20. RESPIRATORY FAILURE

Question 1

Define hypoxia and hypoxaemia.

Answer 1

Hypoxia: O2 deficiency at tissue level
Hypoxaemia: Low pO2 in arterial blood

Question 2

Give examples when hypoxia may be present without hypoxaemia?

Answer 2

Anaemia - reduces the oxygen carrying capacity of blood can result in tissue hypoxia despite a normal pO2.
Poor circulation

Question 3

What is the normal range of O2 saturation and pO2?

Answer 3

• O2 saturation 94 -98%

* paO2 9.3 - 13.3 kPa (UHL)

Question 4

Below what levels of O2 sats and pO2 is tissue damage most likely?

Answer 4

• O2 saturation < 90%

- pO2 < 8 kPa

Question 5

What is respiratory failure?

Answer 5

Impairment in gas exchange causing hypoxaemia with or without hypercapnia

Question 6

When is it possible to have hypoxaemia without hypoxia?

Answer 6

if compensatory mechanisms are adequate – these include elevated haemoglobin and cardiac output, increased capillary bed density, and increased levels of 2,3 DPG to facilitate O2 tissue delivery.

Question 7

What is type 1 respiratory failure?

Answer 7

• low PaO2 < 8 kPa or O2 saturation <90% breathing room air at sea level (Hypoxaemia)
• pCO2 normal or low
• Gas exchange is impaired at the level of aveolar-capillary membrane

Question 8

What is type 2 respiratory failure?

Answer 8

• Low PaO2 AND high PaCO2 > 6.5 kPa breathing room air at sea level
• Reduced ventilatory effort (pump failure) or inability to overcome increased resistance to ventilation entire lung

Question 9

What are the two possible parts that the respiratory system can be divided into?

Answer 9

• the lung, i.e. the gas-exchanging organ,
• the pump, i.e. the part that ventilates the lungs that consists of the chest wall, including the respiratory muscles, the respiratory controllers in the central nervous system (CNS), and the pathways that connect the central controllers with the respiratory muscles (cranial, spinal and peripheral nerves)

Question 10

Give examples of when type 1 and type 2 respiratory failure can be present

Answer 10

• Type 1 RF can progress to Type 2
• Diffuse Pulmonary/Lung Fibrosis - starts as Type 1 Respiratory failure because of impaired oxygen diffusion related to a thickened interstium, but as the disease progress and the lungs get stiffer there is also impaired ventilation (impaired pump) and hence Type 2 Respiratory failure.
• Severe asthma exacerbation leads to both gas-exchange impairment and pump impairment as the central controllers or respiratory muscles, or both, become unable to maintain adequate ventilation.

Question 11

What are the effects of hypoxaemia?

Answer 11

• Impaired CNS function, confusion, irritability, agitation
• Tachypnaea
• Tachycardia
• Cardiac arrhythmias & cardiac ischaemia
• Hypoxic vasoconstriction of pulmonary vessels
• Cyanosis (bluish discolouration of the skin and mucous membranes due to presence of deoxyhaemoglobin (i.e. unsaturated Hb)

Question 12

What is central cyanosis?

Answer 12

Seen in oral mucosa, tongue, lips
Indicates hypoxaemia - occurs when the level of
deoxygenated haemoglobin in the arteries is
below 5 g/dL with oxygen saturation below 85%

Question 13

What is peripheral cyanosis?

Answer 13

In fingers, toes

Poor local circulation - more oxygen extracted by the peripheral tissues

Question 14

What are Common causes central cyanosis?

Answer 14

▪ Pulmonary
▪ Impaired gas exchange secondary to pneumonia
▪ Embolism and ventilation perfusion mismatch
▪ Impaired gas diffusion via the alveoli
▪ High altitude
▪ Anatomic shunts
▪ Right to left shunt in congenital heart disease
▪ Intrapulmonary shunt

Question 15

What 6 things can cause hypoxaemia?

Answer 15

1. Low inspired pO2 -
2. Hypoventilation - (respiratory pump failure)
3. Ventilation/Perfusion mismatch
4. Diffusion defect - problems of the alveolar capillary membrane
5. Intra-lung shunt - Acute Respiratory Distress Syndrome (ARDS)
6. Right to left shunt (eg. Cyanotic heart disease) - extra-pulmonary

Question 16

What are 2 compensatory mechanisms in chronic hypoxaemia?

Answer 16

• increased EPO secreted by kidney, raised Hb (Polycythemia)
• Increased 2,3, DPG - shifts haemoglobin saturation curve so oxygen released more freely
• Increased capillary density

Question 17

What does chronic vasoconstriction of pulmonary vessels result in?

Answer 17

Leads to pulmonary hypertension, increases effort of the right heart, leads to right sided heart failure (cor pulmonale)

Question 18

how does higher altitudes lead to hypoxaemia?

Answer 18

• Partial pressure of oxygen falls the further up we are from sea level
• Therefore partial pressure oxygen falls in alveoli
• Therefore partial pressure oxygen in arterial blood is low
–hypoxaemia
• Fully improves with O2

Question 19

What is the most common cause of hypoxaemia?

Answer 19

ventilation perfusion mismatch

Question 20

What occurs in people acutely exposed to high altitudes?

Answer 20

hypoxaemia due a low inspired pO2 level. The resulting stimulation of the peripheral chemoreceptors
causes hyperventilation with an increase in CO2 washout.
The end result is a low pO2 and a low pCO2

Question 21

Describe the pO2 and pCO2 in by V/Q < 1

Answer 21

perfusion of poorly ventilated alveoli:
• PaO2 is low
• Initially PaCO2 rises UNTIL/UNLESS there is compensatory hyperventilation – then PaCO2 will be either normal or low
• If lung disease severe hyperventilation may not be able to compensate for V:Q <1 and CO2 remains elevated

Question 22

Describe the pO2 and pCO2 in by V/Q > 1

Answer 22

• PaO2 rises (slightly) and PaCO2 falls
• If lungs not healthy the “extra” air going to these parts of the lung is “wasted” – increased dead space (alveoli ventilated but not perfused)

Question 23

Why does hyperventilation only raise pO2 by small amount?

Answer 23

• Can’t dissolve dissolve a lot of O2 in blood as its not very soluble
• Haemoglobin in blood perfusing healthy alveoli are already saturated

Question 24

How does a V/Q < 1 affect rate of ventilation and how?

Answer 24

Rise in CO2 causes drop in pH, detected by central and chemoreceptors and hypoxaemia detected by peripheral chemoreceptors leading to increased rate of ventilation.

Question 25

What happens to partial pressures in mixed blood after increased ventilation in a V/Q < 1?

Answer 25

In blood of unaffected segments: pO2 rises and pCO2 falls

• dissolved oxygen rises only a little, can’t compensate for affected units
• drop in CO2 sufficient to compensate for affected units