168 IM Noon Conf Respiratory Failure

168.1 Summary

Physiology-Based Schemas
Acute Respiratory Failure Goals:
Cargo Cult Thinking: two things occur together → one causes the other
Hypoxemia and Dyspnea
Solution to cargo cult thinking? Try to falsify
Hypoxemia… is a problem though
Refractory Hypoxemia Schema
Pulse Oximetry: Screening Test
SaO2 is low?
RRT 1! Hypoxemia
Urbach-Wiethe Disease: bilateral amygdala damage due to fat deposits.

168.2 Slide outline

168.2.1 Slide 1

Physiology-Based Schemas
Brian Locke MD
P/CCM ### Slide 2
Acute Respiratory Failure Goals:
Construct Schemas for how to think about 2 common situations
[O2] The SpO2 is low and won’t come up with 6L via nasal cannula
[CO2] This patient feels dyspneic
[CO2] This patient is gorked and their serum [HCO3-] is 35 mmHg…. (another time)
Focus on understanding what’s happening (treatment is easy) ### Slide 3
Cargo Cult Thinking: two things occur together → one causes the other
“The first principle is that you must not fool yourself—and you are the easiest person to fool.” ### Slide 4
Hypoxemia and Dyspnea ### Slide 5
Solution to cargo cult thinking? Try to falsify
Still a problem, but not
respiratory distress ### Slide 6
Hypoxemia… is a problem though ### Slide 7
Refractory Hypoxemia Schema
RRT:
SpO2 is 75%,
15L NRB! ### Slide 8
Pulse Oximetry: Screening Test
This issue is not (yet) fixed!
Baseline Accuracy: FDA standard 95% LOA within 2-3%
Situations where accuracy worsens:
Hypoxemia below SaO2 80%
Poor perfusion: hypovolemia, heart failure, vasoconstriction, sepsis
Carbon monoxide poisoning (people who’ve smoked), sickle cell
Non-white skin, nail polish, make-up
Pulse Required!
Lack of venous pulsation required!
Translucency required! ### Slide 9
SaO2 is low?
Causes of hypoxemia that resolve with 6L O2
V/Q Mismatch
Hypoventilation
Low FiO2, Low Barometric Pressure
Causes of hypoxemia that don’t resolve with 6L O2
Shunt: Blood passes from venous to arterial circulation without contacting inhaled air.
Through the lung: opacity should be present on CXR
Through the heart…
In locations where congenital heart disease is identified and fixed, this generally requires systemic PA pressures ### Slide 10
RRT 1! Hypoxemia
68M who is POD 2 after hip replacement.
No DVT prophylaxis.
Hypoxemia (~85%) develops on 6L Patient is hemodynamically stable
Mild tachypnea ~20 breaths per minute
You order a CXR and ABG
Differential? ### Slide 11
Refractory Hypoxemia Schema
Consolidations cause shunts! ### Slide 12
Urbach-Wiethe Disease: bilateral amygdala damage due to fat deposits.
Patient S.M.: No conditioning to aversive stimuli, no fear in life-threatening situations, no recognition of fear in others
35% Inhaled CO2 challenge (↑PaCO2)
Air hunger provoked panic attack
First sensation of fear since childhood
Reproduced in others with U-W Disease
If hypoxemia doesn’t cause dyspnea… what does? ### Slide 13
“16M fell into a … transport wagon being filled with whole wheat grain and was immediately submersed…
…mouth and pharynx were cleared of large amounts of packed grain
Attempts to clear grain provoked instability
…placed in the Trendelenburg position for central line placement, it was noted that …several grain seeds would fall […from] the ETT during expiration.
