ten things everyone needs to know about blood gases

TEN THINGS EVERYONE NEEDS TO KNOW ABOUT

BLOOD GASES

Doug Pursley, M.Ed., RRTProgram Director

Ozarks Technical Community CollegeSpringfield, MO

1. There is a difference between classification and interpretation of blood gases

Classification vs. Interpretation

• Classification – systematic arrangement according to established criteria

Classification: The High/Lo or Arrow System

• 54 year old narcotic overdose:• pH 7.22 (7.40)• PaCO2 70 (40)• HCO3 27 (24)

• Partially compensated respiratory acidosis according to the arrow system

Classification vs. Interpretation• Interpretation – the

act of applying an explanation to the results

Interpretation

• Incorporates calculations, baseline values, electrolytes (Na+/Cl-), and clinical picture into the equation to come up with a more precise explanation of the patient’s acid-base status

2. Hydrolysis happens

Hydrolysis

+ = = +

CO2 + H2O = H2CO3 = H+ + HCO3

• Starting at a PaCO2 of 40 mmHg, HCO3 will increase from normal by 1 for every 10 acute increase in PaCO2

• Starting at a PaCO2 of 40 mmHg, HCO3 will decrease from normal by 2 for every 10 acute decrease in PaCO2

• Simple physiochemical event and occurs almost immediately

Correct interpretation when one knows about hydrolysis…

Starting at a PaCO2 of 40 mmHg, HCO3 will increase from normal by 1 for every 10 increase in PaCO2

• 54 year old narcotic overdose:• pH 7.22• PaCO2 70 (three tens above 40)• HCO3 27 (HCO3 expected to increase by

3)• Expected HCO3 same as Actual HCO3 so…• Acute respiratory acidosis• Base excess is zero in this case

3. Always evaluate the base excess/deficit

• Normal BE/BD is -2 to +2• With the exception of

hydrolysis, tends to follow HCO3 • Values above +2 indicate compensation for

respiratory acidosis or metabolic alkalosis• Values below -2 indicate compensation for

respiratory alkalosis or metabolic acidosis• Normal value for base does not exclude

metabolic acidosis IF there is also a co-existing and competing metabolic alkalosis and visa versa

General rule of thumb

• If the pH and PaCO2 are both out of whack in opposite directions, and the base is normal, start your interpretation out with ACUTE RESPIRATORY…

• Possible exceptions: – Co-existing Metabolic Acidosis AND Metabolic

Alkalosis– Triple acid-base disturbances

4. Indirect metabolic assessment can be performed by comparing the actual pH

with predicted respiratory pH

• Starting at pH of 7.4 and PaCO2 of 40…• pH will increase by 0.1 for each 10 acute

decrease in PaCO2• pH will decrease by 0.06 for each 10 acute

increase in PaCO2• If PaCO2 suddenly changes from 40 to 30, pH

will change from 7.40 to 7.50• If PaCO2 suddenly changes from 40 to 60, pH

will change from 7.40 to 7.28

• If actual pH is greater than the predicted respiratory pH by more than 0.03, the patient will have a base excess

• If actual pH is less than the predicted respiratory pH by more than 0.03, there will be a base deficit

• If both are within 0.03 of each other, then the base will be normal most likely reflecting an acute respiratory disturbance

5. The anion and bicarbonate gaps are important parts of evaluating acid-base disorders

Two types of metabolic acidosis

• High AG metabolic acidosis– Accumulation of acid in the plasma– Lactic acidosis, DKA, azotemic renal failure, toxins

• Normal AG metabolic acidosis– Loss of base from the plasma– Diarrhea– Intestinal drainage tubes– Renal tubular acidosis (RTA)

Anion Gap

• Represents the concentration of unmeasured anions in the plasma

• Unmeasured anions also referred to as “acid anions” such as:– Lactate (lactic acidosis)– Acetoacetate (ketoacidosis)– Sulphate, phosphate (azotemic renal failure)

Anion Gap

• Derived from subtracting measured cations from measured anions

• Clinical equation is:

