/ABG

Interpreting an ABG is a crucial skill for health care professionals, and it is especially important in critically ill patients.

This guide will focus on a commonly used seven-step process, which helps ensure a complete interpretation of every ABG.

In addition, provided is a list of commonly encountered acid-base disorders.

Many guides exist to aid the interpretation of the ABG, some may vary in how they have ordered, or combined their steps. We have put this guide together in a method which appears clinically appropriate and logical to us, however a knowledge of other techniques may still be helpful.

7 Step Approach

STEP 1

Review the patient, including taking a brief history and examination

Before thinking about analysing the ABG, we need some context about the patient’s current clinical status. For instance:

 1. Are they on supplementary oxygen?
     - Although the ABG may look normal, if they are on high-flow oxygen, you may expect their PaO2 to be much higher.

 2. Are they hyper- or hypoventilating?
     - A normal PaCO2 in someone who is hyperventilating is not normal.

 3. Are there any previous ABG results?
     - A chronic problem such as COPD may require a different response to compared to a patient with acute hypoxia.

STEP 2

Is this patient hypoxic?

When first approaching an ABG result, first check the oxygenation status of the patient.
This is assessed via the PaO2.

This is because hypoxia is likely to kill you in minutes, where as a acid-base imbalance is more likely to take hours.
A good example of this is an asthma attack, which presents with a low PaO2, but a relatively normal acid-base balance, however clearly needs rapid action.

The normal range for PaO2 is: 10 - 14kPa.

   <10kPa = hypoxia
   <8kPa   =
severe hypoxia, and respiratory failure (type 1: PaCO2 <6kPa; type 2: PaCO2 >6kPa)

NOTE: If the patient is receiving supplemental oxygen, their PaO2, as a general rule, should be approximately 10kPa less than the % inspired concentration / FiO2.
So a patient on 40% oxygen would be expected to have a PaO2 of approximately 30kPa.

STEP 3

Is there acidemia or alkalemia present?

pH < 7.35  acidemia

7.35 - 7.45 normal

pH > 7.45  alkalemia

Even if the pH is normal however, it does not rule out a pathology.

NOTE: The term acidemia/alkalemia describes the state of low/high blood pH, while acidosis/alkalosis is used to describe the processes leading to these states. However, the terms are sometimes used interchangeably clinically.

STEP 4

Is there a respiratory component? (pCO2)

↑ CO2

Shift in the equilibrium (→) (∴ CO
2 + H2O → H2CO3 HCO3- + H+)

Therefore ↑H+ (and ↑HCO3-, see note below)

↓ pH (acidaemia)

↓ CO2

Shift in the equilibrium (←) (
HCO3- + H+ → H2CO3 → CO2 + H2O)

Therefore ↓H
+ (Increased CO2 is subsequently removed by ↑ventilation)

↑ pH (alkalaemia)

  • The respiratory system alters pH by changing how much CO2 it retains or expels, by changing the bodies ventilation rate.

CO2 + H2O H2CO3HCO3- + H+
Carbon Dioxide + Water Carbonic Acid Bicarbonate + Hydrogen Ion

In more simplistic terms, CO2 forms carbonic acid (and then H+), therefore a high CO2 with lead to an acidemia, and a low CO2, due to a reduction of carbonic acid (and H+) will lead to an alkalemia.

CO2 ≈ Acidotic

So to determine if the cause is respiratory in origin, the pH and PaCO2 must change in opposite directions.

                                                pH        PaCO2
Respiratory acidosis              ↓              ↑
Respiratory alkalosis             ↑              ↓

If they change in the same directions (so that an increase in the acidotic PaCO2 is associated with alkalotic high pH, it may be a compensatory response, explained in  step 6)

The normal range for PaCO2 is: 4.7 - 6kPa.

NOTE: There are very large quantities of HCO3- in the body compared to H+, this means that a large change in the concentration of H+ can occur, with only a very small change in the concentration of HCO3-. So even though the dissociation of H2CO3, leads to an equal number of HCO3- and H+ ions being generated, the change in pH may be significant, whereas the change in HCO3- concentration may be negligible.

STEP 5

Is there a metabolic component? (HCO3-)

↑ HCO3-

Shift in the equilibrium (←) (
HCO3- + H+ → H2CO3 → CO2 + H2O)

Therefore ↓H
+ (as gets converted into CO2 + H2O to be 'blown off' through ventilation)

↑ pH (alkalaemia)

↓ HCO3-

Shift in the equilibrium (→) (
CO2 + H2O → H2CO3 HCO3- + H+)

Therefore ↑H
+

↓ pH (acidaemia)

  • The metabolic system (namely the kidneys) alters pH by retaining or excreting bicarbonate (HCO3-)

CO2 + H2O H2CO3HCO3- + H+
Carbon Dioxide + Water Carbonic Acid Bicarbonate + Hydrogen Ion

HCO3- can be thought to "mop up" H+, increasing pH, therefore a high bicarbonate will lead to an alkalemia, and a low bicarbonate an acidemia.

