Lecture 2 and 5 - Respiratory physiology 1 and 2

Question 1

What is type 1 respiratory failure?

Answer 1

This is where the lungs fail to adequately oxygenate arterial blood (carbon dioxide is normal). It is known as hypoxaemia. The PaO2 (partial arterial pressure of oxygen) is <60mmHg.

Question 2

What is type 2 respiratory failure?

Answer 2

This is where the lungs fail to adequately get rid of carbon dioxide as well as oxygenate the body. It is known as hypercapnia. The PaCO2 (partial arterial pressure of carbon dioxide) is >50mmHg.

Question 3

What are the five mechanisms that cause hypoxaemia (type 1 respiratory failure)?

Answer 3

1. Hypoventilation.
2. Low FiO2.
3. V/Q mismatch.
4. Shunt.
5. Diffusion defect.

Question 4

How does hypoventilation cause hypoxaemia and hypercapnia?

Answer 4

Something is causing our body to not ventilate properly (there is a decrease in ventilation). This means we are unable to get enough oxygen into our lungs and not enough carbon dioxide out. As ventilation decreases carbon dioxide increases.

Question 5

Describe low FiO2?

Answer 5

This is when the air has less oxygen particles, so our ventilation is actually fine it is the atmosphere around us. This is only a type 1 respiratory failure, as we can breathe off the carbon dioxide by hyperventilation. Normally we see low FiO2 in high altitude places. Basically you breathe in less oxygen.

Question 6

Describe a V/Q mismatch?

Answer 6

This is when ventilation (V) does not match perfusion (Q). This causes there to be a decrease in oxygen. The amount of haemoglobin saturated with oxygen can never get past a certain point. It can be due to:

1. Dead space - there is something (e.g. ischaemic alveoli capillaries) which is causing poor perfusion, so V/Q ratio is very high (i.e. 100/0).
2. Shunt - there is something blocking (obstruction) blocking the air, stopping it from being able to reach the capillaries. This causes poor ventilation, so V/Q ratio is 0.

Question 7

Describe a R->L shunts?

Answer 7

This is where deoxygenated blood mixes with oxygenated blood, this causes the overall oxygen content/saturation in the blood to decrease (i.e. from 100% to 75%) - person will have hypoxaemia.

Question 8

How do you tell if a person either has 1) V/Q mismatch OR 2) R-L shunt?

Answer 8

You give the person 100% oxygen and the following will happen depending on what they have?

1) V/Q mismatch - even though you give the pt 100% oxygen, their O2 sats will not change. Something is stopping either the perfusion or ventilation of oxygen so O2 will not get to the tissues hence their O2 sats will not change.
2) R-L shunt - when giving the person 100% O2 their O2 sats will increase.

Question 9

Describe A-a gradient?

Answer 9

This is the gradient between partial pressures of alveolar oxygen and arterial oxygen. PAO2 - PaO2. In normal healthy people this gradient is around 5-10mmHg. A-a gradient is important in determining if hypoaxaemia is intra-pulmonary or extra-pulmonary.

Question 10

What happens if A-a gradient increases?

Answer 10

This means that there is a problem intra-pulmonary. It can be due to:
1. Decreased partial carbon dioxide pressure.
2. Decreased arterial partial oxygen pressure.
Basically something is causing the molecules to take longer to diffuse and get from air into the capillaries.

Question 11

What causes type 2 respiratory failure (hypercapnia)?

Answer 11

1. Increases airway resistance - this can be due to COPD, asthma, suffocation.
2. Decreased work of breathing - drugs, alcohol, brainstem problem (basically something is causing the brain to not respond to ventilation so there is a decrease in ventilation).
3. Less lung to ventilate - chronic bronchitis.
4. Nerve problems - this can cause decreased work of breathing.
5. Abnormal rib cage - kyphosis, scoliosis, obesity.

Question 12

List the oxygen cascade (in order)?

Answer 12

1. Air (PiO2) - this is inspired pressure of O2.
2. Gas (PAO2) - this is alveolar pressure of O2.
3. Capillary.
4. Arterial (PaO2) - this is arterial pressure of O2.
5. Tissues (PtO2).

Question 13

What does PiO2 depend on?

Answer 13

Inspired oxygen depends on altitude and FiO2.

Question 14

What does PAO2 depend on?

Answer 14

Alveolar pressure of oxygen depends on alveolar ventilation (does not include dead space ventilation).

Question 15

What does PaO2 depend on?

Answer 15

Arterial pressure of oxygen depends on gas exchange (measured as A-a gradient and influenced by: V/Q mismatch, R-L shunt and diffusion).

Question 16

What does PtO2 depend on?

Answer 16

Tissue pressure of oxygen depends on delivery of oxygen to tissues (perfusion - Q), content on oxygen in blood (c) and oxygen extraction.

Question 17

What is diffusion limited?

Answer 17

To test if someone is diffusion limited (a problem with diffusion in their lungs) you give them carbon monoxide (CO). This is because CO goes into the blood and binds immediately to Hb, the only variable that affects CO is diffusion. If there is a diffusion problem (i.e. diffusion distance has increased) then the amount of carbon dioxide getting into the blood would be low.

Question 18

What is perfusion limited?

Answer 18

To test if someone is perfusion limited (a problem with perfusion in their lungs) you give them N2. When N2 goes into the blood it doesn’t bind to anything so it stays in the blood, so if perfusion is working then N2 moves along quickly and a gradient is maintained - so N2 in the lungs is always higher than N2 in the capillaries. If blood flow has decreased (poor perfusion - could be due to PE) the gradient will reach a level where there is no gradient (N2 in lungs = N2 in blood). This means that the amount of N2 in the blood will be less than normal. Hence indicating a perfusion limited problem.

Question 19

What does diffusion depend on?

Answer 19

1. Gas.
2. Diffusion distance/thickness.
3. Surface area.
4. (Hb).
5. Capillary volume.

Question 20

What can cause abnormal diffusion?

Answer 20

1. Alveolar-capillary block (diffuse interstitial lung disease).
2. Loss of diffusing surface area (emphysema).
3. Capillary volume/haemoglobin (pulmonary hypertension, anaemia).
4. V/Q mismatch.

Question 21

Define COPD?

Answer 21

Chronic obstructive pulmonary disease - disease state characterised by airflow limitation that is not fully reversible. The airflow limitation is usually both progressive and associated with an abnormal inflammatory response of the lungs to noxious particles or gases (external irritant).

Question 22

What are the causes of COPD?

Answer 22

1. Chronic mucus hyper-secretion (chronic bronchitis) - this is caused by goblet cell hyperplasia, mucus glands hypertrophy.
2. Emphysema (destruction of lung tissue, more collapsible airways especially during expiration) - this is caused by protease imbalance, neutrophil infiltration.
3. Small airway inflammation and obstruction.