Acid Base, Arterial Blood Gas

Basic ABG Interpretation

Jose Socrates ‘DEE’ EvardoneYear Level I

Department of Internal MedicineCebu Doctors University Hospital

arterial blood gas (ABG)• test that measures the:

– 1) oxygen tension (PaO2), – 2) carbon dioxide tension (PaCO2), – 3) acidity (pH), – 4) oxyhemoglobin saturation (SaO2), and– 5) bicarbonate (HCO3) concentration in

arterial blood. • Some blood gas analyzers also measure the

methemoglobin, carboxyhemoglobin levels

ARTERIAL BLOOD GASES• ARTERIAL SAMPLING• Needle puncture

– - Site selection– - Collateral circulation– - Technique– - Complications

• Indwelling catheters• SPECIMEN CARE• TRANSPORT• ANALYSIS• INTERPRETATION

– Normal values

ARTERIAL BLOOD GASES• ARTERIAL SAMPLING:• Common sites include:

– 1) radial, – 2) femoral, – 3) brachial, – 4) dorsalis pedis, or – 5) axillary artery

• the radial artery is used most often because it is – accessible, – easily positioned, and– more comfortable for the patient than the alternative

sites

ARTERIAL BLOOD GASESTechnique

●Local analgesia ●The seal of a heparinized syringe should be broken by pulling its plunger. ●the artery should be punctured with the needle at a 30 to 45 degree angle relative to the skin●rolled between the hands ●pressure applied to the puncture site for five to ten minutes to achieve hemostasis.

30-45-degree angle (for radial artery),

45-60-degree angle (for brachial artery),

45-90-degree angle (for femoral artery) with the bevel of the needle turned up

ARTERIAL BLOOD GASES











ARTERIAL BLOOD GASESSPECIMEN CARE

Gas diffusion through the plastic syringe is a potential source of error.

- Using a glass syringe- placed on ice and analyzed within 15

minutes


The HEPARINdecrease in the pH dilute the PaCO2

heparin solution should be minimized and at least 2 mL of blood should be obtained


Air bubbles that exceed 1 to 2 percent of the blood volume

falsely high PaO2 falsely low PaCO2

gently removing the bubbles without agitation and analyzing the sample as soon as possible

ARTERIAL BLOOD GASESINTERPRETATION

"Oxygenation and mechanisms of hypoxemia“ and

"Simple and mixed acid-base disorders"

ARTERIAL BLOOD GASES"Oxygenation and mechanisms of

hypoxemia“MEASURES OF OXYGENATION:

1) Arterial oxygen saturation (SaO2)2) Arterial oxygen tension (PaO2)3) A-a oxygen gradient4) PaO2/FiO2 ratio5) a-A oxygen ratio6) Oxygenation index

ARTERIAL BLOOD GASESArterial oxygen

saturation (SaO2)

ARTERIAL BLOOD GASESArterial oxygen

tension (PaO2)


A-a oxygen gradientA-a oxygen gradient = PAO2 - PaO2

A-a gradient = 2.5 + 0.21 x age in years


PaO2/FiO2 ratioA normal PaO2/FiO2 ratio is 300 to 500 mmHgless than 300 mmHg indicating abnormal gas exchange less than 200 mmHg indicates severe hypoxemia


a-A oxygen ratioa-A oxygen ratio = PaO2 ÷ PAO2

lower limit of normal is 0.77-0.82 most reliable when the FiO2 is less than 0.55


Oxygenation index OI = [mean airway pressure x

FiO2 ÷ PaO2] x 100

ARTERIAL BLOOD GASESMECHANISMS OF HYPOXEMIA

HypoventilationV/Q mismatch

Right-to-left shuntDiffusion limitation

Reduced inspired oxygen tension


"Simple and mixed acid-base disorders"

ARTERIAL BLOOD GASESNormal Values

(Harrisons)

Normal Values

• pH = 7.35 – 7.45• pCO2 = 35 – 45 mmHg lungs(Reference Value = 40)

