BLOOD gasses

Overview of Arterial Blood Gases (ABG) and Oximetry

  • Arterial Blood Gases (ABGs) are typically drawn from arteries, providing critical information on:

    • Oxygenation levels

    • Ventilation status

    • Acid-base balance

  • Maintaining homeostasis is essential for cell survival. Disruption due to disease can lead to cell death.

Key Parameters of ABG

  • Understanding normal ranges for ABGs and oximetry is crucial:

    • ABG Parameters:

      • pH: 7.35-7.45 (normal)

      • PaCO2: 35-45 mmHg (normal)

      • Bicarbonate (HCO3): 22-26 mmol/L (normal)

    • Pulse Oximetry Parameters:

      • SP02 (saturation of peripheral oxygen) typically >90% for adequate oxygenation.

Indications for Testing

  • Indications for performing ABG and oximetry analysis include:

    • Patient exhibiting cyanosis

    • Abnormal physical exam findings suggesting respiratory or metabolic disturbances

    • Evaluation of treatment efficacy for oxygenation, ventilation, or acid-base status changes.

Measurement Techniques

  • Invasive Methods:

    • Blood sampling via needle puncture or indwelling arterial line.

  • Noninvasive Methods:

    • Pulse oximetry measuring peripheral oxygen saturation.

    • Transcutaneous monitoring for CO2 levels.

  • Pulse Oximetry vs. ABG:

    • Pulse oximetry (SP02) assesses oxygen saturation but does not provide CO2 levels or full acid-base status.

Procedure for Drawing ABGs

  • Precautions Before Puncture:

    • Review patient history and check for bleeding disorders or anticoagulant therapy (e.g., platelet count, INR).

    • Apply pressure for at least 3-5 minutes post-puncture unless otherwise indicated.

  • Preferred Site:

    • Radial artery (thumb side of wrist) due to ease of access and collateral circulation; other sites include femoral and brachial arteries.

  • Modified Allen Test:

    • Used to confirm collateral circulation; patients make a fist while both radial and ulnar arteries are compressed, then released to check for adequate blood flow returning to the hand.

Sample Collection and Handling

  • Ensure no air bubbles in the syringe to avoid false readings. Analyze the sample within 15 minutes or place it in ice if delayed to minimize metabolism-induced changes in results.

Oximetry Data Interpretation

  • Understand different indices for assessing oxygenation:

    • PaO2: Measure of the partial pressure of oxygen; correlates with overall lung function.

    • CaO2: Total oxygen content in the blood; includes both dissolved and hemoglobin-bound oxygen.

    • Hypoxemia Classification:

      • No hypoxemia: 80-100 mmHg

      • Mild: 60-79 mmHg

      • Moderate: 40-59 mmHg

      • Severe: < 40 mmHg

Acid-Base Status Evaluation

  • Using the Henderson-Hasselbalch Equation:

    • Relates changes in pCO2 and bicarbonate to pH.

  • Determine primary disturbance (respiratory or metabolic) based on pH, pCO2, and bicarbonate levels.

  • Normal ranges for interpretation:

    • pH < 7.35: Acidic

    • pH > 7.45: Alkaline

    • PaCO2 > 45: Respiratory acidosis

    • PaCO2 < 35: Respiratory alkalosis

    • HCO3 < 22: Metabolic acidosis

    • HCO3 > 26: Metabolic alkalosis

Compensatory Mechanisms and Disorders

  • Recognize common causes and expected blood gas findings in:

    • Simple respiratory and metabolic disorders

    • Combined and mixed acid-base disorders

  • Understand compensation processes in acid-base disturbances (e.g., pH returning to normal while one component remains abnormal).

Error Prevention and Validity of Measurements

  • Identify preanalytic, analytic, and postanalytic errors and mitigation strategies to ensure accurate ABG results and interpretations.

  • Key Methodologies:

    • Valid measurement relies on correct sample handling, timely analysis, and proper interpretation based on patient history and clinical presentation.

Carboxyhemoglobin and CO Poisoning

  • Carboxyhemoglobin indicates carbon monoxide binding to hemoglobin, impairing oxygen transport. Measurement via co-oximetry is essential in suspected CO poisoning cases.

  • Key facts:

    • Normal carboxyhemoglobin levels are typically <1%.

    • CO's affinity for hemoglobin is 200-250 times greater than oxygen.

Summary of Oxygenation Assessments

  • Recognize how pH, pCO2, and bicarbonate together reveal blood gas status; significant disturbance requires situational interpretation.

  • Incorporate factors that affect hemoglobin saturation to interpret clinical scenarios accurately.

  • Emphasize the importance of understanding and differentiating between conditions like hypoxemia and hypoxia.