ABG Tic-Tac-Toe Interpretation Notes

What is arterial blood gas?

• An arterial blood gas (ABG) is a laboratory test to monitor the patient’s acid–base balance. It determines the extent of compensation by the buffer system and includes measurements of:

  • acidity (pH),

  • levels of oxygen (PaO₂) and carbon dioxide (PaCO₂) in arterial blood, and

  • oxygen delivery and buffering status.

• ABG is obtained from an artery (commonly radial or brachial), unlike venous blood samples.

• It helps assess gas exchange, ventilation, and the buffer systems, guiding clinical decisions.

The 6 components of arterial blood gas

• pH: concentration of hydrogen ions; indicates acidity/alkalinity of body fluids.

  • pH = 7.35 indicates acidosis, pH = 7.45 indicates alkalosis.

  • Normal ABG pH range: $7.35 \le pH \le 7.45$

  • Chemistry note: the bicarbonate buffer system is involved; CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻.

• PaCO₂ (Partial Pressure of Carbon Dioxide): reflects alveolar ventilation and gas exchange.

  • CO₂ cannot escape when alveolar ventilation is impaired; forms carbonic acid (H₂CO₃), contributing to acidosis.

  • Hypoventilation (e.g., COPD) raises PaCO₂ → respiratory acidosis.

  • Hyperventilation lowers PaCO₂ → respiratory alkalosis.

  • Normal PaCO₂ range: $35\text{ mmHg} \le PaCO₂ \le 45\text{ mmHg}$

• PaO₂ (Partial Pressure of Oxygen): indicates oxygen available to bind with hemoglobin.

  • Lower pH can decrease oxygen binding efficiency.

  • Normal PaO₂ range: $75 \le PaO₂ \le 100\text{ mmHg}$

• SaO₂ (Oxygen Saturation): percentage of hemoglobin binding sites carrying oxygen.

  • Can be measured directly by pulse oximetry or inferred from ABG calculations.

  • Normal range: $94\% \le SaO₂ \le 100\%$

• HCO₃⁻ (Bicarbonate): buffer base in the blood; major metabolic buffer.

  • Low bicarbonate suggests metabolic acidosis; high bicarbonate suggests metabolic alkalosis.

  • Normal HCO₃⁻ range: $22 \text{ to } 26\text{ mEq/L}$

• BE (Base Excess): base buffer status; routinely checked with HCO₃⁻.

  • Base excess < −2 indicates acidosis; BE > +2 indicates alkalosis.

  • Normal BE range: $-2 \le BE \le +2\ \text{mmol/L}$

Normal values (reference ranges)

• pH: $7.35 \le pH \le 7.45$

• PaCO₂: $35 \le PaCO₂ \le 45\ \text{mmHg}$

• PaO₂: $75 \le PaO₂ \le 100\ \text{mmHg}$

• HCO₃⁻: $22 \le HCO₃⁻ \le 26\ \text{mmol/L}$

• SaO₂: $94\% \le SaO₂ \le 100\%$

• Note: The values for PaCO₂ are described as a respiratory determinant, while HCO₃⁻ is described as a metabolic determinant.

Goals of ABG analysis

• Determine whether the ABG shows ACIDIC or ALKALOSIS or both (overall acid–base status).

• Determine if the abnormality is METABOLIC or RESPIRATORY (etiology of the disturbance).

• Determine the degree of compensation:

  • UNCOMPENSATED: pH abnormal while PaCO₂ or HCO₃⁻ is normal.

  • PARTIALLY COMPENSATED: both pH and at least one of PaCO₂ or HCO₃⁻ are abnormal.

  • FULLY COMPENSATED: pH within normal range, with abnormal PaCO₂ and/or HCO₃⁻ indicating compensation.

How to draw and use the tic-tac-toe grid (steps overview)

• The tic-tac-toe grid has three columns labeled ACIDOSIS, NORMAL, ALKALOSIS. The rows are populated by pH, PaCO₂, and HCO₃⁻ values as you analyze them; note that PaCO₂ is inverted for this method.

• Step 1: Memorize the normal values (above).

• Step 2: Create your tic-tac-toe grid with the three columns ACIDOSIS | NORMAL | ALKALOSIS.

• Step 3: Determine the acidity or alkalinity based on pH and place pH in the appropriate column.

• Step 4: Determine PaCO₂ status (ACIDOSIS, NORMAL, ALKALOSIS) and place PaCO₂ in the corresponding column. Reminder: PaCO₂ is inverted for this method.

• Step 5: Determine HCO₃⁻ status (ACIDOSIS, NORMAL, ALKALOSIS) and place HCO₃⁻ in the corresponding column.

• Step 6: Goal #1 — determine ACIDOSIS or ALKALOSIS from the pH column.

• Step 7: Goal #2 — determine METABOLIC or RESPIRATORY by comparing the pH column with PaCO₂ and HCO₃⁻ columns.

• Step 8: Goal #3 — determine COMPENSATION type (UNCOMPENSATED, PARTIALLY COMPENSATED, FULLY COMPENSATED).

Step-by-step: Step 3 to Step 5 details (how to categorize values)

• Step 3: Determine acidity/alkalinity from pH

  • If pH is between $7.35$ and $7.39$: interpreted as NORMAL but SLIGHTLY ACIDIC; place under NORMAL with a note toward ACIDOSIS.

  • If pH is between $7.41$ and $7.45$: interpreted as NORMAL but SLIGHTLY ALKALINE; place under NORMAL with a note toward ALKALOSIS.

