AcidAcid--Base Disorders and the ABGBase Disorders and the ABG

BYBYDr/Magda abdelDr/Magda abdel--salamsalam

Lecturer of Thoracic MedicineLecturer of Thoracic Medicine

The Goal :The Goal :

To provide Bedside approach to ABG analysis

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The Blood Gas Report: normals…

pH 7.40 +

0.05

PaCO2

40 +

5 mm Hg

PaO2

80 -

100

mm Hg

HCO3

24 +

4

mmol/L

O2 Sat

>95

Always mention and see FIO2

The essentials

HCO3

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DefinitionsHypoxiadiminished availability of oxygen to the body and tissues

Hypoxemiadeficient oxygenation of the blood

Ischemiadeficiency of blood supply to a part of the body (tissue/organ)

anoxia is ultimate ischemia

Definitions

Acidany substance which donates H+HCl-, H2CO3, CO2Base/Buffersubstances that accept H+minimize changes in pHbicarbonate; proteins; phosphates & hemoglobin

Acidosis

excess H+ in tissuespH: degree of acidity or alkalinitynegative logarithm of hydrogen ion concentrationthe higher the acidity, the lower the pH↑

H+ = ↓

pH

< 7.35

Alkalosis

Excess base (bicarbonate/HCO3)pH: degree of acidity or alkalinitynegative logarithm of hydrogen ion concentrationthe more alkaline, the higher the pH↓

H+ = ↑

pH

> 7.45

Serum bicarbonateSerum bicarbonateThe standard/corrected bicarbonateThe standard/corrected bicarbonateis the value obtained at CO2 levels of is the value obtained at CO2 levels of 40mm Hg and at room temperature.40mm Hg and at room temperature.

The actual bicarbonateThe actual bicarbonate

is the value is the value calculated from the blood gas sample. calculated from the blood gas sample. The. It gives a better estimate of the The. It gives a better estimate of the metabolic problem causing acid base metabolic problem causing acid base imbalanceimbalance

The base deficit/excessThe base deficit/excess is the is the

amount of deviation of the standard amount of deviation of the standard bicarbonate from the normal { actual bicarbonate from the normal { actual bicarbonate}. The metabolic problem could bicarbonate}. The metabolic problem could either be a low (base deficit or metabolic either be a low (base deficit or metabolic acidosis) or high (base excess or acidosis) or high (base excess or metabolic alkalosis) standard bicarbonate metabolic alkalosis) standard bicarbonate normally normally (+/-

2).

HendersonHenderson--Hasselbalch EquationHasselbalch Equation

Primary AcidPrimary Acid--Base DisordersBase Disorders

As dictated by the HendersonAs dictated by the Henderson--Hasselbalch equation, Hasselbalch equation, disturbances in either the respiratory component (pCOdisturbances in either the respiratory component (pCO22

) or ) or metabolic component (HCOmetabolic component (HCO33

--) can lead to alterations in pH.) can lead to alterations in pH.

Metabolic AcidosisMetabolic Acidosis(Too little HCO(Too little HCO33

--))Metabolic AlkalosisMetabolic Alkalosis(Too much HCO(Too much HCO33

--))

Respiratory Acidosis Respiratory Acidosis (Too much CO(Too much CO22

))Respiratory AlkalosisRespiratory Alkalosis

(Too little CO(Too little CO22

))

Respiratory Acidosis

•

decreased respiratory exchange with retention of CO2 results in increased pCO2 which thencauses renal retention of bicarbonate

Respiratory Acidosis: CausesDecreased respiratory exchangeCNS Depressiontrauma/infections/tumorcerebrovascularhydrothoraxpneumothoraxLung disorderbronchial obstructionemphysema (chronicobstructive airwaydisease)severe pulmonary edemaaccidentsdrug overdoseNeuromuscular disordersMyopathiesThoracic disorders

Respiratory Acidosis: Compensation

Problem: increased pCO2 and this results in a decreased blood pH (high H+)[H+] stimulates kidney to generate and retain bicarbonaterespiratory acidosis.is compensated for by the development of a metabolic alkalosis

