Total circulating red cells versus haematocrit as the primary descriptor of oxygen transport by the blood.

Peripheral haematocrit (PCV) is the traditional target and monitor in many transfusion regimens. Without negating the importance of PCV as a determinant of whole blood viscosity, the present article outlines two important reasons why the red cell volume (RCV) should replace PCV in the central target role during blood transfusion in intensive care and other emergency situations: 1. PCV reflects both RCV and plasma volume (PV) and is therefore not directly proportional to the total blood oxygen carrying capacity. At best, the relationship between PCV and RCV is hyperbolic and this is often overlooked when relating the two parameters in practice. At worst, the hyperbolic relationship is unreliable because PV and RCV can vary independently and the PCV is a fluctuating ratio of variable numbers. 2. PCV is not a good indicator of blood volume (BV), which is another important determinant of oxygen delivery to tissues and a crucial parameter in intensively managed patients. BV is directly proportional to RCV and this relationship also is often overlooked in clinical practice. The recommended values for RCV are 30 ml/kg in men. 25 ml/kg in women and between 30 ml/kg and 45 ml/kg in neonates within the first week of life.