British zyxwvutsrqpo Joiirnal of Obstetrics and Gynaecology zyxwvuts June 1990, zyxwvutsrq Vol. zyxwvutsrq 97, pp 393498 The effect of caput succedaneum on oxygen saturation measurements NICOLAS JOHNSON, VALERIE A. JOHNSON, JEFFREY BANNISTER, RICHARD J. LTLFORD Summary. Advances in oximetry have allowed the obstetrician to measure oxygen saturation in the fetus with non-invasive transcutane- ous techniques. The influence of caput succedaneum formation on the oxygen saturation results obtained with a pulse oxinieter was studied in 30 newborn infants. Caput was associated with a reduced oxygen sat- uration reading (mean reduction of 15%; P<n401). This effect is partly due to a true drop in local tissue oxygenation but is exacerbated by a systematic error intrinsic to the physics of spectrophotornetry. If contin- uous intrapartum oximetry is ever to become a part of routine obstetric monitoring then probes that pass through the cervix beyond the caput of the prescnting part will be requircd if erroneously low readings are to be avoided. With the advcnt of reflectance pulse oximetry it has become possible to measure fetal oxygen sat- uration (SaO,) non-invasively through ruptured and intact membranes (Peat et ul., 1988; John- son zyxwvutsrq & Lilford, 1988; Johnson & Johnson, 1989; Gardosi ctal., 1989;Johnson ctal., 1989a, 1989b and 1990). The physical principles of oximetry arc simple but their understanding is vital before the limi- tations of oximetry can be appreciated. Pulse oximcters measure the light absorbed by the pul- satile vascular bed. They commonly consist of light emitting diodes transmitting 700-1 001) pulsesis and the light transmitted through the skin is measured by a single adjacent photo- sensitive cell. If the detected light intensity is low then the oximeter automatically increases the The Fetal Oximetry Research Unit, Department of Obstetrics and Gynaecology, St James's University Hospital, Leeds IS9 7TF. NTCHOLAS JOHNSON VALERIE zyxwvutsr A. JOHNSON JEFFREY BANNISTER RICHARD J. LILFORD Correspondence: Mr N. Johnson power to the light sources. Deoxygenated and oxygenated haemoglobin have different light absorption characteristics; deoxygenated hae- moglobin absorbs more red light than oxygen- ated haemoglobin but at the infra-red range of the spectrum this difference is non-existent or even reversed. Therefore the ratio of absorbed infra-red and red light is related to the oxygen content of haemoglobin. Detected light that has been transmitted through the skin can be con- sidered to consist of two components; a pulsat- ing or alternating Component superimposed on top of an adynamic or direct component. The alternating component represents only the light that has been absorbed by pulsating arterial blood whilst the adynamic component repre- sents light absorption by venous blood, skin and tissue (Figure 1; Trcrnper & Barker, 1989; Wukitsch, 1987; Taylor & Whitwam 1986). Con- ventional oximcters transmit light through tis- sues (transmission oximetry) but the use of wide angle photosensitive cells means transillumi- natcd (backscattcred) light from an adjacent light source can be detected (reflectance oxim- etry). Probes can therefore be applied to flat sur- faces and this represents the potential to be an 493