A New Animal Model of Insulin-Glucose Dynamics in the Intraperitoneal Space Enhances Closed-Loop Control Performance.

Current artificial pancreas systems (AP) operate via subcutaneous (SC) glucose sensing and SC insulin delivery. Due to slow diffusion and transport dynamics across the interstitial space, even the most sophisticated control algorithms in on-body AP systems cannot react fast enough to maintain tight glycemic control under the effect of exogenous glucose disturbances caused by ingesting meals or performing physical activity. Recent efforts made towards the development of an implantable AP have explored the utility of insulin infusion in the intraperitoneal (IP) space: a region within the abdominal cavity where the insulin-glucose kinetics are observed to be much more rapid than the SC space. In this paper, a series of canine experiments are used to determine the dynamic association between IP insulin boluses and plasma glucose levels. Data from these experiments are employed to construct a new mathematical model and to formulate a closed-loop control strategy to be deployed on an implantable AP. The potential of the proposed controller is demonstrated via in-silico experiments on an FDA-accepted benchmark cohort: the proposed design significantly outperforms a previous controller designed using artificial data (time in clinically acceptable glucose range: 97.3±1.5% vs. 90.1±5.6%). Furthermore, the robustness of the proposed closed-loop system to delays and noise in the measurement signal (for example, when glucose is sensed subcutaneously) and deleterious glycemic changes (such as sudden glucose decline due to physical activity) is investigated. The proposed model based on experimental canine data leads to the generation of more effective control algorithms and is a promising step towards fully automated and implantable artificial pancreas systems.

Oxidative stress/reactive metabolite gene expression signature in rat liver detects idiosyncratic hepatotoxicants.

Previously we reported a gene expression signature in rat liver for detecting a specific type of oxidative stress (OS) related to reactive metabolites (RM). High doses of the drugs disulfiram, ethinyl estradiol and nimesulide were used with another dozen paradigm OS/RM compounds, and three other drugs flutamide, phenacetin and sulindac were identified by this signature. In a second study, antiepileptic drugs were compared for covalent binding and their effects on OS/RM; felbamate, carbamazepine, and phenobarbital produced robust OS/RM gene expression. In the present study, liver RNA samples from drug-treated rats from more recent experiments were examined for statistical fit to the OS/RM signature. Of all 97 drugs examined, in addition to the nine drugs noted above, 19 more were identified as OS/RM-producing compounds-chlorpromazine, clozapine, cyproterone acetate, dantrolene, dipyridamole, glibenclamide, isoniazid, ketoconazole, methapyrilene, naltrexone, nifedipine, sulfamethoxazole, tamoxifen, coumarin, ritonavir, amitriptyline, valproic acid, enalapril, and chloramphenicol. Importantly, all of the OS/RM drugs listed above have been linked to idiosyncratic hepatotoxicity, excepting chloramphenicol, which does not have a package label for hepatotoxicity, but does have a black box warning for idiosyncratic bone marrow suppression. Most of these drugs are not acutely toxic in the rat. The OS/RM signature should be useful to avoid idiosyncratic hepatotoxicity of drug candidates.

Effects of hydration on scintigraphic glomerular filtration rate measured using integral and plasma volume methods in dogs with suspected renal disease.

The current standard scintigraphic method for estimating glomerular filtration rate (GFR) in dogs is the integral method, which normalizes renal GFR to body weight. The plasma volume method, that is normalizing GFR to plasma volume, has been reported to be more physiologically correct. The aim of this prospective study was to test the effect of hydration status on GFR measured by these two methods in a group of dogs with suspected renal disease. Eleven dogs were recruited. All dogs underwent standardized scintigraphic examinations before and after 15 ml/kg of fluid was administered intravenously at 5-7 ml/kg/min. Individual kidney GFR estimates (n = 22) were calculated using both methods and a consensus of two observers who were unaware of clinical findings. Individual kidney GFR increased significantly (P = 0.0008) after fluid administration using the integral method and individual kidney GFR using the plasma volume method remained constant. Percentage differences for individual kidney GFR before and after fluid administration were 31.4 ± 58.1% (change ± 95% CI) for the integral method and 0.1 ± 70% (change ± 95% CI) for the plasma volume method. Intravenously administered fluid increased individual kidney GFR from low to normal in 10 of 22 kidneys using the integral method and in 1 of 22 kidneys using the plasma volume method. Findings supported the use of the plasma volume method for scintigraphic calculation of GFR in dogs with suspected renal disease and indicated that errors of kidney status classification may more likely occur when the integral method is used.

