Diagnosis - ABG Flashcards
What important values are obtained via ABG?
pH
PaCO2
PaO2
What is an Allen’s test?
Compression of ulnar and radial arteries
–release each, one at a time, to check for reperfusion
Demonstrates the blood supply to the hand
Used to check that the hard is still well supplied if a thrombus were to occur
What is the normal pH?
7.4
What is the normal PaCO2?
Why is it used?
40
used to detect ventilation problems
What is the normal PaO2?
Why is it used?
100
Used to detect oxygenation problems
Henderson Hassalbalch basic relationship
pH ~ HCO3- / PaCO2
Acidemia
blood pH < 7.35
Acidosisis
a primary physiologic process that, occurring alone, tends to cause academia
Alkalemia
blood pH > 7.45
Akalosis
a primary physiologic process that, occurring alone, tends to cause alkalemia
Primary acid-base disorder
Change that occurs first in the balance of HCO3- and PaCO2
Compensations
Change in HCO3- or PaCO2 that occurs as a result of the primary event
Never over compensate
Respiratory alkalosis
first change - lower PaCO2
causes elevated pH
then, kidney compensates - secondary lowering of HCO3-
Respiratory acidosis
first change - elevation of PaCO2
causes decreased pH
then, kidney compensates - retains HCO3-
Metabolic acidosis
first change - lowering of HCO3- b/c kidney is excreting it
causes decreased pH
then, lung compensate - hyperventilation - lowers PaCO2
Metabolic alkalosis
first change - elevation of HCO3-
causes increased pH
then, lungs compensation - hypoventilation - increases PaCO2
Anion Gap
Na+ - (Cl- + CO2)
Note: CO2 in this equation is the “total CO2” measured in the chemistry lab as part of a routine serum electrolytes
Metabolic acidosis and increased anion gap
MUDPILES Mehtanol Uremia Diabetic ketoacidosis Propylene glycol Isoniziad Lactic acidosis Ethylene glycol Salicylates
Metabolic acidosis and normal anion gap
HARDUPS Hyperalimentation Acetazolamide or carbonic anhydrase inhibitors Renal tubular acidosis Diarrhea Ureteroenteric fistula Pancreaticoduodenal fistual Spironolactone
Causes of metabolic alkalosis
–all of these are considered ..
Chloride responsive
- Vomiting
- Contraction alkalosis
- Diuretics
- Corticosteriods
- Gastric suctioning
Chloride resistant
-hyperaldosterone state, e.g Cushings
Respiratory acidosis causes
CNS depression (e.g. drug overdose) Chest bellows dysfunction (e.g. GBS, MG) Disease of lungs and/or upper airway
Respiratory alkalosis causes
Hypoxemia (includes altitude)
Anxiety
Sepsis
Acute pulmonary insult (e.g. pneumonia, etc.)
Mixed acid-base disorders
more common than single disorders in those that are chronically ill
A increase in PaCO2 by 10 will do what do the pH
–note, this is in an acute setting
decrease it by 0.08
How does compensation change in chronic respiratory settings?
For every increase in PaCO2 by 10, the pH will decreased by 0.03
In chornic resp acidosis, the kidneys compensate by holding onto HCO3 to buffer the pH
These is a 5 pt bicarb rise for every 10 mm PaCO2 rise
FiO2
fraction of inspired oxygen (O2 in air)
SpO2
“pulse-ox” or bound oxygen % (finger probe)
SaO2
Bound oxygen % (measured directly)
PiO2
Pressure inspired oxygen in the trachea
A-a gradient equation
Alveolar oxygen - arterial oxygen
PiO2 - (PaCO2 / R) - PaO2
R = 0.8 = respiratory quotient
Expected normal = Patient age/4 + 4
What is a respiratory quotient?
Ratio of CO2 produced to O2 consumed
What does a normal gradient mean?
A-a gradient will be normal if pure hypoventilation or altitude (not lung problem) is the cause of hypoxia
What does an abnormally elevated gradient mean?
Abnormal (elevated) when gas diffusion is impaired (lung problem)
What two additional relationships are important to know in order to quantify the degree of diffusion or hypoxemia (low O2 concentration)?
- the oxygen saturation to the dissolved oxygen concentration (SpO2 to PaO2)
- the dissolved oxygen concentration to the inspired oxygen concentration (PaO2 to FiO2)
S curve
- what is saturation like
- what kind of curve
At 60 mmHg, you are saturating at 90%
Sigmoidal curve
–due to positive cooperatively of Hg
What causes a right shift?
What does this mean?
O2 is released quicker from hemoglobin
High PCO2
Fever
Acidosis
High 2,3-DPG
What causes a left shift?
What does this mean?
Hemoglobin holds more on to O2
Alkalosis
Hypothermia
Low PCO2
Lower 2-3, DPG
Why is PaO2 to FiO2 useful?
What is a normal ratio?
It compares the concentration of dissolved oxygen (PaO2) to inspired or “inhaled” oxygen (FiO2)
As the severity of diffusion impairment increase this ratio decreases
Normal PaO2:FiO2 on room air = 100:21% = ~475
(short cut - normal is about 5x FiO2)
Decreased V/Q
Areas in the lung that are better perfused than ventilation
aka SHUNT - blood is shunted from R to L w/o being oxygenation
Increased V/Q
Areas that are better ventilated than perfused
aka DEAD SPACE
Lung zones
-what does this lead to?
A normal expected V/Q mismatch
- -at the lower part of the lungs V/Q = 0.6
- -at the top part of the lung V/Q = 3
Zone 1
PA > Pa > Pv
Zone 2
Pa > PA > Pv
Zone 3
Pa > Pv > PA
What diseases cause increased V/Q
Pulmonary embolism
emphysema?
What diseases cause decreased V/Q (shunt)
AV malformation
Pneumonia
Fibrosis
Secretions
What are the two types of shunts?
Extra pulmonary
- R to L cardiac shunts
- e.g. tetralogy of fallot
Intra pulmonary
- Blood is transported through the lungs without taking part in gas exchange
- e.g. atelectasis, pneumonia, AVM
