Lecture 3: Pathophysiology of resp. failure

Question 1

Define respiratory failure:

Answer 1

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

Question 2

What are the types of respiratory failure?

Answer 2

PaO2 < 8kPa with PaCO2 < 6.6 kPa (Hypoxic, type 1)

PaO2 < 8kPa with PaCO2 >6.6kPa (hypercapnic type 2)

Question 3

What are the four things to think about when it comes to respiratory failure?

Answer 3

• Autonomic
• Anatomical / compliance
• Flow limitations
• Alveolar exchange

Question 4

The oxygen dissociation curve:
- Describe its shape
- Whats on its axis
- What shifts it to the right

Answer 4

• Shape: Sigmoidal
• Axis: (x)= PO2, (y) = O2 saturation
• Decreased pH, Increased DPG, Increased temperature, increased PCO2. (check)

Question 5

Describe the importance of the oxygen dissociation curve when it comes to changing oxygen saturation.

Answer 5

Below 8kPa the curve drops dramatically for saturation. Above 8kPa then theres very little gain in saturation

Question 6

Whats hypoxia?

Answer 6

A function of the PO2

Question 7

Whats hypoxaemia?

Answer 7

Total conc. of oxygen in the blood i.e if anaemia then you can have normal PO2 but still be hypoxaemic.

Question 8

Describe the CO2 association curve:

Answer 8

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.

Question 9

How can respiratory failure be divided (again)?

Answer 9

Pump failure (Autonomic, anatomical, flow restriction)

Exchange failure (V/Q mismatch, R-L shunt, diffusion)

Question 10

Whats the oxygen cascade?

Answer 10

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)

Question 11

Describe whats happening in the oxygen cascade:

Answer 11

• 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.

Question 12

In the oxygen cascade what factors affect where?

Answer 12

The first step ‘dilution’ is determined by ventilation

The proceeding diffusion, V/Q shunt, are the product of alveolar exchange.

Question 13

In what situation can PiO2 decrease and describe the effect on the oxygen cascade:

Answer 13

insert slide 10

Question 14

Describe the relationship of PAO2 and Alveolar ventilation:

Answer 14

Increased ventilation and increased kPa

Approaches plateau beyond 10RR

Insert slide 12

Question 15

What does hypoventilation do to the oxygen cascade:

Answer 15

Enhances the dilution step

Hypoventilation or decreased FiO2 leads to lowered PAO2

Question 16

What does hypoventilation lead to?

Answer 16

Excessive work of breathing.
- Fatigued resp muscles
- Reduced elasticity

The flow volume loop becomes massive/

Question 17

Describe the inputs for resp. control?

Answer 17

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

Question 18

Describe what you see on the ventilation alveolar pressure curve for oxygen and CO2:

Answer 18

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.

Question 19

What law governs the alveolar gas exchange?

Answer 19

Ficks law; Diffusion is proportional to SA, Difference in partial pressure and inversely proportional the the thickness of the membrane.

Question 20

Describe the equilibration in the pulm. capillary of O2 and CO2:

Answer 20

It takes 0.25s /1s for PO2 and PCO2 to diffuse to their max partial pressure. (insert graph)

Question 21

What happens in exercise to the equilibration in the pulmonary cap? and how does impaired gas exchange change things?

Answer 21

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.

Question 22

What are some causes of abnormal confusion?

Answer 22

• Alveolar capillary block; Diffuse lung disease
• Loss of diffusing surface; Emphysema
• Capillary volume / heamoglobin; Pulmonary hypertension, anaemia

Question 23

What is a shunt?

Answer 23

Physiological (blood thats not oxygenated)

Anatomical; Contaminated with venous blood.

Question 24

How can shunts be measured?

Answer 24

Measure PAO2 and then do an arterial gas measure PaO2.

Question 25

How can hypoxia be created?

Answer 25

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)

Question 26

Review lecture for slide 29

Answer 26

now

Question 27

What can lead to hypercapnia?

Answer 27

Hypoventilation: Dec resp. drive, neuromuscular blockade, muscle weakness/fatigue, abnormal load (resistive, elastic, chest wall)

Question 28

Insert slide 40

Answer 28

now