Heartworm disease (HWD) is a common mosquito-borne disease in dogs primarily caused by Dirofilaria immitis. Adult heartworms typically reside in the pulmonary arteries (PA), causing inflammation and endothelial damage. Clinical signs associated with the host’s immune response, worm burden, and duration of infection include cough, dyspnea, syncope, exercise intolerance, and lethargy, although some infected dogs remain asymptomatic (1,3-5,14,20,26). The progression of the disease is difficult to predict due to various influencing factors. The American Heartworm Society (AHS) recommends annual screening for all dogs over seven months of age using both an antigen and a microfilaria test, and the year-round administration of FDA-approved preventive medications starting at eight weeks of age to prevent heartworm infection (24).

Several studies have elucidated HWD etiology (1,3,4,14, 20,26). Adult heartworms reside freely within the PA, but cannot move independently. Retrograde migration of heartworms into the right heart occurs when forward flow is reduced, flow resistance increases, and hemodynamics are altered (16). Heartworms mechanically disrupt endothelial cells and their intercellular junctions in the pulmonary artery (2,4,31), leading to proliferative endarteritis and intimal thickening, which in turn result in chronically increased pulmonary pressure, thereby increasing vascular resistance and reducing cardiac output in response to pulmonary hypertension (PH) (7,9,23,28,32). Additionally, the death of an adult worm triggers thromboembolic events that further increase vascular resistance and release vasomediators, impairing cardiac function (17). Other factors contributing to retrograde migration include a high relative worm burden (15), decreased pulmonary arterial endothelium-dependent relaxation (22), and the use of heartworm preventatives, microfilaricides, and adulticides (10,18). Secondary PH and the presence of intracardiac heartworm cause pressure and volume overload. Generally, in pathological states, myocardial hypertrophy is the initial adaptive response to pressure overload, followed by progressive systolic dysfunction and ventricular dilatation to maintain stroke volume through compensatory preload (8). As systolic dysfunction progresses, increased filling pressure, diastolic dysfunction, and decreased cardiac output occur, further aggravated by tricuspid regurgitation due to annular dilatation and leaflet coaptation impairment caused by mechanical disruption from intracardiac heartworms (36). Therefore, the retrograde migration of heartworms, triggered by various factors, exacerbates cardiovascular dysfunction through several pathological processes.

The presence of intracardiac heartworms can lead to a life-threatening complication known as caval syndrome (CS). CS is characterized by abnormally distributed worms within the right heart and acute symptoms such as dyspnea, anorexia, weakness, and jugular distention, along with evidence of hemolysis including hemoglobinuria and anemia. A study revealed that only 25% of dogs with intracardiac heartworm exhibited anemia, hemoglobinuria, and clinical signs consistent with CS (26). CS can also cause heart failure. Heart failure refers to impaired cardiac systolic or diastolic function, a pathophysiological state where the heart is unable to function adequately to meet the metabolic demands of the body’s tissues.

Heartworm detection is achieved by identifying worm burden in the PA and/or right heart via echocardiography, a highly practical method for assessing the structure and function of the right ventricle (RV) in veterinary medicine due to its noninvasive, economical, simple, reproducible nature, and lack of general anesthesia requirement (2,3,28,31,35). Echocardiographic assessment of RV systolic function provides extensive insight into the pathophysiology of heart failure associated with CS (30). Precise evaluation of RV function offers diagnostic and prognostic value across a spectrum of conditions including heart failure, PH, valvular heart disease, pulmonary vascular disease, pericardial disease, and congenital heart disease, as well as HWD (30). However, accurate measurement of RV function using 2D echocardiography presents challenges due to the complex muscle structure of the ventricle, the RV’s anatomical features, and the difficulty in standardizing imaging planes (13,29,33). Cardiac magnetic resonance, 3D echocardiography, and speckle tracking echocardiography surpass 2D echocardiography in evaluating RV function; yet, they are less feasible in veterinary settings due to their time-intensive nature, limited availability, and potentially high costs and anesthesia requirements (33). Given the absence of a definitive single method for precise RV function assessment, comprehensive echocardiographic studies that incorporate all available clinical data are recommended. Commonly used metrics for RV function evaluation include two-dimensional (2D) measures correlating to RV ejection fraction-percent fractional area change (FAC), M-mode derived tricuspid annular plane systolic excursion (TAPSE), and pulsed-wave tissue Doppler imaging (TDI)-derived peak systolic longitudinal myocardial motion velocity of the lateral tricuspid annulus (RV S’).