HCO3- ≈ Alkalotic

So to determine if the cause is metabolic in origin, pH and PaCO2 must change in the same direction.

                                                pH        HCO3-
Metabolic acidosis                ↓              ↓
Metabolic alkalosis               ↑              ↑

If they change in the opposite directions (so that an increase in the alkalotic HCO3- is associated with a acidaemic low pH, it may be a compensatory response, explained in  step 6)

The normal range for HCO3- is: 22 - 26 mmol/L.

STEP 6

Which component is the primary disruption, and is there compensation present?

There may be some cases where both respiratory and metabolic components are present. In this case, commonly one is the primary component, or cause of the disruption, and the other one is a compensatory process, which attempts to correct the pH.

If the cause of the pH imbalance is from the respiratory system, metabolic compensation can either renally excrete or retain HCO3-, to balance the pH and bring it back closer to the normal range.
If the cause of the pH imbalance is metabolic, the respiratory system can compensate by either retaining or expelling ('blowing off') CO2 to balance the pH (via increasing or decreasing ventilation).

When coming to approach if compensation is taking place, assess which value is contradicting the pH.
For instance a high HCO3- (alkalotic) and a low pH (acidemia) suggests metabolic compensation; a high CO2 (acidotic) and a high pH (alkalemia) suggests respiratory compensation. (You may also note that the compensation moves in the same direction as the primary disruption i.e. high PaCO2 and high HCO3-.)

                                                                                                                    pH        PaCO2       HCO3-
Acidosis   Respiratory    with metabolic compensation                      ↓              ↑                ↑
Acidosis   Metabolic       with respiratory compensation                    ↓              ↓                ↓
Alkalosis   Respiratory   with metabolic compensation                      ↑              ↓                ↓
Alkalosis   Metabolic      with respiratory compensation                    ↑              ↑                ↑

Partially compensated = pH still out of normal limits; Fully compensated = pH returned to normal limits

Usually, compensation does not return the pH to normal (7.35 – 7.45), and over compensation never occurs.

NOTE: Respiratory compensation can happen over the course of minutes; metabolic compensation however takes days, therefore if metabolic compensation is present, it can be assumed that the respiratory disruption has been going on for at least a few days.

NOTE: It is possible to have a mixed picture in terms of causes i.e. respiratory & metabolic acidosis / respiratory & metabolic alkalosis. In which case both pathologies are leading to a combined acidemia or alkalemia, and therefore both causes need correcting.

STEP 7

Are there any other deranged values?

 Ensure that you look at all other figures on the analysis, and comment on any abnormalities.

 

/Abg

James Lloyd

Hull York Medical School

/Abg

Interpreting an ABG is a crucial skill for health care professionals, and it is especially important in critically ill patients.

This guide will focus on a commonly used seven-step process, which helps ensure a complete interpretation of every ABG.

In addition, provided is a list of commonly encountered acid-base disorders.

Many guides exist to aid the interpretation of the ABG, some may vary in how they have ordered, or combined their steps. We have put this guide together in a method which appears clinically appropriate and logical to us, however a knowledge of other techniques may still be helpful.

7 Step Approach

STEP 1

Review the patient, including taking a brief history and examination

Before thinking about analysing the ABG, we need some context about the patient’s current clinical status. For instance:

 1. Are they on supplementary oxygen?
     - Although the ABG may look normal, if they are on high-flow oxygen, you may expect their PaO2 to be much higher.

 2. Are they hyper- or hypoventilating?
     - A normal PaCO2 in someone who is hyperventilating is not normal.

 3. Are there any previous ABG results?
     - A chronic problem such as COPD may require a different response to compared to a patient with acute hypoxia.

STEP 2

Is this patient hypoxic?

When first approaching an ABG result, first check the oxygenation status of the patient.
This is assessed via the PaO2.

This is because hypoxia is likely to kill you in minutes, where as a acid-base imbalance is more likely to take hours.
A good example of this is an asthma attack, which presents with a low PaO2, but a relatively normal acid-base balance, however clearly needs rapid action.

The normal range for PaO2 is: 10 - 14kPa.

   <10kPa = hypoxia
   <8kPa   =
severe hypoxia, and respiratory failure (type 1: PaCO2 <6kPa; type 2: PaCO2 >6kPa)

NOTE: If the patient is receiving supplemental oxygen, their PaO2, as a general rule, should be approximately 10kPa less than the % inspired concentration / FiO2.
So a patient on 40% oxygen would be expected to have a PaO2 of approximately 30kPa.

STEP 3

Is there acidemia or alkalemia present?

pH < 7.35  acidemia

7.35 - 7.45 normal

pH > 7.45  alkalemia

Even if the pH is normal however, it does not rule out a pathology.

NOTE: The term acidemia/alkalemia describes the state of low/high blood pH, while acidosis/alkalosis is used to describe the processes leading to these states. However, the terms are sometimes used interchangeably clinically.

STEP 4

Is there a respiratory component? (pCO2)

  • The respiratory system alters pH by changing how much CO2 it retains or expels, by changing the bodies ventilation rate.

