1. Ennis James, M.D. Wednesday May 8th, 2018
Hypoxemia
Ennis James, M.D.
Wednesday May 8th, 2018
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2. Objectives Define hypoxemia
Differentiate the major pathophysiologic categories of hypoxemia
Describe the pathophysiologic
mechanisms of hypoxemia
Apply pathophysiologic mechanisms to clinical care of patients with hypoxemia
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3. A 63yo woman with a h/o DM type 2, GERD, hypertension, and dementia presents with 4 days of cough, fever, and right-sided chest pain. Her temperature is 101◦ F, heart rate is 120bpm, SBP 100mm Hg, SpO2 88% on room air, and RR 34 breaths/minute. She has decreased right basilar lung sounds and right mid-lung crackles. Antibiotics have been ordered and chest x-ray is pending.
What is the most appropriate immediate next step in her care?
A) Perform an arterial blood gas
B) Order an albuterol nebulizer treatment
C) Obtain a STAT chest CT angiogram
D) Initiate supplemental oxygen via full face mask
E) Initiate non-invasive positive pressure ventilation (BiPAP)
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5. (pollev.com/ennisjames849)
What is hypoxemia?
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7. But. . . what determines PaO2 and SaO2?
What is hypoxemia? =
PaO2
SaO2
But. . . what determines PaO2 and SaO2?

8. What is hypoxemia? Alveolar gas equation
PAO2 = FiO2 (PB-PH20) – PaCO2/R
On ambient air at sea level
PAO2 = 150 – (PaCO2 x 1.25)

9. Great, so what is normal?
Alveoli
Arterial blood
Mixed venous blood
PO2
PCO2
100 95 40 40 46
What’s all this talk about oxygen “tension” and “content”?
Tension: the pressure gradient that drives passive diffuse of gas across a membrane (ie, that of the alveolar-capillary type)
Content: more about oxygen transport (involves CO and hemoglobin)

10. PaO2 What is hypoxemia? mild moderate severe (PaO2 60-79) (PaO2 40-59)

11. Oxygen-hemoglobin curve

12. “Supplemental” review
Delivery method
LPM
Nasal Cannula
1-6
Simple mask
5-8
Face Tent
10-15
Venturi Mask
variable
Partial NRB
6-10
NRB
HFNC
10-60
FiO2
(4% rule)
0.40
0.70 – 0.9
1.0 (?PEEP)

13. How does oxygen get from the environment to the blood?
PH2O
PH2O
Patm
PAO2
PAO2
Alveolar-capillary membrane
PaO2
PaO2
PaO2

14. What are causes of hypoxemia?
PH2O
PH2O
Patm
PAO2
PAO2
PaO2
PaO2
PaO2

15. What are causes of hypoxemia?
PH2O
PH2O
Patm
PAO2
PAO2
PaO2
PaO2
PaO2

16. What are causes of hypoxemia?
V/Q mismatch
Diffusion deficit
Shunt
Low PIO2
Hypoventilation
Carboxy- or met-hemoglobin =“histotoxic” hypoxemia

17. SUPER-QUICK CLINICAL QUESTION

18. What is the most appropriate next step in his clinical care?
A 45yo homeless man is brought to the ED after having been found down covered in emesis. He is known to have a history of asthma and EtOH abuse. His SpO2 is 90% on room air, and his respiratory rate is 10 breaths/minute.
ABG on room air:
What is the most appropriate next step in his clinical care? pH
PaCO2
PaO2
A) Start antibiotics for aspiration pneumonia
B) Give bicarbonate for a metabolic acidosis
C) Intubate the patient and initiate invasive ventilation
D) Give steroids
E) Give inhaled bronchodilators
F) Both A & B
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20. Hypoventilation What are causes of hypoxemia? V/Q mismatch
Diffusion deficit
Shunt
Low PIO2
Hypoventilation
Carboxy- or met-hemoglobin =“histotoxic” hypoxemia

21. What are causes of hypoventilation?
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23. Alveolar hypoventilation
PH2O
PH2O
Patm
PAO2
PAO2
PaO2
PaO2
PaO2

