Understanding arterial blood gasses

Definitions

Arterial blood gases

Subset of tests that measure the amount of oxygen (O₂), carbon dioxide (CO₂), and pH of the blood; indicator of the function of lungs, kidneys, and acid- base balance in body

Partial pressure of oxygen (PaO₂)

Pressure of dissolved oxygen in blood

Partial pressure of carbon dioxide (PaCO₂)

Pressure of dissolved carbon dioxide in the blood

pH

Measurement of acidity or alkalinity in blood

Bicarbonate (HCO₃)

Chemical that assists with regulation of pH in blood

Important lab values

• pH
  • Normal/neutral = 7.35-7.45
  • Below 7.35 = acidic
  • Above 7.45 = alkaline

Image #11

https://open.oregonstate.education/aandp/chapter/26-4-acid-base-balance/

-Table gives an example of acidic and alkaline substances in nature.

• Partial pressure of oxygen (PaO₂)

  • Normal= 75-100 mmHg
  • Below 75 = Hypoxemia
  • Above 100 = Hyperventilation

• Partial pressure of carbon dioxide (PaCO₂)- related to respiratory conditions

  • Normal= 35-45 mmHg
  • Below 35 = alkaline
  • Above 45 = acidic

• Bicarbonate (HCO₃)- related to metabolic conditions

  • Normal= 22-26 mEq/L
  • Below 22 = acidic
  • Above 26 = alkaline

Acid-base diseases

• Respiratory acidosis
  • Related diagnosis: COPD, pneumonia, atelectasis, anesthesia, drug overdose
  • Define: the lung does not efficiently remove carbon dioxide from the system causing a buildup of carbon dioxide in the bloodstream
  • Lab values:
    • pH below 7.35
    • PaCO₂ above 45
  • Symptoms: hypoventilation, hypoxia, rapid-shallow breaths, decreased blood pressure, dyspnea, hyperkalemia, muscle weakness
• Respiratory alkalosis
  • Related diagnosis: Anxiety attacks, pulmonary embolism, mechanical ventilation
  • Define: the lung excessively removes carbon dioxide from the system leading to decreased amounts of carbon dioxide in bloodstream
  • Lab value
    • pH above 7.45
    • PaCO₂ below 35
  • Symptoms: hyperventilation, tachycardia, hypokalemia, numbness/tingling of extremities, deep- rapid breathing, seizures, nausea, vomiting, light headedness, lethargy, confusion
• Metabolic acidosis
  • Related diagnosis: Diabetic ketoacidosis, severe diarrhea, renal failure, sepsis (systemic shock)
  • Define: excessive fluid accumulation in the body leading to a buildup of bicarbonate in the bloodstream
  • Lab value
    • pH below 7.35
    • HCO₃ below 22
  • Symptoms: Headache, decreased blood pressure, hyperkalemia, muscle twitching, nausea, vomiting, diarrhea, changes consciousness, kussmal respirations (compensatory hyperventilation)
• Metabolic alkalosis
  • Related diagnosis: Severe vomiting, excessive diuretics usage, excessive antacid usage, excessive gastrointestinal suctioning
  • Define: body has removed too much fluid from the body leading to decreased bicarbonate in bloodstream
  • Lab values
    • pH above 7.45
    • HCO₃ above 26
  • Symptoms: Restlessness, tachycardia, compensatory hypoventilation, confusion, nausea, vomiting, tremors, cramps

Image #14

https://open.oregonstate.education/app/uploads/sites/157/2019/07/2716_Symptoms_of_Acidosis_Alkalosis-768x498.jpg

Treatment of arterial blood gas disorders

Compensation is the body’s way of adjusting one system (respiratory or metabolic) to offset a disturbance in the other.

• If the lungs are the problem, the kidneys try to help.
• If the kidneys are the problem, the lungs try to help.

There are three types of compensation:

Type	pH Status	Compensation Present?
Uncompensated	Abnormal	No compensation
Partially Compensated	Abnormal	Compensation is occurring
Fully Compensated	Normal (7.35–7.45)	Compensation has corrected the pH

Partially compensated arterial blood gases

Definition:

• pH is still outside the normal range, so the imbalance is not fully corrected.
• The opposing system has begun to respond, but not enough to normalize pH.

Example: partially compensated respiratory acidosis::

Value	Result	Interpretation
pH	7.30	Acidotic
PaCO₂	55 mmHg	High (respiratory acidosis)
HCO₃⁻	29 mEq/L	High (kidneys compensating)

Interpretation: Respiratory acidosis with partial metabolic compensation.

Fully compensated arterial blood gas

Definition:

• The pH is back within normal range (7.35–7.45) due to effective compensation.
• One component remains abnormal, indicating that a primary disorder still exists, but the body has corrected the pH.

Example: fully compensated metabolic acidosis:

Value	Result	Interpretation
pH	7.36	Normal (low end)
PaCO₂	30 mmHg	Low (lungs compensating)
HCO₃⁻	18 mEq/L	Low (metabolic acidosis)

Interpretation: Fully compensated metabolic acidosis.