INVESTIGATIONS ON THE USE OF PULSE OXIMET RY IN PRILOCAINE INDUCED METHEMOGLOBINEMIA

During the last 15 years pulse oximetry has become a widely accepted m ethod of monitoring during general and local anaesthesia. Pulse oximet ers measuring with two wavelengths are considerably affected by dyshae moglobin. At concentrations up to 30%, CO-Hb cannot be distinguished f rom O-2-Hb. Met-Hb, even in low concentrations, leads to a constant er ror of measurement; some authors recommended exploiting this for estim ation of the Met-Hb concentration. To prove the aim of the present stu dy was to test whether this error in measurement can be defined with o ne formula for different pulse oximeters. Patients and methods. In a p rospective, randomized, double-blind study, 171 non-smoking patients w ith healthy lungs (ASA 1-3) who had received a plexus block for hand s urgery were investigated. After premedication with 3.75-15 mg medazola m p.o. each patient received a total of 6 lO(2) via a Hudson mask duri ng the investigation. After 10 min the following pulse oximeters were put on the index finger: (1) Ohmeda BIOX 3700e, (2) Critikon Oxyshuttl e, (3) Nellcor N 180. Simultaneously a venous blood sample was taken a nd analysed immediately with a Radiometer OSM3. The procedure was repe ated 15, 30, 60 and 120 min after the plexus block. In 41 patients the plexus block was carried out with lidocaine (6 mg/kg body weight) and in 130 patients, with prilocaine (7 mg/kg body weight). Results. Ther e were no significant differences in age, sex and risk groups between the lidocaine and the prilocaine group. In the lidocaine group we were able to show that hyperoxic conditions can be maintained for 2 h with the method described. In the lidocaine group none of the pulse oximet ers showed a psO(2) less than 99%. Our results show significant differ ences between the three Dulse oximeters. Therefore, in contrast to the convention followed in the literature, the relation between Met-Hb an d psO, under hyperoxic conditions must be described with different for mulas for each pulse oximeter as follows: (1) Ohmeda BIOX 3700e: Met-H b=(101-psO(2)). 0.6 (r=0.94); (2) Critikon Oxyshuttle: Met-Hb=(101-psO (2)). 0.7 (r=0.83); (3) Nellcor N 180: Met-Hb=(101-psO(2)). 0.9(r=0.92 ). Discussion. Our results show that it is not possible to describe th e connection between Met-Hb and psO, for all pulse oximeters with only one formula, but it is possible to set up different formulas with goo d correlations for each of the three pulse oximeters. The reasons for the different sensitivity are probably the different algorithms used b y the manufacturers. In spite of the good correlations we can not reco mmend Met-Hb estimation by pulse oximetry measurement with two wavelen gths, because the distinction of hypoxia and Met-Hb its not possible w hen hyperoxic conditions are not stable as they were in our controlled study. A low psO(2) measured in patients with normal arterial blood g ases can be an indication of Met-Hb, but the exact measurement of dysh aemoglobin is only possibly by using a co-oximeter.