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Arterial Blood Gas - Analysis

last authored: Dec 2009, David LaPierre

Introduction

Arterial Blood Sampling is usually performed via a puncture of the Radial Artery at the wrist or the Femoral Artery at the groin. Although the Dorsalis Pedis and Posterior Tibial Arteries are occasionally used, punctures of other sites are uncommon.

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Evaluation of Arterial Blood Gases

Classify the acid-base disturbance based on pH.

Anion gap

Increased anion gap in metabolic acidosis; decreased in hypoalbuminemia, paraproteinemia, halide ingestion, lab error)

anion gap = Na⁺ = Cl^- - HCO₃ ; normal is <8-12

Assess for compensation

Δ in HCO₃ should equal the Δ in anion gap

Δ in HCO₃ should equal Δ in PCO₂ x correction factor of xxx

If PCO₂ is not available, can calculate by

H⁺ = 24 x PCO₂/HCO₃ or
pH = 6.1 + log(HCO₃^-/0.03) x PaCO₂

If over 10% error, then one of the values is wrong

Converting pH and H⁺

Log rule

pH = -log[H+]

ie pH 7.4 = 10^-7.4 mol/L H+ = 40

pH - [H+] rule of thumb

For a pH range of 7.25-7.55

drop the 7.

subtract from 80

ie pH 7.38 = 42

the 0.1 pH change rule

x 0.8 for every increase of 0.1 in pH

/ 0.8 for every decrease of 0.1 in pH

pH 7.00 = 100 nM H+

ph 7.4 = 40 nM H+

Arterial Blood Gas Analysis

An Arterial Blood Gas provides the clinician with a number of pieces of information, some measured and some calculated using the Henderson Hasselbach equation (which we shall ignore for the moment in the interests of sanity).

Measured - pH Calculated - HCO3

pCO2 Base Deficit (excess)

pO2

pH ( normally 7.36 - 7.44) gives a measure of the overall acid/base status of the blood

pCO2 (normally 38 - 42 mm Hg) is a reflection of ventilation

pO2 (normally 85 - 95 mm Hg) is a measure of oxygenation and independent of Acid Base Status

HCO3 (normally 21 - 27 meq/L) is calculated in an ABG. It is measured in serum electrolytes.

Base Deficit (Excess) is reported in meq/L and is a calculated estimate of the amount of base required to return pH to the normal range.

Rule: pH , pCO2 and HCO3 are related mathematically through the Henderson - Hasselbach Equation (below) which we will not derive here. Any change in pH is therefore a reflection of a change in either pCO2 or HCO3.

pH = 6.1 + log [HCO3]

0.03 x paCO2

↑ pCO2 = ↓ pH ↓ pCO2 = ↑ pH

↑ HCO3 = ↑ pH ↓ HCO3 = ↓ pH

The effect of changes in pCO2 and HCO3 on pH may be predicted. This is useful in determining their relative contributions to an acid base abnormality.

_pCO2 of 10 mm Hg → _pH of 0.08 (Opposite direction)

_HCO3 of 10 mmol/l → _pH of 0.15 (Same direction)

The body will normally respond to a change in pCO2 or HCO3 (and hence pH) in an attempt to return pH towards the normal range.

The body is able to increase or decrease pCO2 by altering the rate and depth of respiration.
The body is able to adjust [HCO3] through cellular shifts, generation and excretion.
Some compensations are more effective and rapid than others. i.e. the body can rapidly change its pCO2 in most cases but changes to [HCO3] occur more slowly.

Simple Acid - Base abnormalities may be thought of in terms of one or more primary disturbances with or without some evidence of compensation. In general, chronic acid - base abnormalities will be more completely compensated than will acute ones.

Simple Acid-Base Disorders

Disorder Primary Disturbance Compensatory Response (timing) Final Status

Respiratory

Acidosis CO2 Retention HCO3 Generation (Days) ↑↑ CO2/↑ HCO3

Alkalosis CO2 Depletion HCO3 Excretion (Weeks) ↓↓ CO2/↓ HCO3

Metabolic

Acidosis HCO3 Depletion Increased Ventilation (Mins - Hrs) ↓↓ HCO3/↓ CO2

Alkalosis HCO3 Retention Decreased Ventilation (Hours) ↑↑ HCO3/↑ CO2

When interpreting the results of arterial blood gases, it is helpful to look at them in a stepwise fashion. Note that ABG’s are always best interpreted in the context of a clinical situation.

First - Look at the pH and determine whether the blood is acidemic, alkalemic or normal range.

Second - Check the pCO2. Comparing the pCO2 to the pH will generally indicate whether you are dealing with a primary respiratory or metabolic problem.(i.e. Is the pCO2 change a cause of or a compensation for the acid base disturbance ?)

Third - Using the four principles on the following page, compare the pCO2 and the HCO3 to determine whether there has been any compensation. (Sometimes there will be a combined acid base disturbance i.e. there are multiple primary abnormalities - as you can imagine this can get complicated.)

Principle #1: Compensation for Respiratory Acidosis takes place over days and is generally effective in restoring a near normal pH. In a compensated case there will be a rise in [HCO3] of ~ 3.5 mmol/l for every 10 mm Hg rise in pCO2. Maximum [HCO3] is usually ~ 38 mmol/l.

Principle #2: Chronic Respiratory Alkalosis is unique in its ability to be fully compensated with a normal pH. For every 10 mm Hg drop in pCO2 there is a corresponding drop of ~ 5 mmol/l in [HCO3].

Principle #3: In a compensated Metabolic Acidosis, the pCO2 will drop by ~ 1.2 mm Hg for every 1.0 mm Hg drop in [HCO3]. The lower limit of pCO2 is usually ~ 10 mm Hg.

Principle #4: In a compensated Metabolic Alkalosis, the pCO2 will rise by up to 0.6 mm Hg for every 1.0 mmol/l rise in [HCO3}. In room air, the maximum compensatory rise in pCO2 is generally ~ 60 mm Hg at which point hypoxia usually supervenes and causes increased respiration.

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