Blood Gas Analysis and Respiratory Failure

Interpretation of Blood Gas Analysis

• Intended Learning Outcomes: Students will understand arterial blood gas principles, acid-base disturbances, and generate differential diagnoses for respiratory and metabolic issues.

Respiratory Failure

• Definition: An acute medical emergency where the respiratory system fails in one or both gas exchange functions (oxygenation and carbon dioxide elimination).

Causes of Respiratory Failure

• Chest Wall Abnormalities: Chest wounds, severe chest deformities
• Lung Tissue Abnormalities: Acute respiratory distress syndrome (ARDS), pneumonia, pulmonary edema, pulmonary fibrosis
• Airflow Obstruction: Chronic obstructive pulmonary disease (COPD), asthma, bronchiectasis, muscle weakness
• Neuromuscular Diseases: Neuromuscular diseases, spinal cord injury,
• Other: Pulmonary embolism, drug/alcohol intoxication, obesity/sleep apnea

Symptoms of Respiratory Failure

• Shortness of breath (dyspnea):
• Rapid breathing (tachypnea):
• Cyanosis (bluish discoloration of lips, fingers, toes)
• Excessive sweating
• Confusion/agitation/restlessness
• Fatigue/drowsiness
• Headache
• Blurred vision

Types of Respiratory Failure

• Type 1 (Hypoxemic): Damage to the lung tissue prevents adequate oxygenation; PaO2 is low (<60mmHg); PaCO2 is normal or low (≤35mmHg)

• Type 2 (Hypercapnic): Alveolar ventilation is insufficient to remove carbon dioxide; PaO2 is normal or low (<60mmHg); PaCO2 is high (>45mmHg)

What is Arterial Blood Gas (ABG)?

• Purpose: Accurate measure of oxygen uptake and carbon dioxide removal by the respiratory system
• Sample Collection: Usually from radial artery or central line while patient is resting quietly with a constant inspired oxygen level (FiO2)
• Assessment: Oxygenation status, ventilation status, and acid-base balance

Blood Gas Data

• pH: Power of hydrogen, indicating acidity/alkalinity of blood
• PaO2: Partial pressure of oxygen in arterial blood
• PaCO2: Partial pressure of carbon dioxide in arterial blood
• HCO3-: Bicarbonate concentration in arterial blood
• BE: Base excess (amount of acid required to normalize blood pH)
• SaO2: Percentage of oxyhemoglobin in the blood

Acid-Base Balance

• Definition: The balance between acids and bases in the body. Maintained by respiratory and renal systems.
• Importance: Necessary for normal cellular and enzymatic functions.
• Normal pH Value: 7.35 - 7.45

Monitoring of Oxygenation Status

• Pulse oximetry (SpO2): Measures oxygen saturation in blood
• Partial pressure of oxygen (PaO2): Measured from ABG analysis; reflects blood oxygenation
• Transcutaneous PO2 (TcPO2): Used in pediatrics
• End expiratory oxygen analyzer (FEO2): Measures oxygen in exhaled gas

Partial Pressure of Oxygen (PaO2)

• Normal Value: 75-100 mmHg (or 10.5-13.5 kPa)
• Interpretation: Reflects blood oxygenation
• Hypoxemia: PaO2 < 75 mmHg (<10.5 kPa)
• Hyperoxemia: PaO2 > 100 mmHg (>13.5 kPa)

Monitoring of Expired CO2

• Transcutaneous CO2 monitoring (tcpCO2): Measures CO2 in the skin
• Partial pressure of carbon dioxide (PaCO2): From blood gas analysis, reflects alveolar ventilation
• End-tidal carbon dioxide (ETCO2): From capnography (a method to measure CO2 exhaled in the breath)

Partial Pressure of Carbon Dioxide (PaCO2)

• Normal Value 35–45 mmHg (or 4.7–6.0 kPa)
• Interpretation Reflects the state of alveolar ventilation; Hypocapnia (low PaCO2) associated with hyperventilation; Hypercapnia (high PaCO2) associated with hypoventilation

Hypercapnia vs. Hypocapnia

• Hypercapnia: Respiratory depression causes retention of CO2; hypoventilation; Symptoms: shallow/slow breathing, confusion/disorientation, lethargy/fatigue, headache, dizziness, nausea.
• Hypocapnia: Occurs with overbreathing that reduces CO2; hyperventilation; Symptoms: rapid breathing, dizziness/lightheadedness, weakness, numbness/tingling, muscle spasms, chest pain, palpitation

Bicarbonate (HCO3⁻)

• Nature: A byproduct of body metabolism, a base
• Regulation: Exhaled through the lungs and regulated by the kidneys; works with other electrolytes to balance the pH
• Normal value: 22-26 mmol/L

Base Excess (BE)

• Definition: Metabolic component of acid-base balance; calculated from blood pH and PaCO2. Represents the amount of acid needed to normalize 1 L of blood to its normal pH at a PaCO2 of 40mmHg.
• Metabolic Alkalosis: BE increases (more positive).
• Metabolic Acidosis: BE decreases (more negative).
• Normal value: -2 to +2 mEq/L

Acid-Base Balance & Buffer Systems

• Regulation: The body regulates acid-base balance using buffers, the respiratory system, and the renal system

• Buffer Action: Work to minimize changes in pH

• Respiratory System: Releases CO2 to regulate pH; controls ventilation rate and depth

• Renal System: Excretes excess acids or bases, requiring several days to take effect.

ABG Interpretation (Steps)

• Step 1: Assess pH (normal, acidic, or alkalotic?)
• Step 2: Evaluate PaCO2 (normal, high, or low?)
• Step 3: Evaluate HCO3⁻ (normal, high, or low?)
• Step 4: Determine if primary disturbance is respiratory or metabolic by matching either PaCO2 or HCO3 with pH
• Step 5: Assess compensation (does the other parameter move in the opposite direction of pH?)
• Step 6: Evaluate PaO2 and O2 saturation (normal, hypoxemic, or hyperoxemic?)

Implications for Physiotherapy

• Ventilation Modifications: Possible modifications to ventilation based on the status of the acid-base
• Compensation: Determine the degree of compensation for respiratory acidosis in patients.
• pH Range: Importance of maintaining within the normal range.