Interpretation:Interpretation:Compensated and Compensated and UncompensatedUncompensated

Blood Gas AnalysisBlood Gas AnalysisJames Barnett, RN, MSNJames Barnett, RN, MSN

Clinical Educator – Neuroscience Clinical Educator – Neuroscience PCCPCC

Vanderbilt University Medical Vanderbilt University Medical CenterCenter

May 2007May 2007

Compensatory Compensatory MechanismsMechanisms

Compensation is the body’s way of Compensation is the body’s way of restoring a normal blood pHrestoring a normal blood pH

Remember: Remember: Acid + Base Acid + Base Neutrality Neutrality

Compensation DOES NOT treat the root Compensation DOES NOT treat the root of the problem – the reason for the acid-of the problem – the reason for the acid-base imbalance is STILL THERE!!!base imbalance is STILL THERE!!!

Compensatory Compensatory MechanismsMechanisms

The body has three means to try to The body has three means to try to compensate for an acid-base compensate for an acid-base imbalanceimbalance

ChemicalChemical

RespiratoryRespiratory

RenalRenal

Chemical CompensationChemical Compensation

Chemicals within the blood act within Chemicals within the blood act within seconds to correct respiratory or seconds to correct respiratory or metabolic imbalancesmetabolic imbalances

Used up quickly – not effective long-Used up quickly – not effective long-termterm

Chemical buffers in the blood includeChemical buffers in the blood include BicarbonateBicarbonate PhosphatePhosphate ProteinsProteins

Respiratory Respiratory CompensationCompensation

Used to compensate for Used to compensate for metabolicmetabolic imbalances onlyimbalances only

Chemoreceptors respond to changes Chemoreceptors respond to changes in H+ concentrations in H+ concentrations alters alters respiratory rate and depthrespiratory rate and depth

Remember CORemember CO22 is an acid is an acid

Respiratory Respiratory CompensationCompensation

Respiratory Rate will…Respiratory Rate will… Increase when blood HIncrease when blood H++ is increased is increased

(acidic pH)(acidic pH) COCO22 is “blown off” is “blown off” Amount of acid in blood is decreasedAmount of acid in blood is decreased

Decrease when HDecrease when H++ is decreased is decreased (alkaline pH)(alkaline pH) COCO22 is retained is retained Amount of acid in blood is increasedAmount of acid in blood is increased

Respiratory Respiratory CompensationCompensation

This meansThis means Metabolic Metabolic acidosisacidosis causes an increase causes an increase

in rate and depth of ventilation as the in rate and depth of ventilation as the body attempts to get rid of acid (CObody attempts to get rid of acid (CO22))

Metabolic Metabolic alkalosisalkalosis causes a decrease causes a decrease in rate and depth of ventilation as the in rate and depth of ventilation as the body attempts to retain acid (CObody attempts to retain acid (CO22))

Renal CompensationRenal Compensation

Used to compensate for Used to compensate for respiratory respiratory imbalancesimbalances

Remember: HCORemember: HCO33-- is a base is a base

Kidneys respond to changes in blood Kidneys respond to changes in blood pHpH Excrete HExcrete H++ and retain HCO and retain HCO33

-- when when acidemia is present (1:1 ratio)acidemia is present (1:1 ratio)

Retain HRetain H++ and excrete HCO and excrete HCO33-- when when

alkalemia is present (1:1 ratio)alkalemia is present (1:1 ratio)

Renal CompensationRenal Compensation

This meansThis means A respiratory acidosis will make the A respiratory acidosis will make the

kidneys excrete acid (Hkidneys excrete acid (H++) and retain ) and retain base (HCObase (HCO33

--))

A respiratory alkalosis will make the A respiratory alkalosis will make the kidneys excrete base (HCOkidneys excrete base (HCO33

--) and retain ) and retain acid (Hacid (H++))

Renal CompensationRenal Compensation

This is the slowest compensationThis is the slowest compensation

May take hours to daysMay take hours to days

Most powerful method of compensationMost powerful method of compensation

Ineffective in patients with renal Ineffective in patients with renal failurefailure

Note on CompensationNote on Compensation

The body is very smart and will not The body is very smart and will not overcompensate for an imbalanceovercompensate for an imbalance

Degrees of CompensationDegrees of Compensation

An acid-base imbalance will be An acid-base imbalance will be compensated for in one of three compensated for in one of three waysways

