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HLT-RESP80-04 · Lesson 7 · 40 clock hours · theory

Arterial Blood Gas Interpretation for Oxygen Therapy

Objective. The student will apply a systematic four-step ABG interpretation method to classify acid-base status and oxygenation, and use PaO2 and PaO2/FiO2 ratio results to adjust oxygen therapy orders in simulated clinical scenarios.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed clinician in the field or by a certification or approval body. It is a study draft and may contain errors. Do not use it to make decisions about any patient or medication. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

Why ABG Interpretation Matters for Oxygen Therapy

Arterial blood gas (ABG) analysis gives you a direct, objective window into a patient's respiratory and metabolic status. As a respiratory therapist, you will use ABG results every day to decide whether to start, adjust, or discontinue oxygen therapy—and to detect life-threatening changes before they become irreversible. Accuracy in reading these values is not optional; misinterpreting an ABG can lead to under-treating dangerous hypoxemia or over-treating in ways that depress ventilatory drive in at-risk patients.

Normal ABG Values

Commit these reference ranges to memory:

Values outside these ranges trigger a structured response, not guesswork.

The Four-Step ABG Interpretation Method

Using a consistent, stepwise approach prevents errors. Apply the following four steps in order every time.

Step 1 — Evaluate pH

Is the pH within normal limits (7.35–7.45)?

Step 2 — Evaluate PaCO₂ (Respiratory Component)

CO₂ is an acid. Ask: does the PaCO₂ explain the pH?

Step 3 — Evaluate HCO₃⁻ (Metabolic Component)

Bicarbonate is a base managed by the kidneys. Ask: does the HCO₃⁻ explain the pH?

Step 4 — Evaluate PaO₂ (Oxygenation Status)

Classify oxygenation independently of acid-base:

A PaO₂ below 60 mmHg is the standard clinical threshold for initiating supplemental oxygen in most patients.

The P/F Ratio: PaO₂/FiO₂

The PaO₂/FiO₂ ratio (P/F ratio) measures how efficiently the lungs transfer oxygen relative to the concentration being delivered.

A falling P/F ratio tells you that the lungs are becoming less efficient even if you are increasing the FiO₂—a critical warning sign requiring escalation of care.

Using ABG Results to Adjust Oxygen Therapy

| Clinical Finding | Action |

|---|---|

| PaO₂ < 60 mmHg; patient not on O₂ | Initiate supplemental oxygen |

| PaO₂ < 60 mmHg on current O₂ | Increase FiO₂ or change delivery device |

| PaO₂ 60–100 mmHg on supplemental O₂ | Titrate down toward lowest effective FiO₂ |

| PaO₂ > 100 mmHg on supplemental O₂ | Wean oxygen to avoid hyperoxia |

| PaO₂ ≥ 60 mmHg on room air, stable | Discontinue supplemental oxygen |

Always correlate ABG data with clinical signs: work of breathing, mental status, SpO₂ trends, and the patient's underlying diagnosis.

Safety Priorities

Severe hypoxemia (PaO₂ < 40 mmHg) is immediately life-threatening. Notify the supervising physician and act without delay—do not wait for further confirmation.

Hypercapnic patients (e.g., chronic COPD with CO₂ retention) may rely on a mild hypoxic stimulus to breathe. In these patients, target SpO₂ of 88–92% and PaO₂ of approximately 60–70 mmHg rather than pushing for higher values, unless directed otherwise by the physician.

Hyperoxia is not harmless. Excessive oxygen can cause absorption atelectasis, worsen hypercapnia in susceptible patients, and cause oxygen toxicity with prolonged high FiO₂ exposure. Always use the lowest FiO₂ that achieves an acceptable PaO₂.

Life-threatening acidosis (pH < 7.20) demands rapid escalation—alert the care team immediately, as ventilatory failure or cardiac arrhythmia may follow.

Scope of Practice Note

As a respiratory therapist, you interpret ABG results and recommend therapy changes. Ordering oxygen therapy is ultimately a physician function in most settings, though protocols and standing orders often authorize RTs to titrate oxygen within defined parameters. Know your facility's policies and your state license requirements before acting independently.