• Na+ - Cl- - HCO3- = Anion Gap

Anion Gap

• Normal value is 12• If value is increased, there is an accumulation

of acid anions in the plasma• If AG is 20-29, there is 67% chance of high AG

metabolic acidosis• If AG is >30, there is 100% chance of high AG

metabolic acidosis

Anion Gap

• If High AG acidosis is the only acid-base disorder, then there should be a 1:1 correlation between rise in the gap and fall in HCO3

• Example: If AG goes from 12 to 24, HCO3 should go from 24 to 12

• A high AG with a normal or high HCO3 indicates extra HCO3 on board from an additional metabolic alkalosis or compensation for respiratory acidosis

Normal AG Metabolic Acidosis

• Due to loss of base rather than accumulation of acids

• Anion Gap will be normal• Common causes are:– Renal tubular acidosis– Intestinal drainage tubes– Diarrhea

Bicarbonate Gap

• Represents the change in AG from normal minus the change in HCO3 from normal

• Normal value is zero, plus or minus 6• Values greater than +6 indicate:– Metabolic alkalosis or compensation for respiratory

acidosis• Values less than -6 indicate:– Normal AG gap metabolic acidosis or compensation

for respiratory alkalosis

Hard way to calculate BG

• ∆ AG - ∆ HCO3• = [AG - 12] – [24 – HCO3]• = [(Na – Cl – HCO3) – 12] – [24 – HCO3]

Easy way to calculate BG

•BG = Na – Cl - 36• Comes from cancelling out the terms in the

equation below:• BG = [(Na – Cl – HCO3) – 12] – [24 – HCO3]

6. Know the rules for compensation

Compensation for respiratory acidosis

• HCO3 will increase by 4 for every 10 chronic increase in PaCO2 above 40

• Kidneys respond to chronic respiratory acidosis by retaining HCO3

• Takes 3-4 days to reach a maximum value

Compensation for respiratory alkalosis

• HCO3 will decrease by 5 for every 10 chronic decrease in PaCO2 below 40

• Kidneys respond to chronic respiratory alkalosis by excreting HCO3

• Takes 3-4 days to reach a maximum value

Compensation for metabolic acidosis

• Expected PaCO2 in metabolic acidosis will follow “One Point Five Plus Eight Rule” or Winter’s Formula: – PaCO2 expected = (1.5 x HCO3) + 8

• Compensation starts immediately and becomes complete in several hours

• Limit of compensation is PaCO2 of about 8-10

Compensation for metabolic alkalosis

• Expected PaCO2 in metabolic alkalosis will follow the “Point Seven Plus 20 Rule” – PaCO2 expected = (0.7 x HCO3) + 20

• Compensation starts immediately and becomes complete in several hours

• Limit of compensation traditional thought to be PaCO2 of 60 but more recent studies indicate CO2 may be linear with the HCO3

7. All acid-base values should calculate out according to the

Henderson-Hasselbalch equation

H-H Equation

30 second accuracy check with a $15 calculator

– Multiply PaCO2 by 0.03 (converts to H2CO3)– Divide HCO3 by H2CO3– Log the result– Add 6.1

• Result should be within 0.03 units of the measured pH

• If not, acid base is inconsistent (could be HCO3 calculation error, sensor error, or transcription error)

8. Total oxygen content (Cao2) is the best index of oxygenation

CaO2

• CaO2 = (1.34 x Hb x SO2) + (PO2 x 0.003)• Best index of blood oxygen because it is a

measure of total oxygen in blood

Other Oxygen indices

PaO2• Measure of oxygen dissolved in plasma• Indication of the amount of oxygen available to

combine with Hb• Can be normal or high in anemia, COHb, and MetHb• PaO2 of 60 is not bad if Hb and pH are normal• Direct correlation with SaO2– Absent dyshemoglobinemia, if a patient’s PaO2 is 60, his

