ABG indications, Parameters, and Interpretations

Understanding

Arterial Blood Gas

Interpretations

Indications

Parameters

when do we need to do ABG?

what kind of interpretations can we derive from the ABG readings?

what are the parameters that we have to look at, and what do they represent?

Contraindications

• An abnormal modified Allen test

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)

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

• Local infection or distorted anatomy at the potential puncture site (eg, from previous surgical interventions, congenital or acquired malformations, or burns)

INDICATIONS

• 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

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

• coagulopathy

Assessment of the response to therapeutic interventions such as mechanical ventilation in a patient with respiratory failure

Measurement of the partial pressures of respiratory gases involved in oxygenation and ventilation

EASY 6-step interpretation of ABG

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

Determination of arterial respiratory gases during diagnostic evaluations

(eg, assessment of the need for home oxygen therapy in patients with advanced chronic pulmonary disease)

when do we consider the "Base Excess"?

when do we consider the "Anion Gap"?

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.

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

is there compensation?

is the HCO3- Normal?

is the pCO2 normal?

is the pH normal?

Match the pH with the CO2 and HCO3- levels

review PaO2, and O2 levels

mnemonic MUD PILES:

M - Methanol, Metformin

U - Uremia,

D - Diabetic Ketoacidosis

P - Paraldehyde

I - Iron, INH

L - Lactate

E - Ethanol, ethylene glycol

S - Salicylate

Anion Gap = [Na+] − ([Cl-] + [HCO3−])

hence, you calculate the Base Excess when you want to correct the patient's pH profile using Sodium Bicarbonate etc.

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.

The anion gap is normally 12 ± 4 mEq/L.

likewise, determine and label the HCO3- level.

first step is to see if the patient is having acidosis or alkalosis.

determine if the level of CO2 is either alkalotic, acidic or normal. label.

does either the CO2 or HCO3 go in the opposite direction of the pH? If so, there is compensation by that system.

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.

If they are below limits there is evidence of hypoxemia.

acidotic

alkalotic

acidotic

alkalotic

7.35

7.45

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