Pathophysiology · ELI Explains: Fluids, Electrolytes & Acid-Base Balance (book 1)

Reading Basic Arterial Blood Gases

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The college version
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The college version

Clinical Orientation

You receive the following ABG results on Ms. Ramirez, a 56-year-old with COPD admitted with increased dyspnea: pH 7.28, PaCO2 65, 58, HCO3 30. You have five numbers. Where do you start? Ms. Ramirez is tachypneic at 32, using accessory muscles, and SpO2 is 86% on room air. She is alert but anxious. Before you interpret the gas, you know she is struggling—the gas will tell you how badly and in what direction. This chapter gives you a repeatable, beginner-safe method to interpret ABGs without memorizing complex formulas. It does not replace clinical judgment or make you a diagnostician—it helps you recognize patterns and communicate findings.

Governing Question: What mechanism links reading basic arterial blood gases to its required bedside findings, tests, red flags, and nursing priorities?

What Is Normal?

Meaning of pH, PaCO2, Bicarbonate, PaO2, Saturation:

pH: 7.35-7.45. The master number—tells you (<7.35) or (>7.45). But a normal pH does not mean "no problem"—it may be fully compensated or mixed.
PaCO2: 35-45 mmHg. The respiratory component. High = hypoventilation (retaining CO2). Low = hyperventilation (blowing off CO2).
Bicarbonate (HCO3-): 22-26 mEq/L (on ABG; may be slightly higher on BMP). The metabolic component. Low = metabolic acidosis or renal for respiratory alkalosis. High = metabolic alkalosis or renal compensation for respiratory acidosis.
PaO2: 80-100 mmHg on room air (varies with age and altitude). Measures oxygenation—how well oxygen is getting from alveoli into blood. Interpret separately from acid-base.
SaO2/SpO2: >95% on room air. Oxygen saturation of hemoglobin. Relationship to PaO2 is described by the oxyhemoglobin dissociation curve.

Arterial Sample Context: An ABG is drawn from an artery (radial, brachial, femoral). It is painful and carries risks (hematoma, arterial injury, thrombosis). The sample must be: arterial (not venous—venous blood has lower pH, higher PaCO2, lower PaO2), free of air bubbles (air equilibrates with the sample, falsely raising PaO2 and lowering PaCO2), properly anticoagulated (heparin), and analyzed promptly (or kept on ice if delayed). Always note the FiO2 (fraction of inspired oxygen) and any ventilator settings.

Acid-Base Ratio: The Henderson-Hasselbalch equation states that pH is proportional to [HCO3-] / [PaCO2]. The body maintains a ratio of about 20:1 (bicarbonate to carbonic acid, where carbonic acid = PaCO2 × 0.03). When bicarbonate falls or PaCO2 rises, the ratio decreases → pH falls. When bicarbonate rises or PaCO2 falls, the ratio increases → pH rises.

What Goes Wrong?

A Primary Metabolic or Respiratory Change Moves pH: The first step is identifying whether the primary problem is metabolic or respiratory. If pH is low and bicarbonate is low → metabolic acidosis. If pH is low and PaCO2 is high → respiratory acidosis. If pH is high and bicarbonate is high → metabolic alkalosis. If pH is high and PaCO2 is low → respiratory alkalosis.

The Other System May Compensate: In metabolic acidosis, the lungs hyperventilate (PaCO2 falls). In metabolic alkalosis, the lungs hypoventilate (PaCO2 rises—but limited by oxygen need). In respiratory acidosis, the kidneys retain bicarbonate (HCO3 rises over days). In respiratory alkalosis, the kidneys excrete bicarbonate (HCO3 falls over days). Compensation is partial—it returns pH toward normal but usually not all the way.

Near-Normal pH May Hide a Compensated or Mixed Process: A pH of 7.36 with PaCO2 60 and HCO3 34 suggests chronic compensated respiratory acidosis. A pH of 7.40 with PaCO2 30 and HCO3 18 suggests chronic compensated respiratory alkalosis—or a mixed metabolic acidosis and respiratory alkalosis where the two processes pull pH in opposite directions, resulting in a normal pH. When the numbers do not match a single disorder with expected compensation, consider a .

The Four-Step Method

Step 1: Assess the patient and verify the sample. Before interpreting the ABG, know: Is this an arterial sample? At what FiO2? On what ventilator settings (if applicable)? What is the patient's clinical status—respiratory rate, work of breathing, mental status, vital signs? A "bad" ABG in a comfortable patient may be a specimen problem. A "good" ABG in a deteriorating patient demands rechecking.