  • If pH < $7.35$: ACIDOSIS; place under ACIDOSIS.

  • If pH > $7.45$: ALKALOSIS; place under ALKALOSIS.

  • Note: ONLY pH exactly equal to 7.40 sits in the middle.

• Step 4: PaCO₂ status

  • Normal PaCO₂: 35–45 mmHg → place under NORMAL.

  • PaCO₂ < 35 mmHg → ALKALOSIS column.

  • PaCO₂ > 45 mmHg → ACIDOSIS column.

  • PaCO₂ is intentionally inverted in the grid for the method.

• Step 5: HCO₃⁻ status

  • HCO₃⁻ < 22 → ACIDOSIS column (left).

  • 22 ≤ HCO₃⁻ ≤ 26 → NORMAL column.

  • HCO₃⁻ > 26 → ALKALOSIS column (right).

Step-by-step: Step 6 to Step 8 (solving the ABG)

• Step 6: Goal #1 — determine ACIDOSIS or ALKALOSIS based on the pH column.

• Step 7: Goal #2 — determine METABOLIC or RESPIRATORY:

  • If the pH column aligns with the PaCO₂ column → RESPIRATORY.

  • If the pH column aligns with the HCO₃⁻ column → METABOLIC.

  • If pH is NORMAL, determine whether the values lean toward ACIDOSIS or ALKALOSIS and interpret accordingly.

• Step 8: Goal #3 — determine compensation:

  • FULLY COMPENSATED if pH is normal.

  • PARTIALLY COMPENSATED if all three values are abnormal.

  • UNCOMPENSATED if PaCO₂ or HCO₃⁻ is normal while the other is abnormal.

Practice problems (practice problems and outcomes)

Practice Problem #1

• Given: pH = 7.26, PaCO₂ = 32, HCO₃⁻ = 18

• Analysis and results (as described):

  • pH 7.26 is ABNORMAL and ACIDOSIS → place pH under ACIDOSIS.

  • PaCO₂ 32 is ABNORMAL and ALKALOSIS (low PaCO₂) → place PaCO₂ under ALKALOSIS.

  • HCO₃⁻ 18 is ABNORMAL and ACIDOSIS → place HCO₃⁻ under ACIDOSIS.

  • Goal #1: pH under ACIDOSIS → ACIDOSIS.

  • Goal #2: pH aligns with HCO₃⁻ column (both ACIDOSIS) → METABOLIC.

  • Goal #3: All three values abnormal → PARTIALLY COMPENSATED ABG.

• Answer: Metabolic Acidosis, Partially Compensated.

Practice Problem #2

• Given: pH = 7.44, PaCO₂ = 30, HCO₃⁻ = 21

• Analysis and results:

  • pH 7.44 is NORMAL but slightly leaning toward ALKALOSIS → place pH under NORMAL with an arrow toward ALKALOSIS.

  • PaCO₂ 30 is ABNORMAL and ALKALOSIS → place PaCO₂ under ALKALOSIS.

  • HCO₃⁻ 21 is ABNORMAL and ACIDOSIS → place HCO₃⁻ under ACIDOSIS.

  • Goal #1: pH under NORMAL but leaning toward ALKALOSIS → ALKALOSIS.

  • Goal #2: pH NORMAL but leaning toward ALKALOSIS and PaCO₂ in ALKALOSIS → RESPIRATORY.

  • Goal #3: pH NORMAL → FULLY COMPENSATED ABG.

• Answer: Respiratory Alkalosis, Fully Compensated.

Practice Problem #3

• Given: pH = 7.1, PaCO₂ = 40, HCO₃⁻ = 18

• Analysis and results:

  • pH 7.1 is ABNORMAL and ACIDOSIS → place pH under ACIDOSIS.

  • PaCO₂ 40 is NORMAL → place PaCO₂ under NORMAL.

  • HCO₃⁻ 18 is ABNORMAL and ACIDOSIS → place HCO₃⁻ under ACIDOSIS.

  • Goal #1: ACIDOSIS.

  • Goal #2: pH is aligned with HCO₃⁻ (ACIDOSIS) → METABOLIC.

  • Goal #3: PaCO₂ is NORMAL while HCO₃⁻ is abnormal → UNCOMPENSATED.

• Answer: Metabolic Acidosis, Uncompensated.

Quick reference: summary of interpretations

• If pH is ACIDOSIS, ABG shows an acidemic state. If pH is ALKALOSIS, the state is alkalemic. If pH is NORMAL, assess direction of lean toward acidosis or alkalosis.

• If the pH column matches the PaCO₂ column → RESPIRATORY process dominates.

• If the pH column matches the HCO₃⁻ column → METABOLIC process dominates.

• Compensation status is determined by whether pH is normal and how the other values shift:

  • Fully compensated: pH normal with both PaCO₂ and HCO₃⁻ abnormal.

  • Partially compensated: pH abnormal with both PaCO₂ and HCO₃⁻ abnormal.

  • Uncompensated: pH abnormal with one of PaCO₂ or HCO₃⁻ normal.

${pH ext{ normal range}} = [7.35, 7.45]$
${PaCO₂ ext{ normal range}} = [35, 45] \ \text{mmHg}$
${PaO₂ ext{ normal range}} = [75, 100] \ \text{mmHg}$
${HCO₃⁻ ext{ normal range}} = [22, 26] \ \text{mEq/L}$
${BE ext{ normal range}} = [-2, +2] \ \text{mmol/L}$

Note: The tic-tac-toe method uses the inverted PaCO₂ placement to facilitate visualization of respiratory versus metabolic disturbances.