Interpreting ABGs Practical Approach

Interpreting ABGs Practical Approach
Muhammad Asim Rana BSc, MBBS, MRCP, SF-CCM, FCCP, EDIC Department of Adult Critical Care Medicine KSMC, Riyadh

Venous Arterial

Arterial Blood Gases pH/PaCO2/PaO2/HCO3 Written in following manner:
pH = arterial blood pH PaCO2 = arterial pressure of CO2 PaO2 = arterial pressure of O2 HCO3 = serum bicarbonate concentration

Part 1 Acid-Base Disorders

Acid-Base Acidosis or alkalosis: Acidemia or alkalemia:
any disorder that causes an alteration in pH Acidemia or alkalemia: alteration in blood pH; may be result of one or more disorders.

Some important concepts
The determinants of extracellular fluid pH indicate that tight control of the pH requires a fairly constant PCO2/HCO3 ratio. Thus, a change in one of the determinants (PCO2 or HCO3) must be accompanied by a proportional change in the other determinant to keep the PCO2/HCO3 ratio (and the pH) constant.

Some important concepts
Thus, an increase in PCO2 (respiratory acidosis) must be accompanied by an increase in HCO3 (metabolic alkalosis) to keep the pH constant. This is how the control system for acid-base balance operates. A respiratory disorder (change in PCO2) always initiates a complementary metabolic response (that alters the HCO3), and vice-versa

Primary Acid-Base Disorders and Associated Compensatory Changes
[H+] = 24 × PCO2/HCO3 Primary Disorder Primary Change Compensatory Change* Respiratory acidosis Increased PCO2 Increased HCO3 Respiratory alkalosis Decreased PCO2 Decreased HCO3 Metabolic acidosis Metabolic alkalosis * Compensatory changes keep the PCO2/HCO3 ratio constant.

Check if data is consistent
{H} = 24 [ PaCO2/HCO3] {H} = (7.8 – pH) x 100 Each 0.01 unit change in pH {H} will change by 1mEq/L {H} = 40+(delta pH) (1mEq/L)/0.01 pH {H}

Check if data is consistent
{H} = 24 [PaCO2/HCO3] {H} = (7.8 – pH) x 100 The {H} in extracellular fluid normally varies less than 10 nEq/L The values of {H} should be within 10 for both calculations ! If it is beyond or more than 10 the blood gas analysis is not interpretable. The reasons may include improper caliberation or others

Here are some examples pH/PaCO2/PaO2/HCO3 Written in following manner:
7.8/36.6/76.4/55.4 7.7/35.5/80.3/50.6 7.54/53.1/63.7/44.6 24 x 36.6/55.4 = 15.85 x 100 = 0 The data is inconsistent 24 x 35.5/50.6 = 16.8 x 100 = 10 The data is inconsistent 24x53.1/44.6 = 28.57 x 100 = 26 The data is consistent

Case Study A 13 years old female presented in ER with pain abdomen and drowsiness. Blood gas revealed 6.87/20.6/88/3.7 Na 140.4, K 4.41, Cl 102

Step-wise Approach Acedemia or Alkalemia
Metabolic or Respiratory (Primary Pathology) For metabolic is it anion gap or non anion gap. For AG acidosis, are there other disturbances. Resp compensation for the metabolic disturbances. For respiratory disturbances is it acute or chronic.

Step 1: Acidemic or Alkalemic?
Acidemic : PH < 7.35 Alkalemic: PH > 7.45 An acid-base abnormality is present if either the PaCO2 or the pH is outside the normal range. (A normal pH or PaCO2 does not exclude the presence of an acid-base abnormality)

Type of disturbance pH 6.87 Acidemia

Primary Acid-Base Disorders
A change in either the PCO2 or the HCO3 will cause a change in the [H+] of extracellular fluid. When a change in PCO2 is responsible for a change in [H+], the condition is called a respiratory acid-base disorder an increase in PCO2 is a respiratory acidosis a decrease in PCO2 is a respiratory alkalosis. When a change in HCO3 is responsible for a change in [H+], the condition is called a metabolic acid-base disorder a decrease in HCO3 is a metabolic acidosis an increase in HCO3 is a metabolic alkalosis.

