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Block 2 > Pathophysiology of respiratory failure > Flashcards

Pathophysiology of respiratory failure Flashcards

1
Q

What is acute respiratory failure?

A

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

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2
Q

What is type 1 respiratory failure?

A

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
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3
Q

What is type 2 respiratory failure?

A

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

May also be hypoxaemic

Caused by hypoventilation

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4
Q

What is end tidal gas?

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

A

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
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5
Q

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

A

Lung pathology:

  • Reduced diffusion
  • V/Q mismatch
  • Shunting
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6
Q

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

A

Changes in:

  • Dead space ventilation
  • Diffusing capacity
  • Lung perfusion
  • Ventilation-perfusion mismatching
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7
Q

How is the pAO2 calculated?

How is the alveolar-arterial oxygen gradient calculated?

What is a normal alveolar-arterial oxygen gradient?

A

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

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8
Q

What is alveolar pressure?

A

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

pAO2 + pACO2 + pAH2O + pAN2

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9
Q

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

A

Hypoventilation:

  • Increased pACO2
  • Decreased pAO2

Hyperventilation:

  • Increased pAO2
  • Decreased pACO2
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10
Q

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

A

Increased pAO2

Unchanged pAH2O, pACO2

Decreased pAN2

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11
Q

Define ventilation

A

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

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12
Q

Define perfusion

A

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

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13
Q

What are the 2 extremes of V/Q mismatch?

A

V/Q ratio = 0

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

V/Q = infinite

  • No perfusion
  • pAO2 = pIO2
  • Dead space
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14
Q

What can cause hypoventilation?

A

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
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15
Q

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

A

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

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16
Q

What are the effects of hypoxic pulmonary vasoconstriction?

A

Increased O2 saturations as blood is moved to well ventilated alveoli

17
Q

What are the causes of shunting?

A

Intrapulmonary:

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

Intracardiac:

  • Right to left shunt (congenital abnormality)
18
Q

What can cause hypoxaemia?

A
  • Low PiO2
  • Hypoventilation
  • V/Q mismatch or Shunting
  • Diffusion abnormality
  • Low Cardiac Output
19
Q

Define oxygen delivery

How is it calculated?

A

The amount of oxygen delivered to the tissues per minute.

= cardiac output x SaO2 x Hb x mls of O2 at 100% saturation

20
Q

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

A
  • 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
Q

What are the clinical features of respiratory failure?

A

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
Q

What are the signs and symptoms of severe respiratory failure?

A

RR >30/min or <8/min

Difficulty completing sentences

Agitation/confusion/comatose

Cyanosis

O2 sats <90%

Deterioration despite therapy

Block 2 (46 decks)

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