Materials and Methods

Animals—Twenty-two client-owned dogs were used in the study. Two groups of Boxers were selected from a cohort of an ongoing study of ARVC for which an ECG, echocardiogram, and 24-hour AECG were obtained annually. One group consisted of 12 Boxers with ARVC diagnosed on the basis of ≥ 1,000 VPCs/24 h on a 24-hour AECG and left ventricle echocardiographic variables typical of those defined for clinically normal dogs of this size.¹⁹ The second group (control Boxers) consisted of 10 Boxers with ≤ 5 VPCs/24 h and left ventricular echocardiographic variables typical of those for clinically normal dogs of this size.¹⁹ The study was conducted in accordance with guidelines of the Animal Care and Use Committee of The Ohio State University College of Veterinary Medicine. Written consent authorizing participation of dogs in the study was obtained from all clients.

Procedures—Results of AECG were used to assign each dog to the 2 groups. A 3-channel transthoracic 24-hour AECG system^a was attached to each dog and removed after 25 hours. A technician under the guidance of a board-certified veterinary cardiologist analyzed the AECG recordings by use of a prospective analysis system. Any recordings that did not have at least 20 hours of useful data were excluded from analysis, and the AECG was repeated. Analysis of AECG included tabulation of the number of VPCs per 24 hours, and arrhythmia was graded on the basis of the highest grade detected (1 = a single uniform VPC; 2 = bigeminy, trigeminy, or both; 3 = ventricular couplets, triplets, or both; and 4 = R wave on T wave, ventricular tachycardia, or both).

Echocardiograms were obtained by use of an echocardiographic system^b with a 3-MHz probe. Echocardiograms were obtained by use of standard clinical techniques in dogs positioned in right and left lateral recumbency without sedation.¹⁹ Dogs were positioned in right lateral recumbency, and the left ventricle was imaged from the right intercostal (parasternal) position at the level of the papillary muscles to obtain a short-axis tomogram. The M-mode measurements of the left ventricular internal diameter during diastole and during systole were obtained, and fractional shortening was calculated. Left apical 4- and 3-chamber views were evaluated by use of color Doppler imaging to screen the mitral and aortic valves for evidence of regurgitation. Dogs with abnormal left ventricular fractional shortening (< 25%), an increase in left ventricular chamber size with respect to body size, or notable regurgitation through the mitral or aortic valves were excluded from the study.¹⁹

Color Doppler imaging was used to guide placement of the sample volume (1 to 3 mm) for all pulse-wave Doppler examinations, which were conducted on the left side. Inflow velocity through the tricuspid valve and right ventricular outflow velocity were recorded separately by use of pulsed-wave Doppler. The transducer was moved 1 intercostal space cranial to the location typically used to obtain an apical 4-chamber view, which enabled us to obtain a modified apical imaging plane to record pulse-wave Doppler recordings of flow through the tricuspid valve. A left cranial image was obtained to record pulse-wave velocity spectra from the right ventricular outflow tract. The sample volume was placed close to the valves so that valve noise was recorded. Baseline filters were kept at a minimum to ensure clear delineation of inflow and outflow.

Echocardiographic information was recorded simultaneously with the ECG, and all raw data were captured digitally to maintain optimal fidelity for off-line measurement to allow sweep speeds of up to 200 mm/s. No attempt was made to match R–R intervals for the inflow and outflow signals. Measurements for dogs that had arrhythmias during echocardiography were obtained from sinus beats preceded by a sinus beat. Twenty cycles of all measured variables were recorded digi tally to allow for analysis of 5 consecutive measurable beats, and the mean heart rate was determined from the 20 cardiac cycles. An IMP for the right ventricle was calculated from the pulsed-Doppler recordings by use of the following equation: (isovolumetric contraction + isovolumetric relaxation)/ET. The IMP was calculated by use of the equation (a − b)/b, where a represents the interval of cessation and onset of inflow across the tricuspid valve and b represents ET across the pulmonic valve¹⁰ (Figure 1). Because right ventricular inflow and right ventricular outflow velocity spectra cannot be recorded from 1 site, a and b were measured in unrelated cardiac cycles.

