Arterial Blood Gas (ABG) Analysis

• Arterial Blood Gas (ABG) analysis measures pH, oxygen (O₂), and carbon dioxide (CO₂) levels in arterial blood.
• ABG analysis assesses acid-base balance and lung function in gas exchange.

Pulmonary Gas Exchange

• ABGs help evaluate gas exchange effectiveness through partial pressure measurements.
• ABG is measured in kPa; to convert to mmHg, multiply by 7.5.
• Example: PaCO2 = 6 kPa equals 45 mmHg

Important Note on Partial Pressure

• PO₂ refers to the partial pressure of oxygen.
• PaO₂ refers to the partial pressure of oxygen in arterial blood.

ABG Sample Site Options

• Radial artery
• Ulnar artery
• Brachial artery
• Femoral artery
• Dorsalis pedis artery

ABG Analysis Five Main Components

• pH
• PaO2
• PaCO2
• Bicarbonate (HCO3)
• Base Excess

Why Use ABG Analysis?

• Establish diagnosis
• Assess illness severity
• Assess treatment effectiveness
• Monitor patient care

Basic Cellular Respiration

• Aerobic respiration: Oxygen + Glucose = Adenosine Triphosphate (ATP) + CO2 + H₂O
• Anaerobic respiration: Glucose = ATP + Lactic Acid

ABG Analysis: pH

• pH indicates blood acidity or alkalinity, determined by hydrogen (H+) ion concentration.

Normal Blood Gas pH

• Normal range: 7.35–7.45
• Below 7.35 indicates acidosis (increased H+).
• Above 7.45 indicates alkalosis (decreased H+).

Normal Partial Pressure of Oxygen

• PaO2 is the partial pressure of O₂ dissolved in plasma.
• 3% of oxygen is carried in plasma, and 97% is carried by hemoglobin.
• It indicates O₂ uptake and efficiency of gas exchange.
• The normal PaO2 in a fit individual is 11-13 kPa (80–100 mmHg).
• PaO2, as a rule of thumb, should be 10 kPa less than the air inspired
• Important to remember: a patient on 50% oxygen should not have a PaO2 of 11 kPa.

Partial Pressure of Carbon Dioxide

• PaCO2 is the respiratory component, the partial pressure of CO2 dissolved in blood.
• When dissolved in water, CO2 becomes carbonic acid
• If carbonic acid is not removed, it causes respiratory acidosis due to H+ accumulation.
• Normal PaCO2 range: 4.7–6.0 kPa (35-45 mmHg).

Compensation

• Compensation starts when the body tries to adjust blood pH.
• Respiratory compensation occurs rapidly, such as during an asthma attack.
• Metabolic compensation takes days, seen in diabetic ketoacidosis.
• Evidence of metabolic compensation with a respiratory issue could be from days of COPD.

Bicarbonate

• HCO₃ (bicarbonate) is the metabolic component
• Generated in the kidneys
• HCO3 buffers hydrogen ions (H+), producing CO2 and H2O
• Normal range: 22–26 mmol/l
• In acid load, the respiratory system increases CO2 excretion first.
• Kidneys are slower to respond.
• HCO3 decreases as it buffers extra H+ ions.
• Once reserves are used, H+ accumulates, and pH decreases

More on Bicarbonate

• Abnormal HCO3 that doesn't match pH suggests a respiratory cause, assessed via CO2.
• Kidney failure that prevents sufficient HCO3 production leads to increased H+, resulting in metabolic acidosis.
• Excessive HCO3 buffers H+ and may cause metabolic alkalosis.

Base Excess Definition

• Base excess (BE) measures acid or base needed to restore normal blood pH.
• It is also an indicator of metabolic acidosis or alkalosis.
• A negative BE signifies metabolic acidosis and a positive BE signifies metabolic alkalosis.
• BE indicates the amount of bicarbonate required to correct the metabolic component of pH.
• High (positive) base excess signifies increased blood bicarbonate, potentially indicating primary metabolic alkalosis or compensated respiratory acidosis.
• Low (negative) base excess signifies decreased blood bicarbonate, potentially indicating primary metabolic acidosis or compensated respiratory alkalosis.
• Bicarbonate mops up hydrogen ions due to increased C02 or increased metabolic components (like hormones).

