543 Week 2 Pulm Diagnostics 2 May 6

Arterial Blood Gas (ABG) Refresher

  • Useful for:
    • Determining acid-base disturbances
    • Assessing efficacy of ventilation
    • Evaluating overall oxygenation
  • ABG Format: pH / pCO2 / O2 / Bicarb (HCO3-)
    • This order is generally followed for documentation and understanding.

Normal Values:

  • pH: 7.35 - 7.45
    • Greater than 7.45: Alkalosis
    • Less than 7.35: Acidosis
  • pCO2: 35 - 45 mmHg (40 mmHg is the middle value)
  • Bicarbonate (HCO3-): Approximately 24 mEq/L

Acidosis vs. Acidemia & Alkalosis vs. Alkalemia

  • Acidosis/Alkalosis: Physiological process
  • Acidemia/Alkalemia: Actual blood state

Systems Controlling Acid-Base Homeostasis

  • Respiratory (Lungs)
  • Metabolic (Kidneys)

Respiratory Acid-Base Balance

  • High CO2: Respiratory Acidosis
  • Low CO2: Respiratory Alkalosis

Metabolic Acid-Base Balance

  • High Bicarbonate: Metabolic Alkalosis
  • Low Bicarbonate: Metabolic Acidosis

ABG Interpretation Steps

  1. Determine the pH: Acidotic or Alkalotic?
  2. Assess CO2: High indicates respiratory cause.
  3. Assess Bicarbonate: Low indicates metabolic acidosis.
  4. If pH is high (alkalotic):
    • Low CO2: Respiratory Alkalosis
    • High Bicarbonate: Metabolic Alkalosis

Clinical Examples

Example 1: Respiratory Acidosis
  • 45-year-old male, found down with agonal breathing.
  • Empty morphine bottle nearby (opioids depress respiratory drive).
  • ABG Results:
    • pH: 7.1 (Acidotic)
    • CO2: 88 (Very High)
    • Bicarb: 27 (Low)
  • Diagnosis: Respiratory Acidosis
Example 2: Respiratory Alkalosis
  • 19-year-old female, tingling sensations, panic attack history, final exam stress.
  • ABG Results:
    • pH: 7.68 (Alkalotic)
    • CO2: 20 (Low)
    • Bicarb: 22 (Normal)
  • Diagnosis: Respiratory Alkalosis
  • Likely hyperventilating, blowing off CO2.

Causes of Respiratory Acidosis

  • Hypoventilation (low respiratory rate or tidal volume)
  • CNS depression (trauma)
  • Airway obstruction (foreign body, laryngospasm)
  • Sleep apnea
  • Asthma
  • Pneumothorax
  • Pneumonia

Causes of Respiratory Alkalosis

  • Hyperventilation
  • CNS disorders
  • Pain
  • Anxiety
  • Salicylates (aspirin)
  • Sepsis
  • Hepatic failure

Metabolic Acidosis - Anion Gap

  • Normal Range: 8-12
  • High Anion Gap with Metabolic Acidosis: Anion Gap Metabolic Acidosis
  • Normal Anion Gap with Metabolic Acidosis: Non-Anion Gap Metabolic Acidosis

Causes of Non-Anion Gap Metabolic Acidosis

  • Bicarbonate loss (diarrhea)
  • Ileal conduit
  • Renal tubular acidosis
  • Acetazolamide therapy

Causes of Anion Gap Metabolic Acidosis - MUDPILS

  • Methanol
  • Uremia
  • Diabetic Ketoacidosis
  • Paraldehyde
  • Isoniazid
  • Lactic Acidosis
  • Salicylates

Compensation for Acid-Base Disorders

  • Lungs compensate for metabolic disturbances (rapid response).
  • Kidneys compensate for respiratory disturbances (takes days).
Expected Compensations in Metabolic Acidosis
  • CO2 is generally down. (Details covered in medicine lectures).

Oxygen-Hemoglobin Dissociation Curve

  • Shift to the Right: Decreased Hemoglobin Affinity for oxygen.
    • Increased CO2
    • Increased temperature
    • Oxygen is released.
  • Shift to the Left: Increased Hemoglobin Affinity for oxygen.
    • Decreased CO2
    • Decreased temperature
    • Oxygen binds more strongly to hemoglobin.

Ventilation-Perfusion (VQ) Scanning

  • Nuclear radiology scan to detect mismatch between ventilation and perfusion.
  • Radioactive chemicals inhaled and injected.
  • Measures distribution of gas and blood flow in the lungs.
  • Not a fast procedure.

Three Phases of VQ Scan:

  • Wash-in
  • Equilibrium
  • Wash-out
  • Looking for gas trapping or obstruction.
Ventilation Phase
  • Radioactive gas (Xenon) inhaled.
  • Monitors how gas distributes within lungs.
Perfusion Phase
  • Radioactive particles attached to albumin are injected.
  • Tracer travels through the right heart into pulmonary vasculature.
  • Areas not perfused indicate potential clots or blockage.
  • A cardiac notch is a normal variant.

Indications for VQ Scans

  • Diagnosing pulmonary embolism (less common now).
  • Pre-operative assessment for lung resection, lung volume reduction, or lung transplant.
  • Assessing suitability for single vs. bilateral lung transplant.

Pulmonary Angiography

  • Historically catheter-based, now more commonly CT-based.

Catheter-Based Pulmonary Angiography

  • Catheter inserted via femoral artery, advanced into pulmonary artery.
  • Contrast dye injected, visualized under fluoroscopy.
  • Invasive procedure with higher risk of bleeding.

CT Pulmonary Angiography (CTPA)

  • CT scan with contrast dye specifically targeting pulmonary vasculature.
  • Gold standard for diagnosing pulmonary embolism (PE).
  • Offers fine detail and is quick and easy to perform.

Identification of Pulmonary Embolism

  • CTPA can show blackening/darkened areas.
  • Extent of the clot, number of lobes affected can be determined/seen.

Pulmonary Embolism (PE) Treatment Considerations

  • Saddle PE cases are often candidates for Percutaneous Endovascular Reperfusion (PERC) therapy.
  • Catheter-directed thrombolysis and mechanical thrombectomy to remove clot.

Complications of Catheter-Based Pulmonary Angiography

  • Death: 0.5%
  • Severe cardiopulmonary compromise: requiring intubation or CPR.
  • Renal failure requiring hemodialysis: 0.3% (contrast-induced).
  • Renal failure not requiring hemodialysis: 0.9% (acute kidney injury).
  • Groin hematomas: potentially common if pressure not held adequately.
  • Blood transfusion: 0.2% (due to significant hematomas).

CT Angiography Benefits

  • Less invasive than catheter-based angiography.
  • Lower risk of complications.
  • Requires less contrast dye.
  • Faster results.