Respiratory Physiology: VQ Mismatches and Disorders

Perfusion and VQ Mismatches

• Ideal Alveolus: Ideally, an alveolus should receive an equal amount of ventilation (V) and pulmonary capillary blood flow (Q).

• Reality: Even in a perfect lung, ventilation and perfusion are not equal.

  • Normal alveolar ventilation: Approximately 4 liters per minute.

  • Normal capillary blood flow (perfusion): Approximately 5 liters per minute.

• Overall Average VQ Ratio: The normal average ratio of ventilation to blood flow is \frac{4}{5} or 0.8. This ratio is a heavily tested concept.

VQ Ratio Throughout the Lungs

• The overall ratio of 0.8 varies throughout the lungs due to gravity and body position.

• Upright Position:

  • More perfusion in the base of the lungs.

  • Alveolar ventilation is moderately increased in the lower regions, but blood flow is greatly increased.

  • This results in the VQ ratio being lower than 0.8 in the bases and progressively decreasing from top to bottom.

  • In the apices (top) of the lungs, there is less blood flow, leading to a higher VQ ratio.

Effect of VQ Ratio on Alveolar Gas Pressures

• The VQ ratio profoundly affects oxygen (PO2) and carbon dioxide (PCO2) pressures in the alveoli (PAO2 and PACO2).

• Normal Alveolar Pressures: Typically, normal PAO2 is approximately 100\text{ mmHg} and PACO2 is approximately 40\text{ mmHg}. These are averages representation, not true for every alveolar unit.

• PAO2 Determination: Determined by the balance between oxygen entering the alveoli and its removal by capillary blood flow.

• PACO2 Determination: Determined by the balance between carbon dioxide diffusing into the alveoli from capillary blood and its removal by ventilation.

High VQ Ratio

• Mechanism: Occurs when ventilation is high relative to perfusion (e.g., increased ventilation, decreased blood flow).

• Effects:

  • Oxygen: Increases the amount of O2 entering the alveolus, increasing alveolar PO2.

  • Carbon Dioxide: Increases the amount of CO2 that washes out, decreasing alveolar PCO2.

• Body's Response: The body automatically starts breathing faster to blow off more CO_2 and gain more oxygen.

• Low Blood Flow Impact: Decreases the O2 diffusion rate and reduces the amount of CO2 returning to the alveolus, which is undesirable as CO_2 needs to be breathed out.

Low VQ Ratio

• Mechanism: Occurs when ventilation is low relative to perfusion (e.g., decreased ventilation, normal/high blood flow).

• Effects:

  • Oxygen: Alveolar PO2 falls because O2 moves out of the alveolus into the pulmonary capillary blood faster than it's replenished by ventilation.

  • Carbon Dioxide: Alveolar PCO2 rises because CO2 moves out of capillary blood into the alveolus faster than it's washed out by ventilation.

• This is an undesirable outcome, as we want high O2 and low CO2.

Pulmonary Capillary Blood Gases and Mixing

• Regional Differences (Upright Lung):

  • Apex (Top): PaO2 = 130\text{ mmHg}, PaCO2 = 30\text{ mmHg}. The capillary blood (PCCO2) will also reflect these pressures initially.

  • Base (Bottom): PaO2 = 80\text{ mmHg}, PaCO2 = 46\text{ mmHg}.

• Venous Admixture: Blood from the upper and lower lung regions mixes as it flows into the pulmonary veins and then to the heart. This mixing lowers the overall arterial PO2 and raises PCO2 values compared to the ideal alveolar gases.

• Result: This is a major reason why we never achieve 100\% oxygen saturation on room air. For example, mixing blood with PaO2 = 100 and PaO2 = 40 might result in an average of approximately 85\text{ mmHg}.

Respiratory Quotient (RQ) and Respiratory Exchange Ratio (RER)

• Respiratory Quotient (RQ): The ratio between the volume of O2 consumed (V{O2}) and the CO2 produced (V{CO2}).

  • Formula: RQ = \frac{V{CO2}}{V{O2}}

  • Example: \frac{200\text{ mL/min}}{250\text{ mL/min}} = 0.8

  • For healthy individuals, the RQ is typically 0.8.