SaO2 will be 90% assuming a normally shaped O2-Hb curve

SaO2calc

• An estimation of the oxygen saturation from the O2Hb curve

• Plots pH with PO2 to determine saturation• Not accurate in COHb and MetHb

SaO2CO-ox

• The actual, fractionalized value of oxygen saturation measure by a CO-oximeter

• Most accurate SaO2

SpO2

• Uses red and infrared wavelengths of light to determine saturation

• Measures “functional” saturation i.e. assumes only oxygen is attached to Hb

• Overestimates saturation in COHb• Migrates towards 85% in MetHb

All saturations

• In severe anemia i.e. Hb 4 g/dl, SaO2calc , SaO2CO-ox , and SpO2 can all be high and the patient could still be have tissue hypoxia

a/A ratio

• Index of shunting• PaO2 ÷ PAO2• Normal value is 0.9 (i.e. PaO2 90 ÷ PAO2 100)• Lower limit of normal is 0.75• The lower the number, the worse the shunt

9. The P/F ratio is a good index of shunting, but it does have a limitation

P/F ratio

• Index of shunting• Simplified version of a/A ratio• PaO2 ÷ FIO2• Normal value is 400-500– PaO2 90 ÷ FIO2 0.21 = 429

• The lower the number, the worse the shunt• Not accurate in severe hypercarbia

10. The alveolar air equation has actual clinical application

Alveolar Air Equation

• PAO2 = (PB – 47 x FIO2) – (PaCO2 x 1.2)• Needed to calculate PaO2/PAO2 ratio and

A-a gradient• A modification of the formula can be used to

estimate device FIO2 in normal subjects

Case 1

• A adult male patient recently brought to the ER has the following ABG on a nasal cannula at 2 l/m:

• 3:49 am• pH 6.83• PaCO2 139• PaO2 138• HCO3 23• Base -13

What is the interpretation?

• A second ABG is taken 23 minutes later while the patient is being manually ventilated:

• 4:12 am• pH 7.16• PaCO2 47• PaO2 92 • HCO3 17• Base -11

Why did the HCO3 drop from 23 to 17 in just twenty-three minutes?

Case 2

• A patient is admitted with the following ABG:• pH 7.47• PaCO2 51• HCO3 35.5• Base +12

Classify the acid-base

What if I told you…

• 78 year old end stage CO2 retainer admitted to the ETC with exacerbation of his COPD. Pneumococcal pneumonia is suspected.

• pH 7.47• PaCO2 51• HCO3 35.5• Base +12

Change your mind? Or stick with the original assessment.

Case 3

• 34 year old aspirin overdose• pH 7.45• PaCO2 9 mmHg• Base -16.4• PaO2115 mmHg breathing room air• Na+ 140 mEQ/L• Cl- 104 mEQ/L• HCO3 6 mEQ/L

Calculate the anion and bicarbonate gaps. What’s the interpretation?

Is the acid-base status accurate?

pH 7.45, PaCO2 9, HCO3 6

– Multiply PaCO2 by 0.03 (converts to H2CO3)

– Divide HCO3 by H2CO3

– Log the result– Add 6.1

Case 4

• 44 year old female, previously healthy, narcotic overdose

• pH 7.19• PaCO2 60• HCO3 22

Interpretation?

Case 5

• 28 year-old in her third trimester of pregnancy is admitted after having severe vomiting for several days.pH 7.58PaC02 31HC03 28Base +6

Interpretation?

Case 6

• The following ABG and electrolytes were taken from a 55 year old male in the ICU with liver disease:

• pH 7.40• PaCO2 20• HCO3 12• Base -12• Na 146• Cl 106

Calculate the anion and bicarbonate gaps and interpret the ABG

Case 7

• 17 year old female seen in the ETC with dizziness and shortness of breath:

• pH 7.60• PaC02 20• HC03 20• Base -1

Interpret the ABG

Case 8

• A 23 year old male has just returned to the US from overseas and is having acute abdominal pain with severe diarrhea.

