PaO2: partial pressure of oxygen in
the blood plasma. The normal level in the adult and child is
80-100 mm Hg.
SaO2: blood saturation of
hemoglobin, data is from arterial blood gas sample. This
value is normally above 95% in the normal adult and child.
But various conditions will result in lesser SaO2 being
considered baseline for that person.
SpO2: the blood saturation of
hemoglobin measured via the pulse-oximeter on the skin, this
figure is generally within a couple of % of the SaO2.
FiO2: fraction of inspired O2, in
units of %. Room air is 20.9% or 21% usually expressed as
0.21. FiO2 can be raised from 0.21 up to 1.00
(100%)
Hypoxemia: O2 in the blood is below
normal (PaO2 is less than 80 mm Hg or SpO2 is less than 92%.)
Hypoxia: O2 levels in the tissue
are lower than needed. This is not routinely measured, but
can be estimated if the level of Hypoxemia is known.
Hypercapnia: high levels of PaCO2.
The plasma’s carbon dioxide levels are higher than normal
(normal is 35-45 mm Hg). Increased CO2 levels create acid.
(Low pH)
Chronic Hypercapnia: PaCO2 has been
above normal for so long that the blood acid-base balance is
back to normal. It has been said that persons whose lung
disease has gone this long are "End stage" and down to there
last few years. They will react to supplementary O2
differently.
Ve: minute ventilation is the
amount of liters breathed in a minute Please note that there
should be a dot over the V. the dot stands for time and the
V stands for flow. The small e stands for exhaled.:
The respiratory rate bpm x the Vt
[tidal volume] = the Ve in lpm.
RR x Vt = Ve
Ventilator drive: the brain controls
breathing by alterations of phrenic nerves impulses to
increase and decrease the Ve.
The Ve alterations keep the blood pH
within a narrow range by increasing or decreasing the PaCO2
. This is quite successful and we keep our PaCO2 within +/-
3 mm Hg all day long. So we normal persons breathe to get
rid of CO2. If this fails, there is a back up system in the
brain that responds to levels of Hypoxemia below 50 mm Hg.
Indications for supplementary oxygen administration in the
acute care setting.
According to the AARC Guidelines,
the use of supplementary oxygen is indicated for the
following:
Documented hypoxemia
·In normal adults and
children [over 28 days of life] give supplemental O2 if
the PaO2 is less than 60 mmHg and/or the SpO2 is less
than 90%
·In neonates, give
additional O2 if the PaO2 is less than 50 mm Hg and the
SpO2 is less than 88% or capillary blood is less than 40
mmHg. [NOTE: Most textbooks say keep SpO2 90-92% even on
neonates to reduce apnea of prematurity. Keep PaO2 about
60-70 mmHg]
Suspected hypoxemia: if s/s of SOB
you may start O2 then check SpO2 or blood gases later
Severe trauma, cardiac arrhythmia,
acute or chronic lung disease
Decrease the work of the heart in
acute myocardial infarction
Short term therapy: post-op
recovery, during procedures such as bronchoscopy.
Contraindications for supplementary
oxygen administration.
There are no contraindications to
O2 therapy, but there are several cautions. SEE HAZARDS.
Hazards of supplementary oxygen
administration in the adult.
Depression of the ventilatory drive
in persons who suffer from chronic Hypercapnia such as:
·End stage COPD and
emphysema
·or long term
neuromuscular disorder in which decreased Ve has
resulted in chronic Hypercapnia
·or morbid obesity in
which decreased Ve has resulted in chronic Hypercapnia
The normal person breathes to get
rid of CO2. If the CO2 is always elevated above normal,
a person’s brain stops reacting to the CO2. If this
reaction happens, there is a fail safe--- the hunger for
O2. Unlike the rest of us, these persons breathe to get
oxygen.
·If we give them more
than the 55-60 mm Hg that they need, they will breathe
more and more shallowly until they fall asleep and die.
