Arterial Blood GasIM 2013 (AVM)

COMPONENTS OF AN ABGpH

Measurement of acidity or alkalinity, based on the hydrogen (H+) ions present.

Negative log of the free H+ ion concentration

The normal range is 7.35 to 7.45

COMPONENTS OF AN ABGPaO2

The partial pressure of oxygen that is dissolved in arterial blood.

The normal range is 80 to 100 mm Hg.

SaO2

The arterial oxygen saturation.

The normal range is 95% to 100%.

COMPONENTS OF AN ABGPaCO2

The amount of carbon dioxide dissolved in arterial blood.

Normal range is 35 to 45 mm Hg (40 + 5)

COMPONENTS OF AN ABGHCO3

The calculated value of the amount of bicarbonate in the bloodstream.

The normal range is 22 to 26 mEq/liter (24 + 2)

B.E.

The base excess indicates the amount of excess or insufficient level of bicarbonate in the system.

The normal range is –2 to +2 mEq/liter (0 + 2).

(A negative base excess indicates a base deficit in the blood.)

Effects of ABG collection errors on pH, paCO2 and paO2

ABG COLLECTION ERROR

pH paCO2

paO2

Dilution with heparin

Air contamination

Venous admixture

Failure to cool blood

STEPWISE APPROACHObtain clues from the clinical setting

Determine primary disorder

Check the compensatory response

Calculate the anion gap

Calculate the delta/deltas

Identify specific etiologies for the acid-base disorder

Prescribe treatment

DETERMINE CLUES FROM THE

CLINICAL SETTING

CLUES FROM CLINICAL SETTING

HIGH ANION GAP METABOLIC ACIDOSIS

HIGH AG, normal Cl

Lactic acidosis

Ketoacidosis

Ingestions; alcohol, INH, methanol, ethylene glycol

Renal failure

Massive rhabdomyolysis

CLUES FROM CLINICAL SETTINGNORMAL ANION GAP METABOLIC ACIDOSIS

Normal AG, HIGH Cl

Diarrhea- GI loss of HCO3

RTA- renal loss of HCO3

Ingestion of ammonium chloride or hyperalimentation fluids

Acetazolamide therapy

CLUES FROM CLINICAL SETTING

METABOLIC ALKALOSIS

(urine Cl < 10 mEq/d)

Vomiting

Remote diuretic use

Post hypercapnea

Chronic diarrhea

Cystic fibrosis

CLUES FROM CLINICAL SETTING

METABOLIC ALKALOSIS

(urine Cl > 10 mEq/d)

