3bpm
pressure: 2 to 5 mbar below
conventional ventilation
HFV frequency: 10 Hz
HFV amplitude: 100%
watch thorax vibrations
HFV volume: about 2 to 2.5 ml/kg
• Troubleshooting during HFOV
• Low PaO2 : Consider:
• • ET tube patency
• • check for chest movement and breath sounds
• • check there is no water in the ETT/T-piece
• • Air leak/pneumothorax
• • chest moving symmetrically?
• • transilluminate
• • urgent chest x-ray
• Sub-optimal lung volume recruitment
Over-inflated lung
• Sub-optimal lung volume recruitment
• • increment MAP
• • consider chest x-ray
• • Over-inflated lung
• • check blood pressure
• • reduce MAP; does oxygenation improve?
• • consider chest x-ray
High PaCO2: Consider:
•
• ET tube patency and air leaks (as above)
•
• Insufficient alveolar ventilation
•
• Increase amplitude, does chest wall movement increase?
•
• Increased airway resistance (MAS, BPD) or non-homogenous lung disease: Is
HFOV appropriate?
•
• Under-inflated lungs, amplitude being delivered on non compliant part of the
pressure(volume curve ie point A in figure 2)
•
• Over-inflated lungs, amplitude being delivered on non compliant part of the
pressure(volume curve ie point C in figure 2)
•
• If all the above seem OK try reducing oscillator frequency; lung impedance and
airway
•
resistance fall, leading to increased VT.
Persisting acidosis/hypotension: Consider:
• • Over-distension
• • reduce MAP; does oxygenation improve?
• • consider chest x-ray
Why are mean airway pressures
higher on HFOV?
•
Mean airway pressure seem to be higher on HFOV because unlike
conventional and jet ventilation, there are no conventional (tidal)
breaths to recruit the lung. Optimal gas exchange occurs when the
lung is at FRC. Depending on the severity of lung disease, the
pressures required to recruit the lung to FRC may seem high.
"Based on the relationships between MAP, compliance, functional
residual capacity, and indexes of ventilation/perfusion matching, we
conclude that increasing MAP to achieve normal FRC... is a simple
method of optimizing lung volume in surfactant depleted subjects
[during HFOV]." .
The choice of strategy is
specific for every
disease:
• 1=to inflate an underinflated lung
• Recruitment maneuver
• 2=to deflate an onerinflated lung
• derecruitment
Non-uniform inflation occurring in non-homogenous lungs
CMV’s with high PIPs aimed at recruiting alveoli
Those pressures are also transmitted to the healthiest regions
of the the lungs
Over-distention and over-pressurization produces volutrauma
and barotrauma
Inflammatory Cascade is triggered
Three Words: Small Tidal Volumes
Small Tidal Volumes allow gas exchange to occur using
extremely small volume displacements of ventilatory gases
Small Tidal Volumes < Anatomical Dead Space
Small Tidal Volumes allow safer use of Optimal PEEP
minimizing the risk of atelectasis and oxygen toxicity
Often referred to as Lung Protective Ventilation/Strategy
Physiologic Basis for Rapid,
Small Tidal-Volume Breathing
Henderson was intrigued
by the shallow breathing
of panting dogs in 1915.
He wondered how dogs
could pant indefinitely
without becoming hypoxic
or hypercapnic.
So, he designed an
experiment to find out.
