Diagnosis - ABG Flashcards by Erica Zanath

What important values are obtained via ABG?

pH
PaCO2
PaO2

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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

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What is the normal pH?

7.4

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What is the normal PaCO2?

Why is it used?

used to detect ventilation problems

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What is the normal PaO2?

Why is it used?

100

Used to detect oxygenation problems

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Henderson Hassalbalch basic relationship

pH ~ HCO3- / PaCO2

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Acidemia

blood pH < 7.35

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Acidosisis

a primary physiologic process that, occurring alone, tends to cause academia

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Alkalemia

blood pH > 7.45

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Akalosis

a primary physiologic process that, occurring alone, tends to cause alkalemia

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Primary acid-base disorder

Change that occurs first in the balance of HCO3- and PaCO2

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Compensations

Change in HCO3- or PaCO2 that occurs as a result of the primary event

Never over compensate

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Respiratory alkalosis

first change - lower PaCO2
causes elevated pH

then, kidney compensates - secondary lowering of HCO3-

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Respiratory acidosis

first change - elevation of PaCO2
causes decreased pH

then, kidney compensates - retains HCO3-

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Metabolic acidosis

first change - lowering of HCO3- b/c kidney is excreting it
causes decreased pH

then, lung compensate - hyperventilation - lowers PaCO2

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Metabolic alkalosis

first change - elevation of HCO3-
causes increased pH

then, lungs compensation - hypoventilation - increases PaCO2

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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

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Metabolic acidosis and increased anion gap

MUDPILES
Mehtanol
Uremia
Diabetic ketoacidosis
Propylene glycol
Isoniziad
Lactic acidosis
Ethylene glycol
Salicylates

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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)?

1. the oxygen saturation to the dissolved oxygen concentration (SpO2 to PaO2) 2. 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