During the subsequent 20 min, the surgeon performed manual chest percussions as the patient was moved alternately from supine to lateral and prone positions”
Patient had no residual neurologic or respiratory signs or symptoms… ### Slide 14
Evolution of the Control of Breathing
Gas Composition in Water
CO2 is 30x more soluble than O2 (low O2)
Gil Breathers
O2 is limiting factor, PaCO2 very low (1-4mmHg)
→ O2 determines respiratory drive in Fish/Amphibians
Gas Composition in Air
21% O2, 0.04% CO2. [and both are equally ‘soluble’]
(Proto-) Lung Breathers
Ventilation only to ensure O2 → CO2 will be too high
→ CO2 determines respiratory drive in mammals (etc.) ### Slide 15
Keep O2 and CO2 Problems Separate!
O2
CO2
Need sufficient O2 gas tension ALWAYS
Won’t cause damage if varies (as long as O2 available)
Body doesn’t keep as close tabs on it as you’d think
Brainstem is EXQUISITELY sensitive to it
We continually screen for low O2 (pulse ox.)
We seldom monitor (except indirectly, by our observation of what the patient’s brainstem is doing) ### Slide 16
Schema for Ventilatory Failure ### Slide 17
Need to exhale CO2: Supply vs Demand
You can understand this with equations, but they’re not required.
Fundamental Rules:
Molecules of CO2 produced Molecules of CO2 exhaled to maintain homeostasis
Amount of CO2 you exhale is proportional to the amount of air that meets blood coming back to the heart from the tissues.
Amount of work is proportional to the amount of air you move past your nose & mouth (RRVt)
𝑉𝐸∝𝑉𝐶𝑂21 −𝑉𝑑𝑉𝑡∗𝑃𝑎𝐶𝑂2
𝑉𝑒𝑛𝑡𝑖𝑙𝑎𝑡𝑖𝑜𝑛 𝑅𝑒𝑞𝑢𝑖𝑟𝑒𝑑∝𝑀𝑒𝑡𝑎𝑏𝑜𝑙𝑖𝑐 𝑅𝑎𝑡𝑒𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦∗𝐵𝑙𝑜𝑜𝑑 𝐶𝑂2 ### Slide 18
Need to exhale CO2: Supply vs Demand
Fundamental Rules:
Amount of CO2 you exhale is proportional to the amount of air that meets blood coming back to the heart from the tissues.
Amount of work is proportional to the amount of air you move past your nose & mouth (RRVt)
Molecules of CO2 produced Molecules of CO2 exhaled to maintain homeostasis
3 things that influence how much air you need to breath:
1. Amount of CO2 production
1. Ventilation Efficiency
1. The concentration of CO2 in the blood
Metabolic Rate (VCO2)
Deadspace Fraction (Vd/Vt)
Respiratory Compensation
VE ∝ VCO2
↑ Size
Hyperthermia/Fever
Work of breathing
Exercise
Carbohydrates (vs Lipids) ### Slide 19
Need to exhale CO2: Supply vs Demand
Fundamental Rules:
Amount of CO2 you exhale is proportional to the amount of air that meets blood coming back to the heart from the tissues.
Amount of work is proportional to the amount of air you move past your nose & mouth (RRVt)
Molecules of CO2 produced Molecules of CO2 exhaled to maintain homeostasis
3 things that influence how much air you need to breath:
1. Amount of CO2 production
1. Ventilation Efficiency
1. The concentration of CO2 in the blood
Metabolic Rate (VCO2)
Deadspace Fraction (Vd/Vt)
Respiratory Compensation
VE ∝ VCO2
↑ Size
Hyperthermia/Fever
Work of breathing
Exercise
Carbohydrates (vs Lipids)
VE ∝ 1/(1-Vd/Vt)
Vd Amount of wasted VE
Vd/Vt Proportion wasted
1-Vd/Vt Proportion not wasted [more efficient] ### Slide 20
L Main PA Occlusive Thrombus
Blood no longer goes to L lung
Inhaled air does
Therefore, 2x the ventilation (mouth) required for same gas-exchange
Vd/Vt ~ ½
Ventilation (past mouth)