·This can happen within a
very few minutes
Keep a person with chronic
hypercapnia at a PaO2 between 55-60 mm Hg, by giving
O2 at a
FiO2 between 24-28%. This will relieve the hypoxemia without
decreasing the ventilatory drive. Their SpO2 is fine at
91-92%
Oxygen Toxicity will damage the lung
tissue: the addition of excessive levels of FiO2 will raise
the tissue and the blood O2 to the point that the O2 free
radicals proliferates.
·O2 free radicals are the
chemical byproducts of cellular respiration. These free
radicals are inactivated by enzymes such as supraoxide
dismutase but excessive O2; even in proscribed amounts
can increase these radicals so that the body can’t
process them.
·Vitamin E and other
antioxidants will defend against this but most important
is to monitor O2 and back off as soon as possible.
Monitor with pulse oximetry to watch that we give just
the O2 that is needed.
·Lung damage from O2
toxicity will result in pulmonary fibrosis and thickened
alveolar walls and capillary walls so that diffusion of
O2 is hampered.
·Type I cells are damaged
and there are more type II cells.
Limit FiO2 of 100% to only
·24 hours
·to above 70% for less
than 2 days
·50% for less than 5
days.
·Generally FiO2 of less
than 40% are considered safe as far as O2 toxicity is
concerned.
Remember: that a person whose PaO2
is less than 50 on FiO2 of 50% or more is in a condition
called refractory hypoxemia. O2 is not enough; it is time to
move to other Rx.
Absorption Atelectasis: In addition
to O2 toxicity, FiO2's above 50% will increase the risk of
alveolar collapse from all the gas in the alveoli leaving
the air sac so that it collapses.
·Nitrogen while not used
in the cellular metabolism does have a function in the
body of keeping the alveoli open when the O2 leaves to
enter the capillaries.
·As the FiO2 increases
the Nitrogen decreases and the volume of the alveoli
drop as O2 is diffused out of the alveoli.
·Nitrogen can be washed
out of the lung in less than 30 minutes.
·Persons at most risk as
those whose Vt is decreased such as post-op. Patients,
sedated persons, or persons with CNS depression.
Additional hazards of supplementary oxygen administration in
the newborn.
Retinopathy of prematurely (ROP) or retrolental fibroplasia
(RLF): if PaO2 rises to above 75 mm
Hg, the more premature infants will develop damage to the
retina of the eye. This complication has been know since the
40-50s
·This is a complex
disorder that is dependent on the infant’s gestational
age as much as the PaO2.
·There is no safe FiO2
for infants. We follow them with the pulse ox [ 90-92%]
or the PaO2 55-65 mmHg.
Bronchopulmonary dysplasia BPD is a
severe form of O2 toxicity that newborns who have been on
mechanical ventilation can get This has been know since the
70s
·This disorder is due to
a combination of O2 toxicity and barotrauma from
mechanical ventilation.
·Minimize this by keeping
the baby’s PaO2 55-65 MmHg and the SpO2 between 90-92%
Note: keep PaO2 below 80 mm Hg. (high 60s is nice) both
neonatal complications can be minimized by keeping SpO2
90-92%.
All infants under 30 days of age,
and who are on O2, should have continuous pulse oximetry
readings and be given an exact FiO2 which is analyzed and
monitored. This has the standard of care in 93% of the USA
nurseries for decades. If a hospitals' nursery cannot afford
O2 analyzers or pulse-ox for each kid, it sure cannot afford
the lawsuits. Everyone's old country granny knows that '
O2
blinds babies.'
Concentration of air at one atmosphere.
·78% Nitrogen N2: makes
up most of the atmosphere.
·The cells do not use N2.
It is medically significant except under hyperbaric
conditions or absorption atelectasis
·20.9% O2: needed for
cellular metabolism
·.93%: Argon
·.03% CO2: this varies a
bit based on the pollution levels
·Other trace gases
O2 delivery devices
Nasal cannula: Double prongs fit into the nares.
