Download Update 11 - Update in Anaesthesia - WFSA

Update in Anaesthesia 11PRACTICAL APPLICATIONS OF PULSE OXIMETRYDr E Hill, Dr MD Stoneham, Nuffield Department of Anaesthetics, Oxford Radcliffe NHS HospitalsHeadington , Oxford OX3 9DU, Email: mark@stoneham1.freeserve.co.ukINTRODUCTIONPulse oximetry is a useful method of monitoring patients inmany circumstances,and in the face of limited resources,the pulse oximeter may represent a wise choice of monitor,as with training it allows for the assessment of severaldifferent patient parameters.Pulse oximeters are now a standard part of perioperativemonitoring which give the operator a non-invasiveindication of the patient’s cardio-respiratory status. Havingbeen successfully used in intensive care, the recovery roomand during anaesthesia, they have been introduced in otherareas of medicine such as general wards apparently withoutstaff undergoing adequate training in their use (1) . Thetechnique of pulse oximetry does have pitfalls andlimitations and it is possible that patient safety may becompromised with untrained staff. This article is thereforeintended for the ‘occasional’ user of pulse oximetry.Pulse oximeters measure the arterial oxygen saturation ofhaemoglobin. The technology involved (2) is complicatedbut there are two basic physical principles. First, theabsorption of light at two different wavelengths byhaemoglobin differs depending on the degree ofoxygenation of haemoglobin. Second, the light signalfollowing transmission through the tissues has a pulsatilecomponent, resulting from the changing volume of arterialblood with each pulse beat. This can be distinguished bythe microprocessor from the non-pulsatile componentresulting from venous, capillary and tissue light absorption.The function of a pulse oximeter is affected by manyvariables, including: ambient light; shivering; abnormalhaemoglobins; pulse rate and rhythm; vasoconstriction andcardiac function. A pulse oximeter gives no indication of apatient’s ventilation, only of their oxygenation, and thuscan give a false sense of security if supplemental oxygen isbeing given. In addition, there may be a delay betweenthe occurrence of a potentially hypoxic event such asrespiratory obstruction and a pulse oximeter detecting lowoxygen saturation. However, oximetry is a useful noninvasivemonitor of a patient’s cardio-respiratory system,which has undoubtedly improved patient safety in manycircumstances.What does a pulse oximeter measure?1. The oxygen saturation of haemoglobin in arterialblood - which is a measure of the average amountof oxygen bound to each haemoglobin molecule.The percentage saturation is given as a digitalreadout together with an audible signal varying inpitch depending on the oxygen saturation.2. The pulse rate - in beats per minute, averaged over5 to 20 seconds.A pulse oximeter gives no information on any of theseother variables:The oxygen content of the bloodThe amount of oxygen dissolved in the bloodThe respiratory rate or tidal volume i.e. ventilation The cardiac output or blood pressureSystolic blood pressure can be estimated by noting thepressure at which the plethysmograph trace reappearsduring deflation of a proximal non-invasive blood pressurecuff.Principles of modern pulse oximetryOxygen is carried in the bloodstream mainly bound tohaemoglobin. One molecule of haemoglobin can carry upto four molecules of oxygen, which is then 100% saturatedwith oxygen. The average percentage saturation of apopulation of haemoglobin molecules in a blood sample isthe oxygen saturation of the blood. In addition, a verysmall quantity of oxygen is carried dissolved in the blood,which can become important if the haemoglobin levels areextremely low. The latter, however, is not measured bypulse oximetry.The relationship between the arterial partial pressure ofoxygen (PaO 2 ) and the oxygen saturation is described bythe haemoglobin-oxygen dissociation curve (see figure 1).The sigmoid shape of this curve facilitates unloading ofoxygen in the peripheral tissues where the PaO 2 is lowand oxygen is required for respiration. The curve may beshifted to the left or right by various patient characteristicse.g. recent blood transfusion, pyrexia.