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Arterial Blood Gas Analysis Waechter

2011 Arterial blood gases, abbreviated ABGs, measure the acid base status and the oxygenation level in the blood. There are multiple depths of analysis, from a very simple, to a fairly detailed level that can be performed. This handout will describe, in stages, the initial levels of analysis in a logical and progressive fashion. To learn the skill of ABG analysis, the skill must be practiced. You will likely discover that you can understand the contents of this handout without too much difficulty, but if you do not practice, it is common to experience difficulty interpreting ABGs when there are no suggestions or learning support provided. This handout will not contain too many ABGs for practice, but the ABG Module (coming soon) in the teachingmedicine.com website will provide you with lots of practice opportunity. Lets begin. The standard format of writing an ABG is:

pH / pCO2 / pO2 / bicarbonate

For example, 7.40/40/100/24 is an example of a normal ABG and means: the pH is 7.40 the partial pressure of carbon dioxide (pCO2) is 40 mmHg the partial pressure of oxygen (pO2) is 100 mmHg the bicarbonate (bicarb) level is 24 mmol/liter

Do not confuse pO2 with the oxygen saturation (SpO2), which is a value expressed in percent. The pO2 and SpO2 are related, but not equal. Step 1: The State of Acid/Base The normal pH of the blood is 7.40 and the normal range is 7.35 to 7.45. Any pH that is lower than 7.35 is considered acidotic. Acidosis, a state of being acidotic, and acidemia, a condition of having acidic blood, are common terms used to describe a low pH in the blood. The stomach has a very low pH and is normally acidic. However, the blood is different. Any pH that is higher than 7.45 is considered alkalotic. Other common terms include alkalosis (a state of having high pH) and alkalemia (high pH in the blood). Of note, the first parts of the small intestine are normally alkalotic (to buffer the acid coming from the stomach).

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Lets practice. What is the acid/base status of the following values:

a) 7.25 / x / y / z

b) 7.55 / x / y / z

c) 6.81 / x / y / z

d) 7.44 / x / y / z Answers at bottom.1 I will assume this task was very easy for you. Lets move along then. Step 2: Metabolic vs. Respiratory Disturbances The equation below shows the relationship between protons (H+), bicarb (HCO3-), water, and CO2. We will discuss this in more detail shortly.

H+ + HCO3- H2O + CO2 Remember too that there is a quantitative relationship between pH, HCO3- and CO2 and it is described by the Henderson Hasselbach equation:

It is not necessary to memorize this equation. However, it is important to understand that there is a fixed and well-described relationship between the 3 variables of pH, HCO3- and CO2. Given 2 of the values, we can calculate or predict what the 3rd value is. Before jumping deeper into this section, you must ensure that you clearly understand the concept of chemical equilibrium and how chemical equations shift to the right or shift to the left. Lets see what happens when one or more factors change within the equilibrium

1 Answers: a) acidosis b) alkalosis c) acidosis d) normal range

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General Principle: Decreasing the concentration of any chemical on the left side of the

equation will drive the equation to the left. Increasing the concentration of any chemical on the right side of the

equation will also drive the equation to the left. Given that pH is defined by the H+ concentration, we are primarily

concerned about how changes in the other chemicals will change the H+ concentration. Since the concentration of water is fixed, the 2 chemicals of interest are HCO3- and CO2.

H+ + HCO3- H2O + CO2

Acidosis Acidosis:

decreasing the bicarb concentration will generate acidosis. When this happens, we call the acidosis a metabolic acidosis. There are many processes which can change the bicarb level.

increasing the carbon dioxide will also generate acidosis. When this happens, we call the acidosis a respiratory acidosis because of the tight relationship between ventilation and CO2 levels.

Everything discussed for acidosis can be applied in reverse, to describe processes that generate alkalosis:

H+ + HCO3- H2O + CO2

Alkalosis Alkalosis:

increasing the bicarb concentration will generate alkalosis. When this happens, we call the alkalosis a metabolic alkalosis.

decreasing the carbon dioxide will also generate alkalosis. When this happens, we call the alkalosis a respiratory alkalosis.