Ventilation participating in gas-exchange (Alveolar Ventilation)
½ of the mouth ventilation
Wasted ventilation (no blood flow to this side) ### Slide 21
Need to exhale CO2: Supply vs Demand
Fundamental Rules:
Amount of CO2 you exhale is proportional to the amount of air that meets blood coming back to the heart from the tissues.
Amount of work is proportional to the amount of air you move past your nose & mouth (RRVt)
Molecules of CO2 produced Molecules of CO2 exhaled to maintain homeostasis
3 things that influence how much air you need to breath:
1. Amount of CO2 production
1. Ventilation Efficiency
1. The concentration of CO2 in the blood
Metabolic Rate (VCO2)
Deadspace Fraction (Vd/Vt)
Respiratory Compensation
VE ∝ VCO2
↑ Size
Hyperthermia/Fever
Work of breathing
Exercise
Carbohydrates (vs Lipids)
VE ∝ 1/(1-Vd/Vt)
Vd Amount of wasted VE
Vd/Vt Proportion wasted
1-Vd/Vt Proportion not wasted [more efficient]
VE ∝ 1/PaCO2
Exhaled [CO2] ∝ blood [CO2]
Why might the body want low PaCO2?
Why might having a high PaCO2 help? ### Slide 22
3 scenarios: why are they breathing hard?
A patient in DKA (Kussmaul Breathing)?
3 things that influence how much air you need to breath:
1. Amount of CO2 production
1. Ventilation Efficiency
1. The concentration of CO2 in the blood
Metabolic Rate (VCO2)
Deadspace Fraction (Vd/Vt)
Respiratory Compensation
VE ∝ VCO2
↑ Size
Hyperthermia/Fever
Work of breathing
Exercise
Carbohydrates (vs Lipids)
VE ∝ 1/(1-Vd/Vt)
Vd Amount of wasted VE
Vd/Vt Proportion wasted
1-Vd/Vt Proportion not wasted [more efficient]
VE ∝ 1/PaCO2
Exhaled [CO2] ∝ blood [CO2]
𝑉𝐸∝𝑉𝐶𝑂21 −𝑉𝑑𝑉𝑡∗𝑃𝑎𝐶𝑂2 ### Slide 23
3 scenarios: why are they breathing hard?
A runner, before they hit their anaerobic threshold?
A runner, after they hit their anaerobic threshold?
3 things that influence how much air you need to breath:
1. Amount of CO2 production
1. Ventilation Efficiency
1. The concentration of CO2 in the blood
Metabolic Rate (VCO2)
Deadspace Fraction (Vd/Vt)
Respiratory Compensation
VE ∝ VCO2
↑ Size
Hyperthermia/Fever
Work of breathing
Exercise
Carbohydrates (vs Lipids)
VE ∝ 1/(1-Vd/Vt)
Vd Amount of wasted VE
Vd/Vt Proportion wasted
1-Vd/Vt Proportion not wasted [more efficient]
VE ∝ 1/PaCO2
Exhaled [CO2] ∝ blood [CO2]
𝑉𝐸∝𝑉𝐶𝑂21 −𝑉𝑑𝑉𝑡∗𝑃𝑎𝐶𝑂2 ### Slide 24
3 scenarios: why are they breathing hard?
A patient with severe pulmonary arterial hypertension?
3 things that influence how much air you need to breath:
1. Amount of CO2 production
1. Ventilation Efficiency
1. The concentration of CO2 in the blood
Metabolic Rate (VCO2)
Deadspace Fraction (Vd/Vt)
Respiratory Compensation
VE ∝ VCO2
↑ Size
Hyperthermia/Fever
Work of breathing
Exercise
Carbohydrates (vs Lipids)
VE ∝ 1/(1-Vd/Vt)
Vd Amount of wasted VE
Vd/Vt Proportion wasted
1-Vd/Vt Proportion not wasted [more efficient]
VE ∝ 1/PaCO2
Exhaled [CO2] ∝ blood [CO2]
𝑉𝐸∝𝑉𝐶𝑂21 −𝑉𝑑𝑉𝑡∗𝑃𝑎𝐶𝑂2 ### Slide 25
Need to exhale CO2: Supply vs Demand
How much air movement can you sustain?
Causes of inability to sustain the needed minute ventilation
Impaired bellows function (Muscle weakness/inefficiency)
Increased respiratory system loads
Decreased ventilatory drive
Neuromuscular disease
Hyperinflation
Respiratory muscle hypoxia
Pleural Disease ### Slide 26
Need to exhale CO2: Supply vs Demand
How much air movement can you sustain?
