ABG Tic-Tac-Toe Method Notes
Normal values to memorize
pH level normal range:
Anything above 7.45 = base/alkalotic
Anything below 7.35 = acid/acidotic
HCO$3$ (bicarbonate) normal range:
Represents metabolic status
> 26 = alkalotic (metabolic alkalosis)
< 22 = acidotic (metabolic acidosis)
PCO$2$ (partial pressure of carbon dioxide) normal range:
Represents respiratory status
PCO$2$ is opposite to pH/HCO$3$ in interpretation
< 35 = respiratory alkalosis (base)
> 45 = respiratory acidosis (acid)
Practical note: memorize these baselines to compare with any ABG problem. They form the foundation for the Tic-Tac-Toe setup.
Tic-Tac-Toe grid setup for ABGs
Visual idea: a simple 3x3 grid labeled with columns for each ABG value and a row/column system to indicate acid, normal, base.
The solver uses two ABG components to determine the primary disorder: pH and either PCO$2$ (respiratory) or HCO$3$ (metabolic).
How the grid is arranged (as described in the video):
Top row labels: Acid, Normal, Base
Columns correspond to the following:
pH column (and also HCO$_3$ as the metabolic column)
PCO$2$ column is treated as the respiratory component and is opposite in interpretation to pH/HCO$3$
Rule of thumb embedded in the method:
If a value is below normal, it goes under Acid; if above normal, under Base; if within normal, under Normal.
PCO$2$ follows the opposite pattern, i.e., it’s the respiratory indicator and is considered in the context of being inverse to pH/HCO$3$ status.
Setup steps: 1) Write Acid, Normal, Base at the top of the grid. 2) Determine status for each value given in the problem:
pH: is it Acid (↓), Normal, or Base (↑)?
PCO$2$: treat as the respiratory component and mark Acid/Normal/Base according to whether it indicates low (alkalotic) or high (acidotic) status, with the understanding that its interpretation is opposite to pH/HCO$3$ in terms of acid/base direction.
HCO$_3$: mark Acid/Normal/Base as metabolic indicator.
3) After the three values are placed, look for a “three in a row” (a line of three in a row) to identify the primary disorder (respiratory vs metabolic).
Important mental model: This method helps you visually align pH with the corresponding respiratory or metabolic driver and then determine compensation.
Worked example from the video
Given values:
pH = 7.23 (acidic)
PCO$_2$ = 50 (respiratory component; high indicates respiratory acidosis)
HCO$_3$ = 30 (metabolic component; high indicates metabolic alkalosis)
Step 1: Fill the Tic-Tac-Toe grid
pH (normal 7.35–7.45): 7.23 → Acid, so place "pH" under the Acid column.
PCO$2$ (normal 35–45): 50 → As a respiratory value, high PCO$2$ is acidotic for respiratory status; place "PCO$_2$" under the Acid column for respiratory interpretation.
HCO$3$ (normal 22–26): 30 → High HCO$3$ is metabolic alkalosis (Base); place "HCO$_3$" under the Base column.
Step 2: Primary disorder determination
The PCO$2$ value is the respiratory driver, and it sits in the Acid row, while HCO$3$ sits in the Base column. This arrangement forms a pattern indicating a respiratory problem with metabolic compensation.
Step 3: Determine acid/base category and compensation
Since pH is acidic (7.23) and the respiratory indicator (PCO$_2$) is also acidotic (50), the primary disorder is a respiratory problem.
The pH is not normal, so this is not fully compensated. The HCO$_3$ is elevated (30), showing the body is trying to compensate metabolically.
Diagnosis: respiratory acidosis with partial (incomplete) metabolic compensation.
Step 4: If the problem were different (swap values)
If HCO$3$ were in the PCO$2$ column and PCO$_2$ in the Base column, the pattern would indicate a metabolic problem with respiratory compensation instead of a respiratory problem with metabolic compensation.
Summary from the example:
Primary disorder: respiratory acidosis
Compensation: partially compensated (metabolic compensation is present but not complete)
Key concepts and definitions
Arterial blood gas (ABG): a test that measures pH, PCO$2$, and HCO$3$ to assess acid-base balance.
