David H. Knight

Pilot study on cardiac troponin I levels in dogs with pericardial effusion.

OBJECTIVES The aim of this pilot study was to assess cardiac troponin I (cTnI) levels in pericardial effusion (PE) and plasma from dogs with PE. BACKGROUND A reliable marker for detecting the etiology of PE in dogs remains undetermined. cTnI is becoming the gold standard marker for detecting myocardial damage in humans. ANIMALS, MATERIALS AND METHODS Twenty-five dogs with PE (21 and 4 secondary to neoplasia and non-neoplasia causes, respectively) and 37 control dogs were studied. RESULTS The median cTnI plasma level from 37 normal dogs versus 15 (out of 25) with PE was 0.03ng/mL and 0.19ng/mL, respectively (p<0.0001). The level of cTnI in PE versus plasma showed a significant correlation (p<0.01) with a Spearman r coefficient of 0.7603. No significant difference could be found upon comparison of dogs with only right atrial tumors (n=14) versus other types of neoplasia (n=7), nor between the group with right atrial tumors (n=14) versus all other cases including neoplasia as well as non-neoplasia (n=11). The median cTnI level in PE from dogs with neoplasia and non-neoplasia was not significantly different. CONCLUSIONS cTnI did rise significantly in both PE and plasma in dogs with PE, but cTnI levels did not help differentiate between etiologies according to this study. One of the study groups is too small to allow final conclusions, and thus further investigation is warranted.

Seasonality of heartworm infection and implications for chemoprophylaxis.

The current emphasis on heartworm prevention reflects the dependable protection provided by the monthly administered macrolide endectocides. This article reviews the prerequisites for heartworm transmission and the importance of daily temperature as a limiting factor in determining the seasonality of the transmission period. The practice of some veterinarians to continuously prescribe monthly chemoprophylaxis exaggerates the actual risk of heartworm transmission in most parts of the country and unnecessarily increases the cost of protection to their clients. Guidelines are provided for making an objective, conservative estimate of the earliest and latest dates for administering monthly chemoprophylaxis; and the use of seasonal projections for other clinical applications such as timing and interpretation of heartworm testing are discussed.

SEASONAL PREVALENCE OF THIRD-STAGE LARVAE OF DIROFILARIA IMMITIS IN MOSQUITOES FROM FLORIDA AND LOUISIANA

Heads of 109,597 mosquitoes collected during 1996 and 1997 from Gainesville, Florida (1996, n = 39,131; 1997, n = 34,209), Bartow, Florida (1996, n = 12,000; 1997, n = 12,000), and Baton Rouge, Louisiana (1996, n = 12,257) were tested by a polymerase chain reaction and Southern hybridization-based test for the presence of third-stage larvae of the canine heartworm Dirofilaria immitis. Mosquito heads were pooled (1–200 heads) by month, locality, and species for testing. The test used was species specific for D. immitis and was capable of detecting DNA from a single larva in a pool of 200 mosquito heads. Specificity for the third larval stage was achieved by probing only mosquito heads. One or more D. immitis-infected mosquito heads were detected in each month of the year from Bartow in both 1996 and 1997. No infected mosquito heads were detected from Gainesville or Baton Rouge in December, January, February, or March. These results are in general agreement with previous sentinel dog and model prediction studies that showed heartworm transmission in the warm temperate Gulf coast region of the United States to be seasonal rather than continuous as previously believed.

Abnormal patterns of embryogenesis in Dirofilaria immitis treated with ivermectin.

The percentage composition and spatial distribution of embryogenic stages in the uteri of female Dirofilaria immitis were examined at various times after treatment with a microfilaricidal dose of ivermectin and compared to nontreated parasites. Worms sampled 42 days post-treatment (PT) exhibited an increased proportion of stretched microfilariae in the distal portion of the uterus. A decreased proportion of developed embryos was noted in the mid body region of worms sampled 42 days PT, and these forms were completely absent from the proximal area of the uterus. Relative numbers and spatial distribution of other stages remained virtually identical to controls. Radical changes in the composition and spatial distribution of embryogenic forms were noted in the uteri of a single worm sampled 80 days PT. Unlike nontreated parasites and worms sampled 42 days PT, stretched microfilariae constituted the predominant form in the distal uterus of this worm, and these stages were found in decreasing numbers throughout the proximal segments. Also, the intermediate embryogenic stages were either rare or absent.

