3. Oxygen Delivery

An organism survives by means of effective oxygen delivery to mitochondria. There is perennial interest in the concept of optimizing oxygen flux both in critically ill patients and in those undergoing major surgery. An understanding of the underlying principles may be linked to the continuing debate about goal-directed therapy

Oxygen is required for energy generation in mitochondria via the process of oxidative phosphorylation. Oxygen delivery (oxygen flux) to the tissues is governed by cardiac output (heart rate [HR] x stroke volume [SV]) and arterial oxygen content.

The formal equation relates delivery to cardiac index (cardiac output/body surface area [BSA]) and so is given by: O2 flux = HRxSV /BSAx SaO2 x hb x 1:34

heart rate (HR), stroke volume (SV), haemoglobin concentration (Hb) and oxygen saturation (SpO2).

Cardiac output: HR and SV are affected by various factors, including venous return and myocardial contractility. Ventricular preload can be improved by optimizing volaemic status, and contractility can be augmented by inotropes.

It will also be influenced by primary pulmonary factors affecting gas exchange, some of which may be amenable to treatment. Conditions that can be improved include chest infections, atelectasis and bronchoconstriction. Supplemental oxygen will increase PaO2, although intra-pulmonary shunting will diminish the effectiveness of increasing the FiO2

given a cardiac output of 5 l min1 and an SpO2 of 100%, O2 delivery at a [Hb] of 100 g l= 670 ml min at 150 g l1 it rises to 1,005 ml min significantly if a low haemoglobin is increased by transfusion. ‘Low’ in the context of anaesthesia and intensive therapy does not of course mean 100 g l1. An oxygen delivery of 670 ml min1 is more than adequate, and few intensivists would wish to transfuse such a patient.

Dissolved oxygen: at atmospheric pressure breathing air, the O2 solubility coefficient that dissolved O2 content is around 0.26 ml dl1. If a subject breathes 100% oxygen, this increases to 1.7 ml dl–1 and, at 3 atmospheres in a hyperbaric chamber, it reaches 5.6 ml dl–1. At this level, dissolved oxygen can make a significant contribution to delivery to the tissues