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Biomedical subjects

R J Sweeney

Publications and source records attributed to R J Sweeney.

7 recordsLinked to original sources
PubMed1991

Characterization of refractory period extension by transcardiac shock.

BACKGROUND: To better understand the refractory period extension (RPE) produced by transcardiac shocks and its possible role in defibrillation, we measured RPE under various experimental conditions. METHODS AND RESULTS: Using ventricular pacing in pentobarbital-anesthetized dogs, we characterized RPE in relation to the anatomic site of pacing, the local voltage gradient (LVG) produced by the shocks at the pacing site, and the pacing rate and pacing current used to make the measurements. We also determined if RPE persisted into the next refractory period after the shock and measured RPE at the end of 30-second episodes of acute ischemia to the pacing site, which were caused by occluding the left anterior descending artery. Each anatomic site tested showed RPE, which increased sharply with increasing LVG at lower levels but less sharply at higher LVG. The RPE versus LVG was approximated with an exponential curve that had an exponential constant of about 5-6 V/cm. At faster pacing rates, RPE occurred earlier in the refractory period but was unchanged when expressed as a percent increase of refractory period. RPE did not vary with the pacing current and was present only in the refractory period during which the shock was delivered. The RPE was not significantly altered by acute ischemia. These results show that transcardiac shocks selectively extend the refractory period of tissue proportional to the LVG and the timing of the shock in the refractory period. They are consistent with the concept that RPE prevents depolarization from tissue directly excited by a shock from propagating to tissue that was refractory to that same shock. CONCLUSIONS: The insensitivity of RPE to short ischemic episodes and the presence of RPE at increased activation rates suggest that RPE might exist under conditions of fibrillation and be a major determinant of the success or failure of defibrillation.

Acute Disease↗
PubMed1990

Ventricular refractory period extension caused by defibrillation shocks.

In pentobarbital-anesthetized dogs, transcardiac shocks of up to 30 J or pacing stimuli were delivered to myocardial tissue at different times in the electrical cycle. When delivered midway or later into electrical systole, shocks, but not pacing stimuli, greatly extended the refractory period as determined by left ventricular pacing. There was a positive correlation between both the shock energy and timing and the amount of delay. A 30-J shock given 10 msec before the end of the refractory period extended the refractory period by 63 +/- 15 msec (p less than 0.001), whereas the same shock given 40 msec earlier produced only 25 +/- 10 msec (p less than 0.001) of extension. By comparison, a 5-J shock given at those times produced 36 +/- 18 (p less than 0.005) and 10 +/- 8 msec (p less than 0.001) of extension, respectively. When delivered early into electrical systole, both a pacing stimulus and a shock had no substantial effect on the tissue refractory period. Because the tissue that is late in electrical systole would otherwise be the first to repolarize if no shock were given, the selective refractory period extension may create a period after the shock during which no tissue is repolarized to a level sufficient for wavefront propagation. Thus, the energy- and time-dependent refractory period extension may help explain the mechanism by which ventricular defibrillation occurs during transcardiac shocks.

Animals↗
PubMedDate not supplied

System designed to improve communication process between patient and technologist.

With the use of more specialized equipment in our radiology departments, we are able to examine more patients with greater efficiency. However, our role as radiologic technologists is only partially fulfilled if our concern over the number of patients and the functional aspects of the equipment restricts us from adequately communicating with the patient during the radiographic examination.

Communication↗
PubMedDate not supplied

More on artifacts.

Explore the source record for details and available documents.

Radiography↗
PubMedDate not supplied

One more look at the mandibular condyle.

A great deal of attention has been given over the years to the development of highly specialized x-ray equipment that, in many instances, has reduced the complex nature of the radiographic procedure. However, it is impossible, and probably always will be, to design equipment that will automatically produce the proper radiographic position. This will remain a distinct function of the technologist, and the individual's skill will have great bearing on the quality of the radiograph. One need only review the literature to realize how innovative our profession has been in developing countless numbers of radiographic positions for demonstrating specific anatomical regions. Moreover, many useful radiographic positions can be found that, for various reasons, never gained the level of acceptance perhaps anticipated by their originators. Consequently, the practical value of these positions often escapes our attention. To illustrate this point, I have selected a radiographic position described some thiry years ago for visualization of the mandibular condyle. It is my intent to demonstrate the practical value of this projection, as well as that of some others not usually thought of, for radiography of the mandibular condyle.

Fractures, Bone↗
PubMedDate not supplied

The use of an inverted Kodak X-Omatic cassette as an improvised grid.

An inverted X-Omatic cassette is particularly useful when substituted for a grid in situations where the employment of a grid would not be practical. It also provides a selective filtration effect during radiography of anatomical regions that exhibit a wide range of subject opacity. With radiology procedures and equipment becoming more complex, the concept of employing an improvised method of controlling radiation scatter may seem antiquated when compared to today's high standards of imaging excellence. However, based on my experience with this method, I feel its practical application should not be overlooked as an alternative method of coping with an age-old problem of radiation scatter control.

Technology, Radiologic↗