Respiratory Acidosis: Compensation

Compensation is complete ([HCO3] levelsout) in 2-4 daysFinal HCO3 can be calculated from thefollowing equation:HCO3 mmol/L = 0.44 X pCO2 mmHg + 7.6 (+/-

2).Limit of compensation is a HCO3 of 45mmol/L

Respiratory Alkalosis

increased respiratory exchange with loss of CO2results in a decreased pCO2 which then stimulates renal excretion of bicarbonate

Respiratory Alkalosis: Causes

Increased respiratory exchangeCNS disturbancesPsychogenic (anxiety)PregnancyHypoxiaDrug toxicity /overdosePulmonary disordersEmbolismEdemaAsthmaPneumonia

Respiratory Alkalosis: Compensation

Problem: decreased pCO2 causing increased blood pH(low H+)Increased pH stimulates the kidney to excrete bicarbonaterespiratory alkalosis is compensated for by the development of a metabolic acidosis

Respiratory Alkalosis: Compensation

If the condition has been present for 7 days or more full compensation may occurCompensation is complete ([HCO3] levels out) in 7-10 daysThe limit of compensation is a HCO3 of 12mmol/L.

Metabolic Acidosis

Increased production or renal retention of H+results in a low pH which stimulates respiration to decreased the pCO2

Metabolic Acidosis: Causes

High Anion GapRenal failuretoxinsketoacidosisNormal anion gap(hyperchloremic)Hyperkalemiaobstructive uropathydiarrhearenal tubular acidosisSome medications

Metabolic Acidosis: Compensation

Problem: decreased [HCO3] causing decreased blood pH(high H+).[H+] stimulates respiration which lowers the blood pCO2metabolic acidosis is compensated for by the development of a respiratory alkalosis

Metabolic Acidosis: Compensation

Compensation is complete (pCO2 levelsout) in 12-24 hours.The final pCO2 can be calculated from thefollowing equation:pCO2 mmHg = 1.5 x [HCO3] (mmol/L) + 8 (+/-

2).The limit of compensation is a pCO2 of 10mmHg

Anion GapAnion Gap

Law of electroneutrality:Law of electroneutrality:––

Blood plasma contains an = number of + and Blood plasma contains an = number of + and ––

charges.charges.The major cation is NaThe major cation is Na++..––

Minor cations are KMinor cations are K++, Ca, Ca2+2+

, Mg, Mg2+2+. .

The major anions are HC0The major anions are HC033

--

and Cland Cl--..

(Routinely measured.)(Routinely measured.)––

Minor anions include albumin, phosphate, sulfate Minor anions include albumin, phosphate, sulfate (called unmeasured anions).(called unmeasured anions).

––

Organic acid anions include lactate and Organic acid anions include lactate and acetoacetate,.acetoacetate,.

Anion Gap

Anion gap = Na -

( Cl + HCO3)Normally, 10-14 mEq/L.Greater than 20 = metabolic acidosis definitely present.Normally due to serum proteins with normal negative chargeAbnormal acids (anions) cause elevated anion gap.Decreased anion gap: hypoalbuminemia,myeloma

Anion gap (cont.)

•

Even

if no apparent metabolic acidosis, aniongap > 20 means a metabolic acidosis is present,just hidden by another disorder.

•

If high anion gap

metabolic acidosis only,DHCO3 should approximate the D anion gapExample, HCO3 20 (D=5) and anion gap 19 (D=5).

•

If not, another disorder is altering the HCO3-

in addition to the metabolic acidosis.

Metabolic Alkalosis

increased production or renal retention of HCO3results in a high pH which inhibitsrespiration to increase the pCO2

Metabolic Alkalosis: Causes

decreased Urinary chlorideGut H+ lossVomiting, suctionRenal H+ lossDiuretic therapyContraction alkalosisincreased Urinary chlorideMineralocorticoidExcess Diuretic therapy

Metabolic Alkalosis: Compensation

Problem: increased [HCO3] causing increased blood pH. (low H+)

Low [H+] suppresses respiration which increases blood pCO2

metabolic alkalosis is compensated for by the development of a respiratory acidosis

Metabolic Alkalosis: Compensation

Compensation is complete (pCO2 levels out) in 12-24 hours

The final pCO2 can be calculated from the following equation:pCO2 mmHg = 0.9 X [HCO3] (mmol/L) + 9 (+/-2)

The limit of compensation is a pCO2 of 60

CompensationCompensation

When a primary acidWhen a primary acid--base disorder exists, the base disorder exists, the body attempts to return the pH to normal via body attempts to return the pH to normal via the the ““other halfother half””

of acid base metabolism.of acid base metabolism.