24. Sites where pathology may cause hypoventilation
Sites at which disease may cause hypoventilation

25. Physiologic relationships
VE = VT x RR = (VA + VD) x RR
Alveolar ventilation equation
PaCO2 = VA
VCO2 x k
VE = minute ventilation; VT = tidal volume; VA = alveolar ventilation; VD = dead space; VCO2 = systemic production of carbon dioxide

26. Anatomic and physiologic dead space
ANATOMY(IC) Dead space
Normal=150mL
Normal VT = ~500mL (150 is VD, 350 VA)
Reduce VT to 250mL, VD doesn’t change but VA goes down (350 to 100mL)…so PaCO2 goes up

27. How does hypoventilation result in hypoxemia?
Small breaths  decrease VA
Decrease VA  pCO2 goes up…
So by definition PAO2 goes down….
PAO2 = FiO2 (PB-PH20) – PaCO2/R

28. PAO2 = FiO2 (PB-PH20) – PaCO2/R
And, as a reminder. . .
Alveolar gas equation
PAO2 = FiO2 (PB-PH20) – PaCO2/R
On RA at sea level
PAO2 = 150 – (PaCO2 x 1.25)

30. Does he have an aspiration pneumonia?
A 45yo homeless man is brought to the ED after having been found down covered in emesis. He is known to have a history of asthma and EtOH abuse. His SpO2 is 90% on room air, and his respiratory rate is 10 breaths/minute.
ABG on room air:
Does he have an aspiration pneumonia? pH
PaCO2
PaO2
A) Yes
B) No
C) I don’t know
D) I don’t care
E) I need more information
F) Both A & B
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32. Your answers

33. TANGENT

34. Pulmonary acid / base tangent
Acidemia (low pH)
PaCO2 high or low?
PaCO2 is high
PaCO2 is low
Respiratory
Acidosis
Metabolic
Acidosis

35. Pulmonary acid / base tangent
Respiratory acidoses
ACUTE
CHRONIC
For every 10mm Hg
increase in PaCO2
For every 10mm Hg
increase in PaCO2
pH decreases by 0.08
pH decreases by 0.03

36. Back to hypoxemia

37. What are causes of hypoxemia?
V/Q mismatch
Diffusion deficit
Shunt
Low PIO2
Hypoventilation
Carboxy- or met-hemoglobin =“histotoxic” hypoxemia

38. Shunt What are causes of hypoxemia? V/Q mismatch Diffusion deficit
Low PIO2
Hypoventilation
Carboxy- or met-hemoglobin =“histotoxic” hypoxemia

39. Shunt pathophysiology
PH2O
PH2O
Patm
PAO2
PAO2
PaO2
PaO2
PaO2
Hypoxemia due to shunt does not improve with supplemental O2.

40. (Please go to pollev.com/ennisjames849)
Which of the following diseases causes hypoxemia by shunt pathophysiology?
A) Acute COPD exacerbation
B) Pulmonary embolism
C) Atelectasis
D) Acute respiratory distress syndrome
E) Acute pulmonary edema
F) Obesity hypoventilation syndrome
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41. Your answers

42. Your answers – take 2

43. V/Q mismatch What are causes of hypoxemia? V/Q mismatch
Diffusion deficit
Shunt
Low PIO2
Hypoventilation
Carboxy- or met-hemoglobin =“histotoxic” hypoxemia

44. Most common cause of hypoxemia (encompasses many diagnoses).
V/Q mismatch
PH2O
PH2O
Patm
PAO2
PAO2
PaO2
PaO2
PaO2
Most common cause of hypoxemia (encompasses many diagnoses).

45. Ventilation to Perfusion ratio
V/Q relationships
Number of lung units
Clearly shunting and dead space ventilation are the extremes of ventilation:perfusion mismatch and less extreme forms of mismatch exist. Indeed acute respiratory failure is characterised by increase in the spread of ventilation perfusion ratios as opposed the normal situation where ratios are closely clustered around 1
Diseased
Normal
Hypoxemia due to V/Q mismatch does improve somewhat with supplemental O2. 1
Ventilation to Perfusion ratio

46. “Gas exchange” deficits
Hypoxemia due to V/Q mismatch does improve somewhat with supplemental O2.