Step 2: Primary disturbance metabolic or Respiratory
If the pH and PaCO2 are both abnormal, compare the directional change. If both change in the same direction (both increase or decrease), the primary acid-base disorder is metabolic, and if both change in opposite directions, the primary acid-base disorder is respiratory. If either the pH or PaCO2 is normal, there is a mixed metabolic and respiratory acid-base disorder (one is an acidosis and the other is an alkalosis). If the pH is normal, the direction of change in PaCO2 identifies the respiratory disorder If the PaCO2 is normal, the direction of change in the pH identifies the metabolic disorder.

Type of disturbance pH 6.87 pH: 6.8 PaCO2: 14.5 Acidemia Metabolic

Step 3 : What is the Anion Gap
Anion gap measures the difference between Anions(-) and Cations(+) present in blood AG = Na – (Cl + HCO3) Normal Anion gap is 12 mEq/L

Anion Gap Difference : 23 – 11 = 12 Unmeasured Anions
Unmeasured Cations Proteins 15 mEq Calcium 5 mEq Organic acid 5 mEq Potassium 4.5 mEq Phosphate 2 mEq Magnesium 1.5 mEq Sulfates 1 mEq Total 23 mEq Total 11 mEq Difference : 23 – 11 = 12

Extra for the experts Albumin carries negative charge .
Hypo-albuminemia causes falsely low AG. To correct for that AG adjusted = AG Observed × (4.5 – pt’s alb) Other causes of low AG Paraproteinemia, Bromism, Profound hypocalcemia, hypomagnesemia hyponatremia lithium toxicity, Other causes of low AG Paraproteinemia, Bromism, lithium toxicity, Profound hypocalcemia, hypomagnesemia and hyponatremia

Extra for the experts In metabolic alkalosis AG can be high but it could be due to unmeasured anions, specifically the albumin.

Type of disturbance pH 7.10 Anion Gap? Na = 140.4 Cl = 104 HCO3 = 3.7
Acidemia Metabolic AG = = 32.3 High AG

Causes Of Anion Gap Acidosis
Methanol Uremia DKA Paraldehyde INH Iron Lactic Acidosis Ethanol Ethylene Glycol Salicylic Acid MUDPILES

Step 4: Is there other metabolic disturbances coexisting with AG Acidosis
In the presence of high AG metabolic acidosis, it is possible that patient may have another metabolic acid base disorder. A normal AG metabolic acidosis or a metabolic alkalosis This can be discovered by comparing the AG excess to the HCO3 deficit.

Delta Anion Gap or ΔAG is sometimes simply called Δgap
Step 4: Is there other metabolic disturbances coexisting with AG Acidosis Delta Anion Gap or ΔAG: Difference between measured and normal AG ΔAG = AG - 12 Delta HCO3 or ΔHCO3: Difference between measured and normal HCO3 ΔHCO3 = 24 – Measured HCO3 Delta Anion Gap or ΔAG is sometimes simply called Δgap

So this patient has pure high AG metabolic acidosis
Step 4: Is there other metabolic disturbances coexisting with AG Acidosis If the disturbance is pure AG Acidosis Δ AG/Δ HCO3 = unity or 1 In our example HCO3= 3.7 so ΔHCO3 = 24 – 3.7 = 20.3 Now Δ AG AG = 32.3 so Δ AG = 32.3 – 12 = 20.3 Δ AG /ΔHCO3 = 20.3/20.3 = 1.0 So this patient has pure high AG metabolic acidosis

Remember ! If Δ AG /ΔHCO3 < 1.0
The decrease in the HCO3 is greater than the increase in the AG and the ratio falls below 1 It means there is accumulation of other acid which does not affect the AG but causes a fall in HCO3 i.e. NON-AG Metabolic Acidosis

