The human body does not care much about pO2 or FiO2. What is important to the body is the amount of oxygen actually being delivered to the tissues. Focusing on the pO2 in the ABG results, or the FiO2 settings of O2 support, without understanding how oxygen is delivered to body tissues can have dire consequences.
The formula below summarizes the various factors that contribute to tissue oxygen delivery, and is worthwhile to remember when thinking about a patient’s oxygen status.
DO2 = CO x (sO2 x ceHb x 1.39) + (PaO2 x 0.03)
- DO2 = total blood oxygen content
- sO2 = oxygen saturation
- ceHb = effective Hb concentration
Oxygen-carrying Capacity of Hb [BO2]
Potential oxygen carrying capacity – determined by total Hb concentration [ctHb, measured in ABG – measures all species of Hb, including carboxyHb].
Actual oxygen carrying capacity [BO2] – maximum amt of Hb-bound oxygen per unit volume of blood [mmol/L], the amt of oxygent present in a volume of blood if all the effective Hb molecules (ceHb) were 100% saturated; or in formula = ceHb x 100%sO2
Therefore BO2 ~ ctHb – cdysHb (non O2-carrying Hb].
BO2 was found to be 1.39 ml/g. [Different textbooks give values between 1.3 to 1.39 g/mL]
The equation usually used to describe the rate of oxygen delivery is as follows:
This equation will not be applicable when levels of dys-hemoglobins (e.g. metHb, carboxyHb, etc) are high. A better formula to describe DO2, taking into account only the concentration of effective Hb would be:
- ceHb = 150g/L
- Wt = 70 Kg
- PaO2 = 100
- SO2 = 100%
- CO = 5 L/min
DO2 = 5 × (1.39 × 150 × 1 + (0.003 × 100)) = 1044 ml/min [15ml, dissolved, 1029ml bound].
Total rate of O2 delivery is usually around 15ml/kg/min.
Implications of this formula:
- Oxygen solubility is poor at body temperature, ~0.03ml/L/mmHg. In simple terms, In 1 L of blood (at an alveolar O2 of 100mmHg), there is only 3ml/L of dissolved oxygen. Or, 1% of total oxygen content is dissolved in blood while the remaining 99% is bound to Hb. Therefore, improving paO2 by itself, has very little effect on tissue oxygenation. [Caveat: Concept does not apply to hypothermia or hyperbaric oxygen therapy]
- At 100% saturation, there is ~1.39 ml of O2 per gram of Hb.
- At a Hb of 150 g/L, there is ~200ml of O2 in every Liter of blood.
- In the presence of inadequate tissue oxygenation – do not think only about increasing PaO2 or FiO2. There are other (more important) factors that can be manipulated, s.a. heart rate, stroke volume, hematocrit and oxygen saturation.
Deranged Physiology,. “The Oxygen – Carrying Capacity Of Whole Blood”. N.p., 2015. Web. 21 Jan. 2016.