Gas Exchange

Gas Exchange

  • For efficient gas exchange:

    • Each region of the lung must receive equal amounts of ventilation and pulmonary perfusion to its alveoli.

    • If ventilation (V) and perfusion (Q) are mismatched, gas exchange is diminished, especially for oxygen.

  • The relationship between ventilation and perfusion is denoted as V/Q ratio, explaining the proportion of ventilation to perfusion for a specific lung region.

Distribution of Pulmonary Perfusion

  • Perfusion is preferentially distributed to gravity-dependent lung regions.

Zones of Pulmonary Perfusion

  • Zone 1: Absence of perfusion.

  • Zone 2: Sporadic perfusion; depends on the difference between alveolar pressure (PA) and arterial pressure (Pa).

  • Zone 3: Constant perfusion; majority of pulmonary perfusion takes place here.

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Zone Descriptions

  • Zone 1:

    • Conditions: PA > Pa; normal humans do not experience pulmonary capillary collapse.

  • Zone 2:

    • Conditions: Pa > PA > Pv; flow depends on the pressure difference.

  • Zone 3:

    • Conditions: Pa > Pv > PA; this zone has the most perfusion.

Distribution of Ventilation

  • Ventilation distribution is not uniform due to gas following the path of least resistance.

  • Functionally Residual Capacity (FRC):

    • Gas remaining in the lungs after exhalation.

    • At normal FRC, more gas remains in upper lung zones than in lung bases.

  • Most of the inspired tidal volume (Vt) is directed to the more compliant alveoli, typically located at the lung bases.

Transpulmonary Pressure

  • Definition: Transpulmonary pressure is the net distending pressure applied to the lung by the inspiratory muscles.

    • It is the positive pressure that keeps the lungs expanded.

  • Calculation:

    Transpulmonary pressure=PAlv−PplTranspulmonary pressure=PAlv−Ppl

  • Intrapleural pressure (Ppl) is negative and lower in the apices than in the bases, contributing to the distention of the alveoli.

Flow of Gas

  • Gas distribution is impacted by changes in lung compliance and resistance:

    • Bronchospasm, secretions in lower airways, and atelectasis particularly affect FRC.

  • Use of mechanical ventilation can alter gas flow dynamics.

  • Airway closure can lead to distribution changes.

Ventilation/Perfusion Ratio

  • Definitions:

    • VAVA​ = alveolar ventilation

    • QCQC​ = pulmonary capillary blood flow

  • Normal resting values:

    • VA≈4 L/minVA​≈4L/min

    • QC≈5 L/minQC​≈5L/min

  • V/Q ratio:

    VA/QC≈0.8VA/QC≈0.8

Ventilation/Perfusion Relationships: Deadspace

  • Deadspace: Ventilation that does not contribute to gas exchange, referred to as "wasted ventilation."

Types of Deadspace

  • Anatomic Deadspace:

    • Air that does not reach the alveoli, held in the conducting airways.

  • Alveolar Deadspace:

    • Air reaching alveoli that lacks perfusion.

  • Mechanical Deadspace:

    • Tubing where gas remains until the next breath occurs.

Physiologic Deadspace

  • Physiologic deadspace combines both anatomic and alveolar deadspace and can be expressed as a percentage of tidal volume.

Equation for Physiologic Deadspace

VD/VT=(PaCO2−PECO2)/PaCO2VD/VT=(PaCO2−PECO2)/PaCO2​

  • Where:

    • PaCO2PaCO2​ = arterial carbon dioxide partial pressure

    • PECO2PECO2​ = expired carbon dioxide.

