L46 | Arterial Blood Gases

What is the most common place of sampling blood for an arterial blood gas (ABG)?

Radial artery

Why is arterial cannulation considered an invasive procesure with a documented potential for morbidity?

1.Hematoma: pool of blood under skin due to injury to a large blood vessel

2.Thrombosis: blood clot

3.Embolism: blood clot that moves in blood vessels

4.Damage to adjacent nerves

What is the 2nd most common site for arterial blood gas measure?

Femoral artery

Why measure arterial blood gases?

ABG’s enable the nurse to evaluate the cardiopulmonary system’s ability to deliver oxygen to and remove carbon dioxide from the cells of the body.

What do ABGs measure?

1. pH - the acidity and/or the alkalinity of the arterial blood

2.PaO₂ - The amount of oxygen dissolved in the arterial blood

3.PaCO₂ - The amount of carbon dioxide dissolved in the arterial blood

4.HCO₃ ^– The amount of bicarbonate ions in the arterial blood

What are the two methods O₂ travels in the blood by?

1.Freely dissolved O₂ in plasma (PaO₂) and expressed in mmHg or torr (100 mmHg)

2.O₂ bound to hemoglobin (SaO₂) as compared to Hgb in the blood and expressed as a percentage (98%)

Once bound to ________, O₂  molecules no longer exert any partial pressure.

Once bound to Hgb, O₂  molecules no longer exert any partial pressure.

__________ doesn’t reveal how much O₂ is in the blood.

SaO₂ doesn’t reveal how much O₂ is in the blood.

The more dissolved O₂ molecules there are (i.e. the greater the PaO₂), the more O₂will bind to the available Hgb, thus _____________ always depends, to a large degree, on the __________.

The more dissolved O₂ molecules there are (i.e. the greater the PaO₂), the more O₂will bind to the available Hgb, thus SaO₂ always depends, to a large degree, on the PaO₂.

___________ is a measurement of acidity or alkalinity in the arterial blood.

pH is a measurement of acidity or alkalinity in the arterial blood.

pH is __________ proportional to the number of hydrogen ions (H⁺) in the blood.

pH is inversely proportional to the number of hydrogen ions (H⁺) in the blood.

Normal blood pH ranges from ___________ to _____________

Normal blood pH ranges from 7.35 to 7.45.

Death is imminent when pH falls below _________ or rises above _________

Death is imminent when pH falls below 6.8 or rises above 7.8

Oxygenation of the arterial blood:

• ___________ → 80-100 mmHg

• ___________ → 60-79 mmHg

• ___________ → 40-59 mmHg

• ___________ → < 40mmHg

Oxygenation of the arterial blood:

• normal → 80-100 mmHg

• mild hypoxemia→ 60-79 mmHg

• moderate hypoxemia → 40-59 mmHg

• severe hypoxemia → < 40mmHg

______________ makes the blood more acidic.

Carbonic acid (H₂CO₃) makes the blood more acidic.

____________ is alkaline, and a vital component of the pH buffering system of the extracellular fluid of the human body

Bicarbonate (HCO₃⁻, base) is alkaline, and a vital component of the pH buffering system of the extracellular fluid of the human body.

• If the disorder is primarily associated with CO₂, it is termed ___________.

• If the disorder primarily affects HCO₃, it is termed ______________.

• If the disorder is primarily associated with CO₂, it is termed respiratory.

• If the disorder primarily affects HCO₃, it is termed metabolic.

Respiratory acidosis is characterized by: 

• ___________ PaCO₂

• ___________ pH

Respiratory acidosis is characterized by: 

• increased PaCO₂

• decreased pH

What causes respiratory acidosis?

• Respiratory depressants (overdose of opiates)

• Lung disease: chronic obstructive pulmonary disease (COPD)

Respiratory alkalosis is characterized by:

• _____________ PaCO₂

• PaCO₂ < _____________

• pH > ________________

Respiratory alkalosis is characterized by:

• decreased PaCO₂

• PaCO₂ < 35 mmHg

• pH > 7.45

What causes respiratory alkalosis?

•Hyperventilation

•Anxiety, pain

•Anemia

•Shock

•Some degrees of Pulmonary disease

What is the treatment for respiratory alkalosis?

•Need to increase CO₂

•Correct underlying disorder

•Rebreathing CO₂ (breathing into a paper bag)

Normal ABG values

• pH → 7.35-7.45

• PaCO₂ → 35-45 mmHg

• HCO₃⁻ → 22-25 mEq/L

• PaO₂ → 80-100 mmHg

Steps to determine whether PaCO₂ or HCO₃⁻ is the cause of the problem or the compensation when interpreting an ABG.

1. Look at the pH

   • pH < 7.35 → Acidosis

   • pH > 7.45 → Alkalosis

2. Decide which value matches the pH problem

• Whichever value goes in the same direction as the pH problem is the cause.

• Whichever value goes in the opposite direction is the compensation.

METABOLIC component (HCO₃⁻)

• Low HCO₃⁻ → metabolic acidosis

• High HCO₃⁻ → metabolic alkalosis

RESPIRATORY component (PaCO₂)

• High CO₂ → respiratory acidosis

• Low CO₂ → respiratory alkalosis

What causes metabolic acidosis?

–increase in H⁺ ion concentration from endogenous or exogenous sources

–inability of the kidneys to excrete H⁺

–loss of bicarbonate from the GI tract (diarrhea)

–Diabetic ketoacidosis (production of ketoacids from fat sources when cellular glucose levels are low)

What are common causes of decreased plasma HCO₃-?

•Renal Failure

•Diabetic Ketoacidosis

•Anaerobic Metabolism (increased lactic acid)

•Starvation

•Salicylate Intoxication

•Diarrhea