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SF3 - Test 2 > 1 - V/Q Matching ( I & II ) > Flashcards

1 - V/Q Matching ( I & II ) Flashcards

1
Q

Objectives: Explain ramification of V/Q mismatches

A
  • V = Ventilation
  • Q = Perfusion (Blood Flow)
  • V/Q = Ratio of ventilation to perfusion; which can be used to represent gas exchange
  • Ideal: 0.8
  • Mismatch: Impaired Gas Exchange
    • ​Airway Obstruction
      • ​V/Q = Zero (R-L Shunt)
      • No Gas Exchange occuring (no V)
      • Blood PO2, PCO2 values ~ venous blood
      • Increased A-a Gradient
    • Pulmonary Embolism
      • ​V/Q = ∞ (Deadspace)
      • No Gas Exchange occuring (no Q)
      • PO2, PCO2 values ~ inspired air
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2
Q

Objectives: What are A-a gradients?

A
  • Measure of efficiency of transfer of O2 from Alveoli to Systemic (arterial) Circulation
  • Equation: A-aGradient = PAO2 - PaO2
    • PAO2 = PO2 in Alveolar Gas
    • PaO2 = PO2 in systemic Arterial Blood
  • Values:
    • Normal: 5 ~ 10
    • Abnormal: >15 = poor exchange
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3
Q

Objectives: What are anatomical and intrapulmonary shunts?

A
  • Anatomical: Does NOT rise form pathologic state
    • Ex: Bronchial Circulation (R-L Shunt)
      • Venous bronchial blood (deoxygenated) drains into Pulmonary Veins (oxygenated)
      • If lung not perfused, results in anatomic shunt
  • Intrapulmonary: Arises from pathologic state
    • ​Ex: Pulmonary Embolus (or partial blockage)
      • ​Can block Q, or V
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4
Q

Objectives: How do V/Q mismatches cause hypoxemia and large A-a gradients?

What is the third result of V/Q mismatches?

(uncompensated)

A
  • Hypoxemia: Result of mixed blood from V/Q mismatch; blood will be averaged for PaO2
    • Average O2 Content for units (weighted for volume)
  • Large A-a Gradients: PAO2 assume normal, but due to hypoxemia (see above), gradient becomes large
    • High (normal) PAO2, Low PaO2 from reduced V or Q
  • High PaCO2: Inverse relationship to V/Q, reduced V or Q leads to poor CO2 unloading at lungs
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5
Q

Objectives: Explain the compensation mechanisms in low V/Q mismatches

A
  • Local Smooth Muscle Mechanisms
    • Alveolar Hypoxia: Vasoconstriction
      • Diverts Perfusion
    • Low PACO2: Bronchial Vasoconstriction
      • Diverts Ventilation
  • Respiratory Control
    • Hypercapnia (High CO2) = High [H+]
      • Stimulates ventilation
    • Hypoxia (PaO2) < 60 mmHg
      • ​Stimultes ventilation
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6
Q

Objectives: Define high V/Q mismatches

A
  • Results in High PAO2 and Low PACO2
  • Causes:
    • Pulmonary Emboli
    • Lung Geometry - High V/Q at top of lung
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7
Q

Objectives: Explain how to diagnose a shunt and shunt equation

A
  • Determined PaO2 breathing normal air
    • Use the tables, average units
  • Determine PaO2 breathing 100% oxygen
    • Use the tables, average units, look for increase
  • Small (~ 10-20 mmHg) improvement = shunt
  • Large (> 100 mmHg) improvement = low V/Q mismatch
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8
Q

Objectives: Illustrate the causes of hypoxemia

A
  • Five Major Causes:
    • Breathing low PO2 Air
      • High altitude
      • Normal A-a gradient
    • Hypoventilation
      • Hypercapnea
      • A-a gradient Normal
    • Shunts
      • Test PaO2 with Normal Air vs. 100% O2
    • Low V/Q Mismatch
      • Test PaO2 with Normal Air vs 100% O2
    • Diffusion Problem (membranes)
      • A-a Gradient Large
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9
Q

Key Point: Why does the bottom (Zone 3) of the lung have a high V/Q ratio, lower PO2, and higher PCO2?