Cardiac output affects the response to pulsed inhaled nitric oxide in mechanically ventilated anesthetized ponies determined by CT angiography of the lung.

OBJECTIVE: To measure changes in regional lung perfusion using CT angiography in mechanically ventilated, anesthetized ponies administered pulsed inhaled nitric oxide (PiNO) during hypotension and normotension. ANIMALS: 6 ponies for anesthetic 1 and 5 ponies for anesthetic 2. PROCEDURES: Ponies were anesthetized on 2 separate occasions, mechanically ventilated, and placed in dorsal recumbency within the CT gantry. Pulmonary arterial, right atrial, and facial arterial catheters were placed. During both anesthetics, PiNO was delivered for 60 minutes and then discontinued. Anesthetic 1: hypotension (mean arterial pressure < 70 mmHg) was treated using dobutamine after 30 minutes of PiNO delivery. Following the discontinuation of PiNO, dobutamine administration was discontinued in 3 ponies and was continued in 3 ponies. The lung was imaged at 30, 60, and 105 minutes. Anesthetic 2: hypotension persisted throughout anesthesia. The lung was imaged at 30, 60, and 90 minutes. At all time points, arterial and mixed venous blood samples were analyzed and cardiac output (Q˙t) was measured. Pulmonary perfusion was calculated from CT image analysis. RESULTS: During PiNO delivery, perfusion to well-ventilated lungs increased if ponies were normotensive, leading to increased arterial oxygenation, reduced alveolar dead space, and reduced alveolar to arterial oxygen tension gradient. When PiNO was stopped and dobutamine administration continued, alveolar dead space and venous admixture increased, in contrast to when dobutamine and PiNO were both discontinued. CLINICAL RELEVANCE: If PiNO is administered to mechanically ventilated, anesthetized ponies with concurrent hypotension and low Q˙t, this must be supported to achieve favorable redistribution of pulmonary perfusion to improve pulmonary gas exchange.

Effects of pulsed inhaled nitric oxide delivery on the distribution of pulmonary perfusion in spontaneously breathing and mechanically ventilated anesthetized ponies.

OBJECTIVE: To measure changes in pulmonary perfusion during pulsed inhaled nitric oxide (PiNO) delivery in anesthetized, spontaneously breathing and mechanically ventilated ponies positioned in dorsal recumbency. ANIMALS: 6 adult ponies. PROCEDURES: Ponies were anesthetized, positioned in dorsal recumbency in a CT gantry, and allowed to breathe spontaneously. Pulmonary artery, right atrial, and facial artery catheters were placed. Analysis time points were baseline, after 30 minutes of PiNO, and 30 minutes after discontinuation of PiNO. At each time point, iodinated contrast medium was injected, and CT angiography was used to measure pulmonary perfusion. Thermodilution was used to measure cardiac output, and arterial and mixed venous blood samples were collected simultaneously and analyzed. Analyses were repeated while ponies were mechanically ventilated. RESULTS: During PiNO delivery, perfusion to aerated lung regions increased, perfusion to atelectatic lung regions decreased, arterial partial pressure of oxygen increased, and venous admixture and the alveolar-arterial difference in partial pressure of oxygen decreased. Changes in regional perfusion during PiNO delivery were more pronounced when ponies were spontaneously breathing than when they were mechanically ventilated. CLINICAL RELEVANCE: In anesthetized, dorsally recumbent ponies, PiNO delivery resulted in redistribution of pulmonary perfusion from dependent, atelectatic lung regions to nondependent aerated lung regions, leading to improvements in oxygenation. PiNO may offer a treatment option for impaired oxygenation induced by recumbency.

Strategies for extended lifetime of implantable intraperitoneal insulin catheters.

