Lecture 2 and 5 - Respiratory physiology 1 and 2 Flashcards
What is type 1 respiratory failure?
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.
What is type 2 respiratory failure?
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.
What are the five mechanisms that cause hypoxaemia (type 1 respiratory failure)?
- Hypoventilation.
- Low FiO2.
- V/Q mismatch.
- Shunt.
- Diffusion defect.
How does hypoventilation cause hypoxaemia and hypercapnia?
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.
Describe low FiO2?
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.
Describe a V/Q mismatch?
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:
- 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).
- 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.
Describe a R->L shunts?
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.
How do you tell if a person either has 1) V/Q mismatch OR 2) R-L shunt?
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.
Describe A-a gradient?
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.
What happens if A-a gradient increases?
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.
What causes type 2 respiratory failure (hypercapnia)?
- Increases airway resistance - this can be due to COPD, asthma, suffocation.
- 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).
- Less lung to ventilate - chronic bronchitis.
- Nerve problems - this can cause decreased work of breathing.
- Abnormal rib cage - kyphosis, scoliosis, obesity.
List the oxygen cascade (in order)?
- Air (PiO2) - this is inspired pressure of O2.
- Gas (PAO2) - this is alveolar pressure of O2.
- Capillary.
- Arterial (PaO2) - this is arterial pressure of O2.
- Tissues (PtO2).
What does PiO2 depend on?
Inspired oxygen depends on altitude and FiO2.
What does PAO2 depend on?
Alveolar pressure of oxygen depends on alveolar ventilation (does not include dead space ventilation).
What does PaO2 depend on?
Arterial pressure of oxygen depends on gas exchange (measured as A-a gradient and influenced by: V/Q mismatch, R-L shunt and diffusion).