blood gas interpretation

Blood Gas Interpretation Review

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Blood Gases• Important diagnostic tool• Reveals:

1. acid-base balance2. oxygenation status

**arterial gases only**

3. abnormalities of ventilation

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Acid- base balance• The body is designed for optimum performance at a

specific pH level• Cell division• Metabolism

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Components of Acid- Base Balance• pH- measures the bloods acidity

– Normal range 7.35- 7.45– Overall H+ from both respiratory and metabolic factors

• pCO2- partial pressure of carbon dioxide in the blood– Normal range 35-45 mmHg– Snapshot of adequacy of alveolar ventilation

• HCO3- the amount of bicarbonate in the blood– Normal range 22- 26 mEq/L

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Acid – Base Balance

Bicarbonate – carbonic acid buffer equation(H+)(HCO3) (H2CO3) (CO2)(H2O)

It’s not that complicated!pH 1 7 14

Acidic Neutral Alkaline

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Acid – Base Balance

• Lungs• Respiratory• CO2 (acid)

• Kidneys• Metabolic• HCO3 ( base/alkaline)

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Making sense of it… pH 7.35 – 7.45

Respiratory Metabolic

CO2=Acidosis HCO3=Acidosis

CO2=Alkalosis HCO3=Alkalosis

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Interpretation: 4 steps

• Normal Values– pH 7.35 – 7.45– pCO2 35 – 45 mmHg– HCO3 22 - 26 mEq/L

• Evaluate each component as Acid or Base

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Step 1…

• Evaluate pH and determine acidosis or alkalosis

7.35 7.40 7.45

Acid Normal BaseAcidosis Alkalosis

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Step 2…

• Evaluate pCO2 (respiratory)

35 40 45

Base Normal Acid

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Step 3…

• Evaluate HCO3 (metabolic)

22 24 26

Acid Normal Base

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Step 4…

• Determine which regulatory system is responsible for the imbalance by checking to see which component matches the pH.– If pH and pCO2 match = respiratory

– If pH and HCO3 match = metabolic

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pH pCO2 HCO3

Resp. Acidosis A(<7.35)

A(>45)

N(22-26)

Resp.Alkalosis

B(>7.45)

B(<35)

N(22-26)

Metabolic Acidosis

A(<7.35)

N(35-45)

A(<22)

MetabolicAlkalosis

B(>7.45)

N(35-45)

B(>26)

ABG Analysis

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Let’s practice…

pH pCO2 HCO3

7.26 55 23

7.54 43 39

7.39 41 25

7.51 29 24

7.29 40 17

7.28 61 18

Respiratory Acidosis

Metabolic Alkalosis

Normal

Respiratory Alkalosis

Metabolic Acidosis

A A N

B N BN N NB B NA N AA A A Mixed Acidosis

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Compensation

• When an acid – base imbalance exists, over time the body attempts to compensate.

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Understanding Compensation

• Uncompensated – the alternate system has not attempted to adjust (remains within normal range), and the pH remains abnormal

• Example– pH 7.30 A – pCO2 60 A– HCO3 25 NUncompensated Respiratory Acidosis

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• Partial Compensation – the alternate system is trying to create a balanced environment and bring the pH back within normal limits, but hasn’t yet succeeded.

• Example– pH 7.34 A – pCO2 59 A– HCO3 28 B Partially Compensated Respiratory Acidosis

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• Fully Compensated – the alternate system has adjusted enough to restore balance and normalize the pH

• Example

– pH 7.36 N (but slightly A)– pCO2 58 A– HCO3 31 BCompensated Respiratory Acidosis

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Let’s Practice Compensation…

pH pCO2 HCO3

7.51 49 40

7.29 53 22

7.37 25 18

7.35 65 28

7.46 22 20

7.34 52 27

Metabolic Alkalosis partially compensated

Respiratory Acidosis uncompensated

Metabolic Acidosis fully compensated

Respiratory Alkalosis partially compensated

Respiratory Acidosis fully compensated

B A BA A N

N B AN A B

B B A

A A B Respiratory Acidosis partially compensated

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A Final Step…

• Determine level of oxygenation (arterial samples only)• Normal = 80 – 100 mmHg• Mild hypoxemia = 60 – 80 mmHg• Moderate hypoxemia = 40 – 60 mmHg• Severe hypoxemia = less than 40 mmHg

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• Excessive CO2 retention• Causes

– Airway obstruction– Depression of respiratory drive

• Sedatives, analgesics• Head trauma

– Respiratory muscle weakness resulting from muscle disease or chest wall abnormalities

– Decreased lung surface area participating in gas exchange

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• Clues– Confusion, restlessness– Headache, dizziness– Lethargy– Dyspnea– Tachycardia– Dysrhythmias– Coma leading to death

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• Solutions– Improve ventilation

• Ensure adequate airway; positioning, suctioning

• Encourage deep breathing and coughing• Frequent repositioning• Chest physio/ postural drainage• Bronchodilators• Decrease sedation/analgesia• Oxygen therapy

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• Excessive CO2 loss due to hyperventilation• Causes

– CNS injury: brainstem lesions, salicylate overdose, Reye’s Syndrome, hepatic encephalopathy

– Aggressive mechanical ventilation– Anxiety, fear or pain– Hypoxia – Fever– Congestive heart failure

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• Clues– Light headedness– Confusion– Decreased concentration– Tingling fingers and toes– Syncope– Tetany

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• Solutions– Decrease respiratory rate and depth

• Sedation/analgesia as appropriate• Rebreather mask• Paper bag• Emotional support/encourage patient to slow

breathing• Calm, soothing environment

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Metabolic Acidosis

• Excessive HCO3 loss, or acid gain• Causes

– Diabetic ketoacidosis– Sepsis/shock– Diarrhea (fluid losses below gastric sphincter)– Renal Failure– Poison ingestion– Starvation– Dehydration

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Metabolic Acidosis

• Clues– Stupor– Restlessness– Kussmaul’s respirations (air hunger)– Seizures– Coma leading to death

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Metabolic Acidosis

• Solutions– Replace HCO3 while treating underlying cause– Monitor intake and output– Monitor electrolytes, especially K+– Seizure precautions

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Metabolic Alkalosis

• HCO3 retention, or loss of extracellular acid, • Causes

– GI losses above gastric sphincter• Vomiting• Nasogastric suction

– Antacids– Diuretic therapy causing electrolyte loss

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Metabolic Alkalosis

• Clues– Weakness, dizziness– Disorientation– Hypoventilation– Muscle twitching– Tetany

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Metabolic Alkalosis

• Solutions– Control vomiting– Replace GI losses– Eliminate overuse of antacids– Monitor intake and output– Monitor electrolytes

blood gas interpretation

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