hello I'm Eric strong from the Palo Alto
Veterans Hospital and Stanford
University this is the sixth lecture in
this series on mechanical ventilation
and the topic is non-invasive positive
pressure ventilation here are the
learning objectives of this lecture
first to understand the indications and
the contraindications for non-invasive
positive pressure ventilation second to
understand the difference between CPAP
and BiPAP as well as no typical starting
settings for each finally to be familiar
with some of the scientific evidence
supporting the use of non-invasive
positive pressure ventilation in
specific situations so what is
non-invasive positive pressure
ventilation usually abbreviated NPPB and
the ppb is a means to support failing
respiratory function by delivering
oxygen enriched gas under pressure
without requiring endotracheal
intubation it is best used as a short
term strategy to buy time for medical
therapy to treat rapidly reversible
causes of respiratory failure these are
some typical masks that can be used to
deliver and PPV because the positive
pressure can leak out if there isn't a
tight seal between the mask and the skin
the masks must be worn very tightly
straps wrapping around the back of the
head between these three options the
nasal mask is the best tolerated by the
patient but the face masks have
physiologic advantages for example with
the nasal mask
pressure can escape via the mouth in
patients who are mouth breathers by
habits in addition the nasal passages
are relatively narrow and add
significant amounts of resistance to
airflow above that from the trachea and
bronchial tree alone however the risk of
aspiration may be diminished with a
nasal mask compared to face masks here's
an example of an unusual means to
deliver an PPV this helmet was designed
within the last several years it
actually allows patients to talk read
and drink through a straw
it is reportedly the most physically
comfortable option for patients however
it is also extremely loud here's a
typical machine that can provide n PPV
this specific model is the BiPAP vision
produced by philips respironics and is
one of the most popular non-invasive
positive pressure ventilator z' in the
United States n PPV can also be
delivered by most full ventilators doing
so provides the advantage of additional
options and provides better monitoring
however some hospitals may have policies
in place restricting use of full
ventilators in acutely ill patients -
only in the ICU
even if eventual intubation is not
expected there are a number of benefits
of n PPV over traditional invasive
mechanical ventilation it avoids the
potential for trauma secondary to
endotracheal intubation such as vocal
cord injury it avoids the need for
sedation it allows the patient to
maintain the ability to communicate it
allows for intermittent eating and
drinking if the mask can be briefly
removed and if the aspiration risk is
felt to be sufficiently low finally it
avoids the risk of ventilator associated
pneumonia there are also a number of
contraindications to n PPV in other
words situations in which the patient
should be intubated and invasively
ventilated cardiac or respiratory arrest
hemodynamic or a rhythmic instability
facial trauma or deformity severe upper
GI bleed severe encephalopathy and
inability to cooperate protect the
airway or clear secretions upper airway
obstruction and finally high aspiration
risk the two major options of
non-invasive positive pressure are CPAP
which stands for continuous positive
airway pressure and BiPAP which stands
for bi-level positive airway pressure
we'll see what the differences are
between these two options in a minute
although most other modes of ventilation
which will be discussed in lecture 7 can
also be delivered non invasively I've
observed this is rarely
in my experience you will likely become
a bit confused by the terminology used
when discussing non-invasive positive
pressure ventilation I want to spend a
minute clarifying two potential sources
of confusion first you should be aware
that some sources do not include CPAP
under the general heading of
non-invasive positive pressure
ventilation and only use the term NPPB
to refer to modes of ventilation which
provide additional positive pressure
during inspiration while the physiologic
effects of CPAP and BiPAP may be quite
distinct from one another I personally
believe that it makes more sense to
group them together here as they use the
same equipment have similar risks
benefits and contraindications and are
considered in the same general type of
patients a second potential source of
confusion is regarding the term BiPAP
unless you have absolutely no prior
familiarity with this topic
you probably pause for a moment when I
list the major options of NP PV as CPAP
or BiPAP instead of CPAP or BiPAP
despite the term by Pat being ubiquitous
only used to describe a general mode of
ventilation the term BiPAP is
technically the correct one what's the
difference
BiPAP with a little i' and BiPAP with a
big i are both the specific types of