Bartter’s syndrome

Severe potassium depletion

Current diuretic use

Hypercalcemia

Hyperaldosteronism

Cushing’s syndrome

CLUES FROM CLINICAL SETTING

RESPIRATORY ACIDOSIS

CHRONIC: COPD

ACUTE: pneumonia

RESPIRATORY ALKALOSIS

Hyperventilation

DETERMINE THE

PRIMARY DISORDER

Characteristics of primary acid base disturbancesDisorder pH Primary

DisturbanceCompensatory Response

Metabolic Acidosis

Decreased Dec HCO3 Dec pCO2

Metabolic Alkalosis

Increased Inc HCO3 Inc pCO2

Respiratory Acidosis

Decreased Inc pCO2 Inc HCO3

Respiratory Alkalosis

Increased Dec pCO2 Dec HCO3

Disorder Primary abnormality

Secondary response

Metabolic acidosis Loss of HCO3 or gain H+

Hyperventilation

Metabolic alkalosis Gain of HCO3 or lose H+

hypoventilation

Resp acidosis hypoventilation HCO3 generation- kidneys

Resp alkalosis hyperventilation HCO3 consumption

Acidosis vs. AlkalosispH Degree of impairment

< 7.20 Severe acidemia

7.20-7.29 Moderate

7.30-7.34 Mild acidemia

7.35-7.45 Normal pH

7.46-7.50 Mild alkalemia

7.51-7.55 Moderate

> 7.55 Severe alkalemia

DETERMINE PRIMARY DISORDERCheck the trend of the pH, HCO3, pCO2

The change that produces the pH is the primary disorder

pH = 7.25 HCO3 = 12 pCO2 = 30

ACIDOSIS ACIDOSIS ALKALOSIS

METABOLIC ACIDOSIS

DETERMINE PRIMARY DISORDERCheck the trend of the pH, HCO3, pCO2

The change that produces the pH is the primary disorder

pH = 7.25 HCO3 = 28 pCO2 = 60

ACIDOSIS ALKALOSIS ACIDOSIS

RESPIRATORY ACIDOSIS

DETERMINE PRIMARY DISORDERCheck the trend of the pH, HCO3, pCO2

The change that produces the pH is the primary disorder

pH = 7.55 HCO3 = 19 pCO2 = 20

ALKALOSIS ACIDOSIS ALKALOSIS

RESPIRATORY ALKALOSIS

DETERMINE PRIMARY DISORDER If the trend is the same, check the percent

difference

The bigger %difference is the 10 disorder

pH = 7.25 HCO3 = 16 pCO2 = 60

ACIDOSIS ACIDOSIS ACIDOSIS

RESPIRATORY ACIDOSIS

(16-24)/24 = 0.33 (60-40)/40 = 0.5

DETERMINE PRIMARY DISORDER If the trend is the same, check the percent

difference

The bigger %difference is the 10 disorder

pH = 7.55 HCO3 = 38 pCO2 = 30

ALKALOSIS ALKALOSIS ALKALOSIS

METABOLIC ALKALOSIS

(38-24)/24 = 0.58(30-40)/40 = 0.25

CHECK THECOMPENSATORY

RESPONSE

COMPENSATED? When a patient develops an acid-base

imbalance, the body attempts to compensate.

Primary buffer response systems in the body: lungs and the kidneys

The body tries to overcome either a respiratory or metabolic dysfunction in an attempt to return the pH into the normal range.

Compensatory ResponsesDISORDER RESPONSE

Metabolic acidosis Dec HCO3 1.2 mmHg dec in PCO2 foe every 1 meq/L fall in HCO3

Metabolic alkalosis Inc HCO3 0.7 mmHg inc in CO2 for every 1 meq/L rise

in HCO3

Respiratory acidosis Inc PCO2 1 meq/L inc in HCO3 for every 10 mmHg

rise in PCO2

Respiratory alkalosis Dec PCO2 2 meq/L dec in HCO3 for every 10 mmHg fall

in PCO2

Compensatory Mechanisms(ex. In acidemia)1. Extracellular buffering primarily by HCO3

- (immediate)

2. Respiratory compensation by an increase in alveolar ventilation (minutes to hours)

3. Intracellular buffering primarily by proteins and phosphates (2 to 4 hours)

4. Renal compensation by an ↑ in H+ excretion and ↑HCO3

- reabsorption (hours to days)

Na+

Regulatory Response to Acidemia

Cl-

H+

Protein- PO4

=,SO4=

Organic acids

normal anion

gap

URINE

HCO3-

NH4+ H2PO4

-

PCT

DT

CompensationIf compensation is < or > predicted then there may be ≥2 disorders:

pCO2 too low: concomitant primary respiratory alkalosis

pCO2 too high: concomitant primary respiratory acidosis

HCO3 too low: concomitant primary metabolic acidosis

HCO3 too high: concomitant primary metabolic alkalosis

CompensationNormal pH but increased pCO2 + increased

HCO3: respiratory acidosis + metabolic alkalosis

Normal pH but decreased pCO2 + decreased HCO3: respiratory alkalosis + metabolic acidosis (e.g., salicylates, DKA)

Normal pH & normal pCO2 & HCO3 but increased AG: HAGMA + metabolic alkalosis (e.g., Alcoholic ketoacidosis w/ vomiting) + respiratory alkalosis (due to hyperventilation of hepatic dysfunction or alcohol withdrawal)

CompensationNormal pH & normal pCO2 & HCO3 & AG: no

disturbance or NAGMA + metabolic alkalosis

Metabolic acidosis + respiratory acidosis: DKA< sedatives.

Cannot have respiratory acidosis & respiratory alkalosis simultaneously (one either hypo- or hyperventilates)

Example 1 If patient presents with pH=7.2 and HCO3=16,

what is the normal compensated value for pCO2?

24-16= 8 meq/L 8 x 1.2 = 9.6 mmHg fall in PCO2

40 mmHg-9.6 mmHg = 30.4 mmHg

Normal compensation PCO2 = 30.4 mmHg

Example 2 If patient presents with pH= 7.23. HCO3= 22

meq/L, and pCO2= 9, what is your interpretation?

Note the pH and tell whether it is acidosis or alkalosis?

Note the HCO3 and pCO2 values to determine which causes the primary disturbance?

Determine the compensatory response

What is our diagnosis?

