Understanding arterial blood gases | Pulmonary system

Definitions

Arterial blood gases: Subset of tests that measure the amount of oxygen ( $O_{2}$ ), carbon dioxide ( $C O_{2}$ ), and pH of the blood; indicator of the function of lungs, kidneys, and acid-base balance in body
Partial pressure of oxygen ( $P a O_{2}$ ): Pressure of dissolved oxygen in blood
Partial pressure of carbon dioxide ( $P a C O_{2}$ ): Pressure of dissolved carbon dioxide in the blood
pH: Measurement of acidity or alkalinity in blood
Bicarbonate ( $H C O_{3}^{-}$ ): 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

Partial pressure of oxygen ( $P a O_{2}$ )
- Normal = $75$ – $100$ mmHg
- Below $75$ = hypoxemia
- Above $100$ = hyperventilation
Partial pressure of carbon dioxide ( $P a C O_{2}$ ) – related to respiratory conditions
- Normal = $35$ – $45$ mmHg
- Below $35$ = alkaline
- Above $45$ = acidic
Bicarbonate ( $H C O_{3}^{-}$ ) – 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 $7.35$
  - $P a C O_{2}$ 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 values:
  - pH above $7.45$
  - $P a C O_{2}$ 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 acids and a reduction in bicarbonate in the bloodstream
- Lab values:
  - pH below $7.35$
  - $H C O_{3}^{-}$ below $22$
- Symptoms: headache, decreased blood pressure, hyperkalemia, muscle twitching, nausea, vomiting, diarrhea, changes in consciousness, Kussmaul respirations (compensatory hyperventilation)
Metabolic alkalosis
- Related diagnosis: severe vomiting, excessive diuretic use, excessive antacid use, excessive gastrointestinal suctioning
- Define: the body has removed too much acid or retained too much bicarbonate, leading to an alkaline shift in the bloodstream
- Lab values:
  - pH above $7.45$
  - $H C O_{3}^{-}$ above $26$
- Symptoms: restlessness, tachycardia, compensatory hypoventilation, confusion, nausea, vomiting, tremors, cramps

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
$P a C O_{2}$	$55$ mmHg	High (respiratory acidosis)
$H C O_{3}^{-}$	$29$ mEq/L	High (kidneys compensating)

Interpretation: Respiratory acidosis with partial metabolic compensation.

Fully compensated arterial blood gases

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)
$P a C O_{2}$	$30$ mmHg	Low (lungs compensating)
$H C O_{3}^{-}$	$18$ mEq/L	Low (metabolic acidosis)

Interpretation: Fully compensated metabolic acidosis.