·FiO2 range between
24-44% at quiet breathing. Decreases as patient's
inspiratory flow rates increase and Ve increases.
·May use without
humidifier at low flows such as less than 4 lpm
·Usual flow rates in
adults range between 1 lpm to 6 lpm.
·Don't use more than 6
lpm because the flow rate into the nose is uncomfortable
·Most common type of O2
delivery device.
·Safest O2 to put on
anyone are 1-2 lpm nasal cannula if you are worried
about blunting the ventilatory drive.
·Contraindicated if the
patient needs more than 40% or more than 6 lpm of flow
·Pressure points on ears
and nares can be a problem
Simple mask: plastic covering over the face increases the
reservoir so that FiO2 at a given liter flow is a bit higher
than with the cannula
·FiO2 between 0.30 to
0.60 with quiet breathing, if the mask is put on tightly
·Should not use at less
than 5 lpm to prevent rebreathing CO2
·Usual flow rates are
5-12 lpm.
·Must use a bubble
humidifier at all flow rates
·This is a low flow
system so air is pulled into the mask as the Ve rises,
the FiO2 to drops
·Patient complain of
claustrophobia, 'hot", or that they cannot eat, or talk
·Pressure points on ear
and bridge of the nose
·Hazard of all masks:
patient can aspirate vomitus
Partial breathing mask: simple mask with the addition of a
bag type of reservoir increased the FiO2
·On exhalation, the first
third of exhaled gases enter the reservoir bag, so that
it increases the FiO2 with minimal rebreathing of CO2,
making it a partial rebreather The first third comes
from the conducting airways (the anatomical Vd) with
higher O2 and lower CO2 than the other 2/3rds.
·Volume of the reservoir
bag is 600-800 mL so that the patients’ inspiration has
plenty of fresh O2 mixed with a little Vd gases.
·There is a minimum of
air entrainment but still the FiO2 is only 40-70%
·This is a low flow
system, because entrainment is still possible.
·The flow rate should be
high enough to keep the reservoir bag partially inflated
during inspiration. Generally between 8-15 lpm, but as
the patient’s Ve rises in distress, this flow rate may
need to be increased.
·The mask should fit
tight
·Exhalation occurs out of
the ports
·The patient can only
inhale from the reservoir bag and must exhale out the
ports so that this is a true none-rebreather
·Minimal flow rate is 10
lpm, but the flow rate must be high enough to keep the
reservoir inflated during the inspiratory phase
·FiO2 60%-80%
·Over time with humidity
the valves can stick
·It is still a low flow
system because some entrainment is possible
Venturi or air-entrainment masks
·These masks have
variable orifices that allow a “controlled air
entrainment”, as well as various entrainment
ports
·As the orifice size
decreases, the gas that leaves it has a higher velocity
and it pulls more air from outside the entrainment port
into the gas flow going to the patient; higher FiO2 have
larger orifices. More O2 comes through and less air is
entrained.
·As the entrainment ports
on the side increase, there is more air pulled into the
O2 flow so that the FiO2 decreases.
·The ratio of entrained
air to O2 flows is exact so that this device can deliver
a high flow with an exact FiO2.
·FiO2 ranges from 24%-70%
with other brands have some but not all the selections
in between
·The set flow rate, which
is the one read on the dial of the meter, will be
determined by the FiO2 because the entrainment device
has restricted orifices that create back pressure if the
flow is excessive.
·Lower FiO2 selections
have smaller, restricted orifices so that one must set
the flowmeter to a lower flow rate
·Higher FiO2 selections
will have larger, less restricted orifices so that the
flow rate can be set higher
·The RCP sets the flow
rate on the meter where the entrainment mask
manufacturer tells them to set it. It will be marked on
the packet and sometimes on the device itself.
