Oxygen status

The Deep Picture is a concept used to ease the interpretation and understanding of the parameters that influence the arterial oxygen status. We divide the parameters related to the arterial oxygen status into these groups:

1. Oxygen uptake

2. Oxygen transport

3. Oxygen release

4. Tissue oxygenation

Lactate is used as an overall monitor of the balance between oxygen demand and oxygen supply, i.e. tissue oxygenation.

1. Oxygen uptake

Oxygen uptake in the arterial blood is used to evaluate the lung function. Oxygen uptake depends primarily on:

• The arterial oxygen tension, pO₂ - the key parameter for evaluation of oxygen uptake
• The alveolar oxygen tension, which is primarily influenced by the ambient pressure, FO₂ (I), and to a lesser degree by pCO₂
• The degree of intra- and extrapulmonary shunting (FShunt)
• The diffusion capacity of the lung tissue

2. Oxygen transport

The oxygen transport of arterial blood is used to evaluate the capability of transporting oxygen from the lungs to the tissue.
Oxygen transport, defined as the amount of oxygen being transported per liter arterial blood, depends primarily on:

• The total content of oxygen in the arterial blood, ctO₂ - the key parameter for evaluation of oxygen transport
• The concentration of hemoglobin in the blood (ctHb)
• The concentration of dyshemoglobins (fraction of oxygenated hemoglobin, O₂Hb)
• The arterial oxygen tension (pO₂)
• The arterial oxygen saturation (sO₂), which again is determined by pO₂ and p50

sO₂ is not the sole indicator of oxygen transport. Presence of dyshemoglobins and/or a low concentration of hemoglobin can cause severe reductions in the oxygen transport capacity of the arterial blood.

3. Oxygen release

Oxygen release is used to evaluate the arterial blood’s capability of releasing oxygen to the peripheral tissues. Oxygen release depends primarily on: 

• The hemoglobin-oxygen affinity expressed by the p50 value - the key parameter for evaluation of oxygen release
• The arterial and end-capillary oxygen tensions and ctO₂

The hemoglobin-oxygen affinity is expressed by the oxygen dissociation curve (ODC), which determines the relationship between oxygen tension, pO₂, and oxygen saturation, sO₂. The position of ODC is determined by the pO₂ value at 50 % saturation, called the p50 value.

4. Tissue oxygenation

Lactate is produced in excess when there is inadequate oxygen supply to the tissues. Lactate thus serves as a marker of critical imbalance between tissue oxygen demand and oxygen supply. In most situations, elevated blood lactate will be caused by hypoperfusion, severely impaired arterial oxygen supply, or a combination of the two.

In general, an elevated or increasing concentration of lactate should alert the clinician. High lactate is often seen several hours before clinical shock or vital organ dysfunction. Consequently, lactate STAT monitoring is important as an early warning indicator. Decreasing or persistently low levels of blood lactate during critical illness signal successful treatment.

For a practical guideline on how to use the parameters above, view the oxygen status flow chart.