*Y Henderson, FP Chillingworth, JL Whitney - American Journal of Physiology, 1915
Henderson filled his mouth with tobacco smoke
and blew it into a glass tube in one quick puff
The smoke shot down the tube in a long spike
He then stuck his tongue over the end of the
tube to stop the flow. Diffusion took over as
flow stopped, and the effect disappeared
We call this phenomenon FLOW
STREAMING, the type of flow we try to create
with our HFJV inspirations
*Y Henderson, FP Chillingworth, JL Whitney - American Journal of
Physiology, 1915
The column of smoke shoots across the
center of the bulb with very little
contamination of the clear air surrounding the
stream
Again, if the flow is stopped, a
complete mixing of smoke and air
occurs almost instantaneously
*Y Henderson, FP Chillingworth, JL Whitney - American Journal of
Physiology, 1915
Henderson concluded that with the proper flow pattern,
“a tidal volume even
much smaller than the
volume of dead space my
thus afford a very
considerable gaseous
exchange”
*Y Henderson, FP Chillingworth, JL Whitney - American Journal of
Physiology, 1915
Evolved from studies of pulmonary physiology using
mathematics, fluid mechanics, and other engineering
principles.
Facilitates gas exchange by sending a steady stream(bulk
flow) of very small tidal volumes into the airways using
relatively low PIPs (peak airway pressures).
The further the gas goes into the airways, the lower those
airway pressures are.
Monitoring and controlling HFV with airway pressure instead
of tidal volumes creates problems for many clinicians
Pressure is a Dependent variable
It depends on gas flow, tidal volume, airways resistance, and
lung compliance
High frequency ventilation
• Techniques
HFPPV
HFJV
HFOV
HFFI
VT
>dead sp
> or < ds
> or <ds
<ds
Exp
passive
passive
passive
active
triangular
triangular
Wave- variable
wave
sine
form
Entrai- none possible
(/min)
ment
Freq.
60-150
none
60-600
none
300-900
Airway Pressures
during HFV
Trachea & Proximal Airways
Distal Airways & Alveoli
20
P
15
HFJV
10
5
20
P
15
10
5
Paw
Amplitude attenuates;
PEEP stays constant, MAP declines
HFOV
Paw
Amplitude attenuates;
PEEP increases; MAP fixed
when I:E = 1:1, less at 1:2
Conventional studies indicate that smaller tidal volumes are
safer than larger tidal volumes *
As Tidal Volumes are pushed smaller and smaller, two things
must be raised:
PEEP/MAP - to keep the lungs open
Rate - to maintain an normal PaCO2
Once you get over 150 bpm, you’re in the domain of high
frequency ventilation
*Kacmaerk RM, Chiche J-D. Resp. Care 1998;43:724-727 & Lee PC, Helemoortel CM,Cohn Sm, Fink MP. Chest 1990;97:430-434
Conventional Ventilation
Alveolar Ventilation = Rate x Tidal Volume
High Frequency Ventilation
Alveolar Ventilation = Rate x Tidal Volume 2
During CV gas exchange
occurs mostly from Bulk
Transport (convective flow)
of the O2 and CO2
molecules between the
central (conducting) airways
and the peripheral airways
The volume of inhaled gas
must exceed the volume of
dead space.
Image Source:www.homehealth-uk.com/ image/lungs.jpg
Bulk Axial Flow - Convection
Interregional Gas Mixing - Pendelluft Effect
Asymmetric Velocity Profiles
Axial and Radial Augmented Dispersion - Taylor Dispersion
Convective Dispersion
Augmented Molecular Diffusion
HFV gas distribution is more affected by
the resistance of the lungs than the
compliance.
Normal, tidal ventilation gas delivery is
more affected by the compliance of the
lungs than resistance.
Gas transport
mechanisms
during HFOV
Bouchut JC et al. Anesthesiology 2004; 100:1007-12
• “Often the most critical factor in
determining optimal gas
exchange (oxygenation) as it
correlates with lung volume.”
• Neonatal/Pediatric Respiratory Care: A Critical Care Pocket
Guide - Dana Oakes
The “Essentials” of HFV
Lungs inflation is essential to
adequate oxygenation
Maintenance of airway patency is
essential to adequate ventilation
Minimizing mechanically delivered VT is
essential to prevention of lung injury
PEEP/
MAP
issues
MAP & HFJV
Don’t let the MAP fall when initiating HFJV !!
