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1

What is acute respiratory failure?

Occurs when the respiratory system is no longer able to meet the metabolic demands of the body 

2

What is type 1 respiratory failure?

Hypoxaemic: paO2 <8kPa (or equal to) when breathing room air

Caused by:

  • Reduced diffusion (e.g. altitude)
  • Reduced diffusion capacity of the lung:
    • Reduced surface area
    • Damage to the alveolar membrane

 

3

What is type 2 respiratory failure?

Hypercapnic respiratory failure: paCO2 >6.7kPa (or equal to)

May also be hypoxaemic

Caused by hypoventilation

4

What is end tidal gas?

Why does it have a slightly higher pO2 than alveolar gas?

Gas at the mouth at the end of exhalation:

  • Combination of dead space gas and alveolar gas
  • This is why the pO2 here is slightly more than alveolar gas

5

What can cause a larger difference in the alveolar-arterial oxygen levels?

Lung pathology:

  • Reduced diffusion
  • V/Q mismatch
  • Shunting

6

What influences the alveolar partial pressure of oxygen (pAO2)?

Changes in:

  • Dead space ventilation
  • Diffusing capacity
  • Lung perfusion
  • Ventilation-perfusion mismatching 

7

How is the pAO2 calculated?

How is the alveolar-arterial oxygen gradient calculated?

What is a normal alveolar-arterial oxygen gradient?

pAO2 = pIO2 - paCO2 (from ABG) / R

pIO2 =  partial pressure of inspired oxygen.

R= constant 

A-aO2 = pAO2 - paO2 (from ABG)

16 years: around 1.1kPa

80 years: around 3.1 kPa

8

What is alveolar pressure?

The partial pressure of all the gases in the alveoli combined:

pAO2 + pACO2 + pAH2O + pAN2

9

What will happen to the alveolar partial pressures of CO2 and O2 during hyperventilation and hypoventilation?

Hypoventilation:

  • Increased pACO2
  • Decreased pAO2

Hyperventilation:

  • Increased pAO2
  • Decreased pACO2

10

What will happen to the partial pressures of the gases in the alveoli if supplementary oxygen is given?

Increased pAO2

Unchanged pAH2O, pACO2

Decreased pAN2

11

Define ventilation

The amount of gas that is exchanged through the lungs in 1 minute

12

Define perfusion

The amount of blood that passes through the lungs in 1 minute

13

What are the 2 extremes of V/Q mismatch?

V/Q ratio = 0

  • Ventilation = 0
  • Shunting
  • pAO2 = paO2 = pvO2

V/Q = infinite

  • No perfusion
  • pAO2 = pIO2
  • Dead space

14

What can cause hypoventilation?

Respiratory disease:

  • COPD
  • Asthma
  • Cystic fibrosis
  • Bronchiolitis

Loss of respiratory drive:

  • Head injury
  • Drug OD
  • Stroke

Altered NM transmission:

  • Myasthenia gravis
  • Damage to anterior horn of spinal cord (MND)
  • Spinal cord injury
  • Demyelination of nerve axon (Guillan Barre)

Muscle disease:

  • Muscular dystrophy

 

15

What are the effects of giving supplementary oxygen to perfused but not ventilated alveoli? (V/Q = 0, shunt)

Little effect as increased pIO2 does not reach poorly ventilated alveoli and blood is already 100% saturated in well ventilated alveoli.

16

What are the effects of hypoxic pulmonary vasoconstriction?

Increased O2 saturations as blood is moved to well ventilated alveoli

17

What are the causes of shunting?

Intrapulmonary:

  • Pneumonia
  • Pulmonary oedema
  • Atelactasis (lung collapse)
  • Pulmonary haemorrhage or contusion

Intracardiac:

  • Right to left shunt (congenital abnormality)

18

What can cause hypoxaemia?

  • Low PiO2
  • Hypoventilation
  • V/Q mismatch or Shunting
  • Diffusion abnormality
  • Low Cardiac Output

19

Define oxygen delivery

How is it calculated?

The amount of oxygen delivered to the tissues per minute.

= cardiac output x SaOx Hb x mls of O2 at 100% saturation

 

20

What are the effects of low cardiac output on oxygen delivery and oxygen saturations?

  • Low Cardiac Output means less oxygen delivery per unit time and tissues have to extract a higher percentage of O2 to meet the demand
    • Blood returning to the lungs is less saturated than normal
  • Decreased cardiac output means more time in pulmonary capillaries for oxygen uptake and so blood arriving at tissues is usually fully saturated

21

What are the clinical features of respiratory failure?

Respiratory compensation

  • Tachypnoea (high RR)
  • Use of accessory muscles
  • Intercostal recession (in infants)
  • Nasal flaring
  • Splinting of accessory muscles

Sympathetic stimulation

  • High BP
  • High HR
  • Sweating

Tissue hypoxia

  • Altered mental state
  • Lactic acidosis (anaerobic metabolism)
  • Low HR and BP (late stage)

Haemoglobin desaturation

  • Cyanosis
  • Low O2 sats

Hypercapnia

  • Flapping tremor
  • Confusion/coma
  • Respiratory acidosis
  • Sympathetic stimulation

22

What are the signs and symptoms of severe respiratory failure?

RR >30/min or <8/min

Difficulty completing sentences

Agitation/confusion/comatose

Cyanosis

O2 sats <90%

Deterioration despite therapy