• HCO3 = 22 – 26 mmol/L kidneys(Reference value = 24)

Definition Of Terms

• Acidemia- Arterial pH < 7.56• Alkalemia- Arterial pH > 7.45• Acidosis- A process that tends to lower the

extracellular pH (Hydrogen ion concentration increases)

• Alkalosis- A process that tends to raise the extracellular pH (hydrogen ion concentration decreases)

Definition Of Terms

• Metabolic acidosis- A disorder that reduces the serum Hc03 and pH

• Metabolic Alkalosis- A disorder that elevates serum Hc03 and pH

• Respiratory Acidosis- A disorder that elevates the arterial pC02 and reduces the pH

• Respiratory Alkalosis- A disorder that reduces the arterial pC02 and elevates the pH

Definition Of Terms

• Simple Acid Base Disorder- Appropriate Respiratory or Renal Compensation for the Disorder

• Mixed Acid Base Disorder- Presence of More than one acid base Disorder

ARTERIAL BLOOD GAS ANALYSIS• 1) Look at the pH

– Acidemia or alkalemia– The cause is in the same direction

• 2) Look at pCO2– Decreased in alkalosis– Increased in acidosis

• 3) Look at HCO3– Decreased in acidosis– Increased in alkalosis

• 4) Remember, the body does NOT overcompensate

• 5) Compensation can be COMPLETE or INCOMPLETE

Example # 1PH – 7.34pCO2 – 52HCO3 - 19





• 4) Remember, the body does NOT overcompensate• 5) Compensation can be COMPLETE or INCOMPLETE


























Calculations for the Medicine Floors

Normal Values(Harrisons)

A 45 y.o. doctor ran a 6:12 mile, then fell down on all 4’s

A 45 y.o. doctor ran a 6:12 mile, then fell

down on all 4’s

ACIDOSIS ALKALOSIS

Metabolic

Respiratory 2

CHRONIC RESP

METABOLIC ACIDOSIS

Winter’s Formula

DETERMINE PRIMARY DISORDER

• Check 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




pH = 7.25 HCO3 = 28 pCO2 = 60

ACIDOSIS ALKALOSIS ACIDOSIS

RESPIRATORY ACIDOSIS




pH = 7.55 HCO3 = 19 pCO2 = 20

ALKALOSIS ACIDOSIS ALKALOSIS

RESPIRATORY ALKALOSIS


• 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


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


• 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 (40-30)/40 = 0.25

CHECK THECOMPENSATORY RESPONSE

• PREDICTION OF COMPENSATORY RESPONSES ON SIMPLE ACID BASE DISORDERS

• Metabolic Acidosis PaCO2 = (1.5 X HCO3) + 8

• Metabolic Alkalosis PaCO2 will increase 0.75 mmHg per meq/L increase in HCO3

• Respiratory Acidosis Acute HCO3 will increase 1 meql/L per 10 mmHg increase

in PaCO2Chronic HCO3 will increase 4 meq/L per 10 mmHg increase

in PaCO2• Respiratory Alkalosis

Acute HCO3 will decrease 2 meq/L per 10 mmHg decrease in PaCO2

Chronic HCO3 will decrease 4 meq/L per 10 mmHg decrease in PaCO2

COMPENSATORY RESPONSE

METABOLIC ACIDOSIS

PaCO2 = (1.5 X HCO3) + 8

HCO3 =12 pCO2 =1.5 X 12 + 8 = 26

pCO2 = 1.5 X 7 + 8 = 18.5HCO3 =7


HCO3 =35 pCO2 =11 X 0.75 = 8.25

= 8.25 + 40 = 48 pCO2 = 52HCO3 =40

METABOLIC ALKALOSIS

PaCO2 will increase 0.75 mmHg per meq/L increase in HCO3


pCO2 =55 HCO3 = 25.5

HCO3 = 28pCO2 =80

ACUTE RESPIRATORY ACIDOSIS

HCO3 will increase 1 meq/L per 10 mmHg increase in PaCO2


RESPIRATORY ALKALOSIS Acute: HCO3 will decrease 2 meq/L per 10 mmHg decrease in PaCO2

