abg made easy

Mar 08 SGT By-names

Simple Method of Acid Base Balance Interpretation

A FOUR STEP METHOD FOR INTERPRETATION OF ABGS

Usefulness

This method is simple, easy and can be used for the majority of ABGs. It only addresses acid-base balance and considers just 3 values.

pH,

PaCO2

HCO3-

Step 1. Use pH to determine Acidosis or Alkalosis.

ph

< 7.357.35-7.45> 7.45

AcidosisNormal or CompensatedAlkalosis

Step 2. Use PaCO2 to determine respiratory effect. PaCO2

< 3535 -45> 45

Tends toward alkalosis

Causes high pH

Neutralizes low pH Normal

or

Compensated Tends toward acidosis

Causes low pH

Neutralizes high pH

Step 3. Assume metabolic cause when respiratory is ruled out.

You'll be right most of the time if you remember this simple table:

High pHLow pH

AlkalosisAcidosis

High PaCO2Low PaCO2High PaCO2Low PaCO2

MetabolicRespiratoryRespiratoryMetabolic

If PaCO2 is abnormal and pH is normal, it indicates compensation.

pH > 7.4 would be a compensated alkalosis.

pH < 7.4 would be a compensated acidosis.

These steps will make more sense if we apply them to actual ABG values. Click here to interpret some ABG values using these steps. You may want to refer back to these steps (click on "linked" steps or use "BACK" button on your browser) or print out this page for reference.

Step 4. Use HC03 to verify metabolic effect

Normal HCO3- is 22-26

Please note: Remember, the first three steps apply to the majority of cases, but do not take into account:

the possibility of complete compensation, but those cases are usually less serious, and

instances of combined respiratory and metabolic imbalance, but those cases are pretty rare.

"Combined" disturbance means HCO3- alters the pH in the same direction as the PaCO2.

High PaCO2 and low HCO3- (acidosis) or

Low PaCO2 and high HCO3- (alkalosis).

Example 1

ABG ValueWhich step?Rationale

pH7.39Step 1Normal pH

PaCO240Step 2Normal PaCO2

InterpretationNormal ABG (acid base is balanced; there are no pH changes, so if the respiratory acid is normal, the metabolic base cannot be causing changes either.)

Example 2


pH7.2Step 1Low pH indicates acidosis

PaCO250Step 2High PaCO2 indicates respiratory cause for acidosis

Interpretationrespiratory acidosis

Example 3


pH7.49Step 1High pH indicates alkalosis

PaCO230Step 2Low PaCO2 indicates respiratory cause for alkalosis (lo respiratory acid is causing higher pH)

Interpretationrespiratory alkalosis

Example 4 ABG ValueWhich step?Rationale

pH7.23Step 1Low pH indicates acidosis

PaCO231Step 3Low PaCO2 rules out respiratory cause for acidosis, therefore metabolic cause. Low respiratory acid is compensating for lower pH.

Interpretationmetabolic acidosis

Example 5


pH7.48Step 1High pH indicates alkalosis

PaCO247Step 3High PaC02 and High pH indicates metabolic cause of alkalosis. Respiratory acid is compensating for high pH.

InterpretationPartially compensated metabolic alkalosis

Example 6


pH7.43Step 1pH is normal but higher than 7.4, therefore compensated alkalosis.

PaCO233Step 3Low PaCO2 causes alkalosis

InterpretationCompensated respiratory alkalosis

Case Studies

The following are examples of clinical situations and the ABGs that may result, as well as causes and solutions for ABG abnormalities.

Case 1Mrs. Puffer is a 35-year-old single mother, just getting off the night shift. She reports to the ED in the early morning with shortness of breath. She has cyanosis of the lips. She has had a productive cough for 2 weeks. Her temperature is 102.2, blood pressure 110/76, heart rate 108, respirations 32, rapid and shallow. Breath sounds are diminished in both bases, with coarse rhonchi in the upper lobes. Chest X-ray indicates bilateral pneumonia.

ABG results are:

pH= 7.44

PaCO2= 28

HCO3= 24

PaO2= 54

Problems: PaCO2 is low.

pH is on the high side of normal, therefore compensated respiratory alkalosis.

Also, PaO2 is low, probably due to mucous displacing air in the alveoli affected by the pneumonia (see Shunting).

Solutions:

Mrs. Puffer most likely has ARDS along with her pneumonia.

The alkalosis need not be treated directly. Mrs. Puffer is hyperventilating to increase oxygenation, which is incidentally blowing off CO2. Improve PaO2 and a normal respiratory rate should normalize the pH.

High FiO2 can help, but if she has interstitial lung fluid, she may need intubation and PEEP, or a BiPAP to raise her PaO2. (Click here to compare BiPAP to other respiratory treatments.)

Expect orders for antibiotics, and possibly steroidal anti-inflammatory agents.

