Lynn G. Mitchell

Median frequency — a new parameter for predicting defibrillation success rate

STUDY HYPOTHESIS Current American Heart Association guidelines recommend immediate defibrillation of ventricular fibrillation. When this is unsuccessful, there are no guidelines to help determine the optimum time at which to defibrillate after the administration of an alpha-adrenergic agonist. Previous studies have shown that the median frequency of the ventricular fibrillation ECG signal correlates with myocardial perfusion during CPR. We hypothesized that median frequency could predict the success of defibrillation and thus accurately determine the most appropriate time at which to defibrillate during ventricular fibrillation. STUDY POPULATION Twenty-two mixed-breed swine weighing more than 15 kg were studied. METHODS Ventricular fibrillation was induced electrically, and the ventricular fibrillation ECG signal was analyzed using fast Fourier analysis. After ten minutes of ventricular fibrillation, mechanical CPR was begun. After three minutes of CPR, the animals received one of three alpha-adrenergic agonists and CPR was continued. Defibrillation was attempted three and one-half minutes after drug administration. The average median frequency 20 seconds before defibrillation was calculated. Sensitivity and specificity of median frequency with respect to defibrillation success were determined. RESULTS A median frequency of 9.14 Hz had a sensitivity of 100% and a specificity of 92.31% in predicting the results of defibrillation in this model. CONCLUSION The median frequency may serve as a valuable parameter to guide defibrillation therapy during ventricular fibrillation.

Surveillance With Duplex Ultrasound In Traumatic Spinal Cord Injury On Initial Admission To Rehabilitation

Abstract Objective: To determine the prevalence of deep vein thrombosis (DVT) by surveillance duplex ultrasound in the traumatic spinal cord injury (SCI) population on admission to rehabilitation. Design: Retrospective sequential case series. Setting: Midwest regional, university-based, Commission on Accreditation of Rehabilitation Facilities-accredited acute rehabilitation center. Methods: Charts of all patients with traumatic SCI admitted and discharged from january 1, 1996 through December 31, 1998 were reviewed. Preadmission data were collected on demographics, severity of injury, and DVT prophylaxis information, along with rehabilitation duplex ultrasound results and incidence of thromboembolic events. Results: Ninety-two participants met the inclusion criteria. There were 68 men and 24 women with a mean age on admission of 3 2.4 years. On admission, 45 participants (49%) were classified as tetraplegic and 47 (51%) were classified as paraplegic; 63 (69%) had motor-complete lesions and 29 (31 %) had motor-incomplete lesions. Of all the participants, 8 (8.7%) were found to have DVT on admission to rehabilitation. There were no statistically significant differences among participants with regard to age, sex, level of injury, or completeness of injury, when comparing those participants with DVT on admission, those without DVT on admission, and those with thromboembolic events diagnosed later in their hospitalization. Of the 84 participants who had negative duplex ultrasounds on admission, 4 individuals (4.8%) were found to have DVT and 4 (4.8%) had pulmonary emboli subsequently. In these 84 participants, DVT prophylaxis with low-molecular-weight heparin was found to be more effective than was adjusted-dose heparin in preventing thromboembolic phenomenon. Conclusion: Incidence of DVT remains high despite prophylaxis in traumatic SCI patients. Two thirds of DVT diagnosed in rehabilitation was identified on admission and one third was diagnosed later. Duplex ultrasound is an effective and valuable tool that assists in the diagnosis of asymptomatic DVT in patients with traumatic SCI who are initiating in -patient rehabilitation.

Regional cerebral blood flow with manual internal cardiac massage versus direct mechanical ventricular assistance

STUDY HYPOTHESIS Previous studies have not discerned the best method for generating regional cerebral blood flow during internal cardiac massage. We hypothesized that regional cerebral blood flow generated by a mechanical method--direct mechanical ventricular assistance (DMVA)--would be superior to manual internal cardiac massage (MAN). STUDY POPULATION Twelve adult Yucatan minipigs weighing more than 44 kg each were studied. METHODS Swine were instrumented for regional cerebral blood flow measurements using tracer microspheres. After 15 minutes of ventricular fibrillation, swine were randomized to receive either MAN or DMVA. Regional cerebral blood flow was measured during normal sinus rhythm and at one minute (VF-1) and six minutes (VF-2) after initiation of circulatory support. Regional cerebral blood flow values were compared using a Wilcoxon rank sum test. RESULTS During VF-1, there was a tendency for DMVA to produce greater regional cerebral blood flow than MAN, although these differences were not statistically significant (DMVA vs MAN as mL/min/100 g): cerebral cortex, 28 versus 11; cerebellum, 49 versus 22; midbrain, 43 versus 16; pons, 55 versus 18; medulla, 55 versus 19; and spinal cord, 33 versus 10. During VF-2, DMVA produced greater regional cerebral blood flows than were produced by MAN: cerebral cortex, 39 versus 12 (P less than .06); cerebellum, 58 versus 20 (P less than 0.5); midbrain, 50 versus 18 (P less than .05); pons, 52 versus 22 (P less than .06); medulla, 53 versus 20 (P less than .05); and spinal cord, 31 versus 12 (P less than .05). CONCLUSION DMVA produces greater regional cerebral blood flow than is produced during MAN after 15 minutes of ventricular fibrillation. DMVA is effective at maintaining regional cerebral blood flow after a prolonged cardiac arrest.

Fluorescein pattern interpretation in keratoconus.

Purpose. The purpose of this study is to assess the effect of disease severity on how accurately contact lens fluorescein patterns can be interpreted in keratoconus by clinician assessment. Methods. Two clinicians evaluated fluorescein patterns on 111 eyes of 60 patients with mild (<45 D, 14 eyes), moderate (45 D to 52 D, 61 eyes,) and severe (>52 D, 36 eyes) keratoconus. The masked clinicians were given six contact lenses in random order, the lens that just cleared the corneal apex (the first definite apical clearance lens), three lenses flatter (in 0.1 mm increments), and two lenses steeper (in 0.1 mm increments) than the first definite apical clearance lens. They ranked the lenses from flattest to steepest, based on the fluorescein patterns. The percentage of lenses correctly ranked was determined using (1) exact match with actual; (2) within 0.1 mm of actual; and (3) within 0.2 mm of actual. Accuracy was assessed as the sum of the squared differences between the actual base curve value and each clinicians ranking. Comparison of the mean percentage correctly ranked and accuracy for each keratoconus severity groups was performed using a mixed linear model. Results. Neither percentage correctly ranked (using any of the three protocols) nor accuracy was found to be related to severity of keratoconus (p > 0.15 for all comparisons). Conclusions. Accuracy of ranking contact lenses in order of base curve radius based on fluorescein pattern assessment by clinicians does not seem to be related to severity of keratoconus. Many factors influence interpretation of fluorescein patterns including all components of the system, fluorescein, tears, cornea, contact lens, external forces, and technique.