CO2 + H2O H2CO3HCO3- + H+
Carbon Dioxide + Water Carbonic Acid Bicarbonate + Hydrogen Ion

↑ CO2

Shift in the equilibrium (→) (∴ CO
2 + H2O → H2CO3 HCO3- + H+)

Therefore ↑H+ (and ↑HCO3-, see note below)

↓ pH (acidaemia)

↓ CO2

Shift in the equilibrium (←) (
HCO3- + H+ → H2CO3 → CO2 + H2O)

Therefore ↓H+ (Increased CO2 is subsequently removed by ventilation)

↑ pH (alkalaemia)

In more simplistic terms, CO2 forms carbonic acid (and then H+), therefore a high CO2 with lead to an acidemia, and a low CO2, due to a reduction of carbonic acid (and H+) will lead to an alkalemia.

CO2 ≈ Acidotic

So to determine if the cause is respiratory in origin, the pH and PaCO2 must change in opposite directions.

                                                pH        PaCO2
Respiratory acidosis              ↓              ↑
Respiratory alkalosis             ↑              ↓

If they change in the same directions (so that an increase in the acidotic PaCO2 is associated with alkalotic high pH, it may be a compensatory response, explained in  step 6)

The normal range for PaCO2 is: 4.7 - 6kPa.

NOTE: There are very large quantities of HCO3- in the body compared to H+, this means that a large change in the concentration of H+ can occur, with only a very small change in the concentration of HCO3-. So even though the dissociation of H2CO3, leads to an equal number of HCO3- and H+ ions being generated, the change in pH may be significant, whereas the change in HCO3- concentration may be negligible.

STEP 5

Is there a metabolic component? (HCO3-)

  • The metabolic system (namely the kidneys) alters pH by retaining or excreting bicarbonate (HCO3-)

CO2 + H2O H2CO3HCO3- + H+
Carbon Dioxide + Water Carbonic Acid Bicarbonate + Hydrogen Ion

↑ HCO3-

Shift in the equilibrium (←) (
HCO3- + H+ → H2CO3 → CO2 + H2O)

Therefore ↓H+ (as gets converted into CO2 + H2O to be 'blown off' through ventilation)

↑ pH (alkalaemia)

↓ HCO3-

Shift in the equilibrium (→) (
CO2 + H2O → H2CO3 HCO3- + H+)

Therefore ↑H+

↓ pH (acidaemia)

HCO3- can be thought to "mop up" H+, increasing pH, therefore a high bicarbonate will lead to an alkalemia, and a low bicarbonate an acidemia.

HCO3- ≈ Alkalotic

So to determine if the cause is metabolic in origin, pH and PaCO2 must change in the same direction.

                                                pH        HCO3-
Metabolic acidosis                ↓              ↓
Metabolic alkalosis               ↑              ↑

If they change in the opposite directions (so that an increase in the alkalotic HCO3- is associated with a acidaemic low pH, it may be a compensatory response, explained in  step 6)

The normal range for HCO3- is: 22 - 26 mmol/L.

STEP 6

Which component is the primary disruption, and is there compensation present?

There may be some cases where both respiratory and metabolic components are present. In this case, commonly one is the primary component, or cause of the disruption, and the other one is a compensatory process, which attempts to correct the pH.

If the cause of the pH imbalance is from the respiratory system, metabolic compensation can either renally excrete or retain HCO3-, to balance the pH and bring it back closer to the normal range.
If the cause of the pH imbalance is metabolic, the respiratory system can compensate by either retaining or expelling ('blowing off') CO2 to balance the pH (via increasing or decreasing ventilation).

When coming to approach if compensation is taking place, assess which value is contradicting the pH.
For instance a high HCO3- (alkalotic) and a low pH (acidemia) suggests metabolic compensation; a high CO2 (acidotic) and a high pH (alkalemia) suggests respiratory compensation. (You may also note that the compensation moves in the same direction as the primary disruption i.e. high PaCO2 and high HCO3-.)

pH        PaCO2       HCO3-
Acidosis   Respiratory    with metabolic compensation
↓              ↑                ↑
Acidosis   Metabolic       with respiratory compensation
↓              ↓                ↓
Alkalosis   Respiratory   with metabolic compensation
↑              ↓                ↓
Alkalosis   Metabolic      with respiratory compensation
↑              ↑                ↑

Partially compensated = pH still out of normal limits; Fully compensated = pH returned to normal limits

Usually, compensation does not return the pH to normal (7.35 – 7.45), and over compensation never occurs.

NOTE: Respiratory compensation can happen over the course of minutes; metabolic compensation however takes days, therefore if metabolic compensation is present, it can be assumed that the respiratory disruption has been going on for at least a few days.

NOTE: It is possible to have a mixed picture in terms of causes i.e. respiratory & metabolic acidosis / respiratory & metabolic alkalosis. In which case both pathologies are leading to a combined acidemia or alkalemia, and therefore both causes need correcting.

STEP 7

Are there any other deranged values?

 Ensure that you look at all other figures on the analysis, and comment on any abnormalities.