CV = larger tidal volumes
• HFJV = Gentle Ventilation
MAP
*
PEEP
Time
*You must raise PEEP to maintain MAP for
Stabilization / Oxygenation.
When HFV rate approaches the
natural or Resonant Frequency of
the lungs, then the airway pressure
needed for proper ventilation can be
minimized.
The natural frequency of premature
infant lungs is around 40 Hz, which is
far beyond the capability of any
mechanical ventilator.
The most basic differences
between HFOV and HFJV are:
HFJV squirt gas into the lungs
faster than Oscillators
Oscillators get the gas back
out faster than Jets by
actively sucking it out
Oscillators with their I:E ratio of 1:2 use symmetrical pressure
waveforms that are very effective in treating homogenous
lung disorders like RDS
HFOV typically uses higher Mean Airway Pressures than
either HFJV or CV because its baseline pressure has to be
raised to counteract the negative, sucking action of its
expiratory mode to avoid airway collapse and gas trapping
Tracer bolus limits
Start of Inspiration
Inspiratory Velocity
Profiles
End of Inspiration
Start of Exhalation
End of Exhalation
Expiratory Velocity
Profiles
Tracer bolus tip in
middle of airway
has moved towards
alveoli
Modified from Haselton et al., Science, 1980
Initial position of
tracer bolus
Deformation of
bolus after a few
HFOV cycles
Modified from Haselton et al., Science, 1980
HFJV uses high velocity gas inspirations of short time
duration (0.02 sec)
Set, Fixed I-Time = Wide Variety of I:E Ratios
These inspirations create substantial momentum which
enable adequate oxygenation using less Mean Airway
Pressure
Allows HFV to use tidal volumes smaller than anatomic dead
space volume because gas in the terminal airways gets
replenished so rapidly that a substantial Partial Pressure
difference is established with alveolar gas.
This partial pressure difference facilitates Diffusion, and good
gas exchange results
HFJV is especially good at creating flow streaming because it
squirts gas into the lungs with a great deal of Velocity.
Amplitude
(ΔP or power)
•
(Choose an Amplitude)
•
Although you can choose an amplitude between 0% and 100%, the amplitude
delivered depends on the MAP – the lower the MAP, the lower the amplitude
before the maximum is reached.
•
The amplitude is calculated the pressure fluctuation as a percentage of the
difference between MAP and 60 mbar.
•
For example, if the MAP is 15, then 100% amplitude would be 45
mbar. Therefore, the pressure would be from -7.5 to 37.5 mbar
•
However, the airway pressure is limited to –4 mbar. Therefore, this limits the
maximum effective MAP – you can set it at 100% but in the example above it will
only deliver -4 to 34 mbar (total 38 mbar) which equals 84%! Therefore, the lower
the MAP the lower the amplitude that will be effective.
•
The formula for this is: Maximum effective amplitude (%) = (2 x (MAP + 4))/(60MAP) x 100
HFV Ventilates so effectively that hypocarbia can
easily be induced, especially when PEEP in
inappropriately low.
Inadvertent PEEP can develop when HFV rate is
inappropriately high. And gas trapping can also occur
when mean airway pressure is too low and airways
become smaller thereby increasing
restriction/resistance.