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Lets do a quick recap. There are 4 primary acid base disturbances:

a) metabolic acidosis b) respiratory acidosis c) metabolic alkalosis d) respiratory alkalosis

These 4 processes will form the fundamental basics of your understanding of ABG analysis. Lets practice a few: For now, I will set the pO2 to be x because it is data that is not required for our analysis and I want to remove visually distracting information. Answers at bottom.2 Step 1: look at the pH. What is the acid/base disturbance? Step 2: look at the bicarb and the pCO2. Which element is driving the pH?

a) 7.29 / 52 / x / 24

b) 7.28 / 40 / x / 18

c) 7.40 / 40 / x / 24

d) 7.59 / 26 / x / 24

e) 7.54 / 40 / x / 33

f) 7.38 / 42 / x / 24 And now maybe a little harder because both the bicarb and the pCO2 will be abnormal:

g) 7.33 / 33 / x / 17

h) 7.47 / 33 / x / 23

i) 7.50 / 48 / x / 36

j) 7.32 / 72 / x / 36

2 Answers: a) respiratory acidosis b) metabolic acidosis c) normal d) respiratory alkalosis e) metabolic alkalosis f) normal (the pH is in normal range) g) metabolic acidosis h) respiratory alkalosis i) metabolic alkalosis j) respiratory acidosis

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Discussion from previous examples: if the pH is acidosis, then you need to look for which process is causing the

acidosis. o If the pCO2 is high, then the pCO2 is causing the acidosis. o If the bicarb is low, then the bicarb is causing the acidosis. o If the PCO2 is low or the bicarb is high (as in examples j-g above), these

values are abnormal but they are alkalotic processes and thus are not driving the acidosis. We will discuss these changes below in Step 3.

similar logic applies in reverse for the analysis of alkalosis. Ok. What if both processes are driving the pH in the same direction at the same time?

when this occurs, we have a mixed acid base disturbance. Analyze the following examples. Answers at bottom3:

a) 7.10 / 47 / x / 14

b) 7.70 / 29 / x / 35 Good. We have now covered analysis of the primary disturbance. Check that off your list. The next step is analysis of the compensatory (secondary) mechanisms. Step 3: Compensation Your body likes to have a normal pH (can you blame it?). Therefore, when one abnormal mechanism starts to push the pH into the abnormal range of either acidosis or alkalosis, a second process will become engaged and try to push the pH back toward a normal value. This mechanism is called secondary compensation. There are some very important distinguishing features of compensatory factors:

the compensatory mechanism is ALWAYS in the opposite direction of the primary disturbance (this was probably obvious to you already).

if the primary disturbance is respiratory, the secondary compensatory mechanism must be metabolic.

if the primary disturbance is metabolic, the secondary compensatory mechanism must be respiratory.

3 Answers: a) mixed respiratory and metabolic acidosis b) mixed respiratory and metabolic alkalosis

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The compensation process never over-corrects the primary disturbance. Therefore, if the primary disturbance is acidosis, the pH will always be < 7.40 with normal compensation. If the primary disturbance is alkalosis, the pH will always be > 7.40.

o If the pH appears to be over-corrected, there is an additional mixed primary disturbance.

Respiratory compensation starts within 30 minutes and is maximal within 12 hours. Metabolic compensation takes about 3-5 days for maximal compensation. Kidneys are a little slower than lungs to make changes.

Shall we examine some examples of compensation? Answers at bottom.4 Step 1: look at the pH. What is the acid/base disturbance? Step 2: look at the bicarb and the pCO2. Which element is driving the pH? Step 3: is there compensation occurring and if yes, what is it?

a) 7.33 / 33 / x / 17

b) 7.47 / 28 / x / 21

c) 7.29 / 52 / x / 24

d) 7.50 / 48 / x / 36

e) 7.32 / 72 / x / 36

f) 7.28 / 40 / x / 18 I got lazy. These are some of the same ABGs I threw at you on page 4. However, you now are analyzing on a deeper level and should have a little more to say about them. Now that you possess the skills of identifying compensation, you should determine if the degree of compensation is appropriate or not. This is analyzed by determining the degree of compensation.

4 Answers: a) metabolic acidosis with respiratory compensation b) respiratory alkalosis with metabolic compensation c) respiratory acidosis with no compensation d) metabolic alkalosis with respiratory compensation e) respiratory acidosis with metabolic compensation f) metabolic acidosis with no compensation

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Step 4: Degree of Compensation It is important to determine the degree of compensation for a couple reasons:

the degree of compensation can be useful in determining if the disturbance is acute. In the short term (an acute process), compensation can be absent or very reduced (incomplete).

if the compensation is too much or too little, it could mean that in addition to a secondary compensatory process, there could be a second primary disturbance.