47. Which of the following diseases causes hypoxemia by V/Q mismatch?
A) Heroin overdose
B) Pneumothorax
C) Atelectasis
D) Right lower lobe pneumonia
E) Advanced idiopathic pulmonary fibrosis
F) Mucus plug
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48. Which of the following diseases causes hypoxemia by V/Q mismatch?
A) Heroin overdose
B) Pneumothorax
C) Atelectasis
D) Right lower lobe pneumonia
E) Advanced idiopathic pulmonary fibrosis
F) Mucus plug
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49. Diffusion Deficit What are causes of hypoxemia? V/Q mismatch
Shunt
Low PIO2
Hypoventilation
Carboxy- or met-hemoglobin =“histotoxic” hypoxemia

50. Diffusion deficit Patm PAO2 PAO2
PH2O
PH2O
Patm
PAO2
PAO2
PaO2
PaO2
Hypoxemia due to diffusion deficit almost always occurs with exertion (and not at rest.)
PaO2

51. Diffusion deficit Due to a reduction in alveolar surface area
(not “increased thickness” of the pulmonary parenchyma)

52. (Please go to pollev.com/pathophysiology)
Which of the following x-rays is most consistent with hypoxemia caused by diffusion deficit?
A) Chest x-ray A
B) Chest x-ray B
C) Chest x-ray C
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53. Chest x-ray A

54. Chest x-ray B

55. Chest x-ray C

56. What are causes of hypoxemia?
V/Q mismatch
Diffusion deficit
Shunt
What differentiates these mechanisms?
Low PIO2
Hypoventilation
Carboxy- or met-hemoglobin =“histotoxic” hypoxemia

57. What is the “A-a gradient”?
PAO2
PaO2
PAO2 – PaO2 = A-a gradient

58. We can calculate the PAO2
Alveolar gas equation
PAO2 = FiO2 (PB-PH20) – PCO2/R
On RA at sea level
PAO2 = 150 – (PCO2 x 1.25)

59. What are the definitions of:
Quick self-review
What are the definitions of:
PAO2
SpO2
PaO2
A-a gradient
FiO2
V/Q mismatch
Shunt
Diffusion deficit
Hypoventilation
Low PIO2

60. (Please go to pollev.com/pathophysiology)
A 55yo man with a h/o severe COPD presents with 3 days of worsening shortness of breath. Chest x-ray reveals a right lower lobe pneumonia. His respiratory rate is 28 breaths/min and his SpO2 is 80%. He is using accessory muscles.
ABG on room air:
He is placed on 60% FiO2, and his SpO2 increases to 96%. What is the most likely cause of his acute hypoxemia? pH
PaCO2
PaO2
A) Low PIO2
B) Alveolar hypoventilation
C) V/Q mismatch
D) Shunt
E) Diffusion deficit
F) Acute respiratory distress syndrome
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62. (Please go to pollev.com/pathophysiology)
A 55yo man with a h/o severe COPD presents with 3 days of worsening shortness of breath. Chest x-ray reveals a right lower lobe pneumonia. His respiratory rate is 28 breaths/min and his SpO2 is 80%. He is using accessory muscles.
ABG on room air:
He is placed on 60% FiO2, and his SpO2 increases to 96%. What is the most likely cause of his acute hypoxemia? pH
PaCO2
PaO2
A) Low PIO2
B) Alveolar hypoventilation
C) V/Q mismatch
D) Shunt
E) Diffusion deficit
F) Acute respiratory distress syndrome
(Please go to pollev.com/pathophysiology)

63. (Please go to pollev.com/pathophysiology)
A 55yo man with a h/o severe COPD presents with 3 days of worsening shortness of breath. Chest x-ray reveals a right lower lobe pneumonia. His respiratory rate is 28 breaths/min and his SpO2 is 80%. He is using accessory muscles.
ABG on room air:
He is placed on 60% FiO2, and his SpO2 increases to 96%. What is the most likely cause of his acute hypoxemia? pH
PaCO2
PaO2
A) Low PIO2
B) Alveolar hypoventilation
C) V/Q mismatch
D) Shunt
E) Diffusion deficit
F) Acute respiratory distress syndrome
(Please go to pollev.com/pathophysiology)

64. Methods of supplementing O2
Regular
~6 Lpm
Nasal cannula
Oximizer
~15 Lpm
Ventimask
~50% FiO2
Face mask
Non-rebreather
~65% FiO2
Non-invasive
100% FiO2
Positive pressure
ventilation
100% FiO2
Endotracheal