Key terms10

pHA measure of hydrogen ion concentration in arterial blood; normal range 7.35–7.45. Values below 7.35 indicate acidosis; above 7.45 indicate alkalosis.
PaCO₂Partial pressure of carbon dioxide in arterial blood; normal 35–45 mmHg. Reflects the respiratory component of acid-base balance; elevated values indicate hypoventilation, low values indicate hyperventilation.
HCO₃⁻ (bicarbonate)The metabolic component of acid-base balance regulated by the kidneys; normal 22–26 mEq/L. Low levels suggest metabolic acidosis; high levels suggest metabolic alkalosis.
PaO₂Partial pressure of oxygen dissolved in arterial blood; normal 80–100 mmHg on room air. The primary measure used to classify oxygenation status and guide oxygen therapy decisions.
HypoxemiaAn abnormally low PaO₂ in arterial blood. Clinically significant hypoxemia is generally defined as PaO₂ < 60 mmHg and is the standard threshold for initiating supplemental oxygen.
PaO₂/FiO₂ ratio (P/F ratio)A calculated index of oxygenation efficiency found by dividing PaO₂ by the FiO₂ (as a decimal). A normal P/F ratio is ≥ 400; progressively lower values indicate worsening lung function.
FiO₂Fraction of inspired oxygen expressed as a decimal (room air = 0.21). Used to calculate the P/F ratio and to describe the concentration of oxygen delivered to the patient.
CompensationThe physiological response in which the system not primarily affected (respiratory or metabolic) adjusts to move pH back toward the normal range. Full compensation returns pH to normal; partial compensation does not.
Respiratory acidosisAn acid-base disorder caused by CO₂ retention (PaCO₂ > 45 mmHg), resulting in a pH below 7.35. Common causes include hypoventilation, COPD exacerbation, and neuromuscular weakness.
HyperoxiaAn excessively elevated PaO₂ resulting from delivery of more oxygen than the patient needs. Can cause absorption atelectasis, oxygen toxicity, and worsened hypercapnia in susceptible patients.

Worked examples4

1. A patient on room air has the following ABG: pH 7.28, PaCO₂ 58 mmHg, HCO₃⁻ 26 mEq/L, PaO₂ 52 mmHg. Classify the acid-base disorder and oxygenation, then state whether oxygen therapy should be initiated.
  1. Step 1 — pH: 7.28 is below 7.35 → Acidosis is present.
  2. Step 2 — PaCO₂: 58 mmHg is above 45 mmHg, and it explains the acidosis (high CO₂ = more acid) → Primary respiratory acidosis.
  3. Step 3 — HCO₃⁻: 26 mEq/L is at the upper limit of normal. It has not risen significantly above 26, so compensation is minimal (acute respiratory acidosis).
  4. Step 4 — PaO₂: 52 mmHg is in the moderate hypoxemia range (40–59 mmHg), well below the 60 mmHg threshold.
  5. Therapy decision: Initiate supplemental oxygen immediately. PaO₂ of 52 mmHg on room air requires intervention. Also notify the physician—respiratory acidosis with hypoventilation may require additional ventilatory support.
Answer: Uncompensated respiratory acidosis with moderate hypoxemia. Initiate supplemental oxygen; escalate to ventilatory support assessment given CO₂ retention.
2. A patient receiving 40% oxygen via air-entrainment mask (FiO₂ 0.40) has a PaO₂ of 72 mmHg. Calculate the P/F ratio and interpret it.
  1. Identify values: PaO₂ = 72 mmHg, FiO₂ = 0.40.
  2. Apply the formula: P/F ratio = PaO₂ ÷ FiO₂ = 72 ÷ 0.40 = 180.
  3. Interpret: A P/F ratio of 180 falls in the moderate impairment range (100–200), indicating the lungs are transferring oxygen significantly less efficiently than normal (normal ≥ 400).
  4. Clinical implication: Despite supplemental oxygen achieving a PaO₂ just above the 60 mmHg threshold, the low P/F ratio signals worsening lung function. The care team should be notified; further increases in FiO₂ or escalation to higher-level respiratory support may be needed.
Answer: P/F ratio = 180, indicating moderate oxygenation impairment. Current therapy is marginally maintaining PaO₂; escalation may be required.
3. A stable post-operative patient is on 2 L/min nasal cannula (approximate FiO₂ 0.28). ABG shows: pH 7.41, PaCO₂ 40 mmHg, HCO₃⁻ 25 mEq/L, PaO₂ 96 mmHg, SaO₂ 98%. What is the appropriate oxygen therapy decision?
  1. Step 1 — pH: 7.41 is within normal range (7.35–7.45) → No acid-base disorder.
  2. Step 2 — PaCO₂: 40 mmHg is normal → No respiratory component issue.
  3. Step 3 — HCO₃⁻: 25 mEq/L is normal → No metabolic component issue.
  4. Step 4 — PaO₂: 96 mmHg is in the normal range (80–100 mmHg). The patient is well above the 60 mmHg threshold and above 100 mmHg is not present, so hyperoxia is not an issue, but supplemental oxygen is delivering more than room air.
  5. Therapy decision: The patient's PaO₂ is normal and acid-base status is normal. Per the principle of using the lowest effective FiO₂, the clinician should attempt to wean or discontinue supplemental oxygen, confirm the patient maintains SpO₂ ≥ 95% on room air, and document the outcome.
Answer: Normal ABG; no acid-base disorder; normal oxygenation. Wean supplemental oxygen per protocol; discontinue if SpO₂ remains ≥ 95% on room air.
4. Calculate the P/F ratio for a patient with PaO₂ of 55 mmHg receiving oxygen via a non-rebreather mask at an estimated FiO₂ of 0.70. Interpret the result.
  1. Identify values: PaO₂ = 55 mmHg, FiO₂ = 0.70.
  2. Calculate: P/F ratio = 55 ÷ 0.70 = 78.6, rounded to approximately 79.
  3. Interpret: A P/F ratio of approximately 79 is severely below normal (normal ≥ 400) and falls in the severe impairment category (< 100).
  4. Clinical implication: Despite a high FiO₂ of 0.70, PaO₂ remains dangerously low at 55 mmHg (moderate hypoxemia threshold). The severely depressed P/F ratio indicates the lungs cannot adequately oxygenate the blood regardless of the oxygen concentration delivered. Immediately notify the physician; this patient likely requires escalation to non-invasive or invasive mechanical ventilation.
Answer: P/F ratio ≈ 79, indicating severe oxygenation failure. Immediate physician notification and likely ventilatory escalation required.

