2.3 Understanding arterial blood gases

Achievable NPTE-PTA

2. Pulmonary system

Our NPTE-PTA course is now in "early access" - get 50% off for a limited time.

Understanding arterial blood gases

7 min read

Font

Discuss

Feedback

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 the 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 $100$ = hyperoxemia (typically supplemental oxygen administration)
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: accumulation of metabolic acids or excessive loss of bicarbonate, lowering blood pH
- 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

The four disorders can present with overlapping symptoms, so the exam expects you to distinguish them by management focus as well as by lab values:

Disorder	Management focus
Respiratory acidosis	Improve ventilation - bronchodilators, positioning, airway clearance, mechanical ventilation if severe
Respiratory alkalosis	Treat the cause of hyperventilation - anxiety management, breathing retraining, treat the underlying illness
Metabolic acidosis	Treat the underlying cause (e.g., insulin for diabetic ketoacidosis, treat sepsis) and replace bicarbonate if severe
Metabolic alkalosis	Correct the cause - replace fluid and electrolytes, stop contributing medications (e.g., diuretics, antacids)

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

To interpret an ABG, work through the values in this order:

Classify the pH as acidotic (below $7.35$ ) or alkalotic (above $7.45$ ).
Compare $P a C O_{2}$ and $H C O_{3}^{-}$ to the direction of the pH shift - whichever one explains the pH shift names the primary disorder - a PaCO2 moving opposite to the pH (high with acidosis, low with alkalosis) points to a respiratory cause, while an HCO3 moving in the same direction as the pH points to a metabolic cause ( $P a C O_{2}$ points to a respiratory cause, $H C O_{3}^{-}$ points to a metabolic cause).
Check the opposing value: if it’s still in the normal range, the disorder is uncompensated; if it has shifted to help but the pH still hasn’t normalized, it’s partially compensated; if the pH has returned to the normal range, it’s fully compensated.

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)

Answer: Respiratory acidosis with partial metabolic compensation.

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)

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)

Answer: 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)

PTA role: if a patient’s ABG values are abnormal, or you observe an adverse response during an intervention - new or worsening dyspnea, chest pain, dizziness, or a drop in systolic blood pressure as workload increases - stop the activity and communicate the finding to the supervising PT. The PTA collects and reports this data; adjusting the plan of care in response to it is the PT’s responsibility.

Definitions & key terms

Arterial blood gases (ABGs): measure $O_{2}$ , $C O_{2}$ , pH; reflect lung, kidney, acid-base function
$P a O_{2}$ : dissolved oxygen pressure; $P a C O_{2}$ : dissolved carbon dioxide pressure
pH: acidity/alkalinity measure; $H C O_{3}^{-}$ : bicarbonate, regulates pH

Important lab values

pH normal: $7.35$ - $7.45$ (below = acidic, above = alkaline)
$P a O_{2}$ normal: $75$ - $100$ mmHg (below = hypoxemia, above = hyperoxemia)
$P a C O_{2}$ normal: $35$ - $45$ mmHg (respiratory marker; below = alkaline, above = acidic)
$H C O_{3}^{-}$ normal: $22$ - $26$ mEq/L (metabolic marker; below = acidic, above = alkaline)

Acid-base diseases

Respiratory acidosis: pH<7.35, $P a C O_{2}$ >45; causes: COPD, pneumonia, overdose; symptoms: hypoventilation, hypoxia, hyperkalemia
Respiratory alkalosis: pH>7.45, $P a C O_{2}$ <35; causes: anxiety, PE, mechanical ventilation; symptoms: hyperventilation, tachycardia, tingling, seizures
Metabolic acidosis: pH<7.35, $H C O_{3}^{-}$ <22; causes: DKA, diarrhea, renal failure, sepsis; symptoms: Kussmaul respirations, hyperkalemia, confusion
Metabolic alkalosis: pH>7.45, $H C O_{3}^{-}$ >26; causes: vomiting, diuretics, antacids, GI suctioning; symptoms: tachycardia, hypoventilation, tremors
Management focus:
- Respiratory acidosis → improve ventilation
- Respiratory alkalosis → treat hyperventilation cause
- Metabolic acidosis → treat underlying cause, replace bicarbonate
- Metabolic alkalosis → correct fluids/electrolytes, stop causative meds

Treatment of ABG disorders / compensation

Compensation: one system (resp/metabolic) adjusts to offset the other’s disturbance
Types:
- Uncompensated: abnormal pH, no compensation
- Partially compensated: abnormal pH, compensation occurring
- Fully compensated: normal pH ( $7.35$ - $7.45$ ), compensation corrected pH
Interpretation steps:
- Classify pH (acidotic/alkalotic)
- Match $P a C O_{2}$ (opposite direction = respiratory cause) or $H C O_{3}^{-}$ (same direction = metabolic cause) to pH shift
- Check opposing value: normal = uncompensated; shifted but pH abnormal = partial; pH normalized = full

Partially vs fully compensated examples

Partially compensated: pH still abnormal, opposing system shifting but insufficient
- Example: pH $7.30$ , $P a C O_{2}$ $55$ , $H C O_{3}^{-}$ $29$ → partially compensated respiratory acidosis
Fully compensated: pH normalized, one value still abnormal indicating primary disorder
- Example: pH $7.36$ , $P a C O_{2}$ $30$ , $H C O_{3}^{-}$ $18$ → fully compensated metabolic acidosis

PTA role/callout

Abnormal ABGs or adverse signs (dyspnea, chest pain, dizziness, BP drop) → stop activity
PTA reports findings; PT adjusts plan of care

Next | 2.4 Interventions for pulmonary system

Understanding arterial blood gases

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 the 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 $100$ = hyperoxemia (typically supplemental oxygen administration)
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: accumulation of metabolic acids or excessive loss of bicarbonate, lowering blood pH
- 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

The four disorders can present with overlapping symptoms, so the exam expects you to distinguish them by management focus as well as by lab values:

Disorder	Management focus
Respiratory acidosis	Improve ventilation - bronchodilators, positioning, airway clearance, mechanical ventilation if severe
Respiratory alkalosis	Treat the cause of hyperventilation - anxiety management, breathing retraining, treat the underlying illness
Metabolic acidosis	Treat the underlying cause (e.g., insulin for diabetic ketoacidosis, treat sepsis) and replace bicarbonate if severe
Metabolic alkalosis	Correct the cause - replace fluid and electrolytes, stop contributing medications (e.g., diuretics, antacids)

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

To interpret an ABG, work through the values in this order:

Classify the pH as acidotic (below $7.35$ ) or alkalotic (above $7.45$ ).
Compare $P a C O_{2}$ and $H C O_{3}^{-}$ to the direction of the pH shift - whichever one explains the pH shift names the primary disorder - a PaCO2 moving opposite to the pH (high with acidosis, low with alkalosis) points to a respiratory cause, while an HCO3 moving in the same direction as the pH points to a metabolic cause ( $P a C O_{2}$ points to a respiratory cause, $H C O_{3}^{-}$ points to a metabolic cause).
Check the opposing value: if it’s still in the normal range, the disorder is uncompensated; if it has shifted to help but the pH still hasn’t normalized, it’s partially compensated; if the pH has returned to the normal range, it’s fully compensated.

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)

Answer: Respiratory acidosis with partial metabolic compensation.

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)

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)

Answer: 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)

Achievable NPTE-PTA

2. Pulmonary system

Our NPTE-PTA course is now in "early access" - get 50% off for a limited time.