Hypoxia, hypoxaemia and impaired oxygenation

The above terms are often used interchangeably but mean different things.

Hypoxia refers to any state in which tissues receive an inadequate supply of O₂ to support normal aerobic metabolism¹ (Figure 9). It may result from either hypoxaemia (see below) or impaired blood supply to tissues (ischaemia). It is often associated with lactic acidosis as cells resort to anaerobic metabolism.

Figure 9 Causes of hypoxia.

Hypoxaemia refers to any state in which the O₂ content of arterial blood is reduced. It may result from impaired oxygenation (see below), low haemoglobin (anaemia) or reduced affinity of haemoglobin for O₂ (e.g. carbon monoxide).

Impaired oxygenation refers to hypoxaemia resulting from reduced transfer of O₂ from lungs to the bloodstream. It is identified by a low PaO₂ (<10.7 kPa; <80 mmHg).

It is important to note the distinction between impaired oxygenation (which results in hypoxaemia) and inadequate oxygenation (which results in hypoxia). Consider a patient with a PaO₂ of 8.5 kPa. He has impaired oxygenation, suggesting the presence of important lung disease. Nevertheless, his PaO₂ would usually result in an SaO₂ > 90% and, provided the haemoglobin and cardiac output are normal, adequate O₂ delivery to tissues.

Type 1 respiratory impairment

Type 1 respiratory impairment¹ is defined as low PaO₂ with normal or low PaCO₂. This implies defective oxygenation despite adequate ventilation. V/Q mismatch is usually responsible and may result from a number of causes (Box 1.3.1). The PaCO₂ is often low due to compensatory hyperventilation.

If the ABG is drawn from a patient on supplemental O₂, the PaO₂ may not be below the normal range, but will be inappropriately low for the FiO₂.

The severity of type 1 respiratory impairment is judged according to the scale of the resulting hypoxaemia and, ultimately, the presence of hypoxia (Table 1.3.1). Here it is important to remember the O₂ dissociation curve. Reductions in PaO₂ as far as 8 kPa have a relatively minor effect on SaO₂ and are well tolerated. Beyond this threshold, we reach the ‘steep part’ of the curve and further reductions in PaO₂ will lead to much greater falls in SaO₂, significantly lowering the O₂ content of arterial blood.

Table 1.3.1 Assessing severity of type 1 respiratory impairment

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