Implantable insulin infusion systems using the intra-peritoneal route have dramatically changed the management of diabetes paving the way toward the realization of the potential "holy grail" of a fully implantable artificial pancreas. However, the wear duration of delivery catheters is compromised by the foreign body-mediated immune response. Both occlusion material present at the distal catheter tip end and fibrotic encapsulation surrounding the catheters influence the controlled and precise delivery of insulin, which eventually leads to the need for surgical intervention. The novel part of the current work is the investigation of the roles of implant physical properties (catheter size and tip configuration), as well as local inflammation control (through utilization of an anti-inflammatory agent) on the host fibrotic response using a previously developed animal model. The cellular and molecular response, the medication delivery efficacy as well as the ability to flush the catheters were examined and further compared among the different mitigation strategies. Reduction in catheter size as well as tuning the tip configuration from a cone shape to a round shape showed delayed host recognition and delayed propagation of the fibrotic response. However, the round shaped tips had an increased occurrence of lumen occlusion as a result of flow change. It became apparent that changing the physical properties of the catheters was not a long-term solution to catheter obstructions caused by the foreign body reaction. In comparison, control of the local inflammatory response through the use of an anti-inflammatory agent demonstrated a promising strategy for maintenance of catheter functionality without any type of obstructions. These finding will have a large impact toward the development of long-term use catheters for continuous intraperitoneal insulin infusion.

Development of a method to measure regional perfusion of the lung in anesthetized ponies using computed tomography angiography and the maximum slope model.

OBJECTIVE: To develop a method based on CT angiography and the maximum slope model (MSM) to measure regional lung perfusion in anesthetized ponies. ANIMALS: 6 ponies. PROCEDURES: Anesthetized ponies were positioned in dorsal recumbency in the CT gantry. Contrast was injected, and the lungs were imaged while ponies were breathing spontaneously and while they were mechanically ventilated. Two observers delineated regions of interest in aerated and atelectatic lung, and perfusion in those regions was calculated with the MSM. Measurements obtained with a computerized method were compared with manual measurements, and computerized measurements were compared with previously reported measurements obtained with microspheres. RESULTS: Perfusion measurements obtained with the MSM were similar to previously reported values obtained with the microsphere method. While ponies were spontaneously breathing, mean ± SD perfusion for aerated and atelectatic lung regions were 4.0 ± 1.9 and 5.0 ± 1.2 mL/min/g of lung tissue, respectively. During mechanical ventilation, values were 4.6 ± 1.2 and 2.7 ± 0.7 mL/min/g of lung tissue at end expiration and 4.1 ± 0.5 and 2.7 ± 0.6 mL/min/g of lung tissue at peak inspiration. Intraobserver agreement was acceptable, but interobserver agreement was lower. Computerized measurements compared well with manual measurements. CLINICAL RELEVANCE: Findings showed that CT angiography and the MSM could be used to measure regional lung perfusion in dorsally recumbent anesthetized ponies. Measurements are repeatable, suggesting that the method could be used to determine efficacy of therapeutic interventions to improve ventilation-perfusion matching and for other studies for which measurement of regional lung perfusion is necessary.

Root cause determination of intraperitoneal catheter obstructions: Insulin amyloid aggregates vs foreign body reaction.

Continuous intraperitoneal insulin infusion, from an implanted insulin pump connected to a catheter that delivers insulin directly to the peritoneal cavity has many clinical advantages for patients with Type 1 diabetes. However, the ongoing incidence of catheter obstructions remains a barrier to the widespread use of this therapy. To date, the root cause of these obstructions remains unknown. Here, a two-year clinical investigation was conducted, along with the development of an animal model to enable a mechanistic investigation into this issue. This novel animal model was able to mimic the catheter obstructions that occur in patients and, fortuitously, at an accelerated rate. This model allowed for independent assessment of each potential cause associated with catheter obstructions to help identify the root cause. Both macroscopic and microscopic analysis were conducted with regards to the onset and progression of catheter obstructions, along with monitoring of insulin delivery. Interestingly, although insulin aggregation occurs in insulin pumps and insulin aggregates were found in some catheter obstructions, insulin is unlikely to be the root cause, since obstructions also occurred in the control groups where only diluent (no insulin) was administered to the animals. Inflammatory cells, different phenotypes of fibroblasts, as well as collagen were observed in all obstructed catheters explanted from the patients and the animals. The presence of these cells and collagen is indicative of a typical foreign body reaction. In addition, the dynamic change in the fibroblasts with respect to morphology, phenotype, and spatial distribution suggests that tissue irritation-mediated epithelial to mesenchymal transition plays a role in catheter obstructions.