Primary metabolic disorder Primary metabolic disorder Respiratory compensationRespiratory compensation

Primary respiratory disorder Primary respiratory disorder Metabolic compensationMetabolic compensation

Compensation (continued)Compensation (continued)Primary DisorderPrimary Disorder Compensatory MechanismCompensatory Mechanism

Metabolic acidosisMetabolic acidosis Increased ventilationIncreased ventilation

Metabolic alkalosisMetabolic alkalosis Decreased ventilationDecreased ventilation

Respiratory acidosisRespiratory acidosis Increased renal reabsorption of HCOIncreased renal reabsorption of HCO33

--

in the proximal tubulein the proximal tubuleIncreased renal excretion of H in the Increased renal excretion of H in the

distal tubule distal tubule Respiratory alkalosisRespiratory alkalosis Decreased renal reabsorption of HCODecreased renal reabsorption of HCO33

--

in the proximal tubulein the proximal tubuleDecreased renal excretion of HDecreased renal excretion of H++

in the in the

distal tubule distal tubule

CO

MP

EN

SIO

N LIM

ITS

METABLIC ACIDOSISPaCO2 = Up to 10 ?

METABOLIC ALKALOSISPaCO2 = Maximum 6O

RESPIRATORY ACIDOSISBICARB = Maximum 40

RESPIRATORY ALKALOSISBICARB = Up to 10

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The Arterial Blood Gas (ABG)The Arterial Blood Gas (ABG)

pH, pCO2

, pO2

–

Measured directly

HCO3-, O2

saturation (usually) –

Calculated from pH, pCO2

, and pO2

Practical ApproachPractical Approach

1. Check the pH1. Check the pH

If the pH < 7.35, acidemia (and at least 1 acidosis) is present.If the pH < 7.35, acidemia (and at least 1 acidosis) is present.

If the pH > 7.45, If the pH > 7.45, alkalemiaalkalemia

(and at least 1 alkalosis) is present. (and at least 1 alkalosis) is present.

Practical ApproachPractical Approach

2. Check the pCO2. Check the pCO22

pH < 7.35 and pCOpH < 7.35 and pCO22

< 40 < 40 metabolic acidosismetabolic acidosispH < 7.35 and pCOpH < 7.35 and pCO22

> 40 > 40 respiratory acidosisrespiratory acidosis

pH > 7.45 and pCOpH > 7.45 and pCO22

< 40 < 40 respiratory alkalosisrespiratory alkalosispH > 7.45 and pCOpH > 7.45 and pCO22

> 40 > 40 metabolic acidosismetabolic acidosis

Practical ApproachPractical Approach3. Choose the appropriate compensation formula3. Choose the appropriate compensation formula

Most prominent Most prominent disorderdisorder

Compensation formulaCompensation formula

Metabolic acidosisMetabolic acidosis pCOpCO22

≈≈

1.5 [HCO1.5 [HCO33

--] + 8 ] + 8

Metabolic alkalosisMetabolic alkalosis pCOpCO22

≈≈

0.9 [HCO0.9 [HCO33

--] + 9 ] + 9

Respiratory acidosisRespiratory acidosis For every 10 For every 10 ΔΔ

in pCOin pCO22

, pH decreases by:, pH decreases by:0.08 (in acute resp. acidosis)0.08 (in acute resp. acidosis)0.03 (in chronic resp. acidosis) 0.03 (in chronic resp. acidosis)

