Abg Intepretation

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RHONDA LUCASSICU, RASHID HOSPITAL

JUNE, 2011


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ABG INTEPRETATION

By the end of this module you will be able to Identify the components of the ABG Interpret ABG values Identify the major causes of acid-base abnormalities Describe interventions to correct acid-base abnormalities

INTRODUCTION

A key component in critical care nursing is being able to understand ABGresults. This self-learning package will help you to analyze ABG results anddescribe actions the healthcare team can take in order to correct acid-basedisturbances.

THE ACID-BASE SYSTEM

The main components in the acid-base system are CO2, an acid, and HCO3-(or bicarb), a base. As with other systems in the body, the trend is towardequilibrium or a neutral state. The body frequently attempts to compensatefor its imbalances by making adjustments in the other direction (think oftachycardia as a response to low cardiac output). It is the same with theacid-base balance if a person has a metabolic acidosis the respiratorysystem will try to compensate by the patient breathing faster to blow offsome of the CO2.

There are two systems in the body responsible for maintaining acid-basebalance:

1. Respiratory System: By changing the rate of respirations, you caneither retain or blow off CO2. The respiratory rate is a rapidcompensator (minutes).

2. Renal System: The kidneys rid the body of various acids (H+) in aneffort to maintain a constant bicarb. It will retain bicarb if it is neededto compensate for an acidotic imbalance in the body, and vice versa.The renal system is a very slow compensator (several hours to severaldays).

There are 4 categories of acid-base disturbances in your body. Regardless ofthe problem, however, treatment is always aimed at resolving the underlyingproblem.

1. Respiratory Acidosis: Always caused by hypoventilation but thecauses of hypoventilation can be numerous (narcotics, cracked ribs,lung disorders, etc.) Treatment is to increase the respiratory rate


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and/or effort by addressing the underlying problem (lower the narcoticrate, bind the ribs, excise the tumour, treat the COPD, etc.)

2. Respiratory Alkalosis: Caused by hyperventilation (pain, fever,pneumonia, heart failure). Treatment is to decrease the respiratoryrate/effort by, once again, addressing the underlying cause.

3. Metabolic Acidosis: This state is caused either by an increase in H+ ora loss of HCO3- (increased catabolism, bicarb loss through diarrhea,starvation, renal failure). Treatment requires you to either excrete thebuildup of acids or retain bicarb. You do this how? You guessed it: Byaddressing the underlying problem!

4. Metabolic Alkalosis: This is caused by a loss of H+ or a retention ofbicarb (loss of potassium through diarrhea or gastric losses, renalfailure). Treatment is addressed once again at fixing the underlyingproblem (replacing potassium losses, administering Diamox to flush

out the bicarb, etc.).

ANALYZING ABGs

Become familiar with normal values for ABGs.

pH 7.35 7.45PaCO2 35 45PaO2 80 100

HCO3-

22 26SaO2 90%Base Excess -2 to +2

If you approach ABG Interpretation systematically, you cant go wrong. Youhave to look at each component one at a time. Always follow these steps inorder:

1. Look at the pH: This is the crucial step because all other interpretationsare dependent upon the pH.

Is it normal? 7. 35 7. 45Is it low? < 7.35 (Acidosis)Is it high? > 7 45 (Alkalosis)


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Keep in mind:If the pH is 7.35 7.39 your patient is closer to acidotic than alkaloticIf the pH is 7.41 7.45 your patient is closer to alkalotic than acidotic


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2. Look at the PaCO2: We now need to determine whether theabnormality is caused primarily by respiratory or metabolic issues:

If the PaCO2 change is in the opposite direction as the pH (pH goes lower

while PaCO2 goes higher) then the cause is primarily respiratory.

Ex. pH = 7. 32 PaCO2 = 48The pH is going down but the PaCO2 is going up, so this is primarily arespiratory acidosis. If the PaCO2 is in the other direction, then theimbalance is primarily metabolic in nature.

3. Look at the HCO3-: If the bicarb is in the same direction as the pH (pHand bicarb both go up) then the cause is primarily metabolic.

Ex. pH = 7.32, HCO3- = 19

Both the pH and the bicarb are acidotic; therefore, the acidosis ismetabolic in nature.

Here is a chart to help you:

Lets see how this works. Using the steps above, interpret the followingabgs:

1. pH 7.22 PaCO2 55 HCO3- 25

The pH is going down; the PCO2 is going up and the HCO3- is normal. Thisis respiratory acidosis

2. pH 7.50 PaCO2 42 HCO3- 33

The pH is up; the PaCO2 is normal but the bicarb is trending up. This ismetabolic alkalosis


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COMPENSATION

The examples above are fairly straightforward. Now lets complicate mattersa bit. What happens when an acid-base imbalance exists over a period of

time? The body tries to compensate for that. So, an ABG result can reflectuncompensated, partially compensated or fully compensated conditions

When an acid-base disorder is either uncompensated or partiallycompensated, the pH remains outside the normal range. In fullycompensated states, however, the pH has returned to within the normalrange, even though the other values may still be abnormal. Be aware thatneither system has the ability to overcompensate.

If you look back to the examples above, these patients are uncompensatedbecause the pH is outside normal limits. The primary cause of the imbalance

is easily identified because the compensatory buffering system (ie either thePCO2 or the HCO3-) remained in the normal ranges, not adjusting itself torestore balance.

So lets now look, then, at ABGs which have evidence of partialcompensation. The same steps apply:

1. Assess the pH. This step remains the same and allows us to determine ifan acidotic oralkalotic state exists. Recall that even when the pH is within normalrange, depending on whether it is above or below 7.40, your results will

usually favouracidosis or alkalosis.