Remember ! If Δ AG /ΔHCO3 > 1.0
When alkali is added in the presence of high AG acidosis, the decrease in serum HCO3 is less than the increase in the AG and the ratio goes above 1 Therefore, in the presence of high AG metabolic acidosis a gap-gap ratio of greater than 1 indicates co-existence of metabolic alkalosis

Concept of corrected HCO3
Add rgap to measured HCO3 If new value becomes normal (22-26) There is no other metabolic problems If it still stays < 22, then there is concomitant metabolic acidosis, non AG metabolic acidosis If it goes > 26, then there is concomitant metabolic alkalosis

Revision rgap + HCO3 = N (Only one disorder i.e.↑AG Met Acid)
rgap + HCO3 = >N (↑AG Met Acid + Meta Alk) rgap + HCO3 = <N (↑AG Met Acid + Nor AG Meta Acid)

Let us apply on our case Corrected HCO3 = HCO3 + Δ AG
Perfect !!

In ↑AG metabolic acidosis
Extend your search further To pin point the diagnosis

In case of high AG acidosis
Always calculate Osmolar gap: Osm gap = measured Osm – Calc Osm Calc Osm = (2 x Na+) + (glucose/18) + (BUN/2.8) Normal Osm gap < 10 mOsm/kg In areas where alcohol is common (2 x Na+) + (glucose/18) +(BUN/2.8) + (EtOH/4.6)

In case of high AG acidosis
↑AG acidosis but N osmolar gap ↑AG acidosis and ↑osmolar gap DKA Uremia Lactic acidosis Salisylates Ethanol Methanol Ehylene Glycol

Causes of non-Anion Gap Acidosis
Hyper Alimentation Acetazolamide Renal Tubular Acidosis Diarrhea Ureterosigmoidostomy Pancreatic Fistula Primary Hyperparathyroidism HARD-UP

Compensatory responses
Compensatory responses are secondary responses designed to limit the change in [H+] produced by the primary acid-base disorder, and this is accomplished by changing the other component of the PaCO2/HCO3 ratio in the same direction.

Secondary Responses If the primary problem is an increase in PaCO2 (respiratory acidosis) The secondary response will involve an increase in HCO3, and this will limit the change in [H+] produced by the increase in PaCO2. Secondary responses should not be called “compensatory responses” because they do not completely correct the change in [H+] produced by the primary acid-base disorder

Secondary/Compensatory responses
If there is a primary metabolic acidosis or alkalosis, use the measured HCO3 to identify the expected PaCO2. If the measured and expected PaCO2 are equivalent, the condition is fully compensated. If the measured PaCO2 is higher than the expected PaCO2, there is a superimposed respiratory acidosis. If the measured PCO2 is less than the expected PCO2, there is a superimposed respiratory alkalosis. Metabolic Acidosis Exp PaCO2 = 1.5 x HCO ± 2 Metabolic Alkalosis Exp PaCO2 = 0.7 x HCO ± 2

Let’s see our case pH 7.10 Acidemia Metabolic High AG
Compensated or ???? Winter’s Formula : Expected PaCO2 = (1.5 x HCO3) + 8 ± 2 Acidemia Applying Winter’s Formula : Expected PaCO2 = (1.5 x 3.7) + 8 ± 2 = Metabolic So in our case it is : Metabolic acidemia is compensated High AG

Mixed Disorders If either the pH or PaCO2 is normal, there is a mixed metabolic and respiratory acid-base disorder (one is an acidosis and the other is an alkalosis). If the pH is normal, the direction of change in PaCO2 identifies the respiratory disorder, and if the PaCO2 is normal, the direction of change in the pH identifies the metabolic disorder.

Mixed Disorders If there is a respiratory acidosis or alkalosis, use the PaCO2 to calculate the expected pH for respiratory acidosis or for respiratory alkalosis. Compare the measured pH to the expected pH to determine if the condition is acute, partially compensated, or fully compensated.

