Arterial Blood Gases Interpretation Part1

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This is a simplified and practical approach to interpret ABGs. In this part only acid-base disturbances are discussed

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Interpreting ABGs Practical Approach:

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

PowerPoint Presentation:

Venous Arterial

Arterial Blood Gases:

Arterial Blood Gases Written in following manner: pH/PaCO 2 /PaO 2 /HCO 3 pH = arterial blood pH PaCO 2 = arterial pressure of CO 2 PaO 2 = arterial pressure of O 2 HCO 3 = serum bicarbonate concentration

Oxygenation:

Oxygenation Hypoxia : reduced oxygen pressure in the alveolus (i.e. P A O 2 ) Hypoxemia: reduced oxygen pressure in arterial blood (i.e. P a O 2 )

Hypoxia with Low PaO2:

Hypoxia with Low PaO 2 Decreased alveolar PO 2 Decreased FiO 2 Hypoventilation High altitude Alveolar diffusion impairment R  L shunt V/Q mismatch

Hypoxia with Normal PaO2:

Hypoxia with Normal PaO 2 Alterations in hemoglobin Anemic hypoxia Carbon monoxide poisoning Methemoglobinemia Histotoxic hypoxia Cyanide Hypoperfusion hypoxia or stagnant hypoxia

Alveolar—Arterial Gradient:

Alveolar—Arterial Gradient Indirect measurement of V/Q abnormalities Normal A-a gradient is 10 mmHg Rises with age Rises by 5-7 mmHg for every 0.10 rise in FiO 2 , from loss of hypoxic vasoconstriction in the lungs

Alveolar—Arterial Gradient:

Alveolar—Arterial Gradient A-a gradient = P A O 2 – P a O 2 P A O 2 = alveolar PO 2 (calculated) P a O 2 = arterial PO 2 ( measured ) P A O 2 = P I O 2 – (P a CO 2 /RQ) P A O 2 = alveolar PO 2 P I O 2 = PO2 in inspired gas P a CO 2 = arterial PCO 2 RQ = respiratory quotient

Alveolar—Arterial Gradient:

Alveolar—Arterial Gradient P I O 2 = FiO 2 (P B – P H2O ) P B = barometric pressure (760 mmHg) P H2O = partial pressure of water vapor (47 mmHg) RQ = V CO 2 /V O 2 RQ defines the exchange of O 2 and CO 2 across the alveolar-capillary interface (0.8)

Alveolar—Arterial Gradient:

Alveolar—Arterial Gradient P A O 2 = Fi O 2 (P B – P H2O ) – (P a CO 2 /RQ) Or P A O 2 = FiO 2 (713) – (P a CO 2 /0.8)

Alveolar—Arterial Gradient:

Alveolar—Arterial Gradient For room air: P A O 2 = 150 – (P a CO 2 /0.8) Assume a normal P a CO 2 (40): P A O 2 = 100 For 100% FiO 2 : P A O 2 = 1 X 713 – (P a CO 2 /0.8) Assume a normal P a CO 2 (40): P A O 2 = 663

Part 1:

Part 1 Acid-Base Disorders

Acid-Base:

Acid-Base Acidosis or alkalosis: 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:

Some important concepts The determinants of extracellular fluid pH indicate that tight control of the pH requires a fairly constant PCO 2 /HCO 3 ratio. Thus , a change in one of the determinants (PCO 2 or HCO 3 ) must be accompanied by a proportional change in the other determinant to keep the PCO 2 /HCO 3 ratio (and the pH) constant.

Some important concepts:

Some important concepts Thus, an increase in PCO 2 (respiratory acidosis) must be accompanied by an increase in HCO 3 (metabolic alkalosis) to keep the pH constant. This is how the control system for acid-base balance operates. A respiratory disorder (change in PCO 2 ) always initiates a complementary metabolic response (that alters the HCO 3 ), and vice-versa

PowerPoint Presentation:

Primary Disorder Primary Change Compensatory Change* Respiratory acidosis Increased PCO 2 Increased HCO 3 Respiratory alkalosis Decreased PCO 2 Decreased HCO 3 Metabolic acidosis Decreased HCO 3 Decreased PCO 2 Metabolic alkalosis Increased HCO 3 Increased PCO 2 Primary Acid-Base Disorders and Associated Compensatory Changes [H+] = 24 × PCO2/HCO3 * Compensatory changes keep the PCO 2 /HCO 3 ratio constant.