Radiographic features of cardiogenic pulmonary edema in dogs with mitral regurgitation: 61 cases (1998-2007).

OBJECTIVE: To evaluate radiographic distribution of pulmonary edema (PE) in dogs with mitral regurgitation (MR) and investigate the association between location of radiographic findings and direction of the mitral regurgitant jet (MRJ). DESIGN: Retrospective case series. ANIMALS: 61 dogs with cardiogenic PE and MR resulting from mitral valve disease (MVD; 51 dogs), dilated cardiomyopathy (9), and hypertrophic cardiomyopathy (1). PROCEDURES: Thoracic radiographs of dogs with Doppler echocardiographic evidence of MR were reviewed for location (diffuse, perihilar, or focal) of PE. Also, direction (central or eccentric) of the MRJ, as evaluated by Doppler color flow mapping (DCFM), and distribution (symmetric or asymmetric) of radiographic findings were evaluated. RESULTS: Diffuse, perihilar, and focal increases in pulmonary opacity were observed in 11 (18.0%), 7 (11.5%), and 43 (70.5%) of 61 dogs, respectively. Radiographic evidence of asymmetric PE in a single lung lobe or 2 ipsilateral lobes was found in 21 dogs, with involvement of only the right caudal lung lobe in 17 dogs. Doppler color flow mapping of the MRJ was available for 46 dogs. Of 31 dogs with a central MRJ, 28 had radiographic findings indicative of symmetric PE. Of 15 dogs with eccentric MRJ, 11 had radiographic evidence of asymmetric PE, and all of these dogs had MVD. CONCLUSIONS AND CLINICAL RELEVANCE: In dogs with cardiogenic PE, a symmetric radiographic distribution of increased pulmonary opacity was predominantly associated with a central MRJ, whereas an asymmetric radiographic distribution was usually associated with eccentric MRJ, especially in dogs with MVD.

Fibrotic Encapsulation Is the Dominant Source of Continuous Glucose Monitor Delays.

Continuous glucose monitor (CGM) readings are delayed relative to blood glucose, and this delay is usually attributed to the latency of interstitial glucose levels. However, CGM-independent data suggest rapid equilibration of interstitial glucose. This study sought to determine the loci of CGM delays. Electrical current was measured directly from CGM electrodes to define sensor kinetics in the absence of smoothing algorithms. CGMs were implanted in mice, and sensor versus blood glucose responses were measured after an intravenous glucose challenge. Dispersion of a fluorescent glucose analog (2-NBDG) into the CGM microenvironment was observed in vivo using intravital microscopy. Tissue deposited on the sensor and nonimplanted subcutaneous adipose tissue was then collected for histological analysis. The time to half-maximum CGM response in vitro was 35 ± 2 s. In vivo, CGMs took 24 ± 7 min to reach maximum current versus 2 ± 1 min to maximum blood glucose (P = 0.0017). 2-NBDG took 21 ± 7 min to reach maximum fluorescence at the sensor versus 6 ± 6 min in adipose tissue (P = 0.0011). Collagen content was closely correlated with 2-NBDG latency (R = 0.96, P = 0.0004). Diffusion of glucose into the tissue deposited on a CGM is substantially delayed relative to interstitial fluid. A CGM that resists fibrous encapsulation would better approximate real-time deviations in blood glucose.

Toward a checklist for exchange and interpretation of data from a toxicology study.

