2.3 Understanding arterial blood gases

Achievable NPTE-PTA

2. Pulmonary system

Understanding arterial blood gases

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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 = $7.35$ - $7.45$
- Below $7.35$ = acidic
- Above $7.45$ = alkaline

Partial pressure of oxygen ( $P a O_{2}$ )
- Normal = $75$ - $100$ mmHg (some references cite $80$ mmHg as the lower limit - this chapter uses the $75$ mmHg cutoff common in PT references)
- 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, dyspnea, 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, numbness/tingling of extremities, light-headedness
Metabolic acidosis
- Related diagnosis: diabetic ketoacidosis, severe diarrhea, renal failure, sepsis or septic shock (lactic acidosis)
- 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, hyperkalemia, nausea/vomiting, 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

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

Disorder	Management focus
Respiratory acidosis	Improve ventilation - positioning, airway clearance, breathing exercises, mechanical ventilation if severe
Respiratory alkalosis	Treat the cause of hyperventilation - anxiety management, breathing retraining
Metabolic acidosis	Primarily medical management of the underlying cause; the PTA monitors the patient’s response and reports findings to the PT
Metabolic alkalosis	Primarily medical management of the underlying cause; the PTA monitors the patient’s response and reports findings to the PT

Compensation and ABG interpretation

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 $7.45$ ). If the pH is within $7.35$ - $7.45$ , use $7.40$ as the midpoint: $7.35$ - $7.39$ leans acidotic and $7.41$ - $7.45$ leans alkalotic. That lean names the primary disorder in a fully compensated ABG.
Find the value that explains the pH shift - that value names the primary disorder. If $P a C O_{2}$ moves opposite the pH (high with acidosis, low with alkalosis), the cause is respiratory. If $H C O_{3}^{-}$ moves in the same direction as the pH (low with acidosis, high with alkalosis), the cause is metabolic.
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 - leans acidotic)

$P a C O_{2}$ $30$ 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 - leans acidotic)
$P a C O_{2}$	$30$ 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, confusion, numbness/tingling, an abnormal breathing pattern, 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.

Arterial blood gas basics

ABGs measure $O_{2}$ , $C O_{2}$ , and pH to assess lung/kidney function and acid-base balance
Key values: $P a O_{2}$ , $P a C O_{2}$ (respiratory marker), $H C O_{3}^{-}$ (metabolic marker), pH

Normal lab value ranges

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

Four acid-base disorders

Respiratory acidosis: $C O_{2}$ retention (pH<7.35, $P a C O_{2}$ >45); causes: COPD, pneumonia, overdose; symptoms: hypoventilation, hypoxia, dyspnea
Respiratory alkalosis: excess $C O_{2}$ loss (pH>7.45, $P a C O_{2}$ <35); causes: anxiety, PE, mechanical ventilation; symptoms: hyperventilation, tachycardia, tingling
Metabolic acidosis: acid buildup/bicarbonate loss (pH<7.35, $H C O_{3}^{-}$ <22); causes: DKA, diarrhea, renal failure, sepsis; symptoms: Kussmaul respirations, hyperkalemia, nausea
Metabolic alkalosis: acid loss/bicarbonate retention (pH>7.45, $H C O_{3}^{-}$ >26); causes: vomiting, diuretics, antacids; symptoms: tachycardia, confusion, compensatory hypoventilation
Management focus: respiratory disorders treated via ventilation/breathing strategies; metabolic disorders managed medically with PTA monitoring/reporting

Compensation and ABG interpretation

Compensation: kidneys help respiratory problems, lungs help metabolic problems
Three states: uncompensated (pH abnormal, no compensation), partially compensated (pH abnormal, compensation occurring), fully compensated (pH normal, compensation corrected it)
Interpretation steps:
- Classify pH as acidotic/alkalotic (use $7.40$ midpoint if normal range)
- Identify cause: $P a C O_{2}$ opposite pH direction = respiratory; $H C O_{3}^{-}$ same direction as pH = metabolic
- Check opposing value to determine uncompensated/partial/full compensation status

PTA role with ABG findings

Report abnormal ABGs or adverse signs (dyspnea, chest pain, dizziness, confusion, tingling, abnormal breathing, BP drop with exertion) to supervising PT
PTA collects/reports data only; 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 = $7.35$ - $7.45$
- Below $7.35$ = acidic
- Above $7.45$ = alkaline

Partial pressure of oxygen ( $P a O_{2}$ )
- Normal = $75$ - $100$ mmHg (some references cite $80$ mmHg as the lower limit - this chapter uses the $75$ mmHg cutoff common in PT references)
- 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, dyspnea, 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, numbness/tingling of extremities, light-headedness
Metabolic acidosis
- Related diagnosis: diabetic ketoacidosis, severe diarrhea, renal failure, sepsis or septic shock (lactic acidosis)
- 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, hyperkalemia, nausea/vomiting, 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

