Anatomy of Pulmonary Shunting

Anatomic Shunting

• Causes of anatomic shunting:
  • Congenital heart disease
  • Intrapulmonary fistula
  • Vascular lung tumors

True Shunt

• True shunt encompasses both anatomic and capillary shunts.
• It is refractory to oxygen therapy.
• True shunt causes:
  • Alveolar collapse (atelectasis)
  • Alveolar fluid accumulation
  • Alveolar consolidation

Shunt-like Effect

• Occurs when capillary perfusion exceeds alveolar ventilation.
• Common causes:
  • Hypoventilation
  • Bronchospasm or mucus plugging
  • Alveolar-capillary membrane (ACM) diffusion defects
• Conditions are corrected with oxygen therapy.

Venous Admixture

• End result of pulmonary shunting.
• Shunt equation: Qs/Qt = (Cco2-Cao2) / (Cco2-Cvo2)
  • Qs = shunt flow
  • Qt = total cardiac output
  • Cco2 = oxygen content in mixed venous blood
  • Cao2 = oxygen content in arterial blood
  • Cvo2 = oxygen content in pulmonary capillary blood
• Necessary information for calculations:
  • Barometric Pressure (PB)
  • Partial Pressure of Oxygen in Arterial Blood (PaO2)
  • Partial Pressure of Carbon Dioxide in Arterial Blood (PaCO2)
  • Partial Pressure of Oxygen in Mixed Venous Blood (PvO2)
  • Hemoglobin (Hb)
  • Partial Pressure of Oxygen in Alveolar Air (PAO2)
  • Fractional Inspired Oxygen (FiO2)
  • Oxygen Saturation in Arterial Blood (SaO2)
  • Oxygen Saturation in Venous Blood (SvO2)

Case Study

• A 38 year old male on volume-cycled ventilation.
  • PB = unknown
  • FiO2 = 0.70
  • Hb = 13g%
  • PaO2 = 50 mmHg (SaO2 85%)
  • PaCO2 = 43 mmHg
  • PvO2 = 37 mmHg (SvO2 65%)
  • PAO2 = 438.35 mmHg
  • CcO2 = (Hb x 1.34) + (PAO2 x 0.003) - CcO2 = 18.73 vol%
  • CaO2 = 14.43 vol%
  • CvO2 = 11.43 vol%
  • Qs/Qt = (18.73 - 14.95) / (18.73 - 11.43) - 0.517 = 52% shunt

Clinical Significance

• Normal shunt: 30%
• High shunt above 30% can be life-threatening.

Factors Increasing Arterial-Venous Oxygen Content Difference (C(a-v)O2)

• Decreased Cardiac Output
• Periods of Increased O2 consumption:
  • Exercise
  • Seizures
  • Shivering
  • Hyperthermia

Factors Decreasing Arterial-Venous Oxygen Content Difference (C(a-v)O2)

• Increased Cardiac Output
• Skeletal Relaxation
• Certain Poisons
• Hypothermia

Factors Increasing Oxygen Consumption (VO2)

• Exercise
• Seizures
• Shivering
• Hyperthermia

Factors Decreasing Oxygen Consumption (VO2)

• Increased Cardiac Output
• Skeletal Relaxation
• Certain Poisons
• Hypothermia

Factors Increasing Oxygen Extraction Ratio (O2ER)

• Decreased Cardiac Output
• Periods of Increased O2 consumption
• Exercise
• Seizures
• Shivering
• Hyperthermia
• Anemia
• Decreased arterial oxygenation

Factors Decreasing Mixed Venous Oxygen Saturation (SvO2)

• Decreased Cardiac Output
• Periods of Increased O2 consumption
• Exercise
• Seizures
• Shivering
• Hyperthermia

Factors Increasing Mixed Venous Oxygen Saturation (SvO2)

• Increased Cardiac Output
• Skeletal Relaxation
• Certain Poisons
• Hypothermia.

Tissue Hypoxia

• Inadequate tissue oxygen levels for cellular metabolism.
• Types of tissue hypoxia:
  • Hypoxic hypoxia
  • Anemic hypoxia
  • Circulatory hypoxia
  • Histotoxic hypoxia

Hypoxic Hypoxia

• Also known as Hypoxemic Hypoxia.
• It can develop from pulmonary shunting and:
  • Low PAO2 (Hypoventilation, High Altitudes, Breathing gas mixtures with < 21% O2)
  • Diffusion Impairment (Interstitial fibrosis, alveolar consolidation, interstitial and/or alveolar edema)
  • Ventilation/Perfusion Mismatch (V/Q): Capillary blood flow is in excess of alveolar ventilation leading to shunt-like effect.

Anemic Hypoxia

• Normal PaO2, but O2 carrying capacity of the blood is inadequate.
• Causes:
  • Low Hb
  • Deficiency in the ability of Hb to carry O2 (CO poisoning)
• Main compensatory mechanism: Increase cardiac output

Circulatory Hypoxia

• Normal PaO2 and CaO2, but inadequate blood flow.
• Causes:
  • Stagnant Hypoxia
  • Arterio-venous shunting

Stagnant Hypoxia

• Peripheral capillary blood flow is slow, decreasing SvO2.
• Causes:
  • Decreased Cardiac output
  • Vascular insufficiency

Arterio-venous Shunting

• Blood bypasses tissue cells and moves into the venous system.
• No O2 delivery to the tissues.