BiPAP delivered by specific ventilator
models produced by individual
corporations to further confuse you a
minority of clinicians but a significant
number of scientific papers use the term
BiPAP and NPP v fully synonymously
when in doubt it's always best to
describe the details whatever mode of
vents aport you wish to provide to
prevent miscommunication on account of
terminology to better understand the
mechanics of non-invasive positive
pressure ventilation let's first take a
closer look at what occurs during normal
spontaneous unassisted breathing here is
a tracing of intra alveolar pressure
relative to atmospheric pressure as a
function of time during inspiration the
diaphragm descends decreasing intra
alveolar pressure whenever intra
alveolar pressure is lower than
atmospheric
sure the gradient between the two drives
air into the lungs then as the diaphragm
relaxes in a sense the intra alveolar
pressure increases above atmospheric
generating a pressure gradient which
drives air back out again in CPAP
ventilation a continuous unchanging
positive pressure is applied to the
Airways by mean of the masks seen
earlier this has a consequence of moving
the intra alveolar pressure versus time
curve up by approximately the same
degree as the value of the extrinsically
apply pressure
however the mechanics of breathing are
otherwise left relatively unchanged the
continuous positive airway pressure acts
similarly to positive end expiratory
pressure or peep which is an important
ventilator setting and using the
approximately equal sign here not to
express mathematical equivalence as much
as conceptual equivalence a more
complete discussion of the consequences
of peep will current lecture 7 & 8 for
now beware that CPAP maintains alveolar
recruitment by preventing their collapse
and can improve intrapulmonary shunting
for these reasons CPAP is predominantly
helpful at improving oxygenation
therefore it is indicated strictly for
pure hypoxic failure as might be seen
with heart failure
CPAP also has more mechanical benefits
in heart failure by reducing afterload
through a reduction in the transmural
pressure across the wall of the heart
this effect will also be discussed more
in lectures 8 and 11 BiPAP is
significantly different with BiPAP there
are two different pressures applied one
is the iPad or inspiratory positive
airway pressure the other is a path or
expert or positive airway pressure here
is a tracing of intra alveolar pressure
as a function of time for BiPAP the
patient triggers an inspiration by a
sudden downward deflection in the airway
pressure caused by contraction of the
diaphragm when the Machine senses this
it delivers positive pressure equal to
the eye pap since the iPad is higher
than the inter alveolar pressure air
flows inward supporting the lungs
ventilatory function as the intra
alveolar pressure approaches
i PAP airflow drops exploration may be
triggered either by the machine sensing
the flow dropping below certain
threshold by the passage of a
predetermined length of time or by the
patient's voluntary control during
expiration the intraocular pressure
asymptotically approaches a path in the
general sense the e PAP applied during
expiration in BiPAP ventilation
functions just like CPAP which also
functions like peep that is it improves
oxygenation ventilation is impacted by
the difference between I PAP and a pap
such that the greater the difference
between the two values the greater the
patient's tidal volume this difference
between AI PAP and a pap is essentially
equivalent to the pressure support which
is an important ventilator setting also
be to be discussed in more detail in
lecture 7 and 8 once again I'm using the
approximately equal sign here to express
conceptual equivalence not necessarily
mathematical equivalence because the
difference between I PAP and a pap can
affect tidal volume and thus alveolar
ventilation BiPAP is indicated for a
hyper Catholic respiratory failure where
ventilation is the primary problem
because BiPAP also includes expert ory
pressure it is also helpful for
oxygenation and therefore is indicated
in mixed hypoxic and hyper catholic
respiratory failure as well unlike with
CPAP some machines that delivered BiPAP
can also provide what's known as a
backup rate whereby the machine itself
will initiate an inspiration if a
certain period of time has elapsed
without a patient triggered breath a
BiPAP mode with a backup rate is
sometimes referred to as st mode for
spontaneous or timed this is to
distinguish it from s mode or
spontaneous only mode where inspiration
can only be triggered by the patient and
not the machine so here's a summary of
CPAP and BiPAP CPAP consists of a single
positive pressure that is continuous
throughout both phases of respiration a
typical starting setting is five
centimeters of water and is typically
titrated upwards as high as 12
centimeters until either patient and
tolerance occurs
or Oh to saturation is above 90%
uninspired oxygen of 60% or less the
reason to start with a relatively low