·Set flow rate should not
be confused with total flow rate, which is the sum of
the O2 flow rate + entrained air flow.
·The more entrained air,
the higher the total flow to the patient FiO2 may be
decreasing but the flow rate is rising
·The RCP can hear the
difference as the device is turned from higher to lower
FiO2; the noise level rises as more gas is entrained.
·The total flow rate to
the patient will differ form the set flow rate coming
out of the flow meter
·With any entrainment
device, the flow rate at the patient’s nose will always
be higher than the set flow rate.
·At a FiO2 of 24% there
is 25 lpm of air entrained for each 1 lpm of O2 on the
meter.
Example
Set O2 flow rate on meter is 3 lpm,
at 24% the Air: O2 ratio is 25:1
this means that [3 x 25 lpm] of air
is entrained so 75 lpm of air is entrained this 75 lpm is added to the original 3 lpm.
The total flow is 78 lpm.
Remember: as FiO2 rises, the total
flow rate drops because less air is entrained. This means
that at FiO2 of more than 40%, the entrainment device is now
a low flow or variable flow device again. The patient can
now entrain air from around the outside of the mask again.
Humidifiers and flow rates
The use of a humidifier is
determined by the flow rate of the device because higher
velocity gas running down the narrow O2 lines creates a lot
of resistance to flow. Backpressure builds up and the
humidifier pop off alarm will go off if the backpressure
gets high enough.
·Nasal cannula below 4
lpm don’t need a humidifier
·All simple masks need
humidifiers because their lowest flow is 5 lpm
·Partial and non-rebreathing
masks need humidifiers but if the pop off alarm goes off
from excessive flow, remove the humidifier. Try it first
with the humidifier but it is more important to get the
O2 to the patient than the humidification.
As a general rule, there is too much
back pressure in an entertainment mask to allow gas flow
through the humidifier without the alarms going off, so
these generally are used without humidifier
O2 devices used in the home care setting
Pendant reservoir cannula : there is a 40 ml reservoir on
the end of a tube that is attached to a nasal cannula
·On inspiration, the
patient pulls O2 from an oversized cannula tubing and
lastly from the reservoir
·The first part of
exhalation goes into the tubing and the reservoir fills
·Once the reservoir
empties, this device acts like a regular cannula
·Disadvantages are size
and weight
Pulse dose O2 delivery: this is an electronic device that
replaces the flowmeter.
·It notes and opens on
the inspiratory effort via an electronic ‘flow sensor’
and it initiates the flow rate as long as the patient is
inhaling
·There is a savings of O2
at home but this flow may not be good enough for
exercise
·The liter flow during
the actual breath may be much higher than the continue
flow would be.
·I.e.: at 2 lpm may get a
pulse of 12 lpm
Moustache-style reservoir cannula:
·Used in the home for
long term O2 patients, the nasal cannula has a huge
reservoir, which holds about 20 mL of O2.