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Representative pulsed Doppler echocardiographic recordings obtained from a Boxer that were used to calculate IMP for the right ventricle by use of the equation (isovolumetric contraction + isovolumetric relaxation)/ET. The IMP was calculated by the equation (a − b)/b, where a represents the interval of cessation and onset of inflow through the tricuspid valve (panel A) and b represents ET through the pulmonic valve (panel B). The ECG at the bottom of each panel was recorded at a rate of 200 mm/s. Unit of measure for the x-axis is millimeters per second and for the y-axis is meters per second.

Citation: American Journal of Veterinary Research 69, 8; 10.2460/ajvr.69.8.1029

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Dogs that were receiving antiarrhythmic medication were not excluded from the study. Four dogs with ARVC were receiving antiarrhythmic medications. Three of these dogs were receiving sotalol (mean ± SD, 2.16 ± 0.12 mg/kg, PO, q 12 h), and the other dog was receiving mexiletine (6.0 mg/kg, PO, q 8 h) and atenolol (0.5 mg/kg, PO, q 12 h).

Statistical analysis—All calculations, statistical analyses, and graphs were accomplished by use of commercial software.^c–e Differences in variables between the 2 groups were identified by use of a Student t test when data were normally distributed or by use of a Mann-Whitney test when data were not normally distributed. A Mann-Whitney test was performed to identify differences for the IMP between affected and unaffected dogs and to identify differences in IMP for Boxers with ARVC that received or did not receive antiarrhythmic treatment. Pearson correlations were calculated to identify correlations between right ventricular IMP and VPC number and between right ventricular IMP and arrhythmia grade for Boxers with ARVC. Significance was set at α ≤ 0.05.

Results

Mean ± SD age differed significantly between Boxers with ARVC (7 ± 2 years) and control Boxers (4 ± 1 years). Mean body weight did not differ significantly (P = 0.16) between Boxers with ARVC (30.0 ± 1.1 kg) and control Boxers (27.3 ± 4.8 kg). Number of VPCs/24 h for Boxers with ARVC (median, 7,876 VPCs/24 h; range, 1,075 to 25,860 VPCs/24 h) was much higher than the number for the control Boxers (mean ± SD, 2 ± 2 VPCs/24 h). Mean ± SD right ventricular IMP did not differ significantly (P = 0.19) between Boxers with ARVC and control Boxers (0.18 ± 0.07 and 0.21 ± 0.04, respectively). Similarly, significant differences were not identified between the groups for right ventricular PEP, right ventricular PEP/ET, right ventricular ET, left ventricular fractional shortening, left ventricular internal diameter during diastole, left ventricular internal diameter during systole, or the R–R interval (Table 1). Right ventricular IMP was not significantly correlated with number of VPCs (r = 0.21; P = 0.5) or with arrhythmia grade (r = −0.3; P = 0.3; Figures 2 and 3).

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Graph of right ventricular IMP (RV IMP) and number of VPCs/24 h for 12 Boxers with ARVC. There was not a significant correlation (r = 0.21; P = 0.5) between the variables.

Citation: American Journal of Veterinary Research 69, 8; 10.2460/ajvr.69.8.1029

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Graph of RV IMP and grade of arrhythmia for 12 Boxers with ARVC. Arrhythmia was graded on the basis of the highest grade detected (1 = a single uniform VPC; 2 = bigeminy, trigeminy, or both; 3 = ventricular couplets, triplets, or both; and 4 = R wave on T wave, ventricular tachycardia, or both). There was not a significant correlation (r = −0.3; P = 0.3) between the variables.

