Arterial Blood Gases Flashcards
facts about ABG
- it’s serious - only get if needed
- remember Allen’s test
- cost is ~ $75
- apply pressure for 5 min. after
- pt is usually being given chest compressions - complicates finding pulse
oxygen tension
measure of molecular oxygen dissolved in blood
hgb saturation
oxygen bound to hgb
what drives oxygen into the tissues?
oxygen tension
when do you get a Hgb saturation of 95-99%?
when PaO2 is 80-90 mmHg (measured w ABG)
During resuscitation, if PaO2 is less than 80 mmHG, what pulmonary causes would you consider?
- 100% oxygen is not being delivered
- incorrect intubation
- aspiration of gastric contents/solids
- pulmonary edema and/or
- pneumothorax
During resuscitation, if PaO2 is less than 80 mmHG, what cardiac causes would you consider?
- poor chest compressions
- dysrhythmias
- pump failure
What determines gas exchange?
CO2 - if there is no blood flow through the lungs, won’t have gas exchange
what is the result of too much O2?
- partial pressure will increase causing free radical formation
- ARDA; IRDS; neuro issues
- maintain O2 sat between 94-99% - not 100%
what percent of hgb is normally saturated at any given time?
-94-99%
What is the relationship between % saturation of hgb and PO2?
- a small drop in hgb saturation will cause a significant drop in pO2
- ex: 80% saturation of hgb = 50% PO2
reading a CO2 detector
- purple = problem
- tan = think
- yellow = yes
-it’s purple to start out, CO2 changes it yellow; if intubated incorrectly like in the right mainstem bronchus, it will be tan
waveform capnography
quantitative measure of CO2
PetCO2
- PCO2 right before you breathe in
- if it drops below 10 mmHg, need to push harder on chest compressions
what is the product of normal metabolism?
- 2 forms of acid:
- respiratory
- metabolic
respiratory acid formed in normal metabolism
-volatile: carbonic
H2CO3 H2O +CO2
metabolic acid formed in normal metabolism
-larger molecular size:
from amino acids, fats, carbs, phosphoric, sulfuric: excreted by kidneys
H+ + HCO3
normal metabolism total equation
H+ + HCO3- H2CO3 H2O + CO2
what happens during anaerobic metabolism?
- ex: cardiac arrest/DKA
- production of lactic acid accumulates, hence metabolic acidosis
- strong acids combine w/ sodium bicarb resulting in a weak acid and neutral salt
- the weak carbonic acid is blown off by the lungs as CO2
anaerobic metabolism equation
HAcid + NcHCO3 –> H2CO3 + NaA NaA + H2O + CO2
CO2 and water forms ________, which can then be broken down into _____ and ___.
CO2 and water forms carbonic acid, which can then be broken down into bicarb and H+.
what are the 2 ways in which you can get rid of acid?
- get rid of H+
- get rid of CO2
how to effect pH through respiratory rate
- getting rid of CO2 leads to no formation of carbonic acid
- retaining CO2 leads to formation of more acid and more H+
buffer systems consist of what?
- a weak acid
- the salf of the weak acid
purpose of buffering systems
to prevent the accumulation of carbonic and metabolic acids that produce marked changes in acidity of the body
What is the most important buffering system in the body?
sodium bicarbonate and carbonic acid system
what is the weak acid in the sodium bicarb/carbonic acid buffer?
carbonic acid
what is another example of a buffer system in the body?
- hgb
- if it dumps all O2 in the periphery and takes nothing but CO2, it’s serving as a buffer for the RBC
pH
- hydrogen ion concentration
- ration of base:acid
what is the ratio in pH
- NaHCO3:H2CO3
- Base:Acid
- 20:1
changes in PaCO2 result in changes of what?
H2CO3 (NOT NaHCO3)
because an acid cannot react w/ its own salt, so only the ratio b/w the two changes
as [H+] double or halves, what is the result?
- pH change of 0.3
- therefore, small pH changes signify marked accumulation/depletion of acid
normal ABG values
- pH: 7.35-7.45 (use 7.4 as reference)
- PaO2: 80-90 mmHg
- PaCO2: 35-45 mmHg (use 40 as reference)
- HCO3: 21-28 mEq/l
golden rule #1
A change in PaCO2 of 10 mmHg = up/down pH change of 0.08 units
golden rule #2
A change in HCO3 of 10 mEql = up/down pH change of 0.15 units
if the calculated pH approximates the real pH, what is the mechanism?
pure respiratory
if the calculation pH do not approximate the real pH, what is the mechanism?
it must have a metabolic component
how do you treat respiratory and metabolic acidosis?
by increasing ventilations and giving a base (ex: NaHCO3)
driving the equation to the right
base deficit is d/t what?
loss of sodium bicarb from ECF
base deficit expresses…
the deficit of sodium bicarb in mEq for a L of ECF
golden rule #3
base deficit X pt. weight in kg / 4
metabolic acidosis is treated with …
IV sodium bicarb
if the ABG is out of whack, but the pH is normal, what is going on?
compensation
compensation
- in chronic disease, lungs/kidneys may compensate for disease in the other
- if pH has returned to 7.35 - 7.45, the compensated
- if not, uncompensated
in COPD, what do the kidneys do?
retain base
in hyperaldosteronism, what do the lungs do?
retain CO2 (increased renal absorption of bicarb)
if pH is increased…
= alkalosis
- if PaCO2 is in same direction = metabolic
- if PaCO2 is in opposite direction = respiratory
if pH is decreased…
= acidosis
- if PaCO2 is in same direction = metabolic
- if PaCO2 is in opposite direction = respiratory