Acid-base terms:
Acidosis: a condition where the body fluids are too acidic (pH below 7.35).
Alkalosis: a condition where the body fluids are too basic/alkalotic (pH above 7.45).
Respiratory component: indicated by PCO$2$; higher PCO$2$ generally drives acidosis, lower PCO$_2$ can drive alkalosis (subject to compensation).
Metabolic component: indicated by HCO$3$; higher HCO$3$ generally drives alkalosis, lower HCO$_3$ can drive acidosis (subject to compensation).
Compensation terms:
Uncompensated: the abnormal pH is not balanced by compensatory changes in the other parameter.
Partially compensated: the compensatory parameter is abnormal, but the pH remains abnormal.
Fully compensated: pH returns to normal range while both PCO$2$ and HCO$3$ are abnormal.
Primary vs compensatory disorder:
The primary disorder is identified by the direction of the abnormal pH and the abnormal parameter of the system (respiratory or metabolic).
Compensation is the body's attempt to restore pH toward normal by altering the other parameter.
How to interpret common ABG patterns (quick references)
Respiratory acidosis: pH low, PCO$2$ high. Partial or full metabolic compensation can raise HCO$3$ level toward normal or above.
Respiratory alkalosis: pH high, PCO$2$ low. Compensation would lower HCO$3$.
Metabolic acidosis: pH low, HCO$3$ low. Compensation would lower PCO$2$ (via faster breathing) toward normal.
Metabolic alkalosis: pH high, HCO$3$ high. Compensation would increase PCO$2$ (hypoventilation) toward normal.
In Tic-Tac-Toe terms, a three-in-a-row along the Acid/Normal/Base categories helps identify whether the primary issue is respiratory (PCO$2$) or metabolic (HCO$3$).
Practical tips for using the Tic-Tac-Toe method
Always memorize the normal baselines before solving problems.
Write all three values down first, then cross (fill in) the grid. Do not jump straight to the crosses.
Use the PCO$2$ value to determine respiratory involvement, since it represents the respiratory component (the “opposite” of pH/HCO$3$ in interpretation).
Check for a three-in-a-row to identify the primary disorder (respiratory vs metabolic).
Determine compensation by evaluating whether the non-primary parameter is altered and whether pH has returned to normal (fully compensated) or remains abnormal (partially/uncompensated).
Connections to broader context
ABG interpretation is foundational for NCLEX-style questions and clinical decision-making in nursing practice.
The Tic-Tac-Toe method provides a structured, visual approach to differentiate acidosis/alkalosis and respiratory vs metabolic etiologies.
The technique aligns with the broader pharmacology and physiology principle of homeostasis: the body attempts to compensate for primary disturbances to restore pH balance.
Ethical, philosophical, and practical implications
Accurate ABG interpretation is critical for patient safety, timely therapeutic interventions, and avoiding misdiagnosis.
Understanding compensation helps clinicians assess whether interventions are effective or if additional treatment is required.
The method emphasizes foundational knowledge (normal values) and logical reasoning over memorization without understanding, aligning with ethical medical education practices.
Quick reference cheat sheet (summary)
Normal ranges: , ,
pH status guide: low => acidotic, high => alkalotic
PCO$_2$ status guide: high => respiratory acidosis, low => respiratory alkalosis
HCO$_3$ status guide: high => metabolic alkalosis, low => metabolic acidosis
Primary diagnosis via Tic-Tac-Toe: look for three-in-a-row with pH status and the corresponding respiratory or metabolic indicator; determine compensation based on whether the second parameter is abnormal and if pH is still abnormal
Additional notes from the presenter
This method is intended to simplify ABG problems for NCLEX-style questions and nursing practice.
A follow-up video dives deeper into disorders and compensation patterns using the Tic-Tac-Toe method.
There is a free ABG quiz on RegisteredNurseRN.com to practice along with other NCLEX quizzes and resources.
Title for the notes
ABG Tic-Tac-Toe Method: Normal Values, Setup, and Worked Example (Respiratory vs Metabolic, Compensation)