Chest physiotherapy and vigorous coughing or suctioning will help the patient clear her airways of excess mucous and increase the number of functioning alveoli.

Case 2Mr. Worried is a 52-year-old widow. He is retired and living alone. He enters the ED complaining of shortness of breath and tingling in fingers. His breathing is shallow and rapid. He denies diabetes; blood sugar is normal. There are no EKG changes. He has no significant respiratory or cardiac history. He takes several antianxiety medications. He says he has had anxiety attacks before. While being worked up for chest pain an ABG is done:

ABG results are:

pH= 7.48

PaCO2= 28

HCO3= 22

PaO2= 85

Problem:

pH is high,

PaCO2 is low

respiratory alkalosis. Solution:

If he is hyperventilating from an anxiety attack, the simplest solution is to have him breathe into a paper bag. He will rebreathe some exhaled CO2.This will increase PaCO2 and trigger his normal respiratory drive to take over breathing control.

* Please note this will not work on a person with chronic CO2 retention, such as a COPD patient. These people develop a hypoxic drive, and do not respond to CO2 changes.

Case 3You are the critical care nurse about to receive Mr. Sweet, a 24-year-old DKA (diabetic ketoacidosis) patient from the ED. The medical diagnosis tells you to expect acidosis. In report you learn that his blood glucose on arrival was 780. He has been started on an insulin drip and has received one amp of bicarb. You will be doing finger stick blood sugars every hour.

ABG results are:

pH= 7.33

PaCO2= 25

HCO3=12

PaO2= 89

Problem:

The pH is acidotic,

PaCO2 is 25 (low) which should create alkalosis.

This is a respiratory compensation for the metabolic acidosis.

The underlying problem is, of course, a metabolic acidosis.

Solution:

Insulin, so the body can use the sugar in the blood and stop making ketones, which are an acidic by-product of protein metabolism.

In the mean time, pH should be maintained near normal so that oxygenation is not compromised III. The Land of ABG *Once upon a time there was a land known as ABG. Everyone there was related with only a limited number of names for the population. They were also very polite and had their own etiquette for learning each others names. Now I would like to introduce you to your patient. Lets figure out what her name is: All of the people in the land of ABG have a first name, a middle name, and a last name. You just have to look at them one name at a time.

A. The Last Name

1. First, look at her pH. (Normal = 7.35-7.45) 2. If her pH is < (less than) 7.35; her last name is ACIDOSIS. 3. If her pH is > (greater than) 7.45; her last name is ALKALOSIS.

(Note: To be an absolutely perfect last name--her pH needs to be 7.40. So, keep in mind, that if her pH is 7.35-7.39--shes thinking about marrying into the ACIDOSIS family. If her pH is 7.41-7.45--shes thinking about marrying into the ALKALOSIS family.)

B. The First Name

Now that you know your patients last name, you would like to also learn her first name.

1. Look at her pH again. 2. If it is 7.35-7.45 (normal) then her first name is COMPENSATED. 3. If the pH is 7.45--then her first name is UNCOMPENSATED.

C. The Middle Name

Now that you know your patients first and last name, you would like to know her middle name.

(Name Alert: These people are all related and you have many patients with the same first and last name. A middle name will give you more information to go on.)

1. First you need to look at the CO2 and HCO3. (Remember: Normal CO2 = 35-45. Normal HCO3 = 22-26) 2. The middle name will either be Respiratory or Metabolic. 3. If the CO2 is 45--her middle name is RESPIRATORY. 4. If the HCO3 is 26--her middle name is METABOLIC.

D. The Family Feud

1. pH and HCO3 are "kissin cousins"--they like to go in the same direction. 2. But CO2 is the "black sheep"--pH runs the opposite direction when it sees him coming.

Therefore:

3. Decreased pH with Decreased HCO3 = ACIDOSIS. 4. Increased pH with Increased HCO3 = ALKALOSIS. 5. Decreased pH with Increased CO2 = ACIDOSIS. 6. Increased pH with Decreased CO2 = ALKALOSIS.

IV. Lets PracticeA. Respiratory Therapy gives you an ABG with the following numbers:

pH = 7.60 CO2 = 30 HCO3 = 22

1. What is her last name? (Her pH is >7.45 so her last name is ALKALOSIS.)

2. What is her first name? (Her pH is not in the normal range of 7.35-7.45 so her first name is UNCOMPENSATED.)

3. What is her middle name? (Her CO2 is 7.45 so her last name is ALKALOSIS.)

2. What is her first name? (Her pH is not in the normal range so her first name is UNCOMPENSATED.)

3. What is her middle name? (Her CO2 is normal but her HCO3 is >26 so her middle name is METABOLIC.)

4. You have now been introduced to UNCOMPENSATED METABOLIC ALKALOSIS. D. pH = 7.35 CO2 = 45 HCO3 = 21

1. What is her last name? (Her pH is normal--but it is

abg made easy

Documents