2. Assess the PaCO2: In an uncompensated state, we have already seen thatthe pH and PaCO2 move in opposite directions when indicating that theprimary problem is respiratory. But what if the pH and PaCO2 are movingin the same direction? That is not what we would expect to see happen.We would then conclude that the primary problem was metabolic.

In this case, the decreasing PaCO2 indicates that the lungs, acting as abuffer response, areattempting to correct the pH back into its normal range by decreasing the

PaCO2 (blowingoff the excess CO2). If evidence of compensation is present, but the pHhas not yet beencorrected to within its normal range, this would be described as ametabolic disorder with apartial respiratory compensation.


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3. Assess the HCO3: In our original uncompensated examples, the pH andHCO3 move in the same direction, indicating that the primary problem wasmetabolic. But what if our results show the pH and HCO3 moving inopposite directions? That is not what we would expect to see. We wouldconclude that the primary acid-base disorder is respiratory, and that the

kidneys, again acting as a buffer response system, are compensating byretaining HCO3, ultimately attempting to return the pH back towards thenormal range.


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These tables will help you understand the relationships:

FULLY COMPENSATED

PARTIALLY COMPENSATED

Notice that the only difference between partially and fully compensatedstates is whether or notthe pH has returned to within the normal range. In compensated acid-basedisorders, the pHwill frequently fall either on the low or high side of neutral (7.40). Makingnote of where thepH falls within the normal range is helpful in determining if the original acid-base disorder was

acidosis or alkalosis.

Lets try some examples:

1. pH 7.32 PaCO2 32 HCO3- 18

This patient is acidotic; normally you would expect the PCO2 to be highas well, but it is not. Therefore, the acidosis is primarily metabolic inorigin. The low PaCO2 indicates that the lungs are trying tocompensate for the metabolic acidosis. However, the pH still remainsout of normal range, so this is apartially compensated metabolic

acidosis.

2. pH 7.35 PaCO2 48 HCO3- 28

The pH is on closer to acidosis than alkalosis (low normal). The PaCO2is high, suggesting a respiratory acidosis. The bicarb is high as well,suggesting it is compensatory. Because the pH is normal, though, thisis a fully compensated respiratory acidosis.


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3. pH 7.43 PaCO2 48 HCO3- 36

The pH is the high side of normal so closer to alkalosis. The highPaCO2 suggests acidosis though. The bicarb, however, is high and so

the PaCO2 must be compensatory. This is a fully compensatedmetabolic alkalosis.

4. pH 7.33 PaCO2 62 HCO3- 35

Can you tell what this one is?

MIXED DISTURBANCES

So far these examples show a clear picture one way or the otherwhat

about the cases when both the PCO2 and the bicarb are abnormal, but inopposite directions? How do we know which one is causing theacidosis/alkalosis?

The answer is that both are contributing to the imbalance. We call thesecases mixed respiratory and metabolic acid-base disorders.

Remember: Compensation for simple acid-base disturbances always drivesthe compensating parameter (ie, the PaCO2 or HCO3-) in the same directionas the primary abnormal parameter (ie, the HCO3- or PCO2 ). For example,if the pH is high and the PaCO2 is high, and the bicarb is high, then this would

be a partially compensated metabolic alkalosis, with the high PaCO2 beingthe compensatory component. But what if the pH is high, the PaCO2 is lowand the bicarb is high?

Whenever the PCO2 and HCO3- are abnormal in opposite directions,ie, one above normal while the other is reduced, a mixed respiratoryand metabolic acid-base disorder exists.

A simple Rule of Thumb:

When the PCO2 is elevated and the [HCO3-] reduced, respiratory acidosis

and metabolic acidosis coexist. When the PCO2 is reduced and the [HCO3-] elevated, respiratory

alkalosis and metabolic alkalosis coexist

In acidosis we are base deficient so SBE will be negative.In alkalosis we are base excess so SBE will be positive.


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But is there one component contributing more than another? There may be.How can you tell?

This is when we have to look at the base excess (also known as standardbase excess or SBE).

If the SBE is in the same direction as the pH (ie both acidotic or bothalkalotic), then the cause is metabolic.

If the SBE is not in the same direction as the pH, then the primaryproblem is respiratory.

So lets look at an example:

1. pH 7. 52 PaCO2 27 HCO3- 29

This is clearly an alkalosis because the PaCO2 and the bicarb both trend inthat direction so there is a mixed picturewhich one is the primary causeand which one is the compensation? We wont know until we look at theBase Excess (also known as Standard Base Excess, SBE).

If the SBE in this case were a positive number, positive suggests there isbase excess, so the primary cause of the alkalosis is metabolic. If the SBEwere negative this would indicate there is a deficit of base; because basecauses alkalosis, if there is a deficit the primary cause of the alkalosiswould therefore have to be respiratory.

2. pH = 7.32 PaCO2 = 48 HCO3- = 19

In this case, everything is acidotic and we dont know whether the causeis metabolic or respiratory. So we must look at the SBE.

SBE = -0.6

This SBE is acidotic (negative number, suggesting a base deficit), so theprimary cause of the acidosis is metabolic. If the SBE was positive thiswould indicate there is an excess of base; an abundance of base cannot

cause acidosis; therefore, the acidosis would be respiratory.

And thats all there is to know about ABGs!!!

There is an ABG Interpretation Quiz in the next clear sheaf. Try to completethe quiz. The answers are attached at the back.


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