Causes of inability to sustain the needed minute ventilation
Muscle weakness/inefficiency
Increased respiratory system loads
Decreased ventilatory drive
Neuromuscular disease
Hyperinflation
Respiratory muscle hypoxia
Pleural Disease
Elevated airway resistance
Stiff Lungs
Low chest wall compliance ### Slide 27
TODO: No text extracted from this slide. ### Slide 28
Need to exhale CO2: Supply vs Demand
How much air movement can you sustain?
Causes of inability to sustain the needed minute ventilation
Muscle weakness/inefficiency
Increased respiratory system loads
Decreased ventilatory drive
Neuromuscular disease
Hyperinflation
Respiratory muscle hypoxia
Pleural Disease
Elevated airway resistance
Stiff Lungs
Low chest wall compliance
Opiates and other sedatives
Chronic compensated hypercapnia
Sleep
Metabolic alkalosis
A talk for another day ### Slide 29
Schema for Ventilatory Failure
𝑉𝐸∝𝑉𝐶𝑂21 −𝑉𝑑𝑉𝑡∗𝑃𝑎𝐶𝑂2
We monitor O2
We infer things about CO2
Patient’s brainstem monitors it for us
When do you need an ABG? ### Slide 30
Schema for Ventilatory Failure
𝑉𝐸∝𝑉𝐶𝑂21 −𝑉𝑑𝑉𝑡∗𝑃𝑎𝐶𝑂2 ### Slide 31
Schema for Ventilatory Failure
𝑉𝐸∝𝑉𝐶𝑂21 −𝑉𝑑𝑉𝑡∗𝑃𝑎𝐶𝑂2
Severity of illness dependent ### Slide 32
Schema for Ventilatory Failure
𝑉𝐸∝𝑉𝐶𝑂21 −𝑉𝑑𝑉𝑡∗𝑃𝑎𝐶𝑂2 ### Slide 33
Schema for Ventilatory Failure
𝑉𝐸∝𝑉𝐶𝑂21 −𝑉𝑑𝑉𝑡∗𝑃𝑎𝐶𝑂2 ### Slide 34
Schema for Ventilatory Failure
𝑉𝐸∝𝑉𝐶𝑂21 −𝑉𝑑𝑉𝑡∗𝑃𝑎𝐶𝑂2 ### Slide 35
Schema for Ventilatory Failure
𝑉𝐸∝𝑉𝐶𝑂21 −𝑉𝑑𝑉𝑡∗𝑃𝑎𝐶𝑂2
Pain and pleural irritation causing hyperventilation ### Slide 36
Schema for Ventilatory Failure
𝑉𝐸∝𝑉𝐶𝑂21 −𝑉𝑑𝑉𝑡∗𝑃𝑎𝐶𝑂2 ### Slide 37
Schema for Ventilatory Failure
𝑉𝐸∝𝑉𝐶𝑂21 −𝑉𝑑𝑉𝑡∗𝑃𝑎𝐶𝑂2 ### Slide 38
Schema for Ventilatory Failure
𝑉𝐸∝𝑉𝐶𝑂21 −𝑉𝑑𝑉𝑡∗𝑃𝑎𝐶𝑂2 ### Slide 39
Schema for Ventilatory Failure
𝑉𝐸∝𝑉𝐶𝑂21 −𝑉𝑑𝑉𝑡∗𝑃𝑎𝐶𝑂2 ### Slide 40
Summary:
Keep O2 and CO2 problems separate in your thinking
Low O2 is not the main cause of dyspnea
If it’s real and it doesn’t get better with 6L, it’s shunting.
Symptoms are driven by CO2 flux
Symptoms occur when it’s hard to sustain what you need
Do they need a lot?
High metabolic rate, Inefficient Ventilation, Low blood CO2
Can they not sustain much?
Bellows don’t work, Breathing control doesn’t work, Loads on system ### Slide 41
TODO: No text extracted from this slide.

168.3 Learning objectives

Physiology-Based Schemas
Acute Respiratory Failure Goals:
Cargo Cult Thinking: two things occur together → one causes the other
Hypoxemia and Dyspnea
Solution to cargo cult thinking? Try to falsify

168.4 Bottom line / summary

Physiology-Based Schemas
Acute Respiratory Failure Goals:
Cargo Cult Thinking: two things occur together → one causes the other
Hypoxemia and Dyspnea
Solution to cargo cult thinking? Try to falsify

168.5 Approach

TODO: Outline the initial assessment or decision point.
TODO: Outline the next diagnostic or management step.
TODO: Outline follow-up or escalation criteria.

168.6 Red flags / when to escalate

TODO: List red flags that require urgent escalation.

168.7 Common pitfalls

TODO: Capture common errors or missed steps.

168.8 References

TODO: Add landmark references or guideline citations.

168.9 Slides and assets

Presentations/IM Noon Conf - Respiratory Failure.pptx

168.10 Source materials

Presentations/IM Noon Conf - Respiratory Failure.pptx