Acute Respiratory
Failure: 5 types of
Hypoxemia
John Heisler, PA-C
Forms of Respiratory Failure
•
Acute Hypoxemic Respiratory Failure
•
Acute Hypercapnic Respiratory Failure
•
Difference between hypoxemia “blood” and hypoxia “cell”
 PaO2: ABG
 SpO2: O2 monitor
Types of Oxygen
5 types of hypoxemia
THE 5 CAUSES OF
HYPOXIA
#1 – High Altitude
(Normal A-a gradient)
#2 – Hypoventilation
#3 – Diffusion Disorder
#4 – Shunt
#5 – VQ Mismatch
(High A-a gradient)
Case 1
You are a medical volunteer at Everest Base camp
clinic (~16,900 ft).
A 27 year old man with no significant PMH, new
climber presents with complaints of throbbing
headache and shortness of breath x 1 day.
On exam patient is tachypneic with bibasilar
crackles. Tachycardic, SpO2 85%. Remainder of
exam normal.
s/p O2
Diagnosis?
Altitude Sickness!
A-a gradient
aka the difference between alveolar and arterial oxygen
𝑨𝒂 𝒈𝒓𝒂𝒅𝒊𝒆𝒏𝒕 = 𝑷𝑨𝑶𝟐 − 𝑷𝒂𝑶𝟐
𝐴𝑎 𝑔𝑟𝑎𝑑𝑖𝑒𝑛𝑡 = [0.21 760𝑚𝑚𝐻𝑔 − 47𝑚𝑚𝐻𝑔 −
150
𝑃𝑎𝐶𝑂2
− 𝑃𝑎𝑂2
0.8
All about the alveolus!
High A-a gradient: Lots of O2 in Alveolus, not a lot in artery
Low/Normal A-a gradient: Little O2 in Alveolus thus little in artery
 Young woman overdosed on
antidepressants and alcohol
 Respiratory rate 8 breaths/min
 Arterial blood gas: pH 7.15, PaCO2
71 mm Hg (9.5 kPa), PaO2 56 mm Hg (7.5 kPa) in room
air
Why is this patient hypoxemic?
Copyright 2016 Society of Critical Care Medicine
Case Study 2
13
Which of the following is the most likely cause
of hypoxemia in this patient?
Hypoventilation
Acidemia
Alveolar hyperventilation
Auto-positive end-expiratory pressure
Copyright 2016 Society of Critical Care Medicine
A.
B.
C.
D.
14
14
Select all of the interventions that would be helpful
in treating this patient’s hypoxemia.
Copyright 2016 Society of Critical Care Medicine
A. Administer supplemental oxygen.
B. Prepare to initiate mechanical ventilation.
C. Treat the patient’s overdose.
D. Encourage the patient to breathe deeply.
16
16
THE 5 CAUSES OF
HYPOXIA
#1 – Low Patm
(Normal A-a gradient)
#2 – Hypoventilation
#3 – Diffusion Disorder
#4 – Shunt
#5 – VQ Mismatch
(High A-a gradient)
#3: Diffusion
O2
CO2
interstitium
1 cell layer thick
Impaired diffusion
Responds to Oxygen
The pearl of clinical truth: Diffusion Dz
• VERY wide differential for
ILD:
 Environmental
 Drug-induced
 Autoimmune dz
 Infection
 Idiopathic
 Malignancy
• Your job?
GOOD H&P
#4: Shunt
70%
SYSTEM
82.5%
95%
What’s the A-a gradient?
What happens with O2?
#4 Shunt
• Two types of shunt:
 Anatomic “cardiac”
 Congenital Heart Defects
 ASD
 VSD
 PDA
 Physiologic “intrapulmonary”
 ARDS
• Does not correct with O2!
#5 CASE STUDY
A 31 year old smoker presents to the ED with acute
onset shortness of breath. Symptoms began ~2 hours
prior to arrival. Syncopal event en route. She also
endorses chest pain. T98.0 HR 125 BP 90/55 SpO2 85%
on 6L NC.
A
R
L
B
ABG
7.56/20/56/24
pH/CO2/O2/HCO3
Diagnosis?
Massive PE!
#5 VQ Mismatch
•
Most common cause of hypoxemia




Pulmonary Embolism
Pneumonia
Pulmonary Edema
COPD
Normal
Dead Space
Summary
Cause of Hypoxia
Remember!
Altitude / low Patm
Low alveolar oxygen
Hypoventilation
Hypercapnia comes
first
Diffusion disorder
Diffusion distance
limited
VQ mismatch
MCC of hypoxia
Shunt
Anatomic or
physiologic
A-a gradient.
↓A-a. Fully corrects w/ O2
↑A-a. Partially corrects w/ O2
↑A-a. O2 WON’T CORRECT
Summary
Cause of Hypoxia
Disease states
Altitude / low Patm
Altitude sickness
Hypoventilation
OSA/OHS
Opioid/drug overdose
Encephalopathy (all comers)
Neuromuscular weakness
Diffusion disorder
Interstitial lung disease (many types)
VQ mismatch
COPD/asthma
PNA
PE
Pulmonary Fibrosis
Pulmonary Edema
Shunt
Anatomic: ASD, VSD, PDA
Physiologic: SCAPE, ARDS