615

discussed in two Lancet papers in August,1991 Z-3 We would like to contribute to this discussion by applying health technology assessment techniques in our analysis (see table). From these data we conclude that the cost/benefit ratio of ondansetron given at the originally recommended total dose of 104 mg per cycle is indeed unfavourable. However, as shown in several phase II studies, including ours, a median dose of 24-32 mg is sufficient. On the basis of these reduced doses a change in antiemetic treatment for high-emetic cancer chemotherapy, from the formerly most effective regimen (metoclopramide 10 mg/kg plus lorazepam) to ondansetron, would increase costs by no more than 6-4%.

Division of Medical Oncology, Ospedale Saint’ Orsola Malpighi, 40138 Bologna, Italy

FRANCO PANNUTI STEPHEN TANNEBERGER GIORGIO LELLI EDERA PIANA

1. Muhlbauer S. Was darf Krebs kosten? Ther Gegenw 1991; 130: 1. 2. Jones AL, Hill A, Soukop M, et al. Comparison of dexamethasone and ondansetron in the prophylaxis of emesis induced by moderately emetogenic chemotherapy. Lancet 1991; 338: 483-87. 3 Editorial. Ondansetron vs dexametasone for chemotherapy-induced emesis. Lancet 1991; 338: 478-79.

Fetal oxygen saturation measurement transmission pulse oximetry

by

SIR,-Pulse oximetry is used for the early detection of hypoxia by continuous measurement of arterial oxygen saturation (SaO), and the technique has been applied in a variety of clinical situations.1 Its application to fetal intrapartum monitoring should have considerable advantages over routine methods of monitoring fetal wellbeing.2The sensor principles used in pulse oximetry are usually reflectance or transmission photometry, depending on which tissue is concerned. Reflectance pulse oximetry has been tried in intrapartum monitoring.3-s However, reflectance pulse oximetry does have drawbacks when basic physiological and physical principles are taken into consideration, so we decided to develop a new optical scalp electrode (OSE) (fig 1), resembling electrical scalp electrodes in form, size, and method of application. The OSE is screwed into the presenting part of the fetus to fix the probe and to create an optical pathway for transillumination of a tiny piece of fetal tissue. The optical pathway consists of two LEDs (light-emitting diodes), emitting into the fetal scalp the red (660 nm) and infrared (940 nm) parts of the spectrum. The emissions are scattered and absorbed by the tissue, and the absorption contains a

Fig 2-Pulsatile waveforms for infra-red light (upper) and red light (middle), corresponding to fetal ECG (lower).

changing (pulsatile) component comprising information on SaO,. A hollow cannula housing an optical fibre receives light at the tip and conducts it to a photodetector. These very small signals are amplified, demodulated, converted from analog to digital, and processed in an AT386 computer, the results being displayed, with additional information from a Hewlett Packard 8040A fetal monitor, on a computer screen and stored on hard disc. This electrode permits transmission pulse oximetry to be applied to the fetus. It functions as a tiny photometer, eliminating the disadvantages of reflectance pulse oximetry. The electrode also permits simultaneous fetal heart rate monitoring. The probe can be applied where cervical dilatation is 1 cm or more. It provides good optoplethysmographic signals up to delivery. The adequacy of the signals is illustrated by heart rate patterns almost identical to those obtained by a standard electrical scalp electrode (fig 2). For calibration, two facts are important. Spectral absorption of fetal and adult haemoglobin, reduced or oxygenated, are so similar that differences can be neglected for pulse oximetry.6 The oxygen binding curves for fetal and adult haemoglobin are very different and this has considerable impact on clinical interpretation. A preliminary calibration was done by comparing the oxygen saturation readings of two pulse oximeters (’Nonin 8500’ and Critikon ’OxiShuttle’) with readings on this device in adult volunteers with values in the range 80-100%; the calibration curve was adjusted accordingly, leading to an error of ± 4% in this range. Since fetal oxygen saturations can be as low as 30%5 careful calibration of the whole range will need to be done. The signals are good, even from oedematous presenting parts. Artifacts from maternal or fetal movements can largely be suppressed by the software. In some cases with very high intrauterine pressure no signal could be detected during the short duration of maximal contraction, probably because of tissue compression disturbing local perfusion. Our preliminary experience with twelve uncomplicated spontaneous deliveries showed fetal oxygen saturation levels between 65% and 90%, calculated according to Forstner.’ The critical levels of oxygen saturation for the induction of anaerobic glycolysis and metabolic acidosis are unknown and we plan to try to answer this clinically important question using this new method. Fetal SaOz monitoring and simultaneous cardiotocography may permit earlier and more specific diagnosis of fetal distress; and this could have an impact on inappropriate caesarean sections and operative vaginal deliveries. Departments of Anaesthesiology and Obstetrics and Gynaecology, Klinikum Grosshadern, University of Munich, D-8000 Munich 70, Germany 1.

Fig 1-Fetal optical scalp electrode.

J. BUSCHMANN G. RALL R. KNITZA

Taylor MB, Whitwarn JG. The current status of pulse oximetry. Anaesthesia 1986; 41:

943. 2. Gardosi J, Carter M, Becket T. Continuous intrapartum monitoring of fetal oxygen saturation. Lancet 1989; ii: 692. 3. Gardosi J, Schram C, Symonds EM. Adaption of pulse oximetry for fetal monitoring during labour. Lancet 1991; 338: 1265. 4. Johnson N, Johnson VA. Continuous fetal monitoring with a pulse oximeter: a case of cord compression. Am J Obstet Gynecol 1989; 161: 1295. 5. Peat S, Booker M, Lanigan C, Ponte J. Continuous intrapartum measurement of fetal oxygen saturation. Lancet 1988; ii: 213. 6. Zijlstra WG, Buursma A, Meeuwsen-van der Roest WP. Absorption spectra of human fetal and adult oxyhemoglobin, deoxyhemoglobin, carboxyhemoglobin and methemoglobin. Clin Chem 1991; 37: 1633. 7. Forstner K. Pulsoxymetrie: Stand und Entwicklung der Technik. Biomed Technik 1988; 33: 6.

Fetal oxygen saturation measurement by transmission pulse oximetry.

615 discussed in two Lancet papers in August,1991 Z-3 We would like to contribute to this discussion by applying health technology assessment techniq...
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