Check if data is consistent:

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} 7.3- -------------- 50 7.2- -------------- 63 7.1- --------------80 7.0--------------100 6.9--------------125 6.8--------------160

Check if data is consistent:

Check if data is consistent { H} = 24 [PaCO 2 /HCO 3 ] { 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:

Here are some examples Written in following manner: pH/PaCO 2 /PaO 2 /HCO 3 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 7.8-7.8 x 100 = 0 The data is inconsistent 24 x 35.5/50.6 = 16.8 7.8-7.7 x 100 = 10 The data is inconsistent 24x53.1/44.6 = 28.57 7.8-7.54 x 100 = 26 The data is consistent

Case:

Case A 69-year-old white male with severe COPD presents to the emergency room with diffuse abdominal tenderness, hypoactive bowel sounds and a history of pain while eating for several months. Abdominal film showed distended bowel. He is evaluated by a surgical colleague who calls you with the following serum chemistry results: serum sodium 140 mEq /L, chloride 105 mEq /L, total venous HCO 3 concentration 15 mEq /L, potassium 6 mEq /L, BUN 24 mg/dl and serum creatinine 1.1 mg/dl. ABGs: pH 7.10, PCO 2 50 mm Hg, PO 2 54 mm Hg (room air). The most appropriate acid base disorder in this individual is: A. Simple metabolic acidosis B. Acute respiratory acidosis C. Combined metabolic acidosis and respiratory alkalosis D. Combined metabolic acidosis and respiratory acidosis E. Combined respiratory acidosis and metabolic alkalosis

Case Study:

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 :

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?:

Step 1: Acidemic or Alkalemic ? Acidemic : PH < 7.35 Alkalemic : PH > 7.45 An acid-base abnormality is present if either the PaCO 2 or the pH is outside the normal range. ( A normal pH or PaCO 2 does not exclude the presence of an acid-base abnormality)

Type of disturbance:

Type of disturbance pH 6.87 Acidemia

Primary Acid-Base Disorders :

Primary Acid-Base Disorders A change in either the PCO 2  or the HCO 3  will cause a change in the [H + ] of extracellular fluid. When a change in PCO 2  is responsible for a change in [H + ], the condition is called a respiratory  acid-base disorder an increase in PCO 2  is a  respiratory acidosis a decrease in PCO 2  is a respiratory alkalosis . When a change in HCO 3  is responsible for a change in [H + ], the condition is called a metabolic  acid-base disorder a decrease in HCO 3  is a  metabolic acidosis an increase in HCO 3  is a metabolic alkalosis .

Step 2: Primary disturbance metabolic or Respiratory :

Step 2: Primary disturbance metabolic or Respiratory If the pH and PaCO 2 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 PaCO 2 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 PaCO 2 identifies the respiratory disorder I f the PaCO 2 is normal , the direction of change in the pH identifies the metabolic disorder.

Type of disturbance:

Type of disturbance pH 6.87 Acidemia Metabolic p H: 6.8 PaCO 2 : 14.5

Step 3 : What is the Anion Gap:

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

Anion Gap:

Anion Gap 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:

Extra for the experts Albumin carries negative charge . Hypo- albuminemia causes falsely low AG. To correct for that AG adjusted = AG Observed + 0.25 × (4.5 – pt’s alb ) Other causes of low AG Paraproteinemia , Bromism , lithium toxicity, Profound hypocalcemia , hypomagnesemia hyponatremia

Extra for the experts:

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:

Type of disturbance pH 7.10 Acidemia Metabolic High AG Anion Gap? Na = 140.4 Cl = 104 HCO 3 = 3.7 AG = 140.4 - 104 - 3.7 = 32.3

Causes Of Anion Gap Acidosis:

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:

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 HCO 3 deficit.

Step 4: Is there other metabolic disturbances coexisting with AG Acidosis:

Step 4: Is there other metabolic disturbances coexisting with AG Acidosis Delta Anion Gap or Δ AG: Difference between measured and normal AG Δ AG = AG - 12 Delta HCO 3 or Δ HCO 3 : Difference between measured and normal HCO 3 Δ HCO 3 = 24 – Measured HCO 3 Delta Anion Gap or Δ AG is sometimes simply called Δ gap

Step 4: Is there other metabolic disturbances coexisting with AG Acidosis:

Step 4: Is there other metabolic disturbances coexisting with AG Acidosis If the disturbance is pure AG Acidosis Δ AG/ Δ HCO 3 = unity or 1 In our example HCO 3 = 3.7 so Δ HCO 3 = 24 – 3.7 = 20.3 Now Δ AG AG = 32.3 so Δ AG = 32.3 – 12 = 20.3 Δ AG / Δ HCO 3 = 20.3/20.3 = 1.0 So this patient has pure high AG metabolic acidosis

Remember !:

Remember ! If Δ AG / Δ HCO 3 < 1.0 The decrease in the HCO 3 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 HCO 3 i.e. NON-AG Metabolic Acidosis

Remember !:

Remember ! If Δ AG / Δ HCO 3 > 1.0 When alkali is added in the presence of high AG acidosis, the decrease in serum HCO 3 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:

Concept of corrected HCO 3 Add r gap to measured HCO 3 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 :

Revision r gap + HCO 3 = N (Only one disorder i.e.↑AG Met Acid ) r gap + HCO 3 = > N (↑AG Met Acid + Meta Alk ) r gap + HCO 3 = < N (↑AG Met Acid + Nor AG Meta Acid)

Let us apply on our case:

Let us apply on our case Corrected HCO 3 = HCO 3 + Δ AG Corrected HCO 3 = 3.7 + 20.3 = 24 Perfect !!

In ↑AG metabolic acidosis:

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

In case of high AG acidosis:

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

In case of high AG acidosis:

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

Causes of non-Anion Gap Acidosis:

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

Extra For The Experts:

Extra For The Experts

Compensatory responses:

C ompensatory 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 PaCO 2 /HCO 3  ratio in the same direction. 

Secondary Responses:

Secondary Responses   If the primary problem is an increase in PaCO 2  (respiratory acidosis) T he secondary response will involve an increase in HCO 3 , and this will limit the change in [H + ] produced by the increase in PaCO 2 .  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:

Secondary/Compensatory responses If there is a primary metabolic acidosis or alkalosis, use the measured HCO 3 to identify the expected PaCO 2 . If the measured and expected PaCO 2 are equivalent, the condition is fully compensated. If the measured PaCO 2 is higher than the expected PaCO 2 , there is a superimposed respiratory acidosis. If the measured PCO 2 is less than the expected PCO 2 , there is a superimposed respiratory alkalosis. Metabolic Acidosis Exp PaCO 2 = 1.5 x HCO 3 + 8 ± 2 Metabolic Alkalosis Exp PaCO 2 = 0.7 x HCO 3 + 21 ± 2

Let’s see our case :

Let’s see our case pH 7.10 Acidemia Metabolic High AG Compensated or ???? Winter’s Formula : Expected PaCO 2 = (1.5 x HCO 3 ) + 8 ± 2 Applying Winter’s Formula : Expected PaCO 2 = (1.5 x 3.7) + 8 ± 2 = 13.5-15.5 So in our case it is : Metabolic acidemia is compensated

Mixed Disorders:

Mixed Disorders If either the pH or PaCO 2 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 PaCO 2 identifies the respiratory disorder, and if the PaCO 2 is normal, the direction of change in the pH identifies the metabolic disorder.

Mixed Disorders :

Mixed Disorders If there is a respiratory acidosis or alkalosis, use the PaCO 2 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:

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.

PowerPoint Presentation:

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. Mixed Disorders

Formulae for secondary responses Predicting Timing by pH change:

Formulae for secondary responses Predicting Timing by pH change Acute Respiratory Acidosis Fall in pH or Δ pH = 0.008 x Δ PaCO 2 Expected pH = 7.40 – [0.008 x ( PaCO 2 – 40)] Chronic Respiratory Acidosis Fall in pH or Δ pH = 0.003 x Δ PaCO 2 Expected pH = 7.40 – [ 0.003 x ( PaCO 2 – 40)] Acute Respiratory Alkalosis Rise in pH or Δ pH = 0.008 x Δ PaCO 2 Expected pH = 7.40 + [0.008 x ( 40 - PaCO 2 )] Chronic Respiratory Alkalosis Rise in pH or Δ pH = 0.003 x Δ PaCO 2 Expected pH = 7.40 + [ 0.003 x ( 40 - PaCO 2 )]

Predicting Timing by response:

Predicting Timing by response

Another way to cram compensatory responses:

Another way to cram compensatory responses Metabolic Acidosis HCO 3 ↓----------------PaCO 2 ↓ PaCO 2 ↓ by 1.3 for each 1 mEq ↓in HCO 3 Metabolic Alkalosis ↑in HCO 3------------------ PaCO 2 ↑ PaCO 2 ↑ by 0.7 for each 1 mEq ↑in HCO 3 Acute Respiratory Acidosis ↑in PaCO 2---- -HCO 3 ↑ HCO 3 ↑ by 1 mEq for each 10 mmHg ↑in PaCO 2 Acute Respiratory Alkalosis ↓in PaCO 2----- HCO 3 ↓ HCO 3 ↓ by 2 mEq for each 10 mmHg ↓in PaCO 2 Chronic Respiratory Acidosis ↑in PaCO 2 --- HCO 3 ↑ HCO 3 ↑ by 3.5 mEq for each 10 mmHg ↑in PaCO 2 Chronic Respiratory Alkalosis ↓in PaCO 2--- HCO 3 ↓ HCO 3 ↓ by 5mEq for each 10 mmHg ↓in PaCO 2