• He is admitted to a NY hospital with the following values:

• pH 7.30, PaCO2 28, HCO3 13, Base -11.7, Na 144, Cl 119

Calculate the anion and bicarbonate gaps and interpret the ABG

Case 9

• A 42 year old man is admitted to the hospital with dehydration and hypotension. No ABG is obtained. Venous electrolytes show:

• Na+ 165• K+ 4.0• Total CO2 32• Cl- 112

Calculate the anion and bicarbonate gaps and make a statement about the patient’s acid-base

status

Note 1. Use Total CO2 as a substitute for HCO3 on a lab report

Note 2: When calculating BG from venous electrolytes, the formula becomes: BG = Na – Cl – 39, since normal value for TCO2 is 27 compared with the HCO3 normal of 24

Case 10

• A 19 year old pregnant insulin dependent diabetic female was admitted to a San Diego hospital (PB = 760 mmHg) with a history of polyuria and thirst.

• She has a history of poor compliance with medical therapy. On examination, she was afebrile, chest was clear, peripheral circulation was adequate with BP 105/65.

Blood gases while breathing room air:

• Na 136, Cl 100• pH 7.17• PaCO2 17 • PaO2 115• HCO3 6• Base -20

Calculate the anion and bicarbonate gaps and interpret the ABG

Case 11 – Oxygen Content

Which patient has better overall oxygenation status?

Mr. Jones • pH 7.48• PaCO2 34• PaO2 85• SaO2 95%• Hb 7 g/dl

Mr. Burton

• pH 7.32• PaCO2 74• PaO2 55• SaO2 85%• Hb 17 g/dl

Case 12 – O2-Hb curve

Which patient will have a higher SaO2?

Mrs. Roberts

• pH 7.04• PaCO2 100• PaO2 80• Hb 15 g/dl

Mrs. Nelson

• pH 7.50• PaCO2 30• PaO2 80• Hb 15 g/dl

Case 13 – Alveolar Air

Assuming PB is 760 mmHg, by the laws of physics, which patient must be breathing supplemental O2?

Patient A

• pH 7.04• PaCO2 100• PaO2 80• Hb 15 g/dl

Patient B

• pH 7.50• PaCO2 30• PaO2 80• Hb 15 g/dl

Case 14 - P/F ratio

A narcotic overdose patient is admitted to the hospital (PB 730) with the following blood gases while breathing room air:

• pH 7.16, PaCO2 80, HCO3 28, PaO2 42, SaO2 63%, Base 0

• Calculations:• PAO247• a/A ratio 0.89• P/F ratio 200

Make a statement regarding this patient’s oxygenation?

Case 15 - Carboxyhemoglobinemia

• A 30 year old man, previously healthy, is brought to the ED after suffering smoke inhalation. He is placed on a NRBM at 15 l/m. COHb is measured at 32%, MetHb 1%, and Hb is 15 g/dl. ABG’s are:

• pH 7.32, PaCO2 32, PaO2 380

All of the following statements are true of this patient EXCEPT:

1. Conventional pulse oximetry will be high

2. SaO2cal will be high3. SaO2CO-ox will be roughly 65-67%4. It is impossible for the PaO2 to

be 3805. There is a metabolic acidosis

If we place this patient on 100% O2, how long will it take for the COHb level to drop to 4%?

Summary

• Always take in to account hydrolysis when evaluating acid-base disorders

• There are rules that govern how pH, PaCO2, and HCO3 change

• Anion and Bicarbonate gaps can be useful in determining proper acid-base interpretation

• Use Winter’s formula to determine the patient’s degree of respiratory compensation in metabolic acidosis

Summary

• There’s a big difference between classification and interpretation of blood gases

• If the base is normal and the pH and PaCO2 are out of whack in opposite directions, you have an acute respiratory alkalosis or acidosis in the vast majority of cases

• There are two kinds of metabolic acidosis: High anion gap and normal anion gap

• All acid-base values should calculate out according to the H-H equation

Summary• Hemoglobin and cardiac output are the keys to

tissue oxygenation• The alveolar air equation has actual clinical

application• The P/F ratio is a good index of shunting except

that it does not take into account changes in PaCO2

• PaO2 can be normal or high in COHb and MetHb• Oxygen saturations (SaO2calc , SaO2CO-ox , and

SpO2) are not created equal