Advances in pulse oximetry

The interaction between light and tissue is the basis of pulse oximetry. This review focuses on the recent development of pulse oximetry, with special emphasis on the reflection oximetry utilized in fetal and cerebral oximetry.     

The value of monitoring arterial oxygenation saturation by pulse oximetry in neuroleptanalgesia

Anaesthesia can induce hypoxaemia. Pulse oximetry gives continuous non invasive monitoring of arterial oxygen saturation. No arterial puncture is needed. Ninety-four patients were monitored by pulse oximetry during neuroleptanalgesia for colonoscopy. Eighteen patients showed desaturation episodes of less than 90%, eight linked to opioid-induced respiratory depression. In all patients, pulse oximetry gave clinicians an immediate awareness of the incident. Treatment was facilitated. Pulse oximetry made anaesthesia safer.     

Potential errors in pulse oximetry

There is no absolute reference for oxygen saturation, although multiwavelength in vitro oximeters are accepted as the 'gold standard'. Regardless of whether fractional or functional saturation is used by manufacturers to calibrate their oximeters, evaluation against fractional saturation is recommended since this is the clinically relevant variable. The use of standard notation and comparisons based on bias and precision is recommended. The accuracy of pulse oximetry is intrinsically limited by the use of only two wavelengths, and is dependent on the initial calibration population. The empirical algorithms used to convert the signal to its 'readout value' and the quality control of hardware may both be important sources of variability between oximeters. Change in blood temperature may introduce errors in pulse oximeter and in vitro oximeter saturation readings, but these will be clinically insignificant. Changes in blood pH should not decrease pulse oximetry accuracy.     

Potential errors in pulse oximetry

The published studies of pulse oximeter performance under conditions of normal, high and low saturation, exercise, poor signal quality and cardiac arrhythmia are reviewed. Most pulse oximeters have an absolute mean error of less than 2% at normal saturation and perfusion; two-thirds have a standard deviation (SD) of less than 2%, and the remainder an SD of less than 3%. Some pulse oximeters tend to read 100% with fractional saturations of 97-98%. Pulse oximeters may be suitable hyperoxic alarms for neonates if the alarm limit chosen is directly validated for each device. Pulse oximeters are poorly calibrated at low saturations and are generally less accurate and less precise than at normal saturations; nearly 30% of 244 values reviewed were in error by more than 5% at saturations of less than 80%. Ear, nose and forehead probes respond more rapidly to rapid desaturation than finger probes, but are generally less accurate and less precise. Ear oximetry may be inaccurate during exercise. Low signal quality can result in failure to present a saturation reading, but data given with low signal quality warning messages are generally no less accurate than those without. Cardiac arrhythmias do not decrease accuracy of pulse oximeters so long as saturation readings are steady.     

Limitations of pulse oximetry.

Pulse oximetry is a noninvasive, accurate, and safe method for the measurement of oxygen saturation during intravenous sedation or general anesthesia. Several factors should be considered with its use, since these variables will either alter the accuracy of the readings or may cause harm to the patient. These factors include changes in the strength of the arterial pulse, body movements, dyshemoglobinemias, plasma lipids and bilirubin, color interferences, venous pulsations, and several physical factors. Awareness of these variations will help the clinician become more knowledgeable in the use of the pulse oximeter.     

Minor thoracotomy for the treatment of spontaneous pneumothorax]

We have recently established the following therapeutic principles for spontaneous pneumothorax, and have obtained favorable results. 1. All patients, including those with a first episode, are subjected to thoracotomy. 2. The standard technique is minor thoracotomy through a skin incision of 5 cm which is aesthetically superior, except complicated cases with chronic obstructive lung disease in elderly patients or giant bullae. 3. Bilateral simultaneous axillary minor thoracotomy is applied to bilateral pneumothorax, whether synchronous or metachronous. 4. Thoracotomy is attempted by other approaches when we preoperatively suspect pulmonary bullae in the lung bases or diaphragm. In the past three years (1988 to 1990), 68 out of 70 cases (97.1%) of spontaneous pneumothorax admitted to our department were operated on. Of them, 55 cases (81% of all surgical cases) underwent minor thoracotomy. Pleural abrasion was performed in order to prevent postoperative recurrence of the pneumothorax. The 55 cases of minor thoracotomy consisted of 51 males and 4 females aged 27.6 years on average (range: 15 to 64). In 22 cases (40%), this was the first occurrence, combination with hemothorax was seen in 4 cases, and bilateral simultaneous axillary minor thoracotomy was performed in 11 cases (20%). The location of the bullae was the upper lobe (or upper segment) in 95.5%, the middle lobe (or lingula) in 6%, and the lower lobe (S6) in 20%.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of methemoglobinemia on pulse oximetry and mixed venous oximetry

The performance of three commercially available pulse oximeters was assessed in five anesthetized dogs in which increasing levels of methemoglobin were induced. Hemoglobin oxygen saturation in each dog was monitored with three pulse oximeters (Nellcor N-100, Ohmeda 3700, and Novametrix 500) and a mixed venous saturation pulmonary artery catheter (Oximetrix Opticath). Arterial and mixed venous blood specimens were analyzed for PaO2, PaCO2, and pHa using standard electrodes. An IL-282 Co-oximeter was used on the same specimens to determine oxyhemoglobin and methemoglobin as percentages of total hemoglobin. Methemoglobin levels of up to 60% were induced by intratracheal benzocaine. As MetHb gradually increased while the dogs were breathing 100% inspired oxygen, the pulse oximeter saturation (SpO2) overestimated the fractional oxygen saturation (SaO2) by an amount proportional to the concentration of methemoglobin until the latter reached approximately 35%. At this level the SpO2 values reached a plateau of 84-86% and did not decrease further. When, at fixed methemoglobin levels, additional hemoglobin desaturation was induced by reducing inspired oxygen fraction, SpO2 changed by much less than did SaO2 (regression slopes from 0.16 to 0.32). Thus, at high methemoglobin levels SpO2 tends to overestimate SaO2 by larger amounts at low hemoglobin saturations. Plots of SpO2 versus functional saturation (oxyhemoglobin/reduced hemoglobin plus oxyhemoglobin) show an improved but still poor relationship (regression slopes from 0.32 to 0.46). The Oximetrix Opticath pulmonary artery catheter behaves similarly but provides somewhat better agreement with functional saturation than do the pulse oximeters in the presence of methemoglobinemia. Pulse oximetry data (SpO2) should be used with caution in patients with methemoglobinemia.     

The current status of pulse oximetry

The history of the development of pulse oximetry is outlined and the principle of how the apparatus works is described. The instrument detects hypoxic hypoxia and the shape of the oxygen dissociation curve means that the minimum saturation alarm should be set at 94% in anaesthetic usage. It is accurate to within 2% and is usually unaffected by racial pigmentation, but accuracy can be affected in low perfusion states, hypothermia and in the presence of abnormal forms of haemoglobin and pigments in the blood. Its clinical evaluation in the operating theatre and intensive care unit is reported. It was found to be useful and reliable and would appear to have logistical and other advantages over current methods of detecting hypoxia. Pulse oximetry may make a significant contribution to the safety of anaesthetic practice.