There is a tendency to overdue alveolar recruitment strategies
With HFOV, MAP is often left inappropriately high which over
expands alveoli and interferes with cardiac output
With HFJV, the ease with which IMV breaths are implemented
leads to not enough PEEP/MAP with inappropriate high CMV
rates and large tidal volumes
Irrational fear of barotrauma can develop even though HFV
tidal volumes are many, many times smaller than CMV
2. Stabilization
1. Recovery
3. Weaning
Recovery Again
High frequency jet
ventilation
(HFJV)
•
high frequency ventilation with
• delivery of a tidal volume (1-3
• mL/kg) at a high flow (jet)
•
originally used for short-term
• ventilation during airway surgery
• (1970s) because of capability to
• ventilate in face of leaks
Baby Judy was born at 27 weeks of gestational age by cesarean
section. Two weeks earlier, her mother had a preterm premature
rupture of membranes (PPROM). She had been started on
antibiotics and tokolysis. However, she developed amnionitis and
went into preterm labour. The babies APGAR score was 3 / 7 / 7 at
1, 5 and 10 minutes, respectively. Her umbilical artery blood pH was
7.27.Initially, she was treated with CPAP but soon developed severe
chest wall distortion, apneic and bradycardic episodes, and required
an FiO2 above 0.60 to maintain her arterial oxygen saturation above
85%. She was therefore intubated 21 minutes after birth and
received exogenous surfactant. Thereafter, oxygenation improved
and Judy was transferred to the NICU where she continued on
conventional controlled mechanical ventilation. To keep her Pa CO2
between 45 and 55 mm Hg, she needed PIPs between 15 and 20
cm H2O with 5 cm H2O of PEEP.
Initiation of HFOV
At day 3 of life her respiratory gas
exchange deteriorated. The FiO2
had to be at 0.8 in order to achieve
an arterial oxygen saturation >
85%. Her tcp CO2 was 65 mmHg
and rising at this point. A second
dose of exogenous surfactant
caused a moderate but transient
improvement in gas exchange so
that she was started on HFOV.
Ventilatory settings on SIMV
immediately before the initiation of
HFOV:
Rate:
60/min
PIP / PEEP:
25 / 5 cmH2O
MAP:
10 cmH2O
FiO2:
0.8
Ventilatory Settings on HFOV:
Rate:
10 Hz
MAP:
12 cmH2O
•
FiO2:
0.6
About 30 minutes after changing
over to HFOV, her gas exchange
improved: The FiO2 came down to
0.4. The Pa CO2 fell quickly into a
target range between 40 and 50
mmHg.
1. X-ray after initiation of HFOV
Unexplained cardiorespiratory
deterioration after 2 days on
HFOV
Figure 2. Chest X-ray, day of life #7.
• Judy remained clinically stable for the next 2 days while on HFOV with
unchanged settings. Towards the end of day 7, however, her urinary
output decreased from 4 cc/kg/h to 1.5 cc/kg/h. She had gained 70
gms of body weight since birth. There was no obvious change in
arterial blood pressure. Arterial oxygen saturation fell to below 90%.
Raising the FiO2 up to 0.9 had little effect on arterial oxygen
saturation. The Pa CO2 remained unaffected.
• The clinical team decided to increase the mean airway pressure under
HFOV. Oxygenation did not improve: if anything, it appeared to
fluctuate more.
Subsequent clinical management
The attending clinician requested a cardiac echo because there was
such a poor response in systemic oxygenation when changing the
FiO2. The echo showed a bidirectional ductal shunt, a right-to-left atrial
shunt, and good contractility of the heart, which had a normal anatomy.
The working hypothesis was that the pulmonary circulation became
compromised while lung compliance had improved (the chest appeared
hyperinflated on X-ray, and lung fields had cleared to some extent).
The applied positive airway pressure during HFOV was initially
appropriate. However, it hyperinflated the chest when lung function
improved.
The baby was changed back to
SIMV in an attempt to allow better
pulmonary perfusion during
expiration with lower expiratory
pressures (4 cm H2O) and a
longer expiratory time. This
immediately resulted in higher
systemic oxygenation. Urinary
output came back to 4–5 cc/kg/h.
There was less right-to-left atrial
shunting and the ductal shunt
became left-to-right only on echo.
Reminder: Transmission of a
positive mean airway pressure
(MAP) onto the pulmonary
circulation depends on lung and
chest wall compliance (Figure 3).
Relationship between MAP pleural
pressure (Ppl). Chest wall
compliance is high in preterm
infants. A larger fraction of the
MAP is transmitted onto the pleural
pressure when lung compliance
(CL) increases. Ppl is a
determinant of the pulmonary
vascular resistance. K,
transmission factor.