L4 Respiratory failure pt 1

Question 1

Define respiratory failure and the two types.

O2 -> delivery to tissues, CO2 –>pH

Answer 1

When the respiratory system is unable to adequately maintain arterial O2 and CO2 within normal limits.

PaO2 > 8kPa -
PaCO2 < 6.6 kPa

1. Hypoxia: O2 is below but CO2 is in normal range
2. Hypercapnic: CO2 is greater, but O2 is in normal range

Question 2

What are the main causes of respiratory failure

Answer 2

1. Pump failure: Hypoventilation

• Neural control
• Mechanical loads: resistive or elastic (airway, chest)

2. Exchange failure: VQ mismatch, R-L shunt, diffusion
   - Diffusive: blood-alveolar relationship

Question 3

Hypoxaemia vs Hypoxic

Answer 3

Hypoxaemia is the amount of O2 being carried to the blood.

whereas Hypoxia is the partial pressure of the gas in blood

Can be normoxic (normal partial presssure) but hypoxaemic if you have anaemia (no Hb)

Question 4

Compare the slope of O2 association and CO2 association curve

Answer 4

Steeper curve = small changes in PaCO2, there is large changes in blood content

The bicarbonate = kidneys (long term-> short term)/ CO2 = lungs (short term/immediate). Coontrolling the Co2 makes the pH constant

O2 curve is relatively flat above 8kpa, but below that it will drop off quite rapidly

Question 5

Where is each drop in the Pressure of O2 in the Oxygen cascade, and what things can disrupt it

Answer 5

1. Start with atmospheric O2: 21.21 kPa.

Drop - interrupted by - ventilation - dilution effect due to pump failure

• Hypoventilation
• starting with reduced PiO2 because of altitude- drops the curve down
  2. Alveoli

Drop - interrupted by Diffusion issues
- surface area -eg. emphysema, diffuse lung disease

• diffusion distance
  eg. pulmonary hypertension, anaemia
• natural lower solubility of O2 vs CO2

Presents as diffusion limitation with increased blood flow in exercise. Only in grossly impaired gas exchange would there be reduced end capillary O2 w/out

3. Pulmonary capillary
   - Drop interrupted by V/Q shunt: mixing of venous blood with arterial blood w/out going through gas exchange in pul. capillaries.

Leads to signif lower O2 but basically same CO2

4. Systemic artery - where it is measured.
   - Drop interrupted by diffusion
5. Tissue/ mitochondria (need 0.66-2.)

Question 6

What is the main relationship between Partial pressure of O2 in the blood (PAO2) to alveolar ventilation (Va) and atmospheric O2 pressure (FiO2)

vs PACO2 to Va

Answer 6

1.PAO2 is proportional to decreased FiO2 which leads to a drop in the curve but no change in its shape: a reduced PaO2

For hypoventilation (decrease in Va) it is a lower intersection of the normal curve

Both get hypoxic.

2. PACO2 is proportional to VCO2- set by metabolic rate/ Va. So reduced Alveolar ventilation (hypoventilation) is the cause of Hypercapnia

Question 7

What are the 6 potential causes of hypoventilation :

leads to hypercapnia / hypoxia

Answer 7

• Reduced respiratory drive (no hypercapnia but low FiO2) because CO2 changes lead to big changes in ventilation, whereas hypoxic drive is low
• Alcohol, drugs, barbituates: depression in respiratory drive
• Mechanical drive lost Phrenic nerve damage, paralysed muscles post Polio
• Congenital reduced respiratory drive
• Excessive work of breathing: Obstructive pulmonary diseases that increase resistance that muscles act against: asthma - noxious
• Increased elastic recoil work of breathing : obesity, kypho-scoliosis. Infants: neonatal respiratory distress syndrome

Question 8

What is the alveolar arterial gradient and what causes it

Answer 8

Difference between the end tidal PAlveolar O2 and the blood gas PaO2.

If it is there, this is due to V/Q shunt
- Pulmonary vascular pathology, cardiac pathology - eg. septal defects.

Question 9

Where is the biggest drop in the Carbon dioxide cascade and why is the rest of it so flat

Answer 9

From Alveoli to Atmospheric air.

This drop is interrupted by
reduced ventilation of the lung (hypoventilation).

1. Diffusion from pulmonary cap–>alveoli only small drop bc of high solubility
2. Transfer from systemic vein –>pulmonary capillary
   - bc of how CO2 is carried, not much effect from V/Q shunt

Question 10

What pH change is related to Hypercapnia and what other test result supports a long term state

Answer 10

Respiratory acidotic state.

If long term, then Bicarbonate should have increased to compensate.

Question 11

What is more distressing, High CO2 or low O2. In what case is it not distressing

Answer 11

High CO2. leads to dyspnoea as a noxious stimulus.
If not distressed there is reduced respiratory drive - substance abuse

O2 can be dropped very low and put people to sleep.