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Arterial Blood Gasses

At the end of this self study the participant will:

1. Correctly identify the following:

a. Respiratory acidosis

b. Respiratory alkalosis

c. Metabolic acidosis

d. Metabolic alkalosis

See BJH Policy: Arterial Blood Gas Sampling

Physician alert levels per BJC Laboratory: (http://www.testcatalog.org/nrr/catalogs/bjc/catalog/)

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• Normal pH: 7.35 to 7.45• For normal metabolism to occur pH must be

maintained in the normal range: • When the pH is below 7.35, blood is acidic. • When the pH is above 7.45, blood is alkalotic. • Physician alert values for blood pH are below 7.2

OR above 7.6– Values outside of that range may interfere with cellular

functioning, and if uncorrected, will lead to death.

pH

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PaCO2 - partial pressure of CO2 • Definition: the amount of carbon dioxide gas dissolved in the

blood• Normal PaCO2 : 35-45 mmHg (torr) • Physician alert value at BJH is 75 mmHg or greater• Carbon Dioxide is acidic

– as PaCO2 , pH levels , (acidic) – as PaCO2 , pH levels , (alkaline)

• Blood carries CO2 to the lungs, where excess CO2 combines with H2O to form carbonic acid (H2CO3). – The level of H2CO3 in blood determines blood pH– H2CO3 triggers the lungs to adjust the rate and depth of

ventilation to stabilize CO2 .

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HCO3 (Bicarb) (Base)

• Normal HCO3 : 22-26 mEq/L • Kidneys excrete or retain bicarbonate (HCO3)

according to pH level. – HCO3 , pH levels : alkaline– HCO3 , pH levels : acidic

• Kidneys may take from hours to days to correct an imbalance.

• Remember: the lungs and kidneys work together.• Goal: maintain blood pH by balancing 1 part acid to

20 parts base.

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PaO2 partial pressure of O2

• Normal PaO2 - 80-105 mmHg • Physician alert value 40 or less• PaO2 (Partial Pressure of Oxygen) amount of oxygen

dissolved in the blood. (The other 97% of oxygen is attached to the hemoglobin) – It primarily measures the effectiveness of the lungs

in pulling oxygen into the blood stream from the atmosphere.

– Critical Threshold <60 mmHg

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PaO2

• Elevated PaO2 levels are associated with:– Increased oxygen levels in the inhaled air – Polycythemia

• Decreased PaO2 levels are associated with:– Decreased oxygen levels in the inhaled air – Anemia – Heart decompensation – Chronic obstructive pulmonary disease – Restrictive pulmonary disease – Hypoventilation

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SaO2: Arterial Oxygen Saturation

• Definition: % of hemoglobin combined with O2 • Normal SaO2 on room air is in excess of 95%.

– deep or rapid breathing can to 98-99%. – Oxygen-enriched air (40% - 100%), can push SaO2 to

100%.• Oxygen Saturation will fall if O2 available:

– increased altitudes. – Airway obstruction (e.g. asthmatic attack) – Alveolar lung disease.

• Oxygen Saturation will rise if:– Deep or rapid breathing occurs oxygen levels (supplemental O2)

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Summary of ABG Acid/BaseRespiratory Acidosis

Respiratory Alkalosis

Metabolic Acidosis

Metabolic Alkalosis

pH

7.35-7.45

<7.35 >7.45 <7.35 >7.45

CO2

35-45

>45 <35 35-45 35-45

HCO3

22-26

22-26 22-26 <22 >26

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Indications for ABG• Inadequate Ventilation?

– Not awake or breathing too fast, (concern about respiratory muscle fatigue)

• Are there concerns about oxygenation that can’t be addressed with pulse ox?– Carbon Monoxide (carboxyhemoglobinemia)– Methemoglobinemia (e.g., benzocaine intoxication)

• Are there concerns about acid-base balance?– Sepsis, DKA, Poly-drug overdose– Chronic CO2 retention

• Miscellaneous– After intubation– Monitor vent settings– Pre-op baseline

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Potential Electrolyte Changes• Goal: serum electoneutrality

• Acidemia: pH below 7.35

– H+ ions enter cells to correct low plasma pH

– K+ ions leave cells

– Result: possible hyperkalemia

• Alkalosis: pH above 7.45 HCO3 will lower H+

– K+ ions enter cell

– Result: possible hypokalemia

Cell

Cell

pH < 7.35

pH > 7.45

H+

K+

K+

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ABG Interpretation• First, pH

– Normal, 7.35-7.45– If normal, quit here

• Second pH– pH < 7.35 Acidosis– pH >7.45 Alkalosis

• Third, Cause– If CO2 is abnormal, respiratory cause– If HCO3 is abnormal, metabolic cause

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Respiratory Alkalosis• pH >7.45• pCO2 <35• Always due to “blowing off” carbon dioxide

– hypoxemia– psychogenic– compensation for metabolic acidosis– stimulation of CNS– inappropriate mechanical ventilation

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Respiratory Alkalosis Physical Assessment

• Dizziness• Tingling• Lightheadedness• Numbness• Hypocalcemia• Hypokalemia (K+ enters cell for electroneutrality)

• Hyperventilation

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Respiratory Alkalosis Treatment

• Treat cause– based on pH– emergent treatment not indicated until pH >7.55

• Decrease ventilation– change vent settings, sedation, pain control

• Supportive measures– Emotional support, relieve stress

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Respiratory Acidosis

• pH <7.35

• PCO2 >45

• Always due to inadequate excretion of carbon dioxide.

