3. Hypoxia

Question 1

Define hypoxia and classify the

causes.

Answer 1

Hypoxia may be defined either

as an inadequate oxygen supply

or

the inability to utilise oxygen at a cellular level.

Causes are divided into four main types:

1. > Hypoxic hypoxia
2. > Anaemic hypoxia
3. > Stagnant hypoxia
4. > Histotoxic hypoxia

Question 2

1
> Hypoxic hypoxia

5 causes

Answer 2

> Hypoxic hypoxia – a PaO2 < 12 kPa

1
• Low FiO2,
e.g. inadvertent hypoxic gas
delivery during anaesthesia

2
• Hypoventilation,
e.g. opiate induced

3
• Diffusion impairment,
e.g. pulmonary oedema, pulmonary fibrosis

4
• Ventilation–perfusion mismatch,
e.g. COPD, asthma, LRTI

5
• Shunt, e.g. atelectasis causing intrapulmonary shunt

Question 3

2. > Anaemic hypoxia

Answer 3

> Anaemic hypoxia –

normal PaO2 but
inadequate oxygen-carrying
capacity

• Low circulating haemoglobin level,
e.g. acute and chronic anaemias

• Normal circulating haemoglobin level
but reduced ability to carry
oxygen, e.g. carbon monoxide poisoning

Question 4

3

> Stagnant hypoxia

Answer 4

> Stagnant hypoxia

normal PaO2
and
oxygen-carrying capacity

but reduced tissue and organ perfusion

• e.g. cardiogenic shock

Question 5

4

> Histotoxic hypoxia

Answer 5

> Histotoxic hypoxia –

normal PaO2,
oxygen-carrying capacity
and tissue perfusion

but an inability of

the tissues to utilise
the oxygen at a cellular
mitochondrial level
• e.g. cyanide poisoning

Question 6

Draw oxyhaemoglobin dissociation
curves showing arterial (♦);
and mixed venous (•); points in
the four types of hypoxia.

Answer 6

3

Oxyhaemoglobin dissociation curve in hypoxic hypoxia
> PaO2 is reduced.
> Pv–O2 is reduced with venous desaturation (<75%).

Fig. 3.3 Oxyhaemoglobin dissociation curve in anaemic hypoxia
> PaO2 remains normal (>13.3 kPa).
> Global oxygen delivery is reduced due to reduced oxygen content.
> Result is increased oxygen extraction and venous desaturation.

Fig. 3.4 Oxyhaemoglobin dissociation curve in stagnant hypoxia
> PaO2 is normal.
> Pv–O2 is normal.
> T issues and organs do not receive the oxygenated blood due to
perfusion failure.

Fig. 3.5 Oxyhaemoglobin dissociation curve in histotoxic hypoxia
> PaO2 is normal.
> Cells are unable to utilise oxygen resulting in high venous saturations.
> C yanide poisoning will also be associated with a left shift of the
oxyhaemoglobin dissociation curve.

Question 7

What is oxygen content?

Answer 7

Oxygen is carried in the blood
in two main ways: combined with
haemoglobin and dissolved in the plasma. Oxygen content is calculated
by combining the proportion of oxygen bound to haemoglobin with that
dissolved

Question 8

Oxygen content =

Answer 8

Oxygen content =
[Bound Oxygen] + [Dissolved Oxygen]

= [Hb · 1.34 · SaO2] +
[PaO2 · 0.0225]

Where:

Hb Haemoglobin g/dL
1.34 Huffner’s constant –
each gram of haemoglobin combines with
1.34 mL oxygen

SaO2 Arterial oxygen saturation as a percentage,
e.g. 96% = 0.96

PaO2 Partial pressure of arterial oxygen
0.0225 mL of oxygen per dL per kPa of oxygen partial pressure

Thus oxygen content may be calculated for arterial (CaO2) and venous (Cv–O2) blood.