o when this occurs, we say that there is a mixed acid base disturbance Before we can begin, we need to define what we mean by appropriate compensation. This is accomplished by analyzing how severe the primary disturbance is, and then comparing that with the amount of change the compensatory mechanism has demonstrated. There are pre-determined levels of compensation for each of the 4 types of primary disturbances (see table below). This section can become a little complicated. Read through slowly and re-read if you need to. The Concept of Delta: In science, change is often called a delta. If the bicarb was 14 (normal is 24), then this would be a delta of 10. We would expect that for this metabolic acidosis process, the compensatory respiratory response would be hyperventilation to lower the pCO2 into an alkalotic range to try to partly offset the acidosis. If the pCO2 was 30 (normal is 40), the delta pCO2 is 10. The ratio of delta bicarb to delta pCO2 is:

10 to 10 = 1:1 = 1.0 The table below shows what the delta ratios are for complete compensation for the given primary disturbances:

Metabolic acidosis 1.0 Metabolic alkalosis 0.7 Respiratory alkalosis 0.5 Respiratory acidosis 0.3

In the acute setting, it is common to have incomplete compensation because respiratory compensation takes about 12 hours and metabolic compensation takes about 3-5 days. Therefore, incomplete compensation can suggest that the process is acute. Incomplete compensation can also suggest an additional mixed disturbance. For example, a primary metabolic acidosis could have incomplete respiratory compensation due to mild hypoventilation secondary to narcotic use (this would be an additional primary respiratory acidosis). This would prevent the patient from hyperventilating enough to reach full respiratory compensation.

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What do these delta ratios numbers mean? Well, given the following examples below, we have the primary disturbance in green and the expected complete compensatory responses in orange: Primary Process Example Primary delta Comp. delta

Metabolic acidosis 1.0 7.29 / 30 / x / 14 10 10 Metabolic alkalosis 0.7 7.48/ 47 / x / 34 10 7 Respiratory alkalosis 0.5 7.42 / 30 / x / 19 10 5 Respiratory acidosis 0.3 7.35 / 50 / x / 27 10 3 Note that with complete compensation, the pH never crosses the 7.40 threshold. With a primary acidosis, the pH is < 7.40 and with a primary alkalosis, the pH is > 7.40 despite complete compensation. Do you think you will have difficulty remembering these compensatory delta ratios? I did. There are a couple little memory tip patterns that might help you remember the last column of ratios:

1. It is easier to change your CO2 levels than it is to change your bicarb levels. Therefore, the respiratory compensation ratios are larger than the metabolic ratios.

2. It is easier to lower a value than it is to raise a value. Therefore, within the respiratory compensation mechanisms, the ratio for lowering CO2 (1.0) is greater than the ratio for raising CO2 (0.7). A similar observation is made for metabolic compensation too: bicarb is lowered more (0.5) than bicarb is raised (0.3).

Jason Waechter 11-8-30 2:12 PM

Jason Waechter 11-8-26 5:00 PMComment: check this one.

Comment: Look up the ratios since COPD'ers always have a much higher bicarb level and so 0.3 seems too small.

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Shall we practice determining if compensation is appropriate or not? Answers below.5 Step 1: look at the pH. What is the acid/base disturbance? Step 2: look at the bicarb and the pCO2. Which element is driving the pH? Step 3: is there compensation occurring and if yes, what is it? Step 4: if compensation is present, is it appropriate?

a) 7.28 / 33 / x / 17

b) 7.47 / 30 / x / 21

c) 7.21 / 52 / x / 20

d) 7.50 / 48 / x / 36

e) 7.32 / 72 / x / 36

f) 7.28 / 40 / x / 18

g) 7.38 / 26 / x / 15 (note this is tricky because the pH is normal)

h) 7.24 / 36 / x / 15 This is the end of this handout. However, it is not the end of learning about ABGs. The Module on ABGs on teachingmedicine.com (coming soon) will give you the opportunity to:

practice and solidify your ABG skills develop differential diagnoses for all acid base disturbances incorporate anion gap analysis into your acid base diagnosis incorporate ABG analysis into a clinical context

5 Answers: a) met acidosis with complete comp (ratio 1.0) b) resp alkalosis with incomplete comp (ratio 0.3) c) mixed met and resp acidosis no comp d) met alkalosis with complete comp (ratio 0.7) e) resp acidosis with complete comp plus little extra primary met alkalosis (ratio 0.4) f) met acidosis with no comp (ratio 0.0) g) met acidosis with complete comp plus extra primary resp alkalosis (ratio 1.5) h) met acidosis with incomplete comp (ratio 0.4)