Data from toxicology and toxicogenomics studies are valuable, and can be combined for meta-analysis using public data repositories such as Chemical Effects in Biological Systems Knowledgebase, ArrayExpress, and Gene Expression Omnibus. In order to fully utilize the data for secondary analysis, it is necessary to have a description of the study and good annotation of the accompanying data. This study annotation permits sophisticated cross-study comparison and analysis, and allows data from comparable subjects to be identified and fully understood. The Minimal Information About a Microarray Experiment Standard was proposed to permit deposition and sharing of microarray data. We propose the first step toward an analogous standard for a toxicogenomics/toxicology study, by describing a checklist of information that best practices would suggest be included with the study data. When the information in this checklist is deposited together with the study data, the checklist information helps the public explore the study data in context of time, or identify data from similarly treated subjects, and also explore/identify potential sources of experimental variability. The proposed checklist summarizes useful information to include when sharing study data for publication, deposition into a database, or electronic exchange with collaborators. It is not a description of how to carry out an experiment, but a definition of how to describe an experiment. It is anticipated that once a toxicology checklist is accepted and put into use, then toxicology databases can be configured to require and output these fields, making it straightforward to annotate data for interpretation by others.

Application of genomics in preclinical drug safety evaluation.

Understanding the response of biological systems to xenobiotics is fundamental to the evaluation of drug safety. Toxicologists have traditionally gathered pathological, morphological, chemical and biochemical information from in vivo studies of preclinical species in order to assess drug safety and to determine how new drugs can be safely administered to the human patient population. In recent years the emerging "-omics" technologies have been developed and integrated into preclinical studies in order to better assess drug safety by gaining information on the cellular and molecular events underlying adverse drug reactions. Genomics approaches in particular have become readily available and are being applied in several stages of drug development. The burgeoning literature on what has become known as "toxicogenomics" has for the most part highlighted successful applications of gene expression profiling in predictive toxicology, enabling decisions to be made on the developability of a compound early in the drug development process. It is also becoming apparent that toxicogenomic approaches are good starting points to develop experiments designed to gain a mechanistic insight into drug toxicities within and across species. Gene expression arrays permit the measurement of responses of essentially all the genes in the entire genome to be monitored, and knowledge of the function of the genes affected can identify the potential mechanisms to then be confirmed using conventional biochemical, toxicological and pathological approaches. As toxicologists put these technologies into practice they build up a knowledge base to better characterize toxicities at the molecular level and to make the search for much needed, novel biomarkers of toxicity more achievable.

The evolution of gene expression studies in drug safety assessment.

Since the identification in the 1950s of deoxyribonucleic acid as the building block of life, the impact of molecular biology has been far-reaching. Understanding the processes of how DNA is replicated, transcribed into RNA and then translated into protein products has not only provided a fundamental knowledge of life but has also spawned a plethora of applications. Molecular biology has been high profile and widespread in research into the biology of disease and in drug discovery. It has additionally found application in understanding the adverse effects, or toxicity, of candidate drugs and how they interfere with biochemical and biological processes. In recent times the biggest impact of molecular biology in toxicology has been through the study of differential gene expression, largely as a result of the advent of genomics. This review seeks to describe how toxicogenomics strategies have been implemented and integrated into nonclinical studies of drug safety.

Drug-induced oxidative stress in rat liver from a toxicogenomics perspective.

Macrophage activators (MA), peroxisome proliferators (PP), and oxidative stressors/reactive metabolites (OS/RM) all produce oxidative stress and hepatotoxicity in rats. However, these three classes of hepatotoxicants give three distinct gene transcriptional profiles on cDNA microarrays, an indication that rat hepatocytes respond/adapt quite differently to these three classes of oxidative stressors. The differential gene responses largely reflect differential activation of transcription factors: MA activate Stat-3 and NFkB, PP activate PPARa, and OS/RM activate Nrf2. We have used gene signature profiles for each of these three classes of hepatotoxicants to categorize over 100 paradigm (and 50+ in-house proprietary) compounds as to their oxidative stress potential in rat liver. In addition to a role for microarrays in predictive toxicology, analyses of small subsets of these signature profiles, genes within a specific pathway, or even single genes often provide important insights into possible mechanisms involved in the toxicities of these compounds.