Step 2: Label acidemia or alkalemia. Look at pH. If <7.35, the patient has acidemia. If >7.45, the patient has alkalemia. If within normal range, the patient may be compensated or have a mixed disorder—look deeper.

Step 3: Identify the . Match the direction of PaCO2 and HCO3 to the pH:

Low pH + high PaCO2 = respiratory acidosis
Low pH + low HCO3 = metabolic acidosis
High pH + low PaCO2 = respiratory alkalosis
High pH + high HCO3 = metabolic alkalosis

Step 4: Assess compensation. Is the "other" system moving in the expected direction? In metabolic acidosis, PaCO2 should be low (the lungs are hyperventilating). In respiratory acidosis (acute), HCO3 should be near normal; in chronic, HCO3 should be elevated. Compensation that is absent, inadequate, or excessive suggests a mixed disorder.

What Happens Inside the Body?

Causal Chain 1: Systematic Interpretation

pH direction → PaCO2 relationship → Bicarbonate relationship → Primary process → Compensation direction → Clinical cause

Start with pH: is the patient acidemic or alkalemic? Then look at PaCO2: does it move in the same direction as pH (respiratory is primary) or opposite (respiratory is compensating)? Then HCO3: does it explain the pH (metabolic primary) or is it compensating? This sequence forces you to think about mechanism, not memorize patterns. For example: pH 7.22 (low), PaCO2 60 (high—explains low pH), HCO3 26 (normal—no renal compensation yet). This is acute respiratory acidosis. The lungs are not ventilating adequately, and the kidneys have not had time to compensate. Key finding: pH tells you the clinical urgency; PaCO2 vs. HCO3 tells you the mechanism; the other component tells you the acuity (compensated or not).

Causal Chain 2: Spotting Complex Patterns

Near-normal pH + abnormal PaCO2 and bicarbonate → Consider compensation or mixed disorder → Compare expected pattern and clinical context

When pH is near normal but PaCO2 and HCO3 are abnormal, ask: Is this a single disorder with full compensation? In chronic respiratory acidosis (COPD), pH may be 7.34-7.36 with PaCO2 60 and HCO3 34—the kidneys have retained bicarbonate over weeks, bringing pH near normal. Or is this a mixed disorder? pH 7.40 with PaCO2 25 and HCO3 15 could be chronic respiratory alkalosis with renal compensation—or it could be a mixed metabolic acidosis plus respiratory alkalosis. The clinical context distinguishes these: a patient with sepsis (lactic acidosis + hyperventilation from fever/cytokines) vs. an asymptomatic pregnant patient (chronic respiratory alkalosis). When the expected and actual compensation do not match, suspect a mixed disorder. Key finding: Mixed disorders are common in critically ill patients—the patient with sepsis may have both lactic acidosis and hyperventilation. The ABG shows what is happening; the clinical picture explains why.

What the Nurse May See

Patient Presentation Governs Urgency: A patient with mild metabolic acidosis (pH 7.32) who is alert with normal vitals is very different from a patient with the same ABG who is obtunded and hypotensive. The ABG provides the mechanism; the patient provides the urgency.

Discordant Breathing or Mental Status: If the ABG shows metabolic acidosis but the patient is NOT hyperventilating—they are compensating poorly or fatiguing. If the ABG shows respiratory acidosis but the patient is alert—they may be a chronic CO2 retainer. Discordance between the ABG and the clinical picture is an important finding.

Signs of Underlying Cause: The ABG does not exist in a vacuum. Look for: ketotic breath (DKA), uremic fetor (renal failure), pinpoint pupils and decreased respiratory rate (opioid toxicity), wheezing and prolonged expiration (COPD/asthma), fever and tachycardia (sepsis), chest pain and dyspnea (pulmonary embolism).

Trend Over Isolated Gas: A single ABG is a snapshot. Serial gases—trending pH, PaCO2, and HCO3 over time—tell you whether the patient is improving, stable, or deteriorating. A pH that was 7.28 and is now 7.20 is worsening.

Tests, Labs, and Monitoring

Sample Type and Collection Issues: Was the sample truly arterial? A venous sample will have lower pH, higher PaCO2, and lower PaO2. Was it collected in a heparinized syringe without air bubbles? Was it analyzed promptly or properly iced? Preanalytic errors are common—always consider whether the ABG makes sense for this patient before acting on it.

pH: The master variable. Acidemia (<7.35), normal (7.35-7.45), alkalemia (>7.45). The absolute value matters, but the trend matters more.