Respiratory alkalosisRespiratory alkalosis For every 10 For every 10 ΔΔ

in pCOin pCO22

, pH increases by:, pH increases by:0.08 (in acute resp. alkaloses)0.08 (in acute resp. alkaloses)0.03 (in chronic resp. alkalosis) 0.03 (in chronic resp. alkalosis)

Practical ApproachPractical Approach

4. Determine if the degree compensation is 4. Determine if the degree compensation is appropriateappropriate

(If it isn(If it isn’’t, a second acidt, a second acid--base disorder is likely present)base disorder is likely present)

Practical ApproachPractical Approach

5. Calculate the anion gap 5. Calculate the anion gap

The difference between [NaThe difference between [Na++] and the sum of [HC0] and the sum of [HC033

--] and [] and [ClCl-- .].]

[Na[Na++] ] –– ([HC0([HC033 --] + [Cl] + [Cl--]) = ]) =

––

140 140 --

(24 + 105) = 11(24 + 105) = 11Normal = 12 Normal = 12 ++

2 2

Clinicians use the anion gap to identify the cause of Clinicians use the anion gap to identify the cause of metabolic acidosis.metabolic acidosis.

Variations of anion gapVariations of anion gapCauses of decreased anion gap includeCauses of decreased anion gap include

hypoalbuminaemia and severe haemodilution. hypoalbuminaemia and severe haemodilution. Rarer causes include increase in minor cation Rarer causes include increase in minor cation concentrations like calcium and magnesium concentrations like calcium and magnesium

. . Causes of a raised anion gap includeCauses of a raised anion gap include dehydration and any cause of raised dehydration and any cause of raised

unmeasurable anions, like lactate, ketones and unmeasurable anions, like lactate, ketones and renal acids, along with treatment with drugs renal acids, along with treatment with drugs given as organic acids such as penicillin, given as organic acids such as penicillin, salicylates and poisoning with methanol, ethanol salicylates and poisoning with methanol, ethanol and paraldehyde. Rarely it may be due to and paraldehyde. Rarely it may be due to decreased minor cation concentrations such as decreased minor cation concentrations such as calcium or magnesiumcalcium or magnesium

Raised anion gap metabolic Raised anion gap metabolic acidosisacidosis

As documented above, accumulation of a number of As documented above, accumulation of a number of acids can result in raised anion gap metabolic acidosis.acids can result in raised anion gap metabolic acidosis.In such cases, the reduction in serum HCO3In such cases, the reduction in serum HCO3--

matches matches

the anion gap.the anion gap.If not, a second acid base disorder should be kept in If not, a second acid base disorder should be kept in mind. When metabolic acidosis and alkalosis coexist, as mind. When metabolic acidosis and alkalosis coexist, as in vomiting and ketoacidosis, the plasma HCO3in vomiting and ketoacidosis, the plasma HCO3--

may be may be

normal, and a raised anion gap may be the initial normal, and a raised anion gap may be the initial evidence of an underlying acidevidence of an underlying acid--base disturbance (2).base disturbance (2).

To differentiate between the many causes of 'increased To differentiate between the many causes of 'increased anion gap metabolic acidosis', we measure the osmolar anion gap metabolic acidosis', we measure the osmolar gap that is the difference between the measured gap that is the difference between the measured osmolarity and the calculated osmolarityosmolarity and the calculated osmolarity

Normal anion gap Normal anion gap metabolic acidosismetabolic acidosis hyperchloraemichyperchloraemic

This usually results from conditions where in This usually results from conditions where in there is a loss of alkali (i.e.HCO3there is a loss of alkali (i.e.HCO3--) or metabolic ) or metabolic equivalent (eg, excretion of salts of organic equivalent (eg, excretion of salts of organic anions in proportionate excess of chloride) or an anions in proportionate excess of chloride) or an accumulation of HCl or metabolic equivalent (eg, accumulation of HCl or metabolic equivalent (eg, NH4Cl and chloride salts of amino acids) (2). NH4Cl and chloride salts of amino acids) (2). Loss of HCO3Loss of HCO3--

can occur either due to GI can occur either due to GI

causes or due to renal causes (renal excretion causes or due to renal causes (renal excretion or insufficient generation). In many surgical or insufficient generation). In many surgical conditions, the cause is usually obvious. conditions, the cause is usually obvious.