Release of (and lessons learned from mining) a pioneering large toxicogenomics database.

AIM: We release the Janssen Toxicogenomics database. This rat liver gene-expression database was generated using Codelink microarrays, and has been used over the past years within Janssen to derive signatures for multiple end points and to classify proprietary compounds. MATERIALS & METHODS: The release consists of gene-expression responses to 124 compounds, selected to give a broad coverage of liver-active compounds. A selection of the compounds were also analyzed on Affymetrix microarrays. RESULTS: The release includes results of an in-house reannotation pipeline to Entrez gene annotations, to classify probes into different confidence classes. High confidence unambiguously annotated probes were used to create gene-level data which served as starting point for cross-platform comparisons. Connectivity map-based similarity methods show excellent agreement between Codelink and Affymetrix runs of the same samples. We also compared our dataset with the Japanese Toxicogenomics Project and observed reasonable agreement, especially for compounds with stronger gene signatures. We describe an R-package containing the gene-level data and show how it can be used for expression-based similarity searches. CONCLUSION: Comparing the same biological samples run on the Affymetrix and the Codelink platform, good correspondence is observed using connectivity mapping approaches. As expected, this correspondence is smaller when the data are compared with an independent dataset such as TG-GATE. We hope that this collection of gene-expression profiles will be incorporated in toxicogenomics pipelines of users.

Gene expression changes in rat liver following exposure to liver growth agents: role of Kupffer cells in xenobiotic-mediated liver growth.

Many xenobiotics are known to cause liver enlargement and hepatocarcinogenesis in rats, although the molecular mechanisms that underlie this effect remain largely undefined. Human exposure to several of these compounds, including glucocorticoids and peroxisome proliferators may be significant, due to their use in both pharmaceutical and industrial processes. It is therefore important to elucidate the molecular mechanisms underlying this abnormal liver enlargement in rats, as this will enable more accurate extrapolation of the possible outcomes of human exposure. Male Sprague-Dawley rats were dosed with the peroxisome proliferator Wy-14,643 and changes in liver gene expression examined using subtractive suppression hybridisation examined either 12 of 24hr later. Twenty-five transcripts were identified which showed differential gene expression in liver following exposure to Wy-14,643. Biochemical indices of liver growth (DNA synthesis, apoptosis) showed that these changes correlated with the initiation of liver enlargement. Rats were next treated with either Wy-14,643, cyproterone acetate and dexamethasone, chemically and mechanistically-distinct hepatomegalic compounds. Carboxylesterase and Kupffer cell receptor mRNA levels were seen to alter in a qualitatively similar fashion for all three compounds, and in a liver specific fashion. In addition, these changes correlated with a decrease in the density of Kupffer cells within the liver, which are known to release mitogenic cytokines, and have been linked to Wy-14,643-induced cell proliferation. We therefore propose that Kupffer cells play a role in a general mechanism of xenobiotic-mediated liver enlargement.

Toxicogenomics in risk assessment: an overview of an HESI collaborative research program.

The value of genomic approaches in hypothesis generation is being realized as a tool for understanding toxicity and consequently contributing to an assessment of drug and chemical safety. In 1999 the membership of the International Life Sciences Institute Health and Environmental Sciences Institute formed a committee to develop a collaborative scientific program to address issues, challenges, and opportunities afforded by the emerging field of toxicogenomics. Experts and advisors from academia and government laboratories participate on the committee, along with approximately 30 corporate member organizations from the pharmaceutical, agrochemical, chemical, and consumer products industries. The committee has designed, conducted, and analyzed numerous toxicogenomic experiments within the broad fields of hepatotoxicity, nephrotoxicity, and genotoxicity. The considerable body of data generated by these programs has been instrumental in increasing understanding of sources of biological and technical variability in the alignment of toxicant-induced transcription changes with the accepted mechanism of action of these agents and the challenges in the consistent analysis and sharing of the voluminous data sets generated by these approaches. Recognizing the importance of standardized microarray data formats and public repository databases as the mechanism by which microarray data can be compared and interpreted by the scientific community, the committee has partnered with the European Bioinformatics Institute to develop a database to house the data generated by its collaborative research.