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

Disorder	Management focus
Respiratory acidosis	Improve ventilation - positioning, airway clearance, breathing exercises, mechanical ventilation if severe
Respiratory alkalosis	Treat the cause of hyperventilation - anxiety management, breathing retraining
Metabolic acidosis	Primarily medical management of the underlying cause; the PTA monitors the patient’s response and reports findings to the PT
Metabolic alkalosis	Primarily medical management of the underlying cause; the PTA monitors the patient’s response and reports findings to the PT

Compensation and ABG interpretation

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 $7.45$ ). If the pH is within $7.35$ - $7.45$ , use $7.40$ as the midpoint: $7.35$ - $7.39$ leans acidotic and $7.41$ - $7.45$ leans alkalotic. That lean names the primary disorder in a fully compensated ABG.
Find the value that explains the pH shift - that value names the primary disorder. If $P a C O_{2}$ moves opposite the pH (high with acidosis, low with alkalosis), the cause is respiratory. If $H C O_{3}^{-}$ moves in the same direction as the pH (low with acidosis, high with alkalosis), the cause is metabolic.
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 - leans acidotic)

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

Achievable NPTE-PTA

2. Pulmonary system

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 = $7.35$ - $7.45$
- Below $7.35$ = acidic
- Above $7.45$ = alkaline

Partial pressure of oxygen ( $P a O_{2}$ )
- Normal = $75$ - $100$ mmHg (some references cite $80$ mmHg as the lower limit - this chapter uses the $75$ mmHg cutoff common in PT references)
- 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, dyspnea, 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, numbness/tingling of extremities, light-headedness
Metabolic acidosis
- Related diagnosis: diabetic ketoacidosis, severe diarrhea, renal failure, sepsis or septic shock (lactic acidosis)
- 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, hyperkalemia, nausea/vomiting, 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

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

Disorder	Management focus
Respiratory acidosis	Improve ventilation - positioning, airway clearance, breathing exercises, mechanical ventilation if severe
Respiratory alkalosis	Treat the cause of hyperventilation - anxiety management, breathing retraining
Metabolic acidosis	Primarily medical management of the underlying cause; the PTA monitors the patient’s response and reports findings to the PT
Metabolic alkalosis	Primarily medical management of the underlying cause; the PTA monitors the patient’s response and reports findings to the PT

Compensation and ABG interpretation

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 $7.45$ ). If the pH is within $7.35$ - $7.45$ , use $7.40$ as the midpoint: $7.35$ - $7.39$ leans acidotic and $7.41$ - $7.45$ leans alkalotic. That lean names the primary disorder in a fully compensated ABG.
Find the value that explains the pH shift - that value names the primary disorder. If $P a C O_{2}$ moves opposite the pH (high with acidosis, low with alkalosis), the cause is respiratory. If $H C O_{3}^{-}$ moves in the same direction as the pH (low with acidosis, high with alkalosis), the cause is metabolic.
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 - leans acidotic)

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

Arterial blood gas basics

ABGs measure $O_{2}$ , $C O_{2}$ , and pH to assess lung/kidney function and acid-base balance
Key values: $P a O_{2}$ , $P a C O_{2}$ (respiratory marker), $H C O_{3}^{-}$ (metabolic marker), pH

Normal lab value ranges

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

Four acid-base disorders

Respiratory acidosis: $C O_{2}$ retention (pH<7.35, $P a C O_{2}$ >45); causes: COPD, pneumonia, overdose; symptoms: hypoventilation, hypoxia, dyspnea
Respiratory alkalosis: excess $C O_{2}$ loss (pH>7.45, $P a C O_{2}$ <35); causes: anxiety, PE, mechanical ventilation; symptoms: hyperventilation, tachycardia, tingling
Metabolic acidosis: acid buildup/bicarbonate loss (pH<7.35, $H C O_{3}^{-}$ <22); causes: DKA, diarrhea, renal failure, sepsis; symptoms: Kussmaul respirations, hyperkalemia, nausea
Metabolic alkalosis: acid loss/bicarbonate retention (pH>7.45, $H C O_{3}^{-}$ >26); causes: vomiting, diuretics, antacids; symptoms: tachycardia, confusion, compensatory hypoventilation
Management focus: respiratory disorders treated via ventilation/breathing strategies; metabolic disorders managed medically with PTA monitoring/reporting

