Interpretation:
Compensated and
Uncompensated
Blood Gas Analysis
James Barnett, RN, MSN
Clinical Educator – Neuroscience PCC
Vanderbilt University Medical Center
May 2007
Compensatory Mechanisms

Compensation is the body’s way of restoring a
normal blood pH

Remember: Acid + Base  Neutrality

Compensation DOES NOT treat the root of
the problem – the reason for the acid-base
imbalance is STILL THERE!!!
Compensatory Mechanisms

The body has three means to try to compensate
for an acid-base imbalance

Chemical

Respiratory

Renal
Chemical Compensation



Chemicals within the blood act within seconds
to correct respiratory or metabolic imbalances
Used up quickly – not effective long-term
Chemical buffers in the blood include
Bicarbonate
 Phosphate
 Proteins

Respiratory Compensation

Used to compensate for metabolic imbalances
only

Chemoreceptors respond to changes in H+
concentrations  alters respiratory rate and
depth

Remember CO2 is an acid
Respiratory Compensation

Respiratory Rate will…

Increase when blood H+ is increased (acidic pH)
CO2 is “blown off ”
 Amount of acid in blood is decreased


Decrease when H+ is decreased (alkaline pH)
CO2 is retained
 Amount of acid in blood is increased

Respiratory Compensation

This means

Metabolic acidosis causes an increase in rate and
depth of ventilation as the body attempts to get rid
of acid (CO2)

Metabolic alkalosis causes a decrease in rate and
depth of ventilation as the body attempts to retain
acid (CO2)
Renal Compensation

Used to compensate for respiratory imbalances

Remember: HCO3- is a base

Kidneys respond to changes in blood pH
Excrete H+ and retain HCO3- when acidemia is
present (1:1 ratio)
 Retain H+ and excrete HCO3- when alkalemia is
present (1:1 ratio)

Renal Compensation

This means

A respiratory acidosis will make the kidneys excrete
acid (H+) and retain base (HCO3-)

A respiratory alkalosis will make the kidneys excrete
base (HCO3-) and retain acid (H+)
Renal Compensation

This is the slowest compensation

May take hours to days

Most powerful method of compensation

Ineffective in patients with renal failure
Note on Compensation
The body is very smart and will not
overcompensate for an imbalance
Degrees of Compensation

An acid-base imbalance will be compensated for
in one of three ways

Uncompensated

Partially compensated

Fully compensated
Degrees of Compensation

Uncompensated


Body has made no attempt to correct the acid-base
imbalance
Partially compensated
Body is attempting to correct the imbalance
 Blood pH remains abnormal in spite of the attempt

Degrees of Compensation

Fully compensated

The body is correcting the imbalance

Blood pH is normal

Other blood gas values remain abnormal until the root
cause is treated and corrected
Uncompensated
Acid-Base
Imbalance
Uncompensated Imbalance

pH abnormal
Either PaCO2 OR HCO3- abnormal
All other values normal

If PaCO2 is abnormal




Problem is respiratory
If HCO3- is abnormal

Problem is metabolic
Uncompensated
Imbalance

Uncompensated
respiratory acidosis

Uncompensated
respiratory alkalosis

pH
PaCO2
HCO3-

pH
PaCO2
HCO3-


< 7.35
> 45
WNL


> 7.45
< 35
WNL
Remember that CO2 is an acid and that the more of it there is the worse is
the acidemia. Notice that with uncompensated respiratory, the HCO3 is
normal – this is because the body has not began to compensate for the
alterations in CO2
Uncompensated
Imbalance

Uncompensated
metabolic acidosis

Uncompensated
metabolic alkalosis

pH
PaCO2
HCO3-

pH
PaCO2
HCO3-


< 7.35
WNL
< 22


> 7.45
WNL
> 26
Remember that HCO3 is a base and that the more of it there is the more
alkalotic you will be. Notice that in the case of uncompensated metabolic
the PaCO2 is normal indicating that the body has not began to compensate.
Partially Compensated
Imbalances

Occur when compensation mechanisms are
activated, but have not had sufficient time to
normalize the blood pH

