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  • Title: Levonorgestrel plasma levels during continuous administration with different models of subdermal implants.
    Author: Alvarez F, Brache V, Faundes A, Johansson ED, Odlind V, Nash H.
    Journal: Contraception; 1983 Feb; 27(2):123-30. PubMed ID: 6406138.
    Abstract:
    Plasma concentrations of levonorgestrel were determined in women using subdermal levonorgestrel implants. One group had 6 Silastic capsules (NORPLANT), one group had 4 and one group had 6 covered Silastic rods for variable lengths of time. Levonorgestrel concentrations remained constant around 0.4 ng/ml up until 6 years of use in the Norplant group. The observation period was shorter for the covered rods; plasma concentrations were, however, constant around 0.55 ng/ml and 0.70 ng/ml for 3.5 years with the 4 rods and 6 covered rods, respectively. The difference in mean levonorgestrel concentrations between the three groups were statistically significant (p less than 0.0005). Plasma levels of levonorgestrel were twice to three times higher in blood obtained from the arm with the implants compared to the levels found in the other arm. The application of a tourniquet for variable periods before blood sampling did not influence the plasma concentrations of levonorgestrel. The study purpose was to follow the plasma concentrations of levonorgestrel in women using the Norplant capsule system or 4 or 6 covered rods to assess the longevity of the implants. An additional purpose was to compare the plasma concentrations of levonorgestrel with the different implant systems. The capsules had an outer diameter of 2.4 mm, an inner diameter of 1.57 mm, and were 34 mm long. The total amount of steroid in each capsule was around 34 mg. The implants were placed subcutaneously in the anterior aspect of the forearm through a 10 gauge trocar under local anesthesia. The same insertion technique was used for all types of implants. Blood samples for the determination of levonorgestrel concentrations were collected at 3 or 6 month intervals from some of the volunteers. The levonorgestrel concentrations in the ipsilateral arm were between 2-3 times higher than those found in the samples from the contralateral arm. A greater variability of plasma levels was also found from the ipsilateral arm. In some volunteers there was only a small difference between the 2 sides; in others the difference was larger, suggesting that the factor causing the higher concentration in the ipsilateral arm may vary from case to case. The prolonged application of the tourniquet before withdrawing the blood sample did not alter the concentration of levonorgestrel. From month 24 and onwards, plasma concentrations of levonorgestrel were stable without any distinct tendency to change at least up to the 72nd month of use. From the initial period of implant use up to 42 months, no tendency for the levonorgestrel levels to decline was found. After 45 months of use of the 4 covered rods, there was an apparent decrease in levonorgestrel plasma levels, which may or may not be real. The mean levonorgestrel concentration with 4 covered rods was 0.55 +or- 0.02 ng/ml, and the mean concentration with 6 covered rods was 0.70 +or- 0.03 ng/ml. This difference in means was highly significant. The study shows that levonorgestrel implants either as capsules as in Norplant or as covered rods provide stable plasma concentrations of the steroid without any tendency to decline for prolonged periods.
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