Oxygen Delivery Equation

Mnemonic

“Delivery is what the heart sends times what the blood carries”:

DO2 = cardiac output x arterial oxygen content

CaO2 = (1.34 x Hb x SaO2) + (0.023 x PaO2)

Normal delivery is about 1000 ml/min, of which only about 250 ml is consumed, giving a healthy reserve.

“Haemoglobin and saturation do the work; dissolved oxygen is negligible.” The dissolved term contributes only about 3 ml per litre at normal pressures, which is why raising PaO2 in an already saturated patient achieves little.

The equation names the three ways to improve delivery, and therefore the three treatments in shock: raise cardiac output (fluids, inotropes), raise haemoglobin (transfusion), raise saturation (oxygen, ventilation).

Anaemia halves delivery far more effectively than desaturation, which is why a haemoglobin of 60 is more dangerous than a saturation of 88 per cent, and why the haemoglobin term dominates the equation.

Expansion

Oxygen delivery (DO2) = cardiac output x arterial oxygen content

Arterial oxygen content (CaO2) = (1.34 x haemoglobin x saturation) + (0.023 x PaO2)

For a typical adult: 5 litres per minute x about 200 ml per litre = about 1000 ml per minute.

Three points follow:

• The dissolved term is trivial at atmospheric pressure, contributing about 3 ml per litre; this is why raising the inspired oxygen concentration does little once saturation is already high
• Anaemia reduces delivery in direct proportion to haemoglobin, and is compensated by raising cardiac output
• Resting consumption is about 250 ml per minute, so the oxygen extraction ratio is about 25 per cent, leaving a substantial reserve

Mixed venous saturation, normally about 75 per cent, is a global marker of the balance between delivery and consumption; a falling value indicates that extraction has risen to compensate for inadequate delivery.