65. (Please go to pollev.com/pathophysiology)
After careful review of his clinical circumstances and ABG results, you have determined that his acute hypoxemia is due to V/Q mismatch due to pneumonia (superimposed on chronic alveolar hypoventilation due to COPD.)
In addition to supplemental oxygen, what is then next most appropriate step in his clinical care?
A) Initiate non-invasive ventilation
B) Intubate him and initiate invasive ventilation
C) Provide high dose steroids for a COPD exacerbation
D) Provide antibiotics for a pneumonia
E) Perform a chest CT angiogram to assess for PE
F) Provide incentive spirometry
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66. (Please go to pollev.com/pathophysiology)
After careful review of his clinical circumstances and ABG results, you have determined that his acute hypoxemia is due to V/Q mismatch due to pneumonia (superimposed on chronic alveolar hypoventilation due to COPD.)
In addition to supplemental oxygen, what is then next most appropriate step in his clinical care?
A) Initiate non-invasive ventilation
B) Intubate him and initiate invasive ventilation
C) Provide high dose steroids for a COPD exacerbation
D) Provide antibiotics for a pneumonia
E) Perform a chest CT angiogram to assess for PE
F) Provide incentive spirometry
(Please go to pollev.com/pathophysiology)

67. The patient is placed on full face mask with a FiO2 of 60%.
As a reminder, his initial ABG on room air was:
And, repeat ABG on 60% FiO2 is:
What is the most likely explanation for he acute increase in his PaCO2 after he was placed on 60% FiO2? pH
PaCO2
PaO2 pH
PaCO2
PaO2
A) Worsening V/Q mismatch
B) The patient stopped breathing
C) Oxygen induced bronchospasm
D) Oxygen toxicity of the lung
E) A shift in the oxygen-hemoglobin dissociation curve
F) Progressive acute respiratory distress syndrome
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68. The patient is placed on full face mask with a FiO2 of 60%.
As a reminder, his initial ABG on room air was:
And, repeat ABG on 60% FiO2 is:
What is the most likely explanation for he acute increase in his PaCO2 after he was placed on 60% FiO2? pH
PaCO2
PaO2 pH
PaCO2
PaO2
A) Worsening V/Q mismatch
B) The patient stopped breathing
C) Oxygen induced bronchospasm
D) Oxygen toxicity of the lung
E) A shift in the oxygen-hemoglobin dissociation curve
F) Progressive acute respiratory distress syndrome
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69. Flow-volume loop

70. Why does your resident recommend doing this?
Based on the patient’s response to delivering a FiO2 of 60%, your resident recommends that you decrease the FiO2 delivered so that the patient’s SpO2 is 90-92%.
Why does your resident recommend doing this?
A) Lower SpO2s will increase the patient’s drive to breath
B) Lower SpO2s will allow you to detect hypoventilation sooner
C) Lower SpO2s result in higher PaO2s
D) Lower SpO2s decrease oxygen toxicity
E) Alveolar hypoventilation is not treated with oxygen
F) Bacteria proliferate more readily with higher PAO2s
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71. Why does your resident recommend doing this?
Based on the patient’s response to delivering a FiO2 of 60%, your resident recommends that you decrease the FiO2 delivered so that the patient’s SpO2 is 90-92%.
Why does your resident recommend doing this?
A) Lower SpO2s will increase the patient’s drive to breath
B) Lower SpO2s will allow you to detect hypoventilation sooner
C) Lower SpO2s result in higher PaO2s
D) Lower SpO2s decrease oxygen toxicity
E) Alveolar hypoventilation is not treated with oxygen
F) Bacteria proliferate more readily with higher PAO2s
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72. Summary – physiology of oxygenation
PH2O
PH2O
Patm
PAO2
PAO2
PaO2
PaO2
PaO2

73. Summary – pathophysiology of hypoxemia
PH2O
PH2O
Patm
PAO2
PAO2
PaO2
PaO2
PaO2

74. Objectives Define hypoxemia
Differentiate the major pathophysiologic categories of hypoxemia
Describe the pathophysiologic
mechanisms of hypoxemia
Apply pathophysiologic mechanisms to clinical care of patients with hypoxemia
(pollev.com/pathophysiology)

75. THANK YOU!