Progress in applying genomics in drug development.

Genomics has had an impact on two areas of drug development, "predictive" toxicology and mechanism-based risk assessment. Predictive toxicology studies are aimed at identifying the potential for a compound to be toxic. By developing databases of expression profiles for a wide variety of toxic compounds and toxic models it has been possible to create statistical and computational methods which provide an indication of the toxic potential of a drug from the pattern of gene expression changes it elicits in in vitro or in vivo systems. Because gene expression is central to many responses to xenobiotics, genomic approaches lend themselves very readily to mechanistic toxicology studies. By examining changes in gene expression in cells and tissues in response to drugs it is possible to generate hypotheses as to the underlying mechanism and in some cases it is possible to evaluate hypotheses of toxic mechanism. Some concerns remain about the use of the technology but toxicogenomics can no longer be regarded as "new" technology in drug development. The investments made in applying the technology are maturing and there is a determined effort to bring the full power of the technology into drug development.

Increased pulmonary transit times in asymptomatic dogs with mitral regurgitation.

Pulmonary transit time (PTT) normalized to heart rate (nPTT) is a measure of the pulmonary blood volume (PBV) to stroke volume ratio (PBV/SV). It is an index of cardiac performance. To determine the effect of compensated mitral regurgitation (CMR) and decompensated mitral regurgitation (DMR) caused by valvular endocardiosis on the index nPTT, we measured nPTT by first-pass radionuclide angiocardiography and ECG in 13 normal dogs, 18 dogs with CMR, and 13 dogs with DMR. PTT was measured as time between onset of appearance of activity at the pulmonary trunk and the left atrium. In the normal dogs, the relationship between PTT and mean R-R interval (mRR) was PTT = 4.08 x mRR + 0.15 (R2 = 0.71). Normal nPTT was 4.4 +/- 0.6 (SD) (range. 3.6-5.3). in CMR, 6.3 +/- 1.6 (SD) (range, 4.0-9.7). and in DMR, 11.9 +/- 3.4 (SD) (range, 8.0-18.8). The differences among all groups were significant. Heart rates were 110 +/- 22 bpm in normal dogs, 111 +/- 20 in dogs with CMR, and 144 +/- 18 in dogs with DMR (P < .001 for difference between DMR group and normal and CMR groups). Increased nPTT in CMR indicates preclinical heart pump dysfunction. Heart rate-normalized pulmonary transit times may be a useful index of heart function in mitral regurgitation.

Efficacy of enalapril for prevention of congestive heart failure in dogs with myxomatous valve disease and asymptomatic mitral regurgitation.

We evaluated the long-term effect of early angiotensin-converting enzyme (ACE) inhibition (enalapril maleate) as monotherapy to postpone or prevent congestive heart failure (CHF) in asymptomatic dogs with mitral regurgitation (MR) attributable to myxomatous valvular disease (MVD) in a prospective, randomized, double-blinded, placebo-controlled multicenter trial involving 14 centers in Scandinavia. Two hundred twenty-nine Cavalier King Charles (CKC) Spaniels with MR attributable to MVD but no signs of CHF were randomly allocated to treatment with enalapril 0.25-0.5 mg daily (n = 116) or to placebo groups (n = 113). Each dog was evaluated by physical examination, electrocardiography, and thoracic radiography at entry and every 12 months (+/-30 days). The number of dogs developing heart failure was similar in the treatment and placebo groups (n = 50 [43%] and n = 48 [42%], respectively; P = .99). The estimated means, adjusted for censored observations, for the period from initiation of therapy to heart failure were 1,150 +/- 50 days for dogs in the treatment group and 1,130 +/- 50 days for dogs in the placebo group (P = .85). When absence or presence of cardiomegaly at the entrance of the trial was considered, there were still no differences between the treatment and placebo groups (P = .98 and .51, respectively). Multivariate analysis showed that enalapril had no significant effect on the time from initiation of therapy to heart failure (P = .86). Long-term treatment with enalapril in asymptomatic dogs with MVD and MR did not delay the onset of heart failure regardless of whether or not cardiomegaly was present at initiation of the study.