3. Oxygen Delivery

Question 1

Intro

Answer 1

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

Question 2

Factors that determine oxygen delivery.

Answer 2

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.

Question 3

Formula for DO2

Answer 3

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

Question 4

Optimization of Oxygen Flux

Answer 4

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

Question 5

Cardiac output

Answer 5

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.

Question 6

Oxygen saturation:

Answer 6

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

Question 7

Haemoglobin concentration

Answer 7

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.

Question 8

Dissolved oxygen

Answer 8

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