PaCO2: The respiratory component. Always interpret PaCO2 in the context of the patient's respiratory rate and effort. A PaCO2 of 50 in a patient breathing 8/min (CNS depression) has a different meaning than PaCO2 50 in a patient breathing 35/min with accessory muscles (COPD exacerbation with fatigue).

Bicarbonate: The metabolic component. Calculated by the blood gas analyzer from pH and PaCO2. On an ABG, the "HCO3" is often the calculated value; on a BMP, it is measured as total CO2 content (which is mostly bicarbonate). These values should be similar but may differ slightly.

Oxygenation Measures: PaO2 (normal ~80-100 on room air, varies with age) and SaO2/SpO2. Interpret oxygenation separately from ventilation. A patient with normal PaCO2 can have severely low PaO2 (pneumonia, ARDS). The A-a gradient (alveolar-arterial oxygen difference) can help differentiate hypoxemia causes (hypoventilation vs. V/Q mismatch vs. shunt) but is beyond the scope of basic interpretation.

Electrolytes: Always check electrolytes alongside ABG. The anion gap (Na - Cl - HCO3) is essential for classifying metabolic acidosis. Potassium reflects acid-base shifts. Renal function determines the kidney's ability to compensate.

Compare with Prior Values: If prior ABGs are available, compare. Has the pH improved or worsened? Has compensation changed? Is this an acute change or a chronic pattern? Prior labs provide the baseline.

Nursing Priorities

First Assess Patient and Sample: Before interpreting, verify the sample is arterial, was properly handled, and corresponds to the clinical picture. Assess the patient: respiratory status, mental status, vital signs, and signs of the underlying cause.

Label Acidemia/Alkalemia: State whether pH is low, normal, or high. This is the starting point for all interpretation.

Identify Matching Respiratory/Metabolic Driver: Does PaCO2 explain the pH change (respiratory primary)? Does HCO3 explain the pH change (metabolic primary)?

Assess Direction of Compensation: Is the "other" system moving in the expected direction? If pH is low from metabolic acidosis, is PaCO2 appropriately low (compensation)? If compensation is absent or inadequate, the patient is deteriorating.

Evaluate Oxygenation Separately: Assess PaO2 and SpO2. Does the patient have hypoxemia requiring supplemental O2 or changes in ventilator settings? This is a separate question from acid-base status.

Escalate Instability or Discordance: Critical ABG values (severe pH, extreme PaCO2, severe hypoxemia), unstable patients, or ABG-clinical discordance require prompt escalation and often repeat gases.

Complications and Red Flags

Red Flag	Why This Is Dangerous
Critical gas with unstable patient	Severe acidemia (pH <7.10), severe alkalemia (pH >7.60), extreme hypercapnia (PaCO2 >80-100), or severe hypoxemia with clinical instability. The patient needs urgent intervention—escalate immediately.
Unexpected rapid change	A PaCO2 that was 45 and is now 75 three hours later, or a pH that dropped from 7.30 to 7.15. Rapid changes indicate active deterioration—the cause must be identified and addressed.
Worsening CO2 and fatigue	Rising PaCO2 in a tachypneic, tiring patient signals impending respiratory failure. Non-invasive or invasive ventilatory support may be urgently needed.
Severe acidemia/alkalemia	pH extremes impair enzyme function, cardiac contractility, and cerebral autoregulation. Mortality increases sharply at pH extremes.
Possible sampling error that conflicts with bedside picture	An ABG showing severe hypoxemia in a comfortable, pink patient, or a normal ABG in a cyanotic, obtunded patient. Question the sample—it may be venous, have air contamination, or belong to a different patient. Redraw before acting on a discordant result.

Patient and Family Teaching

One-Minute Mechanism: "An arterial blood gas is a blood test from an artery—not a vein—that tells us how well you are breathing and how your body's acid-base balance is working. It checks your oxygen level, your carbon dioxide level, and your pH. Getting the blood can be uncomfortable because arteries are deeper and have more nerves than veins, but it provides information we cannot get any other way."

Key Points: The ABG is a momentary snapshot—your condition can change quickly, and repeat tests may be needed. The test requires arterial blood; your nurse or respiratory therapist will check for good blood flow to your hand (Allen test) before drawing from the radial artery at the wrist. Hold pressure on the site for at least 5 minutes afterward to prevent bleeding and bruising. Report any persistent bleeding, swelling, numbness, or color change at the site. If you are on blood thinners, tell the person drawing the blood.