·The reservoir fills with
O2 during exhalation so that at the first part of the
next inspiration the patient will get a little more O2
than he would without a reservoir
·Once this initial bolus
of
O2 is gone, the cannula acts like a regular cannula
·Generally the patient
gets an O2 prescription at a certain flow rate and the
SpO2 is measured as the flow rate is decreased
SCOOP transtracheal catheter: a catheter is placed
surgically with the end sitting in the trachea above the
carina
·The patient’s airway
becomes the reservoir
·The flow rate can be
decreased sometimes as much as by 50%
·The patient can wear the
O2 under his shirt and no
one knows he is on O2
·This can be combined
with pulsed O2 to save even more O2
Disadvantage
·Surgical procedure
·Infection, mucus plugs
·Irritation to trachea
·Hemoptysis [bleeding]
·Requires removal and
cleaning with sterile solutions and a cleaning rod
Troubleshooting O2 delivery devices
Nasal cannula:
·clogged with mucus or
blood
·prongs are mal-placed in
the nose
·O2 line can be kinked or
disconnected
·No backpressure alarm if
no humidifier is used
Simple mask
·O2 line can be kinked or
disconnected
·Increased chance of
rebreathing CO2
Partial breathing mask
·O2 line can be kinked or
disconnected
·watch the fluctuations
of the reservoir during the breath cycle
odeflates on inspiration-
if not, it might be loosened on the face so entrainment
through the sides can happens
ore-inflates on
exhalation- if not the flow is too low or the tubing is
kinked or valves stuck
(Commercial) non rebreathing mask
·O2 line can be kinked or
disconnected
·watch the fluctuations
of the reservoir during the breath cycle
odeflates on inspiration-
if not, it might be loosened on the face so entrainment
through the sides can happens
ore-inflates on
exhalation- if not the flow is too low or the tubing is
kinked or valves stuck
Venturi or entrainment mask
·O2 lines can be kinked
or disconnected
·If there obstruction
downstream from the flow, there will be decreased
entrainment and the FiO2 will rise
·If there is obstruction
upstream the FiO2 will drop as will the total flow rate
·Sometimes the
entrainment port can collect dust like any other intake
valve.
·If the entrainment port
is covered up by the bed sheets, the entrainment drops,
the FiO2 rise and the flow rate drops.
·Humidifier pop-off is
alarming, remove and use without humidifier
Comparing high flow systems to low flow
O2 systems and the
effect of the patient’s Ve on the delivered FiO2 of these
devices.
Low flow O2 delivery devices have a
total flow rate that is less than the patient's inspiratory
needs. Generally a flow rate needs to be more than the
patient's inspiratory flow rate or the patient will have to
pull in room air so that the FiO2 drops as the patient
breaths faster and deeper. So as the patient needs 02 more,
he gets less with the low flow [or variable performance]
O2-delivery device.
Desired flow rate = Ve x [I + E]
So a person's flow rate must be
about 3 x his Ve for the FiO2 to be considered high flow.
Patient's rate is 12 bpm and the Vt is 500, the Ve is 6 lpm
and for a O2 flow to be considered a high flow system, there
must be about 6 x 3 or 18 lpm.
But as the patient gets sicker and
respiratory rate rises to 20 bpm and the Vt rise to 700 the
Ve is now up to 14 x 3 or 42 lpm. Now the person must
entrain more air, which will dilute out the FiO2.
The single most important
disadvantage to the low flow system is that as the patient’s
Ve rises in a response to hypoxemia, the delivered FiO2
drops.
Calculating the potential FiO2 of a nasal cannula at a given
flow rate.
Formula for estimated FiO2 of the
nasal cannula of the adult breathing quietly Start at .20 and add .04% per lpm
Example
1 lpm = [1 x .04] + .20 = .24 or 24%
2 lpm = [2 x .04] + .20 = .28 or 28% 3 lpm = [3 x .04] + .20 = .32 or 32%
4 lpm = [4 x .04] + .20 = .36 or 36% 5 lpm = [5 x .04] + .20 = .40 or 40%
Calculating the entrainment ratio of a given FiO2 via the
‘magic box.
Find the the difference between 100%
and the FiO2 needed and the difference between 20% and the
FiO2 . these differences are placed into a ratio.
20% 100%
40% 60 20
Air flow
O2 flow
At a FiO2 of 40%, there is a ratio of 60 lpm
of air mixed with 20 lpm O2 Or Ratio of 6:2 or 3:1
20% 100%
60%
40 40
Air flow
O2 flow
At a FiO2 of 60%, there is a ratio of 40 of
air mixed into 40 lpm O2 Or Ratio of 40:40 or 1:1
Calculating the total flow at the patient’s interface when
given a FiO2 and a set flow rate from the flowmeter.
·add the O2 flow rate to
the air flow rate
·set flow rate + [O2
flow rate x entrainment ratio]