The math in this lesson1

Learners compute the PaO₂/FiO₂ ratio and classify oxygenation impairment.
6.RP.3
In class: Lecture and guided note-taking on ABG normal values, the four-step method (pH → PaCO2 → HCO3 → PaO2), and clinically significant thresholds; students practice the method on 10 instructor-solved examples. · Independent ABG interpretation drill: students receive 15 ABG result cards and classify each, then identify whether oxygen therapy should be initiated, increased, decreased, or discontinued based on PaO2 and clinical context. · Group simulation: instructor reads aloud a progressive patient scenario with serial ABG values; students call out titration decisions in real time and document them on a flow sheet, then debrief as a class.
Materials: ABG normal values reference card, 15-card ABG drill set, simulated flow sheet, calculator, Egan's Fundamentals (ABG chapter), AARC Clinical Practice Guideline for ABG sampling
Check: 20-question written ABG interpretation test (classification and therapy recommendation); pass mark 75%; all critical safety thresholds (severe hypoxemia, life-threatening acidosis) must be identified correctly to pass.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Clinical Data Analysis
1. A patient's ABG shows pH 7.30, PaCO₂ 50 mmHg, HCO₃⁻ 24 mEq/L, PaO₂ 58 mmHg. Which acid-base classification is correct?
Therapeutic Procedures — Oxygen Therapy
2. At what PaO₂ threshold is supplemental oxygen therapy generally indicated in most adult patients?
Clinical Data Analysis
3. A patient on a 35% air-entrainment mask (FiO₂ 0.35) has a PaO₂ of 70 mmHg. What is the P/F ratio?
Therapeutic Procedures — Oxygen Therapy
4. A patient with known chronic CO₂ retention and COPD has a PaO₂ of 88 mmHg and SpO₂ of 97% on 4 L/min nasal cannula. The most appropriate action is to:
Clinical Data Analysis
5. An ABG result shows pH 7.48, PaCO₂ 30 mmHg, HCO₃⁻ 22 mEq/L, PaO₂ 85 mmHg. Which classification is correct?
Patient Safety and Emergency Management
6. A patient's ABG returns with a PaO₂ of 38 mmHg. The respiratory therapist's immediate priority is to:
Clinical Data Analysis
7. Which ABG pattern is consistent with partially compensated metabolic acidosis?
Therapeutic Procedures — Oxygen Therapy
8. A patient on supplemental oxygen has a PaO₂ of 115 mmHg. The appropriate response is to:
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