Abg

Reading File

Finding Sources

Reading File

Finding Sources

Reading File

Here is a thorough guide to ABG (Arterial Blood Gas) interpretation, drawn from Current Surgical Therapy 14e, Fishman's Pulmonary Diseases, and Murray & Nadel's Respiratory Medicine:

Arterial Blood Gas (ABG) - Complete Guide

What an ABG Measures

An ABG provides the following directly measured values:

pH - reflects overall acid-base balance
PaCO2 - partial pressure of arterial CO2 (ventilation)
PaO2 - partial pressure of arterial oxygen (oxygenation)

And calculated values:

HCO3- - calculated via the Henderson-Hasselbalch equation
Base excess/deficit - amount of base needed to titrate 1L blood to pH 7.4 at 37°C

Normal Values

Parameter	Normal Value	Range
pH	7.40	7.35 - 7.45
PaCO2	40 mmHg	35 - 45 mmHg
PaO2	100 mmHg	75 - 100 mmHg
HCO3-	24 mEq/L	22 - 26 mEq/L
Base excess	0	-2 to +2 (abnormal if >+4 or <-4)

5-Step Systematic Interpretation

Step 1 - Evaluate pH

pH < 7.35 = Acidemia
pH > 7.45 = Alkalemia

Step 2 - Evaluate PaCO2 (respiratory component)

In respiratory disorders: PaCO2 moves opposite to pH (↑ CO2 → ↓ pH)
In metabolic disorders: PaCO2 moves in the same direction as pH

Step 3 - Evaluate HCO3- (metabolic component)

HCO3- < 22 mEq/L = metabolic acidosis
HCO3- > 26 mEq/L = metabolic alkalosis

Step 4 - Evaluate Base Excess

Positive = alkalosis (metabolic component)
Negative (base deficit) = acidosis (metabolic component)

Step 5 - Evaluate PaO2

Check for concurrent hypoxemia (PaO2 < 80 mmHg)
Don't overlook hypoxemia even when focused on acid-base

The Four Primary Disorders

1. Metabolic Acidosis

HCO3- < 22 mEq/L
Compensation: lungs hyperventilate to lower CO2
Winter's Formula (expected PaCO2): PaCO2 = 1.5 × [HCO3-] + 8 ± 2
Work up with Anion Gap: AG = Na - (Cl + HCO3-); normal < 12

Anion Gap causes (MUDPILES):

M - Methanol, Muscle injury (rhabdomyolysis), Metformin
U - Uremia
D - Diabetic ketoacidosis
P - Propylene glycol, Paracetamol (acetaminophen)
I - Isoniazid, Iron
L - Lactic acidosis
E - Ethylene glycol
S - Salicylates

Non-anion gap causes: diarrhea, RTA, saline excess

2. Metabolic Alkalosis

HCO3- > 26 mEq/L
Compensation: hypoventilation raises CO2
Expected PaCO2 = 0.7 × [HCO3-] + 20 ± 5
Causes: vomiting, NG suction, diuretics, hyperaldosteronism

3. Respiratory Acidosis

PaCO2 > 45 mmHg (hypoventilation)
Causes: COPD, obesity hypoventilation, neuromuscular disease, sedation

Type	Expected HCO3- compensation
Acute	↑ HCO3- = ΔPaCO2 / 10
Chronic	↑ HCO3- = 4 × (ΔPaCO2 / 10)

4. Respiratory Alkalosis

PaCO2 < 35 mmHg (hyperventilation)
Causes: pain, anxiety, sepsis, PE, fever, pregnancy, salicylate toxicity (early)

Type	Expected HCO3- compensation
Acute	↓ HCO3- = 2 × (ΔPaCO2 / 10)
Chronic	↓ HCO3- = 4 × (ΔPaCO2 / 10)

Mixed Acid-Base Disorders

If the calculated compensation differs from expected, a mixed (concurrent) disorder is present. Example: a patient in septic shock can have simultaneous respiratory acidosis (poor ventilation) and metabolic acidosis (lactic acidosis). Always compare measured vs. expected compensatory values using the formulas above.

Key Clinical Points

HCO3- on ABG is calculated - confirm with a serum chemistry panel
If osmol gap is elevated (measured - calculated osmolality > 10), consider toxic ingestion (methanol, ethylene glycol)
Pulse oximetry cannot detect hypoventilation in patients on supplemental oxygen - ABG/capnography is needed
ABG is the gold standard for oxygenation assessment and can identify carboxyhemoglobin and methemoglobin that SpO2 cannot

Sources: Current Surgical Therapy 14e, p. 1607-1610; Fishman's Pulmonary Diseases and Disorders; Murray & Nadel's Textbook of Respiratory Medicine

Make a visual flowchart poster for ABG interpretation steps

Generating Image

A clean, professional medical flowchart poster titled "ABG Interpretation: Step-by-Step" with the following sequential steps connected by downward arrows: STEP 1 box (blue): "Evaluate pH" → branches: pH < 7.35 = ACIDEMIA (red label), pH 7.35–7.45 = NORMAL (green label), pH > 7.45 = ALKALEMIA (orange label) STEP 2 box (blue): "Evaluate PaCO2 (Respiratory Component)" → Normal: 35–45 mmHg. Arrow note: "↑ PaCO2 = Respiratory Acidosis | ↓ PaCO2 = Respiratory Alkalosis" STEP 3 box (blue): "Evaluate HCO3– (Metabolic Component)" → Normal: 22–26 mEq/L. Arrow note: "↓ HCO3– = Metabolic Acidosis | ↑ HCO3– = Metabolic Alkalosis" STEP 4 box (blue): "Evaluate Base Excess" → Normal: –2 to +2. "Positive = Alkalosis | Negative (deficit) = Acidosis" STEP 5 box (blue): "Evaluate PaO2 (Oxygenation)" → Normal: 75–100 mmHg. "< 80 mmHg = Hypoxemia" Below that, a 2x4 summary table titled "The 4 Primary Disorders" with columns: Disorder | pH | PaCO2 | HCO3– Rows: Metabolic Acidosis (↓ ↓ ↓), Metabolic Alkalosis (↑ ↑ ↑), Respiratory Acidosis (↓ ↑ normal/↑), Respiratory Alkalosis (↑ ↓ normal/↓) Bottom section: Compensation Formulas box: • Winter's Formula (Met. Acidosis): PaCO2 = 1.5×[HCO3–] + 8 ± 2 • Met. Alkalosis: PaCO2 = 0.7×[HCO3–] + 20 ± 5 • Acute Resp. Acidosis: ΔHCO3– = ΔPaCO2/10 • Chronic Resp. Acidosis: ΔHCO3– = 4×(ΔPaCO2/10) White background, color-coded boxes, medical infographic style, clear readable fonts, suitable for a hospital reference poster

Here's your ABG interpretation flowchart poster. It covers all 5 steps in sequence, the 4 primary disorders at a glance, and the key compensation formulas - ready to print or use as a quick bedside reference.

This is a shared conversation. Sign in to Orris to start your own chat.