• Root of problem is Inadequate Alveolar Ventilation– pH term is acidosis

– PCO2 term is hypercarbia

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Respiratory Acidosis Physical Assessment

• Hypoventilation/Dyspnea• Mental status changes• Lethargy• Headache• Cough/Wheezing

• Acute Respiratory Acidosis Indicates Sudden Failure To Breathe and Requires Immediate Investigation

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Causes of Respiratory Acidosis • Depressed respiratory drive

– CNS depression

– CNS depressants

– head injury

– respiratory diseases

• Respiratory muscle fatigue / dysfunction

– ARDS

– neuromuscular diseases

– chest trauma

• Increased dead space

– pulmonary emboli

– chronic bronchitis/ emphysema

• Increased CO2 production

– elevated temperature

– high carbohydrate diet

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Respiratory Acidosis Treatment• Treat cause

– based on pH– Bronchodilators if warranted by underlying

condition• Supportive Measures

– Airway– Artificial Ventilation

• Consider Bag/Valve• Consider intubation (laryngeal mask

airway (LMA) shown)– Oxygen Supplementation

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Metabolic Alkalosis • pH >7.45• HCO3 >26• Too much alkalinity

– excess bicarb administration– antacid overuse– alkaline drugs– Lactated Ringers administration

• Metabolic acid loss– vomiting– NG suctioning– diuretics– steroids

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Metabolic Alkalosis Physical Presentation• Hypoventilation

• Electrolyte Disturbances

• Cardiac Dysrythmias

• Alkalosis ionizes calcium, so may exhibit S&S hypocalcemia– tetany, Chvostek/Trousseau’s signs, seizures

Chvostek’s sign

Trousseau’s sign

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Metabolic Alkalosis Treatment

• Lungs can provide limited compensation• Treat underlying cause• Correct electrolytes

– Diamox (carbonic anhydrase inhibitor)

• Inhibition reduces reabsorption of NaHCO3

– Ammonium chloride• metabolized into urea and HCL

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Metabolic Acidosis

• pH <7.35• HCO3 <22

• Increased HCO3 loss

– Diarrhea (GI preps)– GI fistula

• Increased H+ production– lactic acidosis– ketoacidosis– renal failure

• External influences– Salicylate intoxication– Ethanol, methanol or

isopropyl alcohol intoxication

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Metabolic Acidosis Physical Assessment

• Hyperventilation

– Kussmaul respiratory pattern

• Headache

• Mental Status Changes

• Hyperkalemia

– acidosis draws K+ out of the cells

• Hypercalcemia

– Increased parathyroid hormone causes hypercalcemia and increased bicarb excretion

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Metabolic Acidosis Treatment• Treat underlying condition

– based on pH – if pH <7.2 may need IV bicarbonate until cause is controlled

– Lactic acidosis, ketoacidosis• If kidney disease is cause, may require hemodialysis• Acute metabolic acidosis may indicate tissue hypoxia

– requires further assessment– consider oxygen therapy

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ABG Practice

• pH 7.55

• PaCO2 28

• HCO3 24

• PaO2 84

• Interpretation?

Answer: Respiratory Alkalosis

pH is above 7.45, PaCO2 is below 35

A. Normal

B. Respiratory Acidosis

C. Respiratory Alkalosis

D. Metabolic Acidosis

E. Metabolic Alkalosis

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ABG Practice

• pH 7.28

• PaCO2 54

• HCO3 24

• PaO2 59

• Interpretation?

Answer: Respiratory Acidosis

pH is below 7.35, PaCO2 is above 45

A. Normal

B. Respiratory Acidosis

C. Respiratory Alkalosis

D. Metabolic Acidosis

E. Metabolic Alkalosis

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ABG Practice

• pH 7.42

• PaCO2 38

• HCO3 24

• PaO2 89

• Interpretation?

Answer: Normal ABG

A. Normal

B. Respiratory Acidosis

C. Respiratory Alkalosis

D. Metabolic Acidosis

E. Metabolic Alkalosis

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ABG Practice

• pH 7.31

• PaCO2 38

• HCO3 19

• PaO2 89

• Interpretation?

Answer: Metabolic Acidosis

pH is below 7.35, HCO3 is below 22

A. Normal

B. Respiratory Acidosis

C. Respiratory Alkalosis

D. Metabolic Acidosis

E. Metabolic Alkalosis

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ABG Practice

• pH 7.50

• PaCO2 42

• HCO3 31

• PaO2 82

• Interpretation?

Answer: Metabolic Alkalosis

pH is above 7.45, HCO3 is above 26

A. Normal

B. Respiratory Acidosis

C. Respiratory Alkalosis

D. Metabolic Acidosis

E. Metabolic Alkalosis

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Next module:ABG - Compensation

CO2 HCO3

pH 7.35-7.45