Check for Secondary Acid Base Disorders

Primary Acid Base Disorder

Compensation Secondary Base Disorder

Metabolic Acidosis Actual reduction of pC02 from baseline is HIGHER than that of calculated compensation

Secondary RESPIRATORY ALKALOSIS is present

Actual reduction of pC02 from baseline is LESS than that of calculated compensation

Secondary RESPIRATORY ACIDOSIS is present




Metabolic Alkalosis Actual increase of Pc02 from baseline is HIGHER than that of calculated compensation


Actual reduction of pC02 from baseline is LESS than that of calculated compensation





Respiratory Acidosis

Actual increase of Hc03 from baseline is HIGHER than that of calculated compensation

Secondary Metabolic Alkalosis is present

Actual increase of Hc03 from baseline is LESS than that of calculated compensation

Secondary METABOLIC ACIDOSIS is present




Respiratory Alkalosis

Actual decrease of Hc03 from baseline is HIGHER than that of calculated compensation

Secondary Metabolic Acidosis is present

Actual decrease of Hc03 from baseline is LESS than that of calculated compensation

Secondary METABOLIC Alkalosis is present

ANION GAP

Na – (HCO3 + Cl) = 10-12 mmol/L

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

AG = 135 – 112 = 23

ANION GAP

Na – (HCO3 + Cl) = 10-12

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

Corrected AG = Na + RBS mg% -100 x 1.4

100

AG = 135 + 5.6 – 112 = 28.6

DETERMINE CLUES FROM THE

CLINICAL SETTING

CLUES FROM CLINICAL SETTINGHIGH ANION GAP METABOLIC ACIDOSIS

(HAGMA)M Methanol

U Uremia

D Diabetic Ketoacidosis

P Paraldehyde

I Isoniazid, Iron

L Lactic Acidosis

E Ethylene Glycol, Ethanol

S Salicylates

CLUES FROM CLINICAL SETTINGNORMAL ANION GAP METABOLIC ACIDOSIS

(NAGMA)H Hyperalimentation

A Acetazolamide

R Renal Tubular Acidosis

D Diarrhea

U Ureteropelvic shunt

P Post Hypocapnia

CLUES FROM CLINICAL SETTING

METABOLIC ALKALOSIS Vomiting

Remote diuretic usePost hypercapneaChronic diarrhea

Cystic fibrosisAcute alkali administration


METABOLIC ALKALOSIS

Bartter’s syndromeSevere potassium depletion

Current diuretic useHypercalcemia

HyperaldosteronismCushing’s syndrome

Gastric aspiration



CHRONIC: COPD, intracranial tumors ACUTE: pneumonia, head trauma, general

anesthetics, sedatives

RESPIRATORY ALKALOSIS

Hyperventilation, Pregnancy, Liver failure, Methylxanthines

CASE 1

56F with vomiting and diarrhea 3 days ago despite intake of loperamide. Her last urine

output was 12 hours ago.

PE showed BP = 80/60, HR = 110, RR = 28. There is poor skin turgor.

CASE 1

serum Na = 130 pH = 7.30 K = 2.5 pCO2 = 30 Cl = 105 HCO3 = 15 BUN = 42 pO2 = 90 crea = 2.0 RBS = 100

BCR = BUN / crea = 21 PRE-RENAL

CASE 1


pH = acidosis, pCO2 =alk, HCO3 = acidosis

Metabolic Acidosis

CASE 1


pCO2 = 15 x 1.5 + 8 = 30.5Compensated

Metabolic Acidosis

CASE 1


AG= 130 – (105+15) = 10 NAGMA

CASE 2

19F, fashion model, is surprised to find her K=2.7 mmol/L because she was normokalemic 6

months ago. She admits to being on a diet of fruit and vegetables but denies vomiting and

the use of diuretics or laxatives. She is asymptomatic. BP = 90/55 with subtle signs of

volume contraction.