Compensation and ABG interpretation

Compensation: kidneys help respiratory problems, lungs help metabolic problems
Three states: uncompensated (pH abnormal, no compensation), partially compensated (pH abnormal, compensation occurring), fully compensated (pH normal, compensation corrected it)
Interpretation steps:
- Classify pH as acidotic/alkalotic (use $7.40$ midpoint if normal range)
- Identify cause: $P a C O_{2}$ opposite pH direction = respiratory; $H C O_{3}^{-}$ same direction as pH = metabolic
- Check opposing value to determine uncompensated/partial/full compensation status

PTA role with ABG findings

Report abnormal ABGs or adverse signs (dyspnea, chest pain, dizziness, confusion, tingling, abnormal breathing, BP drop with exertion) to supervising PT
PTA collects/reports data only; 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 = $7.35$ - $7.45$
- Below $7.35$ = acidic
- Above $7.45$ = alkaline

Partial pressure of oxygen ( $P a O_{2}$ )
- Normal = $75$ - $100$ mmHg (some references cite $80$ mmHg as the lower limit - this chapter uses the $75$ mmHg cutoff common in PT references)
- 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, dyspnea, 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, numbness/tingling of extremities, light-headedness
Metabolic acidosis
- Related diagnosis: diabetic ketoacidosis, severe diarrhea, renal failure, sepsis or septic shock (lactic acidosis)
- 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, hyperkalemia, nausea/vomiting, 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

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

Disorder	Management focus
Respiratory acidosis	Improve ventilation - positioning, airway clearance, breathing exercises, mechanical ventilation if severe
Respiratory alkalosis	Treat the cause of hyperventilation - anxiety management, breathing retraining
Metabolic acidosis	Primarily medical management of the underlying cause; the PTA monitors the patient’s response and reports findings to the PT
Metabolic alkalosis	Primarily medical management of the underlying cause; the PTA monitors the patient’s response and reports findings to the PT

Compensation and ABG interpretation

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 $7.45$ ). If the pH is within $7.35$ - $7.45$ , use $7.40$ as the midpoint: $7.35$ - $7.39$ leans acidotic and $7.41$ - $7.45$ leans alkalotic. That lean names the primary disorder in a fully compensated ABG.
Find the value that explains the pH shift - that value names the primary disorder. If $P a C O_{2}$ moves opposite the pH (high with acidosis, low with alkalosis), the cause is respiratory. If $H C O_{3}^{-}$ moves in the same direction as the pH (low with acidosis, high with alkalosis), the cause is metabolic.
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 - leans acidotic)

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

Key points

Arterial blood gas basics

ABGs measure $O_{2}$ , $C O_{2}$ , and pH to assess lung/kidney function and acid-base balance
Key values: $P a O_{2}$ , $P a C O_{2}$ (respiratory marker), $H C O_{3}^{-}$ (metabolic marker), pH

Normal lab value ranges

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

Four acid-base disorders

Respiratory acidosis: $C O_{2}$ retention (pH<7.35, $P a C O_{2}$ >45); causes: COPD, pneumonia, overdose; symptoms: hypoventilation, hypoxia, dyspnea
Respiratory alkalosis: excess $C O_{2}$ loss (pH>7.45, $P a C O_{2}$ <35); causes: anxiety, PE, mechanical ventilation; symptoms: hyperventilation, tachycardia, tingling
Metabolic acidosis: acid buildup/bicarbonate loss (pH<7.35, $H C O_{3}^{-}$ <22); causes: DKA, diarrhea, renal failure, sepsis; symptoms: Kussmaul respirations, hyperkalemia, nausea
Metabolic alkalosis: acid loss/bicarbonate retention (pH>7.45, $H C O_{3}^{-}$ >26); causes: vomiting, diuretics, antacids; symptoms: tachycardia, confusion, compensatory hypoventilation
Management focus: respiratory disorders treated via ventilation/breathing strategies; metabolic disorders managed medically with PTA monitoring/reporting

Compensation and ABG interpretation

Compensation: kidneys help respiratory problems, lungs help metabolic problems
Three states: uncompensated (pH abnormal, no compensation), partially compensated (pH abnormal, compensation occurring), fully compensated (pH normal, compensation corrected it)
Interpretation steps:
- Classify pH as acidotic/alkalotic (use $7.40$ midpoint if normal range)
- Identify cause: $P a C O_{2}$ opposite pH direction = respiratory; $H C O_{3}^{-}$ same direction as pH = metabolic
- Check opposing value to determine uncompensated/partial/full compensation status

PTA role with ABG findings

Report abnormal ABGs or adverse signs (dyspnea, chest pain, dizziness, confusion, tingling, abnormal breathing, BP drop with exertion) to supervising PT
PTA collects/reports data only; PT adjusts plan of care