Mixed Disorders For respiratory acidosis
If the measured pH is lower than the expected pH for the acute, uncompensated condition, there is a superimposed metabolic acidosis If the measured pH is higher than the expected pH for the chronic, compensated condition, there is a superimposed metabolic alkalosis.

Mixed Disorders For respiratory alkalosis
If the measured pH is higher than the expected pH for the acute, uncompensated condition, there is a superimposed metabolic alkalosis If the measured pH is below the expected pH for the chronic, compensated condition, there is a superimposed metabolic acidosis.

Formulae for secondary responses Predicting Timing by pH change
Acute Respiratory Acidosis Fall in pH or Δ pH = x ΔPaCO2 Expected pH = 7.40 – [0.008 x ( PaCO2 – 40)] Chronic Respiratory Acidosis Fall in pH or Δ pH = x ΔPaCO2 Expected pH = 7.40 – [0.003 x ( PaCO2 – 40)] Acute Respiratory Alkalosis Rise in pH or ΔpH = x ΔPaCO2 Expected pH = [0.008 x ( 40 - PaCO2 )] Chronic Respiratory Alkalosis Rise in pH or Δ pH = x ΔPaCO2 Expected pH = [0.003 x ( 40 - PaCO2 )]

Predicting Timing by response

Another way to cram compensatory responses
Metabolic Acidosis HCO3 ↓ PaCO2 ↓ PaCO2 ↓ by 1.3 for each 1 mEq ↓in HCO3 Metabolic Alkalosis ↑in HCO PaCO2 ↑ PaCO2 ↑ by 0.7 for each 1 mEq ↑in HCO3 Acute Respiratory Acidosis ↑in PaCO HCO3 ↑ HCO3 ↑ by 1 mEq for each 10 mmHg ↑in PaCO2 Acute Respiratory Alkalosis ↓in PaCO HCO3 ↓ HCO3 ↓ by 2 mEq for each 10 mmHg ↓in PaCO2 Chronic Respiratory Acidosis ↑in PaCO2 ---HCO3 ↑ HCO3 ↑ by 3.5 mEq for each 10 mmHg ↑in PaCO2 Chronic Respiratory Alkalosis ↓in PaCO2---HCO3 ↓ HCO3 ↓ by 5mEq for each 10 mmHg ↓in PaCO2

Causes Of metabolic Alkalosis
Volume contraction (Vomiting, diuresis, ascities) Hypokalemia Alkali ingestion Excess gluco-mineralocorticosteroids Bartter’s Syndrome

Let’s Solve PH 7.02/PaCO219/HCO32.8, Na 141, Cl 111
Status Acedmia Met/ Resp Metabolic AG= =27 Other disorder? Anion Gap Compensation?? Corrected HCO3= 3 + (15) = 18 Additional Non AG acidosis PaCO2 = 1.5 × ± 2 = 12.5 ± 2 Resp Acidosis

7.50/21.9/88.7/20.3/98.2% Respiratory Status Alkalemia Met/ Resp
Acute Respiratory Alkalosis Rise in pH or ΔpH = x ΔPaCO2 Expected pH = [0.008 x ( 40 - PaCO2 )] Chronic Respiratory Alkalosis Rise in pH or Δ pH = 0.003 x 40 – ΔPaCO2 [0.003 x ( 40 - PaCO2 )] Met/ Resp Respiratory Acute/Chronic Acute Respiratory Alk Exp HCO3= 24+0.2x = 27.6 Compensation?? ΔHCO3 = 0.2 x ΔPaCO2 Exp HCO3 = 24 + [ 0.2 x (40 – PaCO2)] Acute Resp alkalosis with metabolic acidosis? Acute Resp alkalosis not yet compensated? PaCO2 = 0.7 x HCO ± 2

not yet fully compensated
7.23/58/96/24 Acidosis Acute Chronic Respiratory Δ PH=0.003 x 18 = 0.054 Δ PH= x (58-40) =0.08 x 1.8 = 0.144 PH=7.236 PH=7.326 Compensated or? Exp HCO3 = 24 + [0.1(PaCO2-40)] Acute resp acidosis not yet fully compensated Exp HCO3 = 24 + [0.1(58-40)] Answer = 25.8