setting for CPAP and for BiPAP as well
that matter isn't just to prevent
physiologic complication it also
provides some time for the patient to
adjust to the additional support
breathing with positive pressure feels
very unnatural at first and when
combined with the claustrophobia induced
by the tight-fitting mask patient
intolerance can be a significant problem
as mentioned previously CPAP is
indicated for isolated hypoxic failure
in contrast the CPAP BiPAP consists of
two different levels of positive
pressure with a higher level occurring
during inspiration a typical value would
be a star that would be 10 were 5 where
10 refers to the eye path and 5 refers
to the EEP app this is titrated upwards
as high as about 20 over 12 or so until
tidal volume minute ventilation and/or
arterial co2 tension have reached
desired goals or until patient
discomfort develops BiPAP is indicated
for either isolated piper capping
failure or mixed hypoxic and hypercapnia
failure now I'm going to switch gears a
bit and touch upon the evidence in the
literature for using n PPV first let's
talk about CHF or slightly more
specifically acute cardiogenic pulmonary
edema there is a 2006 meta-analysis of
17 studies comparing CPAP and or BiPAP
to standard therapy or each other here's
a summary of the effect CPAP had on
overall mortality based on 10 individual
trials presented as absolute risk
differences as you can see while seven
of the 10 trials showed no statistically
significant effect on mortality when the
data from all 10 trials are pooled
together
mortality was improved by 13% with CPAP
an improvement that was statistically
significant in other words this
improvement was unlikely to be due to
chance alone when it came to the effect
on the need for intubation and in vase
mechanical ventilation CPAP reduced this
by 22% over standard medical care also
statistically significant there had
previously been concerned that CPAP
might increased risk of myocardial
ischemia when used during acute
exacerbations of CHF but this
meta-analysis demonstrated that this did
not occur when it came to BiPAP in CHF
there was a statistically nonsignificant
7% reduction in mortality and there was
a significant 18 percent reduction in
the need for intubation so what's the
bottom line in treatment of acute
cardiogenic pulmonary oedema use of CPAP
is superior to use of BiPAP which is
probably superior to standard medical
care alone there are some people who
believe that patients with CHF who
present with concurrent hypercapnia
comprises specific subgroup which would
benefit more from BiPAP than CPAP while
this makes sense based on physiology
there's only limited evidence supporting
that hypothesis as of now what about
BiPAP and COPD in 2008 chest published a
meta-analysis of 14 studies involving
979 patients in which patients
presenting with COPD exacerbations who
did not need emergent intubation and
outset were randomized to be PAP versus
standard care BiPAP reduced the risk of
eventually requiring intubation it
reduced in hospital mortality from all
causes and it reduced Hospital length of
stay by almost two days per patient you
could say that the impact of BiPAP on
COPD exacerbations is quite dramatic I
apologize that this graph may seem
excessively busy and may be illegible
depending upon the video resolution
however essentially what it this
demonstrates is that the reduction in
risk of intubation was closely related
to initial arterial pH in other words if
the initial arterial pH was normal there
was no benefit from BiPAP but the
benefit was more pronounced the lower
the pH was the authors of the study
didn't provide a specific cutoff
but from eyeballing the graph it appears
to be about 7.35 in other words the
lower limit of normal so what's the
bottom line in COPD BiPAP improves risk
of intubation survival and the hospital
stay the greatest benefit is seen with
the lowest initial arterial pH there are
four more important indications for NPV
of which you should be aware but which
will not be discussed in detail at this
specific point first use of NPP be in
the ICU has been associated with
improved outcomes in certain populations
of immune compromised patients
presumably due to decreased risk of
ventilator associated pneumonia NPP B is
also used to facilitate excavation in
some patients in order to limit the need
for emergent re-intubation this is most
commonly done during treatment for COPD
exacerbations and will be discussed more
in lecture 10 discontinuing mechanical
ventilation finally nocturnal NPP V is
standard of care in the chronic
treatment of obstructive sleep apnea and
obesity hyperventilation syndrome along
with advanced neuromuscular diseases
such as ALS a discussion of these
indications falls outside the scope of
this lecture series which is focused on
positive pressure ventilation in the
management of acute respiratory failure
only
I hope you have found this lecture on
non-invasive positive pressure
ventilation to be both interesting and
useful please continue to lecture seven
on ventilator modes
you