UncompensatedUncompensated

Partially compensatedPartially compensated

Fully compensatedFully compensated

Degrees of CompensationDegrees of Compensation

UncompensatedUncompensated Body has made no attempt to correct Body has made no attempt to correct

the acid-base imbalancethe acid-base imbalance

Partially compensatedPartially compensated Body is attempting to correct the Body is attempting to correct the

imbalanceimbalance Blood pH remains abnormalBlood pH remains abnormal in spite of in spite of

the attemptthe attempt

Degrees of CompensationDegrees of Compensation

Fully compensatedFully compensated

The body is correcting the imbalanceThe body is correcting the imbalance

Blood pH is normalBlood pH is normal

Other blood gas values remain Other blood gas values remain abnormalabnormal until the root cause is treated until the root cause is treated and correctedand corrected

UncompensatedUncompensatedAcid-BaseAcid-BaseImbalanceImbalance

Uncompensated Uncompensated ImbalanceImbalance

pH abnormalpH abnormal Either PaCOEither PaCO22 OROR HCO HCO33

-- abnormal abnormal All other values normalAll other values normal

If PaCOIf PaCO22 is abnormal is abnormal Problem is Problem is respiratoryrespiratory

If HCOIf HCO33-- is abnormal is abnormal

Problem is Problem is metabolicmetabolic

UncompensatedUncompensatedImbalanceImbalance

Uncompensated Uncompensated respiratory respiratory acidosisacidosis

pHpH < 7.35< 7.35 PaCOPaCO22 > 45> 45

HCOHCO33-- WNLWNL

Uncompensated Uncompensated respiratory respiratory alkalosisalkalosis

pHpH > 7.45> 7.45 PaCOPaCO22 < 35< 35

HCOHCO33-- WNLWNL

Remember that CO2 is an acid and that the more of it there is the worse isthe acidemia. Notice that with uncompensated respiratory, the HCO3 is normal – this is because the body has not began to compensate for thealterations in CO2

UncompensatedUncompensatedImbalanceImbalance

Uncompensated Uncompensated metabolic acidosismetabolic acidosis

pHpH < 7.35< 7.35 PaCOPaCO22 WNLWNL

HCOHCO33-- < 22< 22

Uncompensated Uncompensated metabolic alkalosismetabolic alkalosis

pHpH > 7.45> 7.45 PaCOPaCO22 WNLWNL

HCOHCO33-- > 26> 26

Remember that HCO3 is a base and that the more of it there is the morealkalotic you will be. Notice that in the case of uncompensated metabolicthe PaCO2 is normal indicating that the body has not began to compensate.

Partially CompensatedPartially CompensatedImbalancesImbalances

Occur when compensation mechanisms Occur when compensation mechanisms are activated, but have not had sufficient are activated, but have not had sufficient time to normalize the blood pHtime to normalize the blood pH

NOTE: Some people say that there is no NOTE: Some people say that there is no such thing as “partially” compensated – such thing as “partially” compensated – it is kind of like being “a little pregnant” it is kind of like being “a little pregnant” – but it is indicative of a part of the – but it is indicative of a part of the process called compensationprocess called compensation

Partially CompensatedPartially CompensatedImbalancesImbalances

pH is abnormalpH is abnormal Both PaCOBoth PaCO22 and HCO and HCO33

-- are abnormal in are abnormal in the same direction (increased or the same direction (increased or decreased from normal)decreased from normal) If PaCOIf PaCO22 is high (↑ acid), HCO is high (↑ acid), HCO33

-- will also be will also be high (↑ alkaline) to neutralize the high (↑ alkaline) to neutralize the environmentenvironment

If PaCOIf PaCO22 is low (↓ acid), HCO is low (↓ acid), HCO33-- will also be will also be

low (↓ alkaline) to neutralize the low (↓ alkaline) to neutralize the environment environment

Partially CompensatedPartially CompensatedImbalancesImbalances

Partially Partially Compensated Compensated Respiratory Respiratory AcidosisAcidosis

pHpH < 7.35< 7.35 PaCOPaCO22 > 45> 45

HCOHCO33-- > 26> 26

Partially Partially Compensated Compensated Respiratory Respiratory AlkalosisAlkalosis

pHpH > 7.45> 7.45 PaCOPaCO22 < 35< 35

HCOHCO33-- < 22< 22

In the case of Partially Compensated Resp Acidosis, the pH is low, indicating anacid environment…when you look at the PaCO2, it too is acidic, which is how youknow that you have a respiratory acidosis. With the HCO3 being high, you candeduce that the body is raising its base to counteract the acid represented by the pH;therefore, partially compensated respiratory acidosis.

