Week 5 Lesson36 Interpretation of Blood Gases_

Systems 2: Urinary Tract and Water Balance

Lesson 36: Interpretation of Blood Gases

Presented by Melinda J. Wilkerson, RSITAS, St. George's University, School of Veterinary Medicine, Grenada, West Indies.

Learning Outcome

By the end of this session, students will be able to:

• Integrate the significant use of blood gases in clinical practice to accurately identify and differentiate between primary and compensatory mechanisms of the most common clinically relevant examples of acid/base disturbances (VM520, UTWB 2-2.8).

Blood Sample and Tube Type for Blood Gas Analysis

Recommended Options include various blood types drawn for specific analyses:

• A. Venous blood and purple top tube: for complete blood count (CBC), plasma total protein, and fibrinogen measurements.

• B. Venous blood and red top tube: clot tube used for serology and chemistry tests.

• C. Arterial blood and purple top tube: employed for specific analyses not requiring heparin.

• D. Arterial blood and red top tube: generally avoided for blood gases due to lack of heparin.

• E. Arterial blood and green top tube: recommended option for blood gas analysis, using heparin to prevent clotting (samples must be obtained anaerobically to avoid artifactual results).

Assessing PO2

Activities influencing PO2 assessment include:

• Aerobic and anaerobic physical activities such as bungee jumping, running, hiking, and rowing, which can lead to variances in oxygen saturation levels.

• Other less strenuous activities such as video gaming and weight lifting, which may require less immediate oxygen uptake but still impact overall respiratory function.

• Environmental factors during activities such as ice fishing and swimming, where external temperatures can affect oxygen levels.

Factors Affecting pH in Respiratory Difficulty

In patients experiencing respiratory complications, a decrease in pH indicates:

• Analyte Contribution: A rise in PCO2 as a result of inadequate ventilation, leading to hypercapnia and consequent acidosis.

Acid Base Status Terminology

• Alkalemia: Defined as a blood pH greater than the established reference interval, often indicative of excess bicarbonate or reduced carbon dioxide.

• Acidemia: Defined as a blood pH lower than the reference interval, often indicative of metabolic or respiratory failures.

Common Acid Base Disorders:

1. Metabolic Alkalosis: Associated with increased bicarbonate concentration or total carbon dioxide, occurring with alkalemia and a corresponding increased pH.

2. Metabolic Acidosis: Characterized by decreased bicarbonate concentration or total carbon dioxide, resulting in acidemia (decreased pH).

3. Respiratory Acidosis: Indicated by elevated PCO2 coupled with a decrease in pH, frequently stemming from respiratory failure or hypoventilation.

4. Respiratory Alkalosis: Marked by lowered PCO2 and a resultant increase in pH, often due to hyperventilation or anxiety disorders.

Normal Metabolic Pathways and Acidosis

• Chemical Reaction Insight: H+ ions produced from cellular metabolism combine with bicarbonate to form carbonic acid, which dissociates into water and carbon dioxide, playing a crucial role in maintaining acid-base balance in the body.

• Emphasis on the significance of urine pH, influenced by dietary intake, metabolic activity, and homeostatic mechanisms that regulate blood [H+].

Steps of Blood Gas Interpretation

1. Confirm blood sample type: Whether arterial or venous to assess appropriate protocols in treatment.

2. Identify change in pH: Determine whether the patient is acidemic or alkalemic.

3. Determine analyte contribution: Focus on bicarbonate (HCO3) and carbon dioxide (PCO2) levels to establish the primary pathology affecting acid-base status.

4. Identify primary condition: Differentiate if the disturbance is metabolic or respiratory.

5. Determine compensation: Check if the pH is within reference intervals and if two analytes contribute to the acid/base condition, noting compensatory responses either by renal or respiratory pathways.

Summary: Classification of Acid-Base Disorders

Key Data for Acid-Base Disorders:

• Blood gas evaluations should accurately reflect bicarbonate levels alongside PCO2 variances.

• Clinical representations of acid-base disorders may include:

  • Respiratory acidosis: pH decreases, increased PCO2, HCO3 levels remain within reference interval.

  • Metabolic acidosis: pH decreases, normal PCO2 levels, decreased HCO3 levels indicate possible renal failure or metabolic derangement.

  • Respiratory alkalosis: pH increases, decreased PCO2, HCO3 levels remain within reference threshold, often presenting in conditions such as hyperventilation.

  • Metabolic alkalosis: pH increases, normal PCO2 levels, increased HCO3 levels, often due to excessive vomiting or diuretics usage leading to loss of acid.

Examples of Acid-Base Disorders

• Acidemia: pH < normal reflects potential reasons like decreased HCO3 or increased PCO2 levels platefully leading to various compensatory mechanisms, highlighting the role of the kidneys and lungs in maintaining homeostasis.

Hypoxemia and Hypoxia

• Hypoxemia: Refers specifically to the deficiency of dissolved oxygen in blood, clinically examined through arterial blood gas (decreased PaO2).

• Hypoxia: Represents a broader condition where tissues are deprived of adequate oxygen regardless of blood oxygen levels.

• Hematologic Issues: Conditions such as anemia can cause hypoxia despite normal or varied hypoxemia.

Gas Exchange

• Ventilation: The volume of air successfully reaching the alveoli is paramount for oxygen uptake.

• Perfusion: The volume of blood flow to the alveoli is equally crucial for optimal gas exchange.

• Proper gas exchange is essential for maintaining arterial PaO2 and PaCO2 levels, influencing overall patient health.

Clinical Case Studies

Example Case Evaluations:

• Dog 1: Demonstrated signs of sudden anemia leading to tachycardia and tachypnea, emphasizing the need for timely analyses of blood gas data.

• Dog 4: Investigated respiratory acidosis related to hypoventilation issues, necessitating interpretation of changes in blood gas measures.

• Overall Insight: The analysis of blood gas data is critical for diagnosing and interpreting acid-base disorders in clinical veterinary practice.

Break Time

• Encourage a 10-minute break to maintain focus and refresh, allowing students to process the information rigorously.

Next Steps & Questions

• Recommend reviewing materials in preparation for Lesson 37 to deepen understanding of content related to blood gases and acid/base balance.