Metabolic vs Respiratory

• If pH aligns with PaCO2, it's a respiratory issue.
• If pH aligns with HCO₃, it's a metabolic issue.

Mixed Acidosis and Alkalosis

• This is when one has both respiratory and metabolic acidosis/alkalosis
• CO₂ and HCO3- move in opposite directions
• For example, there is increased CO₂ and decreased HCO3- in mixed respiratory and metabolic acidosis
• Treatment corrects each primary acid-base disturbance.

Interpreting Results: Four-Step Approach

• Step 1: Check SpO2 and PaO2 to determine if the patient is receiving enough oxygen
• Step 2: Check the pH - is it high indicating alkalosis or low indicating acidosis?
• Step 3: Check for PaCO2 - high indicates respiratory acidosis, whereas low indicates respiratory alkalosis
• Step 4: Check for the level of HCO3- - is it increased (alkalosis) or high (acidosis)?

ABG Ranges

• PaO2: 11-13 kPa or 80–100 mmHg for a fit person in room air
• pH: 7.35–7.45
• PaCO2: 4.7–6.0 kPa or 35–45 mmHg
• HCO3: 22–26 mmol/l
• Base excess: Between -2 mmol/l to +2 mmol/l

Scenario 1

• A patient named Rita is experiencing shortness of breath post-surgery.
• RR = 46, SpO2 = 87%, with audible wheezing
• HR = 105, BP = 100/76, CRT = 3 seconds
• ABG without oxygen administration reveals pH 7.47, PaO2 8.2 kPa, PaCO2 4.1 kPa, HCO3 23, Base Excess +1
• pH is high, indicating alkalosis.
• PaO2 is decreased, therefore oxygenation is inadequate.
• PaCO2 is low = respiratory alkalosis
• HCO3 and base excess are normal
• Decreased H+ caused by hyperventilation.

Scenario 2

• Cheryl is admitted for emergency surgery and shows deterioration
• RR = 25, SpO2 = 87% after deteriorating on ward
• HR 120, BP = 70/45, CRT = 5 seconds
• High-flow oxygen at 15l/min via non-rebreather mask and 80% oxygen is administered.
• ABG Results: pH 7.31, PaO2 60 kPa, PaCO2 8.6 kPa, HCO3 22, Base Excess -2.
• pH is low, indicating acidosis.
• PaO2 is decreased, therefore oxygenation is inadequate.
• PaCO2 is high = respiratory acidosis.
• HCO3 and base excess are normal
• Increased H+ due to lactic acid caused by sepsis.

Scenario 3

• Paul, a type 1 diabetic, reports feeling unwell for three days in the Emergency Department
• RR = 30, SpO2 = 90%, equal bilateral air entry
• HR = 90, BP = 87/50, CRT = 4 seconds
• Pale, clammy, and cool.
• High-flow oxygen at 15l/min and 90% administered.
• ABG Results: pH 7.31, PaO2 70 kPa, PaCO2 4.1 kPa, HCO3 18, Base Excess -4
• pH is low, indicating acidosis.
• PaO2 is decreased, therefore oxygenation is inadequate.
• PaCO2 is low = respiratory alkalosis
• HCO3 is low = metabolic acidosis
• Metabolic acidosis with compensating respiratory alkalosis
• Increased H+ because of increased ketones.

Explore key indicators such as negative Base Excess (BE), PaO2, PaCO2, and HCO3. Understand how they shift in acid-base disorders, including respiratory and metabolic acidosis. Learn about normal ranges, compensation types, and the kidneys' role in bicarbonate regulation.