Partially CompensatedPartially CompensatedImbalancesImbalances

Partially Partially Compensated Compensated Metabolic AcidosisMetabolic Acidosis

pHpH < 7.35< 7.35 PaCOPaCO22 < 35< 35

HCOHCO33-- < 22< 22

Partially Partially Compensated Compensated Metabolic AlkalosisMetabolic Alkalosis

pHpH > 7.45> 7.45 PaCOPaCO22 > 45> 45

HCOHCO33-- > 26> 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 lowbase (HCO3), so you know you have a metabolic acidosis. You know it is partially compensatedbecause the PaCO2 is low indicating that CO2 (an acid) is being lost from the body to correctfor the low pH.

Compensated ImbalancesCompensated Imbalances

Occur when compensatory Occur when compensatory mechanisms have been able to mechanisms have been able to fully fully

normalize blood pHnormalize blood pH

Compensatory Compensatory MechanismsMechanisms

Both PaCOBoth PaCO22 and HCO and HCO33-- are abnormal, are abnormal,

but in the same directionbut in the same direction

If PaCOIf PaCO22 is high (↑ acid), HCO is high (↑ acid), HCO33-- will also will also

be high (↑ alkaline)be high (↑ alkaline)

If PaCOIf PaCO22 is low (↓ acid), HCO is low (↓ acid), HCO33-- will also will also

be low (↓alkaline)be low (↓alkaline)

Compensated ImbalancesCompensated Imbalances

Compensated Compensated Respiratory Respiratory AcidosisAcidosis

pHpH WNL but WNL but

closer to closer to 7.357.35

PaCOPaCO22 > 45> 45

HCOHCO33-- > 26> 26

Compensated Compensated Respiratory Respiratory AlkalosisAlkalosis

pHpH WNL but WNL but

closer to closer to 7.457.45

PaCOPaCO22 < 35< 35

HCOHCO33-- < 22< 22In 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), butThe HCO3 is also high, indicating that the body has compensated and normalized the low pH.

Compensated ImbalancesCompensated Imbalances

Compensated Compensated Metabolic AcidosisMetabolic Acidosis

pHpH WNL but WNL but

closer to closer to 7.357.35

PaCOPaCO22 < 35< 35

HCOHCO33-- < 22< 22

Compensated Compensated Metabolic AlkalosisMetabolic Alkalosis

pHpH WNL but WNL but

closer to closer to 7.457.45

PaCOPaCO22 > 45> 45

HCOHCO33-- > 26> 26

Mixed ImbalancesMixed Imbalances

Occur when patient has both Occur when patient has both metabolic and respiratory disorders metabolic and respiratory disorders that cause an acid-base imbalancethat cause an acid-base imbalance

Examples:Examples: Diabetic KetoAcidosis (metabolic Diabetic KetoAcidosis (metabolic

acidosis) with decreased respiratory acidosis) with decreased respiratory drive (respiratory acidosis)drive (respiratory acidosis)

Severe vomiting (metabolic alkalosis) Severe vomiting (metabolic alkalosis) with high fever (respiratory alkalosis)with high fever (respiratory alkalosis)

Mixed ImbalancesMixed Imbalances

pH will be normalpH will be normal

PaCOPaCO22 and HCO and HCO33-- will be abnormal will be abnormal

PaCOPaCO22 will be high with low HCO will be high with low HCO33-- (both (both

tend toward acid side)tend toward acid side)

PaCOPaCO22 will be low with high HCO will be low with high HCO33-- (both (both

tend toward base side)tend toward base side)

Mixed ImbalancesMixed Imbalances

Mixed acidosisMixed acidosis

pHpH < 7.35< 7.35 PaCOPaCO22 > 45> 45

HCOHCO33-- < 22< 22

Mixed alkalosisMixed alkalosis

pHpH > 7.45> 7.45 PaCOPaCO22 < 35< 35

HCOHCO33-- > 26> 26

Notice with the mixed acidosis that you have an acidic pH (less than 7.35, with otherParameters indicating an acid environment. High PaCO2 (too much acid). Low HCO3(too little base – an acidic environment). This is classic mixed acidosis.

FinishedFinished

You have finished this in-service.You have finished this in-service. Please go to the next in-service Please go to the next in-service

titled:titled: Effects of Acid Base on OxygenationEffects of Acid Base on Oxygenation