7.35/48/69/29 Mixed Disorder Acute Chronic Respiratory
Δ PH= x (48-40) =0.003 x 8 = 0.024 7.40 – = Δ PH= x (48-40) =0.008 x 8 = 0.064 7.40 – = PH=7.336 PH=7.37 Compensated or? Exp HCO3 = 24 + [0.4(PaCO2-40)] Chronic compensated resp acidosis With metabolic alkalosis Exp HCO3 = 24 + [0.4(48-40)] Answer = 27.2

More Examples Introduction Acid Base Physiology
Cases Acid Base Abnormalities Acid Base Physiology Pearls Metabolic Alkalosis Metabolic Acidosis ◄ Back Mixed Acid Base Disorders Respiratory Alkalosis Respiratory Acidosis When to suspect a mixed acid base disorder: Mixed acid base disorders occur when there is more than one primary acid base disturbance present simultaneously. They are frequently seen in hospitalized patients, particularly in the critically ill. The expected compensatory response does not occur Whenever the PCO2 and [HCO3-] becomes abnormal in the opposite direction. (i.e. one is elevated while the other is reduced). In simple acid base disorders, the direction of the compensatory response is always the same as the direction of the initial abnormal change. Compensatory response occurs, but level of compensation is inadequate or too extreme pH is normal but PCO2 or HCO3- is abnormal Mixed metabolic disorders In simple acid base disorders, the compensatory response should never return the pH to normal. If that happens, suspect a mixed disorder. In anion gap metabolic acidosis, if the change in bicarbonate level is not proportional to the change of the anion gap. More specifically, if the delta ratio is greater than 2 or less than 1. Anion Gap and Normal Anion Gap Acidosis. This mixed acid base disorder is identified in patients with a delta ratio less than 1 which signifies that the reduction in bicarbonate is greater than it should be, relative to the change in the anion gap. Thus, implicating that there must be another process present requiring buffering by HCO3-, i.e a concurrent normal anion gap acidosis. Example: Progressive Renal Failure Lactic acidosis superimposed on severe diarrhea. (note: the delta ratio is not particularly helpful here since the diarrhea will be clinically obvious) Anion Gap Acidosis and Metabolic Alkalosis This mixed acid base disorder is identified in patients with a delta ratio greater than 1, which signifies a reduction in bicarbonate less than it should be, relative to the change in the anion gap. This suggests the presence of another process functioning to increase the bicarbonate level without affecting the anion gap, i.e. metabolic alkalosis. Examples: Type IV RTA and DKA DKA during treatment Lactic acidosis, uremia, or DKA in a patient who is actively vomiting or who requires nasogastric suction. Normal Anion Gap Acidosis and Metabolic Alkalosis This diagnosis can be quite difficult, because the low HCO3- and low PCO2 both move back toward normal when metabolic alkalosis develops. Also, unlike elevated anion gap acidosis, the anion gap will not indicate the presence of the acidosis. Example: Patient with lactic acidosis or DKA given sodium bicarbonate therapy. Mixed respiratory and respiratory–metabolic disorders Having a good knowledge of compensatory mechanisms and extent of compensation will aid in identifying these disorders. Remember; compensation for simple acid-base disturbances always drives the compensating parameter (ie, the PCO2, or [HCO3-]) in the same direction as the primary abnormal parameter (ie, the [HCO3-] or PCO2). Whenever the PCO2 and [HCO3] are abnormal in opposite directions, ie, one above normal while the other is reduced, a mixed respiratory and metabolic acid-base disorder exists. In patients who are vomiting and with diarrhea (note: all acid base parameters may fall within the normal range) When the PCO2 is elevated and the [HCO3-] reduced, respiratory acidosis and metabolic acidosis coexist. Rule of thumb: The above examples both produce very extreme acidemia or alkalemia and are relatively easy to diagnose. However more often, the disorder is quite subtle. For example, in cases of metabolic acidosis, the HCO3- is low and PCO2 low. If the PCO2 is normal or not aqequately reduced, this may indicate a subtle coexisting respiratory acidosis. When the PCO2 is reduced and the [HCO3-] elevated, respiratory alkalosis and metabolic alkalosis coexist Mixed acid base disorders usually produce arterial blood gas results that could potentially be explained by other mixed disorders. Oftentimes, the clinical picture will help to distinguish. It is important to distinguish mixed acid base disorders because work up and management will depend on accurate diagnosis. Chronic Respiratory Acidosis with superimposed Acute Respiratory Acidosis Example: COPD patient with worsening hypoventilation secondary to oxygen therapy or sedative administration Acute exacerbation of COPD secondary to acute pneumonia Chronic Respiratory Acidosis and Anion Gap Metabolic Acidosis Example: Chronic Respiratory Acidosis and Metabolic Alkalosis Example: COPD patient who develops shock and lactic acidosis Pulmonary insufficiency and diuretic therapy Respiratory Alkalosis and Metabolic Acidosis Example: or COPD patient treated with steroids or ventilation (important to recognize as alkalemia will reduce acidemic stimulus to breathe) Acute cardiopulmonary arrest Gram negative sepsis Salicylate intoxication Severe pulmonary edema Please note that it is impossible to have more than one respiratory disorder in the same mixed disorder(i.e. concurrent respiratory alkalosis and respiratory acidosis)