Key takeaways and summary

Summary

Normal → Change → Consequence → Finding → Priority: ABG interpretation follows a structured approach: verify the sample, label pH, identify the primary disorder by matching pH with PaCO2 or HCO3, and assess compensation. The patient's clinical status determines urgency. Compensation is partial and follows predictable patterns—deviations suggest mixed disorders. The nursing priority is to recognize abnormal patterns, correlate with the patient, trend serial gases, and escalate critical values or discordant findings.

Causal Chain 1: pH direction → PaCO2 relationship → bicarbonate relationship → primary process → compensation direction → clinical cause. Causal Chain 2: Near-normal pH + abnormal PaCO2 and HCO3 → consider compensated or mixed disorder → compare expected pattern and clinical context.

If You Remember Nothing Else:

Always assess the patient before interpreting the ABG—the patient determines urgency.
pH tells you acidemia or alkalemia; PaCO2 and HCO3 tell you whether it is respiratory or metabolic.
Compensation never overcorrects—it brings pH toward normal but not past it.
Red flag: Rising PaCO2 with tiring respirations in metabolic acidosis signals failing compensation.
Test limitation: Venous samples, air bubbles, and delayed analysis cause misleading ABG results—verify the sample.

One-Minute Teach-Back: "Walk me through the four steps of ABG interpretation using this example: pH 7.24, PaCO2 58, HCO3 26."

Common Student Mistakes

Mistake: "ROME alone replaces understanding." ROME (Respiratory Opposite, Metabolic Equal) is a mnemonic for the direction of pH vs. PaCO2/HCO3 in primary disorders. It is a starting point, not a complete understanding. It does not help with compensation, mixed disorders, or the clinical picture. Use it as a check, not your only tool. Always think about mechanism: why is this value changing?

Mistake: "A normal pH means no urgent disorder." Wrong. A pH of 7.36 with PaCO2 70 and HCO3 38 is compensated respiratory acidosis—the pH is near normal because the kidneys have been working overtime for weeks. But the PaCO2 of 70 indicates severe hypoventilation. Do not be reassured by a normal pH when PaCO2 and HCO3 are markedly abnormal. Also, mixed disorders can produce a normal pH while two dangerous processes are occurring simultaneously.

Eli explains

The same idea, in plain words

Explain it like I’m 10

The Story: A pool test strip has several pads—pH, alkaline buffer, and acidic gas level. You use them in the same order every time to avoid confusion. But before you trust the strip, you walk to the pool and look at the water. Is it clear or cloudy? Is the pump running? Are there swimmers in distress? The test strip confirms what you suspect from looking at the pool—it does not replace looking at the pool. The strip can also be wrong if it was stored improperly or read incorrectly. Always correlate the strip with the pool.

Mapping:

Analogy Element	Real Physiology
Test strip pads	pH, PaCO2, HCO3, PaO2—each test on the ABG
Same order every time	Systematic ABG interpretation method
Walk to the pool first	Assess the patient before interpreting the ABG
Strip can be wrong	Preanalytic errors—venous sample, air bubbles, delayed analysis
Swimmers in distress	The patient's clinical status

Where the Analogy Stops: The pool test strip gives independent readings. In the body, pH, PaCO2, and HCO3 are mathematically related through the Henderson-Hasselbalch equation. You cannot change one without affecting the others. The ABG is not a set of independent tests—it is a picture of a single equilibrium.

Check yourself

12 review questions from the chapter. Try each one, then open the answer.

