Impact of Ketorolac on Opioid Consumption after Knee Arthroscopy

PURPOSE

The objective of this study was to examine postoperative opioid consumption in outpatients undergoing knee arthroscopy after a single dose of intravenous ketorolac.

METHODS

Patients ages 18-65 years old, weighing over 50kg and scheduled for knee arthroscopy were randomized to one of the four groups of preoperative ketorolac (0mg, 7.5mg, 15mg, 30mg). The primary outcome measured was postoperative opioid consumption. Secondary outcomes included visual analog scale pain scores, patient satisfaction scores, side effects and total postoperative anesthesia care unit time. Equivalency between ketorolac groups in opioid reduction relative to placebo was evaluated for each dose pair (7.5 vs. 15mg, 7.5 vs. 30mg, and 15 vs. 30mg). Linear regression models were used to examine associations between ketorolac dose with postoperative length of stay and patient satisfaction. A linear mixed model was used to evalaute the association between ketorolac dose and pain scores over time.

RESULTS

A total of 112 patients with comparable patient and procedural characteristics were enrolled. Equivalency in opioid reduction relative to placebo was not demonstrated between any examined ketorlac doses (7.5 vs. 15mg, P = 0.167; 7.5 vs. 30mg, P = 0.451; 15 vs. 30mg, P = 0.515). Compared to placebo, all ketorlac doses decreased postoperative pain scores (global P=0.012). Patient satisfaction and postoperative duration did not vary with ketorolac dose.

CONCLUSIONS

Although all ketorolac doses decreased PACU pain scores, equivalency in PACU opioid reduction between ketorolac doses was not demonstrated.

Calculation of electric fields and currents induced in a millimeter-resolution human model at 60 Hz using the FDTD method

The finite-difference time-domain (FDTD) method has previously been used to calculate induced currents in anatomically based models of the human body at frequencies ranging from 20 to 915 MHz and resolutions down to about 1.25 cm. Calculations at lower frequencies and higher resolutions have been precluded by the huge number of time steps that would be needed in these simulations. This paper describes a method used to overcome this problem and efficiently calculate induced currents in an MRI-based, 6-mm-resolution model of the human under a high-voltage transmission line. This model is significantly higher resolution than the 1.31-cm-resolution model previously used; therefore, it can be used to pinpoint locations of peak current densities in the body. Proposed safety guidelines would allow external electric fields of 10 kV/m and 25 kV/m for exposure to 60 Hz fields of the general public and workers, respectively. For this external electric field exposure of 10 kV/m, local induced current densities as high as 20 mA/m2 are found in the head and trunk with even higher values (above 150 mA/m2) in the legs. These currents are considerably higher than the 4 or even 10 mA/m2 that have been suggested in the various safety guidelines, thus indicating an inconsistency in the proposed guidelines. In addition, several ratios of E/H typical of power line exposures were examined, and it was found that the vertical electric field couples strongly to the body, whereas the horizontal magnetic field does not.

Power deposition in the head and neck of an anatomically based human body model for plane wave exposures

At certain frequencies, when the human head becomes a resonant structure, the power absorbed by the head and neck, when the body is exposed to a vertically polarized plane wave propagating from front to back, becomes significantly larger than would ordinarily be expected from its shadow cross section. This has possible implications in the study of the biological effects of electromagnetic fields. Additionally the frequencies at which these resonances occur are not readily predicted by simple approximations of the head in isolation. In order to determine these resonant conditions an anatomically based model of the whole human body has been used, with the finite-difference time-domain (FDTD) algorithm to accurately determine field propagation, specific absorption rate (SAR) distributions and power absorption in both the whole body and the head region (head and neck). This paper shows that resonant frequencies can be determined using two methods. The first is by use of the accurate anatomically based model (with heterogeneous tissue properties) and secondly using a model built from parallelepiped sections (for the torso and legs), an ellipsoid for the head and a cylinder for the neck. This approximation to the human body is built from homogeneous tissue the equivalent of two-thirds the conductivity and dielectric constant of that of muscle. An IBM SP-2 supercomputer together with a parallel FDTD code has been used to accommodate the large problem size. We find resonant frequencies for the head and neck at 207 MHz and 193 MHz for the isolated and grounded conditions, with absorption cross sections that are respectively 3.27 and 2.62 times the shadow cross section.

