Lecture 3: Pathophysiology of resp. failure Flashcards
Define respiratory failure:
When the respiratory system is nuable to adequately maintain arterial O2 and CO2 within normal limits.
i.e PaO2 >8kPa
i.e PaCO2 <6.6kPa
What are the types of respiratory failure?
PaO2 < 8kPa with PaCO2 < 6.6 kPa (Hypoxic, type 1)
PaO2 < 8kPa with PaCO2 >6.6kPa (hypercapnic type 2)
What are the four things to think about when it comes to respiratory failure?
- Autonomic
- Anatomical / compliance
- Flow limitations
- Alveolar exchange
The oxygen dissociation curve:
- Describe its shape
- Whats on its axis
- What shifts it to the right
- Shape: Sigmoidal
- Axis: (x)= PO2, (y) = O2 saturation
- Decreased pH, Increased DPG, Increased temperature, increased PCO2. (check)
Describe the importance of the oxygen dissociation curve when it comes to changing oxygen saturation.
Below 8kPa the curve drops dramatically for saturation. Above 8kPa then theres very little gain in saturation
Whats hypoxia?
A function of the PO2
Whats hypoxaemia?
Total conc. of oxygen in the blood i.e if anaemia then you can have normal PO2 but still be hypoxaemic.
Describe the CO2 association curve:
Near linear and very steep. Therefore much more CO2 in blood content per increase in kPa and elevated CO2 will lead to acidosis. (acutely can be bad)
CO2 curve goes up indef, O2 flattens off very quickly.
How can respiratory failure be divided (again)?
Pump failure (Autonomic, anatomical, flow restriction)
Exchange failure (V/Q mismatch, R-L shunt, diffusion)
Whats the oxygen cascade?
It describes how the partial pressure drops as oxygen passes through various tissues to get through to the blood i.e
- PiO2 ~20kPa
- Ventilation, PO2 ~ 14kPa
- Diffusion, PO2 ~13.9kPa
- V/Q shunt, PO2 ~ 13.3kPa (in blood)
Describe whats happening in the oxygen cascade:
- Oxygen passes from air to alveoli (dilution phase) [ventilation]
- Oxygen passes from alveoli to pulm. cap (diffusion phase)
- Oxygenated blood passes from pul. cap to systemic artery (V/Q shunt, Dilution)
- Oxygen is passed from blood to tissues (diffusion)
Different insults alter the level of kPa drop in various places i.e anaemia drops it significantly when it passes from the pul. cap to system art.
In the oxygen cascade what factors affect where?
The first step ‘dilution’ is determined by ventilation
The proceeding diffusion, V/Q shunt, are the product of alveolar exchange.
In what situation can PiO2 decrease and describe the effect on the oxygen cascade:
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Describe the relationship of PAO2 and Alveolar ventilation:
Increased ventilation and increased kPa
Approaches plateau beyond 10RR
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What does hypoventilation do to the oxygen cascade:
Enhances the dilution step
Hypoventilation or decreased FiO2 leads to lowered PAO2
What does hypoventilation lead to?
Excessive work of breathing.
- Fatigued resp muscles
- Reduced elasticity
The flow volume loop becomes massive/
Describe the inputs for resp. control?
In the lung:
- J receptors (+ive)
- Deflation reflex (+ive)
- Hering breurer reflex (-ive)
Muscular:
- External intercostals (+ive)
- Receptors in muscles & joints (+ive)
Chemoreceptors
- Central (+ive) (CO2)
- Peripheral (+ive)(Dec O2, Inc CO2, H ions)
++ Pain, emotions, temp
Describe what you see on the ventilation alveolar pressure curve for oxygen and CO2:
As the partial pressure of CO2 changes the RR : O2 curve shifts down and left as CO2 decreases. However both curves plateau around 8kPa for PO2. (insert charts)
As the PO2 shifts the steepness of the RR:PCO2 decreases.
What law governs the alveolar gas exchange?
Ficks law; Diffusion is proportional to SA, Difference in partial pressure and inversely proportional the the thickness of the membrane.
Describe the equilibration in the pulm. capillary of O2 and CO2:
It takes 0.25s /1s for PO2 and PCO2 to diffuse to their max partial pressure. (insert graph)
What happens in exercise to the equilibration in the pulmonary cap? and how does impaired gas exchange change things?
Exercise reduces the transit time down from 0.75 to 0.25s.
Impaired gas exchange flattens the sigmoidal curve thus impairing exercise as insufficient time to increase PO2. Therefore can use exercise to test for impaired gas exchange as it pronounces the effect.
What are some causes of abnormal confusion?
- Alveolar capillary block; Diffuse lung disease
- Loss of diffusing surface; Emphysema
- Capillary volume / heamoglobin; Pulmonary hypertension, anaemia
What is a shunt?
Physiological (blood thats not oxygenated)
Anatomical; Contaminated with venous blood.
How can shunts be measured?
Measure PAO2 and then do an arterial gas measure PaO2.
How can hypoxia be created?
Decrease PiO2; Altitude
Hypoventilation: Dec resp. drive, neuromuscular blockade, muscle weakness/fatigue, abnormal load (resistive, elastic, chest wall)
Shunt V/Q: Pulmonary vascular pathology, Cardiac pathology (R->L shunt)
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What can lead to hypercapnia?
Hypoventilation: Dec resp. drive, neuromuscular blockade, muscle weakness/fatigue, abnormal load (resistive, elastic, chest wall)
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