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Respiratory (AM) > 13b - Clinical Correlations of Hypoxemia > Flashcards

13b - Clinical Correlations of Hypoxemia Flashcards

1
Q

What are the factors that affect the flux of gas?

A

Fick’s law: Flux of gas is related to the concentration present at steady state

**Gas moves from regions of high concentration to those of low concentration with a magnitude directly proportional to the concentration gradient

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2
Q

What are 4 clinical causes of pulmonary diffusion defects?

A

Both acute and chronic conditions:

  1. thickening or destruction of the interstitial space, alveolar wall, or capillary
  2. pulmonary edema
  3. interstitial lung disease (sarcoid, idiopathic pulmonary fibrosis, ARDS)
  4. emphysema
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3
Q

What do you see on this xray?

A

Trick question: this is a normal xray (this makes a diffusion defect less likely)

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4
Q

A patient has the following vitals:

pH 7.45, pCO2 25, pO2 70, HCO3- 24, 88% O2 saturation

What is her A-a gradient?

A

A-a gradient= 150 - (5/4)(pCO2) - (pO2)

150 - (5/4)(25) - (70) = 49

**This A-a is elevated!

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5
Q

What are 3 possible differential diagnoses with an elevated A-a gradient?

A
  1. diffusion defect
  2. V/Q mismatch
  3. shunting
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6
Q

What is an ideal V/Q ratio?

A

V/Q= 1 where ventilation and perfusion are perfectly matched

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7
Q

What is an explanation for an area of persusion with low ventilation?

A

Pulmonary shunt

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8
Q

What is an explanation for an area of ventilation with low perfusion?

A

Dead space

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9
Q

What are 5 ventilation problems that would decreased the V/Q ratio?

A

decreased ventilation:

  1. asthma
  2. COPD
  3. pulmonry edema
  4. pleural effusions
  5. mucous plugging *barf sound*
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10
Q

What are 2 perfusion problems that would decreased the V/Q ratio?

A

increased perfusion:

  1. hepatopulmonary syndrome
  2. anatomic shunt
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11
Q

What are 3 reasons the V/Q ratio would be increased?

A

perfusion decreased or ventilation increased:

  1. PE (most common reason)
  2. hyperventilation (rarely pathologic)
  3. dead space
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12
Q

Describe hepatopulmonary syndrome

A
  • normal V, increased Q (decreased V/Q)
  • shortness of breath and hypoxemia caused by vasodilation in the lungs of patients with liver disease
  • due to arteriovenous malformations (AVMs)
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13
Q

Decribe an intrapulmonary shunt

A
  • normal Q, decreased V
  • alveoli are perfused but no ventilation occurs
  • main cause of hypoxemia in pulmonary edema and conditions where the lungs become consolidated (e.g. pneumonia)
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14
Q

Describe an anatomic shunt

A
  • normal Q, decreased V
  • bronchial arteries and coronary arteries return blood to circulation without passing by the alveoli to participate in gas exchange
  • usually accounts for less than 3% of the total circulation
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15
Q

What components of the respiratory system comprise the physiologic dead space?

A
  • mouth
  • pharynx
  • larynx
  • trachea
  • conducting airways
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16
Q

Describe anatomic dead space

A
  • areas of the respiratory system that do not participate in gas exchange
  • typically ~150 ml
17
Q

Describe alveolar dead space

A
  • not all alveolar units are as efficient in exchanging gas as they should be
  • in a normal, healthy person, alveolar dead space is minimal
18
Q

What does this V/Q scan show?

A
  • pulmonary embolism
  • compare the ventilation (left) and perfusion (right) scans, if they don’t show the same defects then there is V/Q mismatch
19
Q

What does this V/Q scan show?

A

Trick question! It’s normal!

20
Q

If you place a patient with a PE on 100% O2 what would you expect in regards to the arterial oxygen level?

A

Increased arterial oxygen! You CAN overcome V/Q mismatching with 100% FIO2 (it won’t affect a shunt however)

21
Q

What are 4 causes of systemic shunts?

A
  1. Genetic conditions (Hereditary hemmorhagic telangiectasisa/HHT, VonHippel Lindau/VHL)
  2. Right to left cardiac blood flow (ASD, VSD, congenital)
  3. Pulmonary arteriovenous malformations (AVMs)
  4. Systemic AVMs (liver, kidney, spleen, GI, etc)
22
Q

What is the Berggren Shunt Equation?

A

Used to compare oxygenation of a patient on room air versus 100% O2 (pulmonary shunt fraction= Qs/Qt)

23
Q

How can you analyze an intracardiac shunt? Extracardiac?

A
  • Intracardiac
    • “Bubble study”
  • Extracardiac
    • Nuclear medicine scan
    • PFT lab/Berggren shunt equation
24
Q

How can you use a bubble study to locate a shunt?

A
  • When bubbles return to left side of the heart WITHIN 4 cardiac cycles= intracardiac shunt
  • When bubbles return to left side of the heart in MORE THAN 4 cardiac cycles= extracardiac shunt (e.g. hepatopulmonary shunt)
25
Q

What factor has the largest effect on the oxygen content of arterial blood? What is the equation for CaO2?

A

Hemoglobin (Hgb) has the most effect:

CaO2= (Hgb x 1.34) x (sPO2) + (PaO2 x 0.003)

sPO2= oxygen saturation

PaO= dissolved oxygen

**Normally, CaO2= 20 g/dl

26
Q

How can you calculate oxygen delivery?

A

Delivery (DaO2) = CaO2 x CO

CaO2= oxygen content

CO= cardiac output

27
Q

What does this xray show?

A

Fluid in the left lung: V/Q mismatch

28
Q

What happens to a patient’s oxygen saturation following the removal of a mucous plug?

A

Decrease for a short time, then gradually increase to normal (due to hypoxic pulmonary vasoconstriction; diverting blood to lung areas being properly ventilated)

29
Q

Describe hypoxic pulmonary vasoconstriction

A
  • onset in seconds in response to hypoxia, max intensity in minutes
  • reversible when O2 levels restored
  • mitochondria sense O2 reduction and trigger response through redox sensing (avoids “wasting perfusion”)
  • can be affected by volatile anesthetics, vasodilators, hypothermia, and CCBs
30
Q

What endothelium-derived products increase vasoconstriction?

A

Thromboxane and endothelin

31
Q

What endothelium-derived products inhibit vasoconstriction?

A

Nitric oxide and prostacyclin

Respiratory (AM) (28 decks)

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