A
  • The base (Zone 3) of the lung is ventilated and perfused more than the top
  • It is overperfused, and shunt-like
  • The top (Zone 1) is over ventilated, and has a high V/Q ratio (contributes to dead space)
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10
Q

How does lung transplatation affect V/Q ratios in fibrosis and emphysema patients?

A
  • Fibrosis: Old lung “shunts” ventilation away (ventilation goes toward new lung), V/Q matching will be okay
  • Emphysema: Old lung easier to distend (soft) and ventilation will be shunted away from new lung, emphysema also destroys vessels (reduced Q)
    • ​V/Q mismatched; lung transplants not good treatment (back in the day at least)
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11
Q

How does V/Q affect Partial Pressures of CO2?

A

V/Q α 1/PACO2 (inverse)

V/Q α 1/PaCO2 (inverse​)

High V/Q (Zone 1 - Apex) = Low PCO2 (inverse relationship)

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12
Q

What is the best treatment for Right Heart Failure?

A
  • Inhale Nitric Oxide (NO) to reverse hypoxic pulmonary vasoconstriction
    • ​Selectively dilates ventilated blood vessels only
  • Nitroglycerin / Nitroprusside (NTP) dilate al segments, including thos not well ventilated, leading to poor V/Q match
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13
Q

How can compensatory methods go wrong?

A

In chronic lung disease, with systematicall low PaO2 (<60 mmHg), all lung tissue will vasoconstrict

This can lead to pulmonary hypertension, and right heart failure

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14
Q

What is the paradox of compensation?

Why do patients with severe low V/Q disorders not present with this?

A
  • PaCO2 reduces to normal, PaO2 remains low
  • Once Hb is fully saturated in “good” units, compensation will direct changes their direction due to the reasons causing V/Q mismatch
    • Increasing O2 to these regions will not alter the averaged values as much as PaCO2
  • If disorder is severe enough, they will have both low PaO2 and elevated PaCO2
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15
Q

What type of diseases limit ventilation and what is the result in V/Q?

What is the ultimate low V/Q mismatch?

A
  • Obstructive lung diesase limites ventilation
    • Emphysema
    • Bronchitis
    • Asthma
  • Shunt is ultimate low V/Q mismatch
    • Alveoli fill will fluid or foreign material
    • Pneumonia of lungs
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16
Q

What causes V/Q mismatches in normal individuals? How is this affected by body position?

How does this change with age?

A
  • Gravity and anatomical shunts cause normmal V/Q mismatching in health individuals
    • The larger the difference in V/Q the greater the A-a Gradient–a persin in supine position has less gravitional impact due to the reduced vertical distances in the lung tissue
  • A-a Gradients increase with age due to accumulated wear and tear of the lungs; perfused areas but not ventilated
17
Q

How can one clinically diagnose a fluid filled shunt in the lungs?

A
  • Determined PaO2 breathing normal air
    • Use the tables, average units
  • Determine PaO2 breathing 100% oxygen
    • Use the tables, average units, look for increase
  • Small (~ 10-20 mmHg) improvement = shunt
  • Large (> 100 mmHg) improvement = low V/Q mismatch
18
Q

What are the secondary affects of a pulmonary emboli that cause further lung damage?

A
  • Initial are becomes dead space, right heart has to work harder
  • Secondary:
    • Clot attracts immune cells = inflammation
    • Inflammation causes pain (deep breaths), discomfort, anxiety
    • Capillary permeability of neighbors altered, causes edema, destroys surfactant, local collapse (atelectasis)
  • Result: Areas of high and low V/Q mismatches, and shunts
19
Q
A

SF3 - Test 2 (13 decks)

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