NOTE: Some people say that there is no such
thing as “partially” compensated – it is kind of
like being “a little pregnant” – but it is indicative
of a part of the process called compensation
Partially Compensated
Imbalances


pH is abnormal
Both PaCO2 and HCO3- are abnormal in the
same direction (increased or decreased from
normal)
If PaCO2 is high (↑ acid), HCO3- will also be high (↑
alkaline) to neutralize the environment
 If PaCO2 is low (↓ acid), HCO3- will also be low (↓
alkaline) to neutralize the environment

Partially Compensated
Imbalances

Partially Compensated
Respiratory Acidosis

Partially Compensated
Respiratory Alkalosis

pH
PaCO2
HCO3-

pH
PaCO2
HCO3-


< 7.35
> 45
> 26


> 7.45
< 35
< 22
In the case of Partially Compensated Resp Acidosis, the pH is low, indicating an
acid environment…when you look at the PaCO2, it too is acidic, which is how you
know that you have a respiratory acidosis. With the HCO3 being high, you can
deduce that the body is raising its base to counteract the acid represented by the pH;
therefore, partially compensated respiratory acidosis.
Partially Compensated
Imbalances

Partially Compensated
Metabolic Acidosis

Partially Compensated
Metabolic Alkalosis

pH
PaCO2
HCO3-

pH
PaCO2
HCO3-


< 7.35
< 35
< 22


> 7.45
> 45
> 26
With partially compensated metabolic acidosis, you notice first that the pH is low (acidosis).
Ask yourself, which number is representative of an acid condition. In this case it is the low
base (HCO3), so you know you have a metabolic acidosis. You know it is partially compensated
because the PaCO2 is low indicating that CO2 (an acid) is being lost from the body to correct
for the low pH.
Compensated Imbalances
Occur when compensatory mechanisms have been
able to fully normalize blood pH
Compensatory Mechanisms

Both PaCO2 and HCO3- are abnormal, but in
the same direction

If PaCO2 is high (↑ acid), HCO3- will also be high (↑
alkaline)

If PaCO2 is low (↓ acid), HCO3- will also be low
(↓alkaline)
Compensated Imbalances

Compensated
Respiratory Acidosis

Compensated
Respiratory Alkalosis

pH

pH

PaCO2
HCO3-

PaCO2
HCO3-

WNL but
closer to
7.35
> 45
> 26

WNL but
closer to
7.45
< 35
< 22
In compensated respiratory acidosis, the pH tends to range between 7.35 and 7.39 – still acidic,
But in the normal pH range. When you look at the PaCO2, you notice that it is high (acidic), but
The HCO3 is also high, indicating that the body has compensated and normalized the low pH.
Compensated Imbalances

Compensated Metabolic
Acidosis

Compensated Metabolic
Alkalosis

pH

pH

PaCO2
HCO3-

PaCO2
HCO3-

WNL but
closer to
7.35
< 35
< 22

WNL but
closer to
7.45
> 45
> 26
Mixed Imbalances


Occur when patient has both metabolic and
respiratory disorders that cause an acid-base
imbalance
Examples:
Diabetic KetoAcidosis (metabolic acidosis) with
decreased respiratory drive (respiratory acidosis)
 Severe vomiting (metabolic alkalosis) with high fever
(respiratory alkalosis)

Mixed Imbalances

pH will be normal

PaCO2 and HCO3- will be abnormal

PaCO2 will be high with low HCO3- (both tend toward
acid side)

PaCO2 will be low with high HCO3- (both tend toward
base side)
Mixed Imbalances

Mixed acidosis

Mixed alkalosis

pH
PaCO2
HCO3-

pH
PaCO2
HCO3-


< 7.35
> 45
< 22


> 7.45
< 35
> 26
Notice with the mixed acidosis that you have an acidic pH (less than 7.35, with other
Parameters indicating an acid environment. High PaCO2 (too much acid). Low HCO3
(too little base – an acidic environment). This is classic mixed acidosis.
Finished


You have finished this in-service.
Please go to the next in-service titled:

Effects of Acid Base on Oxygenation