• Respiratory Exchange Ratio (RER): The quantity of O2 and CO2 exchanged during a period of one minute.

  • It is a ratio definition.

• Relationship: Normally, the respiratory exchange ratio and respiratory quotient are equal.

• Other Definitions for Matching:

  • External Respiration: Gas exchange between pulmonary capillaries and the alveoli.

  • Internal Respiration: Gas exchange between systemic capillaries and tissues (not explicitly defined in transcript but mentioned as a matching option).

Respiratory Disorders Affecting the VQ Ratio

Disorders that Increase the VQ Ratio (Low Perfusion or High Ventilation)

• Low Perfusion: Conditions where blood flow is diminished.

  • Pulmonary Embolism: Partial or complete obstruction in the pulmonary artery or arterioles.

  • Atherosclerosis: Hardening and narrowing of arteries, commonly seen in elderly individuals.

  • Extrinsic Pressure on Pulmonary Vessels: Conditions that compress vessels from outside.

    • Pneumothorax: Air in the pleural space.

    • Hydrothorax: Fluid in the pleural space.

    • Tumor: Any growth compressing vessels.

  • Destruction of Pulmonary Vessels: Loss of capillaries.

    • Emphysema: Hyperinflated air sacs (blebs) that eventually burst and damage surrounding vessels.

  • Decreased Cardiac Output: Any condition that reduces the heart's pumping ability, leading to reduced overall perfusion.

• Compensatory Mechanism: The body increases ventilation to compensate for low perfusion, attempting to maintain homeostasis.

Disorders that Decrease the VQ Ratio (Low Ventilation or High Perfusion)

• Low Ventilation: Conditions where air movement into/out of the alveoli is impaired (perfusion may be normal or higher relative to ventilation).

  • Obstructive Lung Disorders (Obstructive to Flow): Characterized by difficulty exhaling air.

    • Cystic Fibrosis (CF)

    • Bronchiectasis

    • Asthma

    • Bronchitis

    • Emphysema

    • (Mnemonic: CBAVE or CBABD)

  • Restrictive Lung Disorders (Restrictive to Volume): Characterized by difficulty inhaling air due to reduced lung expansion.

    • Pneumonia

    • Silicosis

    • Pulmonary Fibrosis: A severe, often terminal, disease where lung tissue becomes scarred and thickened, making gas exchange impossible.

  • Hypoventilation from Any Cause:

    • Neuromuscular Diseases: Affecting respiratory muscles (e.g., Muscular Dystrophy, ALS, Guillain-Barré, Myasthenia Gravis).

    • It's generally not advisable to put patients with severe pulmonary fibrosis on a ventilator, as it's unlikely to help due to irreversible lung damage.

Alveolar VQ Mismatch Units

• Normal Unit: PAO2 = 100\text{ mmHg}, PACO2 = 40\text{ mmHg}.

• Shunt Unit (Low VQ):

  • Represents areas with perfusion but no or very little ventilation (e.g., blocked airway, consolidated alveolus).

  • Typical values: PAO2 = 40\text{ mmHg}, PACO2 = 46\text{ mmHg}.

  • Key characteristic: Cannot be corrected by administering 100\% oxygen or increasing Positive End-Expiratory Pressure (PEEP).

  • Causes venous admixture, meaning venous blood (low O2, high CO2) is sent back to the heart without proper gas exchange.

• Dead Space Unit (High VQ):

  • Represents areas with ventilation but no or very little perfusion.

  • Typical values: PAO2 = 150\text{ mmHg}, PACO2 = 0\text{ mmHg}.

  • CO_2 of zero is detrimental for brain perfusion pressure.

  • Often correctable (e.g., addressing a pulmonary embolus).

Clinical Applications and Patient Management

• Patient Positioning: Always position patients at least at a 30 degree head-of-bed elevation (semi-Fowler's position) unless contraindicated.

  • Rationale: Prevents reflux and significantly reduces the risk of aspiration pneumonia and Ventilator-Associated Pneumonia (VAP).

• VAP Bundles: A set of evidence-based interventions to reduce VAP incidence.

  • Oral care.

  • Blood sugar management (high blood sugar increases pneumonia risk).

  • Prophylactic anti-reflux medication (e.g., Protonix, Pepcid).