Constriction of the Fetal Ductus Arteriosus Induced by Oxygen, Acteylcholine, and Norepinephrine in Normal Dogs and those Genetically Predisposed to Persistent Patency

Functional closure of the isolated perfused ductus arteriosus was studied in normal dog fetuses and in fetuses receiving various known proportions of their genes from dogs with patent ductus arteriosus (PDA). Constriction of the ductus in response to O2, acetylcholine (ACh), and norepinephrine (NE) was evaluated by plotting changes in input conductance against time. Ductuses from the control (normal dog) fetuses constricted in response to high PO2, ACh, and NE, indicating that in the dog the response of the ductus arteriosus to these agents is qualitatively similar to that in other mammals. Hypoxia relaxed the O2 constricted ductus. After two to three cycles of oxygenation and hypoxia, the ductus became refractory to O2 but remained responsive to ACh and NE. Ductuses from PDA-related fetuses tended to be more widely patent when hypoxic and did not respond to O2, ACh, and NE to a degree comparable to the control fetuses. Both the proportion of subnormal ductal responses and degree of abnormality in the PDA-related group increased as the degree of relationship to dogs with PDA increased. These experimental results are consistent with the polygenic mode of inheritance previously demonstrated for canine PDA. It is suggested that the liability to defective ductal closure in the dog results from a quantitatively inherited inability of the ductus to constrict.

Efficacy of Inotropic Support of the Failing Heart

The clinical literature on the subject of inotropic therapy of heart failure, particularly use of digitalis glycosides, is full of contradictions. Most of this disparity can be accounted for if not reconciled by taking the methodology of the clinical trials into consideration. Because drug interventions may produce subtle effects requiring a subjective determination, the questions being asked in these studies cannot be answered without removing as many sources of bias as possible from the patient management and data analysis. If a study has not been adequately randomized, double-blinded, and placebo-controlled, the clinical findings will be inconclusive at best. Systolic myocardial dysfunction plays a pivotal role in the pathogenesis of CHF in many patients and is a prerequisite for the use of cardiotonic drugs. Although the clinical signs of heart failure may be relieved initially by diuretics and vasodilators, compensation may require the addition of a positive inotrope, particularly in advanced cases. In veterinary medicine, the choice of positive inotrope is limited to digoxin, digitoxin, dobutamine, or amrinone. Digoxin possesses superior pharmacokinetics and is the cardiac glycoside of choice for use in the dog. Dobutamine and amrinone are more potent inotropes, but since they must be administered by continuous intravenous infusion, their use is limited to critical care therapy. At the present time, only digoxin can be administered orally for sustained long-term maintenance therapy. Milrinone, a more potent derivative of amrinone, also offers this option, but it has not been available since its brief trial debut as an investigational drug. None of the nonglycoside alternatives couples the benefits of positive inotropic and negative chronotropic effects. Consequently, digoxin remains the mainstay for chronic inotropic support of the heart. Atrial fibrillation with a rapid ventricular response rate is the prime indication for digoxin. In the last few years, evidence from methodologically sound clinical trials on humans has also restored faith in the efficacy of digoxin for treating heart failure in patients with normal sinus rhythm. From these studies, the profile of a digitalis responsive heart failure patient has emerged. Digoxin is most likely to be efficacious when heart failure is associated with chronic, severe ventricular systolic dysfunction, which has resulted in ventricular dilatation. The most reliable clinical marker is the presence of a third heart sound (gallop rhythm). Although the patients in the worst heart failure generally have the shortest survival time, they may also have the most dramatic short-term clinical benefit. However, once cardiac reserve is exhausted in the terminal stages of failure, cardiotonic stimulation ceases to be effective.(ABSTRACT TRUNCATED AT 400 WORDS)

Hemodynamic Phenomena in Fusiform Aneurysms - I: Steady Flow Conditions

Pressure and flow distributions are reported at different steady flows for four latex fusiform aneurysm models. The accuracy of results by pitot-static techniques is examined in terms of experimental evidence and theoretical considerations. Results indicate that pressure distributions are associated with the Reynolds number, the degree of wall divergence and the degree of dilation of the sac, and that these parameters are interactive. The Reynolds number was found to have some predictive value for pressure distribution in the aneurysm, but the Bernoulli principle, based on geometrical variation and laminar flow, was usually inapplicable. Also, disturbed flow (eddies, reversal, turbulence) can occur as functions of the interactive parameters.

Hemodynamic Phenomena in Fusiform Aneurysms-II: Pulsatile Flow Conditions

Pressure and flow distributions are reported at different pulsatile flows for four latex fusiform aneurysm models. The accuracy of results by Pitot-static techniques is examined in terms of experimental evidence and theoretical considerations. Pressure distributions within the aneurysm are associated with the Reynolds number, the degree of wall divergence, and the degree of sac dilation, and these parameters are interactive. The Bernoulli principle was usually inapplicable, and higher peak pressures may be seen where measured velocities may lead one to predict lower pressures. The presence of such localized high pressures depends on whether disturbed flow occurs at that location. There may be little qualitative difference in the patterns of velocity and pressure within an aneurysm between pulsatile and steady flow conditions. However, with pulsatile flow, the presence of distributed flow may be more obvious and commence closer to the proximal end of the aneurysm.