Histotoxic Hypoxia

• Impairment of tissue cells ability to utilize O2.
• Causes:
  • Cyanide poisoning
• Normal PaO2 and CaO2 but tissue cells are hypoxic.
• PvO2, CvO2, and SvO2 are elevated because oxygen is not utilized.

Cyanosis

• Occurs in cases of severe hypoxemia.
• Visible purplish discoloration of mucous membranes, fingertips, and toes.
• Present when blood has at least 5g% of reduced Hb.
• Normal Hb 14-15g%, PaO2 97-100 mmHg, 20 vol%.

Polycythemia

• Abnormal increase in RBC number.
• Stimulation of erythropoietin by chronic hypoxemia in pulmonary disorders.
• Increased viscosity of the blood, higher pressure needed to overcome resistance causing right or left ventricular hypertrophy that can lead to heart failure.

Oxygen transport Studies

• Oxygen Transport studies measure the body's ability to transport oxygen.
• Key studies include: total oxygen delivery, arterial-venous oxygen content difference, oxygen consumption, oxygen extraction ratio, mixed venous oxygen saturation, and pulmonary shunting.

Total Oxygen Delivery (DO2)

• DO2 is the amount of oxygen delivered to the tissues per minute.
• It depends on the heart's ability to pump blood (cardiac output) and the oxygen-carrying capacity of the blood (hemoglobin concentration).
• DO2 is calculated using the formula: DO2 = QT x (CaO2 x 10), where QT is cardiac output, CaO2 is arterial oxygen content, and 10 is a conversion factor.

Arterial-Venous Oxygen Content Difference (C(a-v)O2)

• C(a-v)O2 is the difference between the oxygen content in arterial blood (CaO2) and venous blood (CvO2).
• It represents the amount of oxygen extracted by the tissues from each 100mL of blood.
• Normal C(a-v)O2 is 5 vol%, meaning 5mL of oxygen is extracted from each 100mL of blood.

Oxygen Consumption (VO2)

• VO2 is the amount of oxygen the tissues extract from the blood per minute.
• It is calculated using the formula: VO2 = QT [C(a-v)O2 x 10].
• VO2 is typically indexed to the patient's body surface area (BSA).

Oxygen Extraction Ratio (O2ER)

• O2ER represents the fraction of oxygen delivered to the tissues that is actually used.
• Also known as the oxygen coefficient ratio.
• Calculated using the formula: O2ER = C(a-v)O2/CaO2.
• Normally, 25% of delivered oxygen is metabolized by the tissues, resulting in a normal O2ER of .25.

Mixed Venous Oxygen Saturation (SvO2)

• SvO2 reflects the oxygen saturation in mixed venous blood, a good indicator of changes in C(a-v)O2, VO2, and O2ER.
• Normal SvO2 is 75%, but values above 65% are acceptable.

Pulmonary Shunting

• Portion of the cardiac output that bypasses the lungs, either not participating in gas exchange (true shunt) or exchanging gas but not obtaining a normal PaO2 (shunt-like effect).
• Causes hypoxemia.

Types of Shunts

• Anatomic Shunt: A small portion of the cardiac output (2-5%) bypasses the lungs through bronchial, pleural, or Thebesian veins.
• Capillary Shunt: Caused by alveolar collapse, fluid accumulation, or consolidation.
• True shunt: Combined anatomic and capillary shunts, is refractory to oxygen therapy.
• Shunt-like effect: When blood flow exceeds alveolar ventilation, caused by hypoventilation, bronchospasm, or diffusion defects. It is generally correctable with oxygen therapy.

Venous Admixture

• The end result of pulmonary shunting.

Shunt Equation

• Qs/Qt:Cco2-Cao2/Cco2-Cvo2
• Requires several variables: partial pressure of blood gases, hemoglobin concentration, and oxygen saturations.

Clinical Significance of Shunt

• Normal shunt is around 3% of cardiac output.
• Shunt greater than 30% can be life-threatening.

Factors Influencing Oxygen Consumption

• Increased Oxygen Consumption: Exercise, seizures, shivering, hyperthermia.
• Decreased Oxygen Consumption: Increased cardiac output, skeletal relaxation, certain poisons, hypothermia.

Tissue Hypoxia

• Inadequate oxygen supply for cellular metabolism.
• Types:
  • Hypoxic hypoxia: Low PaO2 and CaO2.
  • Anemic Hypoxia: Normal PaO2, but impaired oxygen carrying capacity of the blood.
  • Circulatory Hypoxia: Normal PaO2 and CaO2, but inadequate blood flow.
  • Histotoxic Hypoxia: Impaired ability of tissue cells to utilize oxygen.

Cyanosis

• Purplish discoloration of the skin and mucous membranes due to low blood oxygen levels.
• Present when blood has at least 5g% of reduced hemoglobin.

Polycythemia

• Increased number of red blood cells.
• Occurs in response to chronic hypoxemia, increasing the blood's viscosity and leading to potential heart strain.