7.27/87.4/83.5/40.1 Acidemia Respiratory Acute or Chronic ? ΔpH:
Acute: x ΔPaCO2 = x 47 = 0.379 Expected pH = 7.40 – = 7.021 Chronic: x ΔPaCO2 = x 47 = 0.142 Expected pH = 7.40 – = 7.258 Chronic Respiratory Acidosis Compensation: 3.5 x 47 / 10 = 16.59 Expected HCO3 = = 40.59 For each 10 mmHg CO2 rise HCO3 rises by 3.5

HCO3 will fall by 1 with each 10 mmHg rise in CO2
7.24/62/58/22 Acidemia Primary …. Respiratory acidosis as PaCO2↑ Acute or Chronic ? ΔpH: Acute: x ΔPaCO2 = x 22= 0.176 Expected pH = = 7.224 Chronic: x ΔPaCO2 = x 22 = 0.066 Expected pH = = 7.334 So it is Acute Respiratory Acidosis Compensation for acute resp acidosis Expected ↓ in HCO3 = 22/10 = 2.2 Expected HCO3 = 24 – 2.2 = 21.8 HCO3 will fall by 1 with each 10 mmHg rise in CO2

7.365/22/110/12.3 Mixed Disorder ….. pH (N) and CO2 ↓
Respiratory alkalosis as PaCO2↓ ΔpH: Acute: x ΔPaCO2 = x 18 = 0.114 Expected pH = = 7.514 Chronic: x ΔPaCO2 = x 18 = 0.054 Expected pH = = 7.454 In both cases the pH should be higher than what we have !! So there is concomitant metabolic acidosis !! Expected HCO3 for respiratory alkalosis Expected HCO3 for acute = 20.4 Expected HCO3 for chronic = 15 Acute: HCO3 ↓by 2 for each 10 mmHg ↓ in CO2 Chronic: HCO3 ↓by 5 for each 10 mmHg ↓ in CO2 Although the last calculation is not required !!

See you in part 2 Oxygenation Status
Thank you See you in part 2 Oxygenation Status

Interpreting ABGs Practical Approach

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