Causes Of metabolic Alkalosis:

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:

Let’s Solve PH 7.02/PaCO 2 19/HCO 3 2.8, Na 141, Cl 111 Acedmia AG=141-111-3=27 Corrected HCO 3 = 3 + ( 15 ) = 18 Additional Non AG acidosis PaCO 2 = 1.5 × 3 + 8 ± 2 = 12.5 ± 2 Resp Acidosis Metabolic Status Met/ Resp Anion Gap Other disorder? Compensation??

7.50/21.9/88.7/20.3/98.2%:

7.50/21.9/88.7/20.3/98.2% Alkalemia Acute Respiratory Alk Exp HCO 3 = 24+0.2x40-21.9= 27.6 Respiratory Status Met/ Resp Acute/Chronic Compensation?? Acute Respiratory Alkalosis Rise in pH or Δ pH = 0.008 x Δ PaCO 2 Expected pH = 7.40 + [0.008 x ( 40 - PaCO 2 )] Chronic Respiratory Alkalosis Rise in pH or Δ pH = 0.003 x 40 – Δ PaCO 2 Expected pH = 7.40 + [0.003 x ( 40 - PaCO 2 )] Δ HCO 3 = 0.2 x Δ PaCO 2 Exp HCO3 = 24 + [ 0.2 x (40 – PaCO2)] Acute Resp alkalosis with metabolic acidosis? Acute Resp alkalosis not yet compensated? PaCO2 = 0.7 x HCO3 + 21 ± 2

7.23/58/96/24:

7.23/58/96/24 Acidosis Respiratory Δ PH= 0.008 x (58-40) = 0.08 x 1.8 = 0.144 Δ PH=0.003 x 18 = 0.054 PH=7.326 PH=7.236 Chronic Acute Compensated or? Exp HCO3 = 24 + [0.1(PaCO2-40)] Exp HCO3 = 24 + [0.1(58-40)] Answer = 25.8 Acute resp acidosis not yet fully compensated

7.35/48/69/29:

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

More Examples:

More Examples

7.27/87.4/83.5/40.1:

7.27/87.4/83.5/40.1 Acidemia Respiratory Acute or Chronic ? ΔpH : Acute: 0.008 x Δ PaCO2 = 0.008 x 47 = 0.379 Expected pH = 7.40 – 0.379 = 7.021 Chronic: 0.003 x Δ PaCO2 = 0.003 x 47 = 0.142 Expected pH = 7.40 – 0.142 = 7.258 Chronic Respiratory Acidosis Compensation: 3.5 x 47 / 10 = 16.59 Expected HCO3 = 24 + 16.59 = 40.59 For each 10 mmHg CO2 rise HCO3 rises by 3.5

7.24/62/58/22:

7.24/62/58/22 Acidemia Primary …. Respiratory acidosis as PaCO 2 ↑ Acute or Chronic ? ΔpH : Acute: 0.008 x Δ PaCO2 = 0.008 x 22= 0.176 Expected pH = 7.40 - 0.176 = 7.224 Chronic: 0.003 x Δ PaCO2 = 0.003 x 22 = 0.066 Expected pH = 7.40 + 0.066 = 7.334 So it is Acute Respiratory Acidosis Compensation for acute resp acidosis Expected ↓ in HCO 3 = 22/10 = 2.2 Expected HCO 3 = 24 – 2.2 = 21.8 HCO 3 will fall by 1 with each 10 mmHg rise in CO 2

7.365/22/110/12.3:

7.365/22/110/12.3 Mixed Disorder ….. pH (N) and CO 2 ↓ Respiratory alkalosis as PaCO 2 ↓ ΔpH : Acute: 0.008 x Δ PaCO2 = 0.008 x 18 = 0.114 Expected pH = 7.40 + 0.114 = 7.514 Chronic: 0.003 x Δ PaCO2 = 0.003 x 18 = 0.054 Expected pH = 7.40 + 0.054 = 7.454 In both cases the pH should be higher than what we have !! So there is concomitant metabolic acidosis !! Expected HCO 3 for respiratory alkalosis Expected HCO 3 for acute 24 - 3.6 = 20.4 Expected HCO 3 for chronic 24 - 9 = 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 !!

Thank you :

Thank you See you in part 2 Oxygenation Status

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