Priority patient.** Which ABG result requires the most immediate action?
Show answer
pH 7.36, PaCO2 48, HCO3 28 (compensated respiratory acidosis) B. pH 7.09, PaCO2 55, HCO3 12 (severe mixed acidosis), patient obtunded, HR 130, BP 72/40 C. pH 7.48, PaCO2 32, HCO3 23 (mild respiratory alkalosis) D. pH 7.38, PaCO2 39, HCO3 24 (normal)
First assessment.** An ABG shows pH 7.30, PaCO2 50, HCO3 24. What should the nurse assess first?
Show answer
Urine output B. Respiratory status—rate, depth, breath sounds, work of breathing, SpO2 C. Bowel sounds D. Skin turgor
Mechanism.** An ABG shows pH 7.46, PaCO2 48, HCO3 34. What is this?
Show answer
Acute respiratory acidosis B. Metabolic alkalosis with respiratory compensation C. Respiratory alkalosis D. Mixed respiratory and metabolic acidosis
Trend interpretation.** Serial ABGs: 0800: pH 7.33, PaCO2 48, HCO3 24. 1200: pH 7.28, PaCO2 56, HCO3 24. What is happening?
Show answer
The patient is improving—pH is stabilizing B. The patient has worsening acute respiratory acidosis without renal compensation C. The patient is developing metabolic compensation D. The patient has primary metabolic acidosis
Expected vs. unexpected.** A patient with DKA has pH 7.15, HCO3 6, PaCO2 12. The PaCO2 of 12 is:
Show answer
Unexpected—it should be elevated B. Expected—this is appropriate respiratory compensation for severe metabolic acidosis C. Unexpected—PaCO2 should be unchanged D. Expected—this is primary respiratory alkalosis
Clinical deterioration.** A patient being treated for DKA had ABG: pH 7.10, PaCO2 15, HCO3 5. After several hours, repeat ABG: pH 7.05, PaCO2 30, HCO3 6. What has changed?
Show answer
The metabolic acidosis is resolving B. The patient is developing respiratory fatigue—PaCO2 is rising despite ongoing acidosis, indicating failing compensation C. Renal compensation has started D. The patient is hyperventilating more effectively
Patient teaching.** A patient asks why they need "another blood gas" after already having one this morning. Best response?
Show answer
"It is hospital policy." B. "Blood gases can change quickly, especially when you are sick. Repeating the test helps us see whether you are improving or getting worse, and whether our treatments are working." C. "The first one was probably wrong." D. "Don't worry about it."
Scope/delegation.** A nursing assistant hands the nurse an ABG result that was just faxed from the lab. What should the RN do?
Show answer
File the result in the chart B. Review the result in the context of the patient's current clinical status, compare with prior values, and act on any critical findings C. Ask the assistant to interpret the result D. Discard the result—it is not relevant
Answer: B. pH 7.09 is severe acidemia with both metabolic (HCO3 12) and respiratory (PaCO2 55—should be low but is high, indicating ventilatory failure) components. The patient is in shock (tachycardic, hypotensive) with altered consciousness. This is a peri-arrest situation. (A) is stable COPD. (C) is mild and not life-threatening. (D) is normal.
Show answer
B.** This ABG shows acute respiratory acidosis (pH low, PaCO2 high, HCO3 normal → no renal compensation yet). The primary threat is ventilation. Assess the patient's respiratory status immediately—are they breathing effectively? Is the airway patent? Is there obstruction or CNS depression? (A), (C), and (D) are secondary.
Answer: B. pH is high (alkalemia). HCO3 is high (explains the alkalosis—metabolic origin). PaCO2 is high—this is the respiratory system compensating for metabolic alkalosis by retaining CO2. The elevated PaCO2 is in the same direction as bicarbonate (both high), which is expected in metabolic alkalosis compensation. (A) would have low pH. (C) would have low PaCO2.
Show answer
B.** PaCO2 is rising (48 → 56), pH is falling (7.33 → 7.28), and bicarbonate is unchanged at 24—no renal compensation has occurred. This is worsening acute respiratory acidosis. The patient is not improving (A). There is no evidence of compensation (C). The primary problem is respiratory, not metabolic (D).
Answer: B. In metabolic acidosis, the expected respiratory compensation is hyperventilation to lower PaCO2. A PaCO2 of 12 is extreme but appropriate for the severity of this acidosis (pH 7.15, HCO3 6). This is Kussmaul breathing. Winter's formula (expected PaCO2 = 1.5 × HCO3 + 8 ± 2) predicts PaCO2 ≈ 1.5(6) + 8 = 17 ± 2, so 12 is close to expected. The respiratory system is compensating maximally.
Show answer
B.** In the second ABG, PaCO2 has risen from 15 to 30 despite ongoing severe acidosis (HCO3 still 6). Expected PaCO2 for HCO3 6 is ~15-19. The rising PaCO2 indicates the patient is tiring and can no longer maintain the extreme hyperventilation needed to compensate. This is respiratory fatigue/failure superimposed on metabolic acidosis—a dangerous development. (A) is wrong. (C) takes days and is not the cause of PaCO2 rising. (D) is the opposite.
Answer: B. Serial ABGs track the trajectory of acid-base and oxygenation status. Changes in pH, PaCO2, and PaO2 guide treatment decisions. This answer is honest and educational. (A) is dismissive. (C) undermines confidence without basis. (D) is patronizing.
Show answer
B.** The RN receives, reviews, and acts on laboratory results within nursing scope. This involves interpreting the ABG in clinical context, comparing with prior values, recognizing critical results, and escalating as needed. (A) without review is negligent. (C) is inappropriate delegation. (D) is wrong.