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ABG indications, Parameters, and Interpretations

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by

Tatsuki Abe

on 27 October 2013

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Transcript of ABG indications, Parameters, and Interpretations

Understanding
Arterial Blood Gas

Indications
Interpretations
Parameters
when do we need to do ABG?
Identification of
respiratory, metabolic, and mixed acid-base disorders
,
with or without physiologic compensation
, by means of
pH ([H+]) and CO2 levels (partial pressure of CO2)
Measurement of the partial pressures of
respiratory gases
involved in oxygenation and ventilation
Monitoring of acid-base status, as in patient with
diabetic ketoacidosis
(DKA) on insulin infusion; ABG and venous blood gas (VBG) could be obtained simultaneously for comparison, with VBG sampling subsequently used for further monitoring
Assessment of the response to therapeutic interventions such as mechanical ventilation in a patient with
respiratory failure
Determination of arterial respiratory gases during
diagnostic evaluations

(eg, assessment of the need for
home oxygen therapy in patients with advanced chronic pulmonary disease
)
what are the parameters that we have to look at, and what do they represent?
what kind of interpretations can we derive from the ABG readings?
INDICATIONS
Contraindications
The presence of
arteriovenous fistulas
or vascular grafts, in which case arterial vascular puncture should not be attempted
Known or suspected severe
peripheral vascular disease
of the limb involved
Local infection
or
distorted anatomy at the potential puncture site
(eg, from previous surgical interventions, congenital or acquired malformations, or burns)
An abnormal
modified Allen test

very simply put, the 3 parameters we mainly look at - as far as pH is concerned, are ;

pH value

CO2 value

HCO3 value
coagulopathy
7.35
7.45
35
45
22
26
acidotic
acidotic
alkalotic
alkalotic
alkalotic
acidotic
Quantitating the
partial oxygen pressure (PO2) or O2 saturation
during the administration of 100% oxygen -
distinguishing cyanosis produced primarily by heart disease or by lung disease.

In
cyanotic heart disease,
the
partial arterial oxygen pressure (PaO2) increases very little
when 100% oxygen is administered over the values obtained while breathing room air. However,
PaO2 usually increases significantly when oxygen is administered to a patient with lung disease
EASY
6-step interpretation of ABG

is the
pH
normal?
is the
pCO2
normal?
is the
HCO3-
Normal?
Match the
pH
with the
CO2
and
HCO3-
levels
is there
compensation
?
review
PaO2
, and
O2
levels
when do we consider the "
Base Excess
"?
Calculation of the base excess or base deficit may be very useful in determining the
therapeutic measures
to be administered to a patient. The base excess or base deficit is the
number of milliequivalents of acid or base needed to titrate 1 L of blood to pH 7.4 at 37°C if the PaCO2were held constant at 40 mm Hg.
hence, you calculate the Base Excess when you want to
correct the patient's pH profile using Sodium Bicarbonate
etc.
when do we consider the "Anion Gap"?
Calculation of the
anion gap
can be helpful in
determining the cause of a patient's
metabolic acidosis
. It is determined by
subtracting the sum of a patient's plasma chloride and bicarbonate concentrations
(in mEq/L) from his or her plasma sodium concentration
Anion Gap = [Na+] − ([Cl-] + [HCO3−])

The anion gap is normally
12 ± 4 mEq/L
.
metabolic acidosis
with an abnormally great anion gap (i.e.,
greater than 16 mEq/L
) would probably be caused by;

lactic acidosis or ketoacidosis
;

ingestion of organic anions such as salicylate, methanol, and ethylene glycol;

or renal retention of anions such as sulfate, phosphate, and urate.
mnemonic MUD PILES:
M - Methanol, Metformin
U - Uremia,
D - Diabetic Ketoacidosis
P - Paraldehyde
I - Iron, INH
L - Lactate
E - Ethanol, ethylene glycol
S - Salicylate
As we look at the patient's ABG reading, we would like to interpret;

the patient's blood pH profile
whether the pH derangement (if any) has been caused by a respiratory or metabolic cause (or Mixed)
whether there is any compensatory mechanism
whether the patient is oxygenated adequately
7.35
7.45
acidotic
alkalotic
first step is to see if the patient is having acidosis or alkalosis.
35
45
alkalotic
acidotic
determine if the level of CO2 is either alkalotic, acidic or normal. label.
acidotic
alkalotic
22
26
likewise, determine and label the HCO3- level.
now, compare the pH with the CO2 and HCO3- level

if the pH is acidic and CO2 acidic, it is a respiratory acidosis;
pH acidic and HCO3- acidic, metabolic acidosis.
does either the CO2 or HCO3 go in the opposite direction of the pH? If so, there is compensation by that system.
If they are below limits there is evidence of hypoxemia.
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