Citation: American Journal of Veterinary Research 69, 8; 10.2460/ajvr.69.8.1029

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Discussion

In the study reported here, use of the IMP did not enable us to identify a significant difference in right ventricular function between Boxers with ARVC and control Boxers. Additionally, right ventricular IMP values for both groups were within or less than the 95% confidence interval (0.20 to 0.32) reported for clinically normal non-Boxer dogs that weighed between 15.1 and 35 kg.⁸ Although this could indicate that the IMP is not a sensitive indicator of ventricular myocardial function, IMP has been used to assess myocardial function in dogs with left or right ventricular disease. The IMP has been used to evaluate left ventricular function in dogs with dilated cardiomyopathy and mitral valve regurgitation.^11,13 In another study,¹⁸ investigators used IMP to assess right ventricular function in dogs with regurgitation through the tricuspid valve or heartworm disease. In those studies, a decrease in ventricular function was associated with an increase in IMP, which is consistent with an increase in isovolumetric contraction time or relaxation time.^12,18 Identification in our study of IMP values that are similar to those detected in clinically normal dogs may indicate that Boxers with ARVC do not have coexistent myocardial dysfunction of the right ventricle and ventricular arrhythmias. However, it could also mean that the dogs chosen for our study were not affected severely enough to have identifiable functional abnormalities.

The selection criterion for affected dogs for the study was based on detection of at least 1,000 VPCs/24 h, which was an arbitrary cutoff point used to select Boxers that have convincing evidence of this arrhythmic disease. Ventricular premature complexes have been considered to be a hallmark of this myocardial disease in Boxers and a likely consequence of the fibrofatty infiltration, but a correlation of VPCs with the degree of fibrofatty infiltrate into the myocardium has not been determined, and such a correlation is unlikely to be a linear relationship. Neither the number of VPCs nor the grade of ventricular arrhythmia was correlated with IMP in the study reported here. It could be assumed that the degree of fibrofatty infiltration into the myocardium would have the greatest correlation with myocardial function and be the best marker of disease severity. Selection of study dogs on the basis of the severity of fibrofatty infiltration may have been more informative; however, it is extremely difficult to noninvasively assess fibrofatty infiltration into the myocardium, and methods to quantify infiltration are lacking. Magnetic resonance imaging has had value in humans with ARVC for use in the detection of ventricular fatty infiltration but has poor specificity and is difficult to quantify.²⁰ Evaluation of endomyocardial biopsy specimens may also be used to identify fibrofatty infiltrate in the myocardium of the right ventricle but cannot be used to assess the extent of infiltrate in the ventricle. Therefore, we currently could not definitively assess the extent of the right ventricular disease in these Boxers.

A significant difference was identified in the age of dogs in the 2 groups. The selection criterion for the control group was detection of ≤ 5 VPCs/24 h. This was also an arbitrary cutoff point that was chosen in an attempt to avoid selection of dogs with identifiable disease in the control group; however, this inadvertently resulted in selection of a younger population of dogs. It is possible that at least some of the dogs in the control group would eventually develop ARVC because they were significantly younger than the affected dogs and ARVC is an adult-onset disease. Therefore, it could be argued that the failure to identify a difference in IMP between the 2 groups was attributable to the fact that some of the control dogs could have been in the early stages of ARVC. However, because the IMP for both groups did not differ from that for the non-Boxer control dogs reported in another study,⁸ it would appear that the difference in age of dogs in the study reported here was not important for the interpretation of the results.

Dogs that were receiving antiarrhythmic medication were not excluded from the study because it was assumed that the substrate (fibrofatty infiltrate) that has been hypothesized to alter ventricular function would not regress as a result of administration of β-adrenoreceptor blockers or class I antiarrhythmic agents and because of concerns for the well-being of the dogs. The effects of β-adrenoreceptor blockade associated with the antiarrhythmic medications on global right ventricular function are not fully understood; however, it would seem likely that β-receptor blockade would alter left and right ventricular systolic and diastolic function. In this study, left ventricular function was within the anticipated range in all dogs, and right ventricular IMP was the same for the subset of 4 dogs receiving anti-arrhythmics and the untreated dogs; thus, it could be assumed that the influence of antiarrhythmic drugs on the myocardial function of the dogs in our study was not relevant to the interpretation of the results.

We concluded that Boxers with ARVC do not have an increase in right ventricular IMP. This would suggest that right ventricular dysfunction does not develop in Boxers with ARVC or that it does not develop until later during the disease. Additional studies that use more sensitive techniques to evaluate myocardial function may be warranted.