Example 3pH 7.14

HCO3 9

pCO2 25

Example 3pH 7.14

HCO3 9

pCO2 25

Metabolic acidosis with respiratory acidosis

Example 4pH 7.2

HCO3 15

pCO2 40

Example 4pH 7.2

HCO3 15

pCO2 40

Metabolic acidosis with respiratory acidosis

Example 5pH 7.5

HCO3 15

pCO2 20

Example 5pH 7.5

HCO3 15

pCO2 20

Respiratory alkalosis with metabolic acidosis

Example 6pH 7.36

HCO3 26

pCO2 65

Example 6pH 7.36

HCO3 26

pCO2 65

Respiratory acidosis, compensated

Example 7pH 7.379

HCO3 15

pCO2 25.1

Example 7pH 7.379

HCO3 15

pCO2 25.1

Metabolic Acidosis with Respiratory Alkalosis

CALCULATE THE ANION GAP

Calculate Anion GapIf with metabolic acidosis, check for other existing metabolic derangements; compute for the anion gap

AG = Na – (Cl + HCO3) = normal 10-12

Represents unmeasured anions in the plasma

Na136

Cl100

AG 12

HCO3

24

NORMAL

Unmeasured anionsProtein- PO4

=,SO4=

Organic

acids

ANION GAP

Na – (HCO3 + Cl) = 12 + 4

Na = 135 HCO3 = 15 Cl = 97 RBS = 100 mg%

AG = 135 – 112 = 23

ANION GAP

Na – (HCO3 + Cl) = 12 + 4

Na = 135 HCO3 = 15 Cl = 97 RBS = 500 mg%

Corrected Na = Na + RBS mg% -100 x 1.6

100

AG = 135 + 6.4 – 112 = 29.4

ANION GAP IN MAJOR CAUSES OF METABOLIC ACIDOSIS

High Anion GapA. Lactic acidosis: LactateB. Ketoacidosis: B-hydroxybutyric acidC. Renal failure: Sulfate, phosphate, urateD. Ingestions

1. Salicylate: ketones, lactate, salicylate2. Methanol or formaldehyde3. Ethylene glycol: glycolate, oxalate

Normal Anion GapE. Gastrointestinal loss of HCO3-

1. DiarrheaB. Renal HCO3- loss

1. Type I and Type II Renal Tubular AcidosisC. Ingestion:

1. Ammonium Chloride

Na+

States of Systemic Acidosis

Cl-

High anion gap

H+

Protein- PO4

=,SO4=

Organic acids

HCO3-

M- methanol U- uremia D- DKA P- paraldehyde I- iron, INH L- lactic acidosis E- ethylene glycol

S- salicylates

CHECK THE DELTA / DELTA

Na136

Cl100

AG 12

HCO3

24

NORMAL

Na136

Cl100

AG 26

HCO3 10

HIGH GAP METABACIDOSIS

Increased when acidosis due toIncrease in fixed acids (HCO3 actsas buffer so it is depleted and theunmeasured anions increase to preserve neutrality)

Na136

Cl114

AG 12

HCO3 10

NORMAL GAP METABACIDOSIS

Gap is normal if metab acidosis due to loss of base (when HCO3 lost,Cl- anions increased to maintain Neutrality)

Na136

Cl100

AG 12

HCO3

24

NORMAL

Na136

Cl94

AG 22

HCO3

20

COMBINED HAGMET. ACIDOSIS& MET. ALKALOSIS

AG HCO3

= 10 4

Na136

Cl106

AG 22

HCO3 8

COMBINED HAG& NAG MET. ACIDOSIS

AG HCO3

= 1016

Na136

Cl100

AG 22

HCO3

14

SIMPLE HAGMETABOLICACIDOSIS

AG HCO3

= 1010

HAGMA: DELTA AG/DELTA HCO3

HAGMAΔ AG = Δ HCO3 pure HAGMA

Δ AG < Δ HCO3 HAGMA + NAGMA

Δ AG > Δ HCO3 HAGMA + metabolic alkalosis

Na136

Cl100

AG 12

HCO3

24

NORMAL

Na134

Cl110

AG 10

HCO3

14

SIMPLE NAGMETABOLICACIDOSIS

Cl HCO3

= 1010

Na128

Cl110

AG 10HCO3 8

COMBINED NAG & HAG MET. ACIDOSIS

Cl HCO3

= 1016

Na140

Cl110

AG 10

HCO3

20

COMBINED NAGMET. ACIDOSIS& MET. ALKALOSIS

Cl HCO3

= 10 4

For Normal Gap: DELTA Chloride/DELTA HCO3

NAGMAΔ Cl = Δ HCO3 pure NAGMA

Δ Cl < Δ HCO3 NAGMA + HAGMA

Δ Cl > Δ HCO3 NAGMA + metabolic alkalosis

MISCELLANEOUS

Assess the PO2Classification PaO2 (mmHg)

Hyperoxemia > 100Normoxemia 80-100Mild hypoxemia 60-79Moderate hypoxemia

45-59

Severe hypoxemia < 45

Room air, patient < 60 y.o.Mild hypoxemia paO2 < 80 mm HgModerate paO2 < 60 mm HgSevere paO2 < 40 mm

Hg

For each year > 60 y.o., subtract 1 mm Hg for limits of mild and moderate hypoxemia

At any age, a paO2 < 40 mm Hg indicates severe hypoxemia