  • Normal values for physiologic deadspace percentage:

    • Spontaneously breathing: 20%-40%

    • Mechanically ventilated: 40%-60%

Physiologic Deadspace Example Calculation

  • Given:

    • PaCO2=42 mmHgPaCO2​=42mmHg

    • PECO2=30 mmHgPECO2​=30mmHg

  • Calculation:

    VD/VT=(PaCO2−PECO2)/PaCO2×100VD/VT=(PaCO2−PECO2)/PaCO2​×100

    =(42−30)/42×100≈28.6 %=(42−30)/42×100≈28.6%

Deadspace Disorders

  • Increased work of breathing (WOB) may occur due to wasted ventilation, characterized by:

    • Anatomic deadspace: Rapid, shallow breathing.

    • True deadspace: Present in pulmonary emboli and decreased cardiac output.

    • Relative deadspace: Commonly seen in COPD and positive pressure ventilation.

  • Deadspace can be reduced with bronchodilators and positive end-expiratory pressure (PEEP).

Ventilation/Perfusion Relationships: Shunt

  • Definition: Shunt is the passage of blood through the lungs without engaging in external respiration, resulting in blood entering and leaving the lungs with identical gases.

Types of Shunt

  1. True/Absolute Shunt:

    • Absolute blood flow without ventilation.

  2. Anatomic Shunt:

    • Definition: Blood that bypasses pulmonary capillaries; 2% of cardiac output.

    • Examples: Thebesian veins, pleural veins, bronchial veins leading to right-left shunts via congenital abnormalities or vascular lung tumors.

Capillary (Intrapulmonary) Shunt

  • Definition: Blood passes through an alveolar-capillary unit that lacks fresh alveolar ventilation, often seen in collapsed or fluid-filled alveoli.

  • True (Absolute) Capillary Shunt:

    • V/Q = 0; does not respond to oxygen treatment.

    • Examples: Atelectasis and pneumonia lead to silent units.

  • Relative Capillary Shunt:

    • V/Q between 0 and 1; responds to oxygen therapy but insufficient oxygen is present to fully oxygenate blood.

Physiologic Shunt

  • Defined as the combination of all shunt types: anatomic + relative + true capillary shunt.

  • Normal values for shunt percentages:

    • True capillary shunt: 0%

    • Relative capillary shunt: 1%

    • Anatomic shunt: 2%

    • Normal physiologic shunt: 3% of cardiac output.

Physiologic Shunt Calculation

  • Equation:

    Qsp=(CcO2−CaO2)⋅QT(CcO2−CvO2)Qsp=(CcO2−CvO2)(CcO2−CaO2)⋅QT​

  • Where:

    • QspQsp​ = volume of blood per minute that is shunted physiologically,

    • QTQT​ = total cardiac output.

Oxygen Content of Capillary Blood

  • Formula:

    CcO2=(Hb⋅1.34⋅1.0)+(PAO2⋅0.003)CcO2=(Hb⋅1.34⋅1.0)+(PAO2⋅0.003)

  • Where:

    • HbHb = hemoglobin concentration

    • PAO2PAO2​ = partial pressure of oxygen in the alveoli.

Physiologic Shunt Example Calculation

  • Given the information to calculate the physiologic shunt:

    • PaO2=145 mmHgPaO2​=145mmHg

    • PvO2=34 mmHgPvO2​=34mmHg

    • SaO2=99 %SaO2​=99%

    • SvO2=63 %SvO2​=63%

    • Hb=13 g/dLHb=13g/dL

  • Calculation steps:

  1. Calculate arterial content of oxygen (CaO2).

    CaO2=(13×1.34×0.99)+(145×0.003)CaO2=(13×1.34×0.99)+(145×0.003) CaO2=17.25+0.435=17.69 mL/dLCaO2=17.25+0.435=17.69mL/dL

  2. Calculate mixed venous content of oxygen (CvO2).

    CvO2=(13×1.34×0.63)+(34×0.003)CvO2=(13×1.34×0.63)+(34×0.003) CvO2=10.97+0.102=11.07 mL/dLCvO2=10.97+0.102=11.07mL/dL

  3. Use PAO2 formula and calculate:

    PAO2=FiO2(PB−47)−PaCO2RQPAO2=FiO2(PB−47)−RQPaCO2​​

  4. Finally, calculate shunt fraction (Qsp/QT).

Clinical Significance of Pulmonary Shunt

  • Shunting % – Clinical Significance – Treatment:

    • < 10%: Normal lung status; no treatment necessary.