CASE 2

serum Na 138 63 K 2.7 34 Cl 96 0 HCO3 30 0 pH 7.45 5.6 pCO2 45

Metabolic Alkalosis

Plasma Urine

pH = alk, pCO2 =acidosis HCO3 = alkalosis


HCO3 =35 pCO2 =11 X 0.75 = 8.25

= 8.25 + 40 = 48 pCO2 = 52HCO3 =40

METABOLIC ALKALOSIS

PaCO2 will increase 0.75 mmHg per mmol/L increase in HCO3

CASE 3

AG= 138 – (96+30) = 12 NAG

Plasma Urineserum Na 138 63

K 2.7 34 Cl 96 0 HCO3 30 0 pH 7.45 5.6 pCO2 45

CASE 3Plasma Urine

serum Na 138 63 K 2.7 34 Cl 96 0 HCO3 30 0 pH 7.45 5.6 pCO2 45

What is the cause of the acid base disorder?

CASE 4

73M with long standing COPD (pCO2 stable at 52-58 mmHg), cor pulmonale, and peripheral

edema had been taking furosemide for 6 months. Five days ago, he had anorexia,

malaise, and productive cough. He continued his medications until he developed nausea.

Later he was found disoriented and somnolent

CASE 4

PE: BP 110/70, HR 110, RR 24, T=40respiratory distressprolonged expiratory phasepostural drop in BPdrowsy, disorientedscattered rhonchi and rales BLFsdistant heart soundstrace pitting edema

CASE 4admission after 48 hrs

pH = acidosis pCO2 =acidosis, HCO3 = alk


serum Na 136 139 K 3.2 3.9 Cl 78 86 HCO3 40 38 pH 7.33 7.42 pCO2 78 61 pO2 43 56


pCO2 =55 HCO3 = 25.5

HCO3 = 28pCO2 =80

ACUTE RESPIRATORY ACIDOSIS

HCO3 will increase 1 mmol/L per 10 mmHg increase in PaCO2

serum Na 136 139 K 3.2 3.9 Cl 78 86 HCO3 40 38 pH 7.33 7.42 pCO2 78 61 pO2 43 56

CASE 4admission after 48 hrs

HCO3 = 25.5

Respiratory Acidosis & M. Alkalosis





Actual increase of Hc03 from baseline is HIGHER than that of calculated compensation

Secondary Metabolic Alkalosis is present

Actual increase of Hc03 from baseline is LESS than that of calculated compensation

Secondary METABOLIC ACIDOSIS is present

CASE 5

42M, alcoholic, brought to the ER intoxicated. He was found at Rizal park in a pool of

vomitus. PE showed unkempt and incoherent patient with a markedly contracted ECF

volume. T=390 C with crackles on the RULF.

serum Na = 130 pH = 7.53 K = 2.9 pCO2 = 25 Cl = 80 HCO3 = 20 BUN = 34 pO2 = 60 crea = 1.4 alb = 38 RBS = 15 mmol/L

CASE 5

PRE-RENALBCR = 24

serum Na = 130 pH = 7.53 K = 2.9 pCO2 = 25 Cl = 80 HCO3 = 20 BUN = 12 pO2 = 60 crea = 120 alb = 38 RBS = 15 mmol/L

CASE 5

Respiratory Alkalosis

%pCO2 =38, %HCO3 = 18


RESPIRATORY ALKALOSIS Acute: HCO3 will decrease 2 mmol/L per 10 mmHg decrease in PaCO2


CASE 5

Compensated Respiratory

AlkalosisHCO3 = 21


CASE 5

HAGMA + RAlkAG = 130 – (80 + 20) = 30

QUESTIONS?

Acid Base, Arterial Blood Gas

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