Question 9

O2 content in arterial blood

Answer 9

E.g.
In arterial blood: Hb 15 g/dL,
SaO2 100% and
PaO2 13.3 kPa

Arterial oxygen content = [15 · 1.34 · 1.0] + [13.3 · 0.0225]

= [20.1] + [0.3]

= 20.4 mL of oxygen per dL

Question 10

O2 content in venous blood

Answer 10

E.g. In venous blood:
Hb 15 g/dL, Sv–O2 75% and Pv–O2 5.3 kPa
Venous oxygen content =
15 · 1.34 · 0.75] + [5.3 · 0.0225]

= [15] + [0.2]

= 15.2 mL of oxygen per dL

Note that the difference between arterial and venous oxygen content is just under 5 mL of oxygen per dL.

If oxygen content is multiplied by cardiac output, oxygen delivery is obtained.

Question 11

Discuss arterial and venous oxygen content in the four types of hypoxia.

1. Hypoxic

Answer 11

Hypoxic hypoxia

E.g. 
Altitude: Hb 15 g/dL , 
SaO2 85%, 
PaO2 6.5 kPa,
Pv–O2 3.0 kPa,
Sv–O2 45%

CaO2 = [15 · 1.34 · 0.85] + [6.5 · 0.0225] = 17 mL O2/dL
Cv–O2 = [15 · 1.34 · 0.45] + [3.0 · 0.0225] = 9 mL O2/dL

Note arterial oxygen content is reduced and there is increased oxygen extraction resulting in a lower venous oxygen content.

Question 12

Discuss arterial and venous oxygen content in the four types of hypoxia.

Anaemic hypoxia

Answer 12

E.g.

Haemorrhage: 
Hb 7 g/dL, 
SaO2 100%, 
PaO2 13.3 kPa, 
Pv–O2 4.0 kPa,
Sv–O2 50%

CaO2 = [7 · 1.34 · 1.0] + [13.3 · 0.0225] = 10 mL O2/dL

Cv–O2 = [7 · 1.34 · 0.5] + [4.0 · 00225] = 5 mL O2/dL

Significant reduction in arterial oxygen content and hence oxygen delivery to the tissues.

There will be a resultant increase in cardiac work in an attempt to maintain oxygen delivery to the tissues.

Question 13

Discuss arterial and venous oxygen content in the four types of hypoxia.

Stagnant hypoxia

Answer 13

Stagnant hypoxia

E.g. Cardiogenic shock:

Hb 15 g/dL, 
SaO2 100%, 
PaO2 13.3 kPa, 
Pv–O2 5.3,
Sv–O2 75%

CaO2 = [15 · 1.34 · 1.0] + [13.3 · 0.0225] = 20 mL O2/dL

Cv–O2 = [15 · 1.34 · 0.75] + [5.3 · 0.0225] = 15 mL O2/dL

Note arterial oxygen content is normal. However, circulatory dysfunction results in inadequate oxygen delivery to organs and venous saturations may
even be increased.

Question 14

Discuss arterial and venous oxygen content in the four types of hypoxia.

Histotoxic hypoxia

Answer 14

Histotoxic hypoxia

E.g.

Cyanide poisoning:
Hb 15 g/dL, SaO2 100%, PaO2 13.3 kPa, Pv–O2
8.0 kPa, Sv–O2 90%

CaO2 = [15 · 1.34 · 1.0] + [13.3 · 0.0225] = 20 mL O2/dL

Cv–O2 = [15 · 1.34 · 0.9] + [8.0 · 0.0225] = 18 mL O2/dL

Arterial oxygen content is normal.

However, at a cellular level there is an inability to utilise oxygen, resulting in high venous oxygen content.

This picture may also be seen in severe sepsis where despite adequate oxygen delivery cellular hypoxia remains with high central venous saturations.

Question 15

Delivery of O2

Answer 15

If circulating volume for a 70 kg

man is 80 mL/kg (5600 mL),

this equates to an arterial oxygen content
of just over 1000 mL

and a venous oxygen content of approximately
750 mL.