A memory efficient method of calculating specific absorption rate in CW FDTD simulations

Specific absorption rate (SAR) distributions in man models are often calculated using the finite-difference time-domain (FDTD) method. The traditional method of calculating SAR requires calculation and storage of the three electric field components in each cell and is therefore very time- and memory-intensive. A new algorithm, based on the mass-normalized time-averaged energy distribution, is presented in this paper. This new method of calculating SAR requires 1/6 of the memory and a small fraction of the computer time of the traditional method. The accuracy of the two methods is shown to be virtually identical. In addition to improving the efficiency of SAR distribution calculations, the memory requirements are virtually eliminated for calculations of layer-averaged or organ-averaged SAR.

Evaluation and optimization of the electromagnetic performance of interstitial antennas for hyperthermia

Effective and realistic evaluation of the performance of interstitial antennas for microwave hyperthermia involves: (a) calculations of the radiation characteristics of these antennas, including those of multi-section designs, and (b) the development of 3-dimensional models of tumors and the computation of the power deposition patterns in these tumors due to their irradiation by an array of interstitial antennas. In this paper, we address both of these issues. Specifically, we developed numerical models for calculating the radiation characteristics of multi-section insulated antennas, and we utilized a 3-dimensional model based on a volume integral equation formulation for calculating the power deposition pattern in tumors. Numerical results were verified by comparing them with experimental data available in the literature. New numerical data are presented to illustrate the advantages of multi-section designs over uniformly-insulated interstitial antennas and to show the feasibility of using amplitude and phase steering capabilities to control the power deposition patterns of an interstitial antenna array. The role of each section in a multi-section antenna design in guiding versus radiating the electromagnetic energy and hence in controlling the power deposition pattern of multi-section antennas is also described.

Three-dimensional electromagnetic power deposition in tumors using interstitial antenna arrays

Interstitial arrays of insulated antennas have shown promise for microwave hyperthermia treatment of deep-seated tumors. Available analytical techniques for predicting the electromagnetic (EM) power deposition of these antennas have been limited to the case of a homogeneous conductive medium surrounding the array. Since tumors and host tissue may differ in their electrical characteristics, it is necessary to consider the impact of this variation in electrical properties and the geometry of the tumor in the calculation of the EM field distribution and power deposition pattern when modeling interstitial antennas. In this paper a three-dimensional model of a tumor of arbitrary shape subjected to the fields of an interstitial antenna array is developed to predict the EM power deposition in an inhomogeneous tumor-tissue medium. The volume integral equation for the imbedded tumor is developed and solved by method of moments. The incident fields are calculated based on the available formulation of interstitial antennas in homogeneous media. The accuracy of the developed computer code was checked by comparing the results from the volume integral approach with the Mie solution for the special case of spherical tumors. Good comparison was obtained for tumors with properties approximately 25 percent different from those of the surrounding tissue. Comparisons of results from models of antenna arrays with and without imbedded tumors show significant differences in their predictions of the EM power deposition in the tumor. Hyperthermia protocols generally specify uniform temperature distribution within the tumor. The developed inhomogeneous model was used to examine the feasibility of controlling the uniformity of the power deposition pattern in large tumors by adjusting the amplitude or relative phase between the array elements. Results are presented to show that a phase lead of +90 degrees or relative amplitude of 4.0 on one antenna in a square array of four antennas could be used to shift the power deposition pattern to sequentially heat outer portions of a 2 cm diameter tumor, thereby achieving a more uniform time-averaged temperature distribution in the tumor.