ABG’S

Differential Diagnosis of Hypoxemia

• Low PaO2 and high A-aDO2 indicates hypoventilation, diffusion defect, or V/Q mismatch
• V/Q mismatch responds favorably to positioning and CPAP/PEEP application

Diffusion Defect Case

• Patient presents with a history of left heart failure, respiratory distress, and crackles in the bases
• ABG results show low PaO2 consistent with a diffusion defect and improve after oxygen therapy
• Interpretation of ABG results is based on comparing PaO2, A-aDO2, PaCO2 and response to oxygen therapy
• Diffusion defects respond well to oxygen therapy
• Pulmonary edema, specifically related to left heart failure, is a leading cause of diffusion defect

Hypoventilation Case

• Patient presents with respiratory distress, a low pH, and increased PaCO2.
• The patient's condition improves after receiving oxygen therapy.
• Hypoventilation, a common cause of hypoxia, can be diagnosed by examining PaO2, A-aDO2, and PaCO2.
• Oxygen therapy is effective in improving hypoventilation and PaCO2 levels.

Shunting Case

• Shunting is a condition where blood bypasses the alveoli and mixes with oxygenated blood.
• Shunting is difficult to correct even with oxygen therapy.
• Severe respiratory issues are characterized by increased A-aDO2 difference.
• Shunting does not always improve with oxygen therapy.

Clinical Assessment of Oxygenation

• Hypoxia is a condition where tissues are not adequately oxygenated.
• Hypoxia can be characterized by shortness of breath, tachycardia, tachypnea, hypertension, and cyanosis.
• Hypoxemia can be present even with a normal A-aDO2.
• An FiO2 greater than 0.60 is highly suggestive of a ventilation-perfusion mismatch issue.
• Hypoxia can occur even with a normal PaO2 if the patient is anemic because oxygen content is reduced.
• A pulse oximeter cannot distinguish carboxyhemoglobin from oxyhemoglobin.
• Oxygen therapy can be effective for hypoventilation or diffusion defects, but may not work for patients with shunting.

Alveolar Air Equation

• PAO2 is the partial pressure of oxygen in the alveoli.
• The equation is used to calculate PaO2 and assess the effectiveness of oxygen therapy.

PaO2

• A PaO2 below 80 mmHg is considered hypoxemia.
• Hypoxia is a broader term referring to inadequate tissue oxygenation.

SpO2 Measurement

• Conventional pulse oximeters cannot distinguish between oxyhemoglobin and carboxyhemoglobin.
• Advanced pulse oximeters utilize multiple wavelengths of light to better measure oxygen saturation.

Clinical Assessment of Tissue Oxygenation

• Tissue oxygenation is better assessed through physical exam rather than ABG results.
• The most accurate indicator of oxygenation is CaO2, which depends on hemoglobin levels.

Anemic Patient Case

• A patient with a normal PaO2 but reduced hemoglobin levels is hypoxic.
• Anemia lowers the oxygen-carrying capacity of the blood, leading to hypoxia even with a normal PaO2.

Complications of ABG sampling

• Thrombosis: Clot formation can block blood flow.
• Hemorrhage: Blood loss can occur in patients on anticoagulants or with low platelet counts.
• Hematoma: Blood leakage into the tissues is common in elderly individuals.
• Arteriospasm: Spasms caused by pain can affect results.
• Pain: Sampling can be uncomfortable.
• Infection: Contamination can occur during sampling.
• Peripheral nerve damage: Injury to nerves can happen.
• Vasovagal responses: Can trigger distress, anxiety, nausea, and respiratory distress.

ABG Arterial Line Placement

• Indwelling Arterial Catheter: Used for frequent sampling and continuous BP monitoring during acute hypotensive crisis.
• Complications: Infection and thrombosis.

Interpretation of Blood Gas

• ABG analysis reveals:
  • Oxygenation status (PaO2, SaO2, CaO2, PvO2)
  • Acid-base balance (pH, PaCO2, HCO3-, BE)
  • Adequacy of ventilation (PaCO2)

Assessment of Oxygenation

• Involves evaluating oxygen content in the blood and tissue oxygenation.
• Oxygen transport mainly occurs through binding to hemoglobin.

Partial Pressures of Oxygen: PaO2

• PaO2: Pressure of dissolved oxygen in arterial blood.
• Reflects the lungs' ability to transfer oxygen from inspired gas into circulating blood.
• PaO2 is influenced by factors such as:
  • Partial pressure of oxygen in inspired air
  • Lung function (age, disease status)
  • Barometric pressure

Partial Pressure of Oxygen in Arterial Blood: PaO2

• Normal value: 80-100 mmHg on room air.

Arterial Blood Gases (ABG’s)

• ABGs measure acid-base balance and lung function.
• Venous blood analysis is less informative for oxygenation assessments.
• ABG analysis should be performed only when clinically indicated.

Arterial Blood Sampling

• Always review:
  • Physician's order
  • Patient's condition
• Modified Allen's Test: Evaluates collateral circulation before radial artery sampling.
• Sample: Obtained by inserting a needle into a major artery (arteriotomy), commonly the radial, dorsalis pedis, brachial, or femoral artery

Arterial vs Venous Blood

• Arterial blood provides more information about oxygenation, acid-base balance, and lung function.

ABG Sampling Sites

• Radial: Most common site.
• Dorsalis pedis: Used when radial artery is inaccessible.
• Brachial: Used in emergencies or when radial and dorsalis pedis are not accessible.
• Femoral: Used in situations where other sites are not accessible, but carries higher risk of complications.

Modified Allen's Test

• Assesses collateral blood flow.
• Positive test indicates adequate collateral blood supply.
• Negative test suggests poor collateral circulation, requiring investigation of alternative sampling sites.

Arterial Blood Sampling Procedure

• Remove air bubbles from the sample before analysis.
• Store samples in an ice bath and deliver to the lab within 30 minutes, especially for patients with elevated white blood cell counts.
• Point-of-care devices analyzed immediately provide rapid results.
• Apply pressure to the puncture site until bleeding stops.

This quiz explores the differential diagnosis of hypoxemia, focusing on conditions like V/Q mismatch and diffusion defects. Through case studies, learners will interpret ABG results and understand how treatments like oxygen therapy affect patient outcomes. Ideal for students in advanced respiratory medicine.