    • 10% - 20%: Insignificant pulmonary abnormality; consider supplemental oxygen.

    • 20% - 30%: May be life-threatening; require supplemental oxygen, non-invasive ventilation (NIV), consider intubation.

    • > 30%: Serious, life-threatening; requires cardiopulmonary support, intubation, high PEEP, and cardiac drug support.

  • Increased shunt occurrences seen in conditions such as pneumonia, hypoventilation, bronchospasm, emphysema, congestive heart failure (CHF), ARDS, congenital heart defects.

Causes of Hypoxia

  • Various types and causes of hypoxia including:

    • Hypoventilation

    • Absolute shunting

    • Relative shunting

    • Diffusion defects

Etiologies of Hypoxemia

  • Examples of conditions leading to hypoxemia:

    • Impaired diffusion (e.g., pulmonary fibrosis).

    • V/Q mismatch (e.g., pulmonary edema, interstitial lung diseases).

    • Shunt (e.g., R→L intracardiac shunt, pneumonia, COPD, ARDS/ALI, pulmonary embolism).

Determining Shunt: P(A-a)O2

  • Definition: Alveolar-arterial oxygen tension gradient.

  • Use: Quantifies the efficiency of oxygen transfer, assessing the degree of shunting and V/Q mismatch.

  • Interpretation: An increased P(A-a)O2 indicates physiologic shunting.

  • Calculation:

    P(A−a)O2=(FiO2(PB−PH2O)−PaCO2)−PaO2P(A−a)O2=(FiO2(PB−PH2O)−PaCO2)−PaO2

Determining Shunt: PaO2/PAO2

  • Definition: Ratio of arterial O2 pressure to alveolar O2 pressure, evaluating oxygen transfer efficiency.

  • Stability: More stable during changing FiO2 levels than P(A-a)O2.

  • Normal limit: Acceptable values \ge 0.75 indicate sufficient transfer of alveolar oxygen to arterial blood.

Determining Shunt: PaO2/FiO2 Ratio

  • Definition: Ratio of arterial oxygen pressure to inspired oxygen concentration.

  • Normal is 380-475 (PaO2 of 80-100 mmHg with FiO2 of 0.21).

  • Categories:

    • PaO2/FiO2 < 300 = mild ARDS

    • PaO2/FiO2 < 200 = moderate ARDS

    • PaO2/FiO2 < 100 = severe ARDS.

  • Notes: Changes in PaCO2 may provide false low values concerning the shunt indicator.

Hypoxia Classifications: HASH Mnemonic

  • Types of hypoxia include:

    • Hypoxic hypoxia: Related to low alveolar oxygen levels (PAO2).

    • Anemic hypoxia: Related to reduced hemoglobin concentration.

    • Stagnant hypoxia: Resulting from hypoperfusion.

    • Histotoxic hypoxia: Related to cellular utilization failures (e.g., poisons).

Additional Notes on Hypoxia Types

  • Hypoxic Hypoxia:

    • Causes include hypoventilation, shunting, and V/Q mismatch.

  • Anemic Hypoxia:

    • Causes are hemoglobin loss or dysfunction, treated by blood transfusion.

  • Stagnant Hypoxia:

    • Related to decreased blood flow; oxygen therapy is beneficial but limited.

  • Histotoxic Hypoxia:

    • Caused by poisoning of cellular mechanisms; treated with specific antidotes.

Diffusion Defects

  • Key concept: Sufficient time for gas diffusion is critical for equilibrium between alveolar and pulmonary capillary blood.

  • Factors affecting diffusion:

    • Thickening of the alveolar-capillary membrane increases diffusion time (e.g., pulmonary edema).