6. Mixed Acid 6. Mixed Acid -- Base DisordersBase Disorders Compensated AppropriatelyCompensated Appropriatelymet acidosismet acidosis

expected pCO2 = 1.5(HCO3) + 8

met alkalosismet alkalosis

expected pCO2 = .9(HCO3) + 9Acute resp acidAcute resp acid

each increase in pCO2 of 1, pH should decr by .008

Acute resp alkAcute resp alk each decrease in pCO2 of 1, pH should incr by .008

Chronic resp acidChronic resp acid

each increase in pCO2 of 1, pH should decr by .003

Chronic resp alkChronic resp alk

each decrease in pCO2 of 1, pH should incr by .003

7.7. The Triple DisorderThe Triple Disorder Calculate the Delta GapCalculate the Delta GapIf If AGAG

acidosis is present: to determine if other acidosis is present: to determine if other

underlying condition presentunderlying condition presentTake the change in AG= (AG Take the change in AG= (AG --

12)12)

Add to HCO3Add to HCO3––

ieie

(AG (AG --

12) + Serum HCO312) + Serum HCO3

if <23if <23

= non gap acidosisif >30if >30

= metabolic alkalosis

Practical ApproachPractical Approach

8. If an elevated gap acidosis is present, calculate 8. If an elevated gap acidosis is present, calculate the deltathe delta--delta ratio, to determine if a second delta ratio, to determine if a second metabolic disorder is present.metabolic disorder is present.

DeltaDelta––Delta = Delta = Measured anion gap Measured anion gap ––

Normal anion gapNormal anion gapNormal [HCONormal [HCO33

--] ] ––

Measured [HCOMeasured [HCO33

--]]

Differential Diagnosis for AcidDifferential Diagnosis for Acid--Base DisordersBase Disorders

Summary of the Approach to Summary of the Approach to ABGsABGs

1.1.

Check the pHCheck the pH2.2.

Check the pCOCheck the pCO22

3.3.

Select the appropriate compensation formulaSelect the appropriate compensation formula4.4.

Determine if compensation is appropriateDetermine if compensation is appropriate

5.5.

Check the anion gapCheck the anion gap6.6.

If the anion gap is elevated, check the deltaIf the anion gap is elevated, check the delta--deltadelta

7.7.

If a metabolic acidosis is present, check urine pHIf a metabolic acidosis is present, check urine pH8.8.

Generate a differential diagnosisGenerate a differential diagnosis

Case 1Case 1

A 26 year old man with unknown past medical history A 26 year old man with unknown past medical history is brought in to the ER by ambulance, after friends is brought in to the ER by ambulance, after friends found him unresponsive in his apartment. He had last found him unresponsive in his apartment. He had last been seen at a party four hours prior.been seen at a party four hours prior.

ABG:ABG:

pH pH 7.257.25

Chem. 7:Chem. 7:

NaNa++

137137PPCOCO22

6060

KK++ 4.54.5

HCOHCO33

--

2626

ClCl-- 100100

PPOO22

55 55 HCOHCO33

--

2525

Case 2Case 2A 67 year old man with diabetes and early diabetic A 67 year old man with diabetes and early diabetic nephropathy (without overt renal failure) presents for a nephropathy (without overt renal failure) presents for a routine clinic visit. He is currently asymptomatic. routine clinic visit. He is currently asymptomatic. Because of some abnormalities on his routine blood Because of some abnormalities on his routine blood chemistries, you elect to send him for an ABG.chemistries, you elect to send him for an ABG.

ABG:ABG:

pH pH 7.357.35

Chem. 7:Chem. 7:

NaNa++

135135PPCOCO22

3434

KK++ 5.15.1

HCOHCO33

--

1818

ClCl-- 110110

PPOO22

92 92 HCOHCO33

--

1616CrCr

1.41.4

Urine pH:Urine pH:

5.05.0

Case 3Case 3

A 68 year old woman with metastatic colon cancer A 68 year old woman with metastatic colon cancer presents to the ER with 1 hour of chest pain and presents to the ER with 1 hour of chest pain and shortness of breath. She has no known previous shortness of breath. She has no known previous cardiac or pulmonary problems.cardiac or pulmonary problems.

ABG:ABG:

pH pH 7.497.49

ChemChem

7:7:

NaNa++

133133PPCOCO22

2828

KK++ 3.93.9

HCOHCO33

--

2121

ClCl-- 102102

PPOO22

5252

HCOHCO33

--

2222

Case 4Case 4

A 6 year old girl with severe gastroenteritis is admitted A 6 year old girl with severe gastroenteritis is admitted to the hospital for fluid rehydration, and is noted to to the hospital for fluid rehydration, and is noted to have a high [HCOhave a high [HCO33

--] on hospital day #2. An ABG is ] on hospital day #2. An ABG is ordered:ordered:

ABG:ABG:

pH pH 7.477.47

ChemChem

7:7:

NaNa++

130130PPCOCO22

4646

KK++ 3.23.2

HCOHCO33

--

3232

ClCl-- 8686

PPOO22

96 96 HCOHCO33

--

3333

Urine pH:Urine pH:

5.85.8

Case 5Case 5A 75 year old man with morbid obesity is sent to the A 75 year old man with morbid obesity is sent to the ER by his skilled nursing facility after he developed a ER by his skilled nursing facility after he developed a fever of 103fever of 103°°

and rigors 2 hours ago. In the ER he is and rigors 2 hours ago. In the ER he is

lucid and states that he feels lucid and states that he feels ““terribleterrible””, but offers no , but offers no localizing symptoms. His ER vitals include a heart rate localizing symptoms. His ER vitals include a heart rate of 115, and a blood pressure of 84/46.of 115, and a blood pressure of 84/46.

ABG:ABG:

pH pH 7.127.12

ChemChem

7:7:

NaNa+ + 138138PPCOCO22

5050

KK++ 4.24.2

HCOHCO33

--

1313

ClCl-- 9999

PPOO22

5252

HCOHCO33

--

1515

Urine pH:Urine pH:

5.05.0

Case 6Case 6

A 25 year old man with type I diabetes presents to the A 25 year old man with type I diabetes presents to the ER with 24 hours of severe nausea, vomiting, and ER with 24 hours of severe nausea, vomiting, and abdominal pain.abdominal pain.

ABG: pH ABG: pH 7.157.15

ChemChem

7:7:

NaNa++

138138PPCOCO22

3030

KK++ 5.65.6

HCOHCO33

--

1010

ClCl--

8888PPOO22

8888

HCOHCO33

--

1111CrCr

1.11.1

Urine pH:Urine pH:

5.05.0

Case 7Case 7A 62 year old woman with severe COPD comes to the A 62 year old woman with severe COPD comes to the ER complaining of increased cough and shortness of ER complaining of increased cough and shortness of breath for the past 12 hours. There are no baseline breath for the past 12 hours. There are no baseline ABGsABGs

to compare to, however, her HCOto compare to, however, her HCO33

--

measured measured during a routine clinic visit 3 months ago was 34 during a routine clinic visit 3 months ago was 34 mEqmEq/L./L.

ABG:ABG:

pH pH 7.217.21

ChemChem

7:7:

NaNa++

135135PPCOCO22

8585

KK++ 4.04.0

HCOHCO33

--

3333

ClCl-- 9090

PPOO22

47 47 HCOHCO33

--

3434

Urine pHUrine pH

5.55.5

Case 8Case 8

A 36 year old man with a history of alcoholism is A 36 year old man with a history of alcoholism is brought to the ER after being found on the floor of his brought to the ER after being found on the floor of his apartment unresponsive, soiled with vomit, and with an apartment unresponsive, soiled with vomit, and with an empty pill bottle nearby.empty pill bottle nearby.

ABG:ABG:

pH pH 7.037.03

ChemChem

7:7:

NaNa++

134134PPCOCO22

7575

KK++ 5.25.2

HCOHCO33

--

1919

ClCl--

9090PPOO22

48 48 HCOHCO33

--

2020

Urine pHUrine pH

5.05.0