This study aimed to assess the impact of intracardiac heartworm on RV function by utilizing echocardiographic indices of RV systolic function such as FAC, TAPSE, and RV S’. It was hypothesized that intracardiac heartworms would exacerbate RV dysfunction and cause discernible differences in RV systolic function compared to HW-free or non-intracardiac HW dogs.

Animals

This study included dogs who attended Jeonbuk National University Veterinary Medical Teaching Hospital from July 2020 to August 2022 and were enrolled retrospectively. All subjects were either client-owned or local shelter dogs, This study was approved by the Institutional Animal Care and Use Committee (IACUC) under approval number NON2022-089.

A total of 36 dogs were included in this study. Of the 36 dogs, 12 were healthy and 24 had heartworm. All dogs with heartworm were classified according to AHS heartworm guidelines. Dogs with no clinical signs or a mild cough were classified as class 1 and consisted of two dogs. If they showed coughing, activity intolerance, or respiratory distress, they were classified as class 2 and comprised 15 dogs. Class 3 consisted of nine dogs and included coughing, activity intolerance, dyspnea, ascites, hepatomegaly, and syncope. Class 4 consisted of 10 dogs with caval syndrome. All dogs were divided into three groups to evaluate the effect of intracardiac heartworm on RV systolic function: Group 1 (HW-free, n = 12), Group 2 (HW resided in PA, n = 15), and Group 3 (intracardiac HW, n = 9). Group 3 included dogs with heartworms present in the right atrium and right ventricle, as well as those with heartworms located in the right atrium, right ventricle, and pulmonary artery. Group 1 comprised healthy dogs that tested negative for heartworm and showed no abnormality in physical examination, blood test, radiography, echocardiography and did not showed any clinical sign. Dogs with naturally acquired heartworm infection characterized by hyperechoic parallel lines within the right heart on echocardiography were assigned to Group 3. Group 3 included five dogs with myxomatous mitral valve disease (MMVD). According to the American College of Veterinary Internal Medicine (ACVIM) classification, four dogs were classified as stage B1 and one as stage B2. Those with infection located only in the PA were included in Group 2. Group 2 did not include dogs with MMVD. The diagnosis of right congestive heart failure (R-CHF) relied on the presence of ascites, jugular venous distension, hepatomegaly, and subjectively dilated caudal vena cava. Data including signalment, clinical signs, underlying disease, and diagnostic tests were recorded for all dogs.

Heartworm screening test

The heartworm screening included a heartworm antigen test and microscopic examination for microfilaria. The IDEXX SNAP^®4Dx^® Test Kit was employed for the antigen test.

Echocardiography

Image acquisition was performed by one trained observer using a Philips EPIC 7C echocardiography machine equipped with 3-8 and 4-12 MHz phased-array transducers. A comprehensive echocardiographic examination consisting of transthoracic two dimension, spectral, M-mode, and color flow Doppler imaging was conducted from both right and left parasternal positions in all dogs. To characterize RV systolic function, parameters including FAC, TAPSE, and RV S’ were measured from the left apical four-chamber view optimized for the right heart (35).

Measurements of RV area relative to FAC were obtained by tracing the RV endocardial border at end-diastole (onset of Q-wave) and end-systole (end of T-wave) using Simpson’s method, as illustrated in Fig. 1. The RV FAC was calculated using the formula: FAC = [(RV diastolic area − RV systolic area)/RV diastolic area] × 100 (27,35). The normalized FAC (FACn) was calculated as the ratio of FAC to body weight (BW) using allometric scaling (FACn = FAC/BW−0.097) as previously determined (35).

Figure 1. Representative measurement of 2-dimensional (2D) correlate to RV ejection fraction-percent fractional area change (FAC). Measurements of RV area were obtained by tracing the RV endocardial border (dotted lines) at end-diastole (RV EDA) and end-systole (RV ESA) from left apical 4-chamber view optimized for the right heart. The RV FAC was calculated using the formula: FAC = [(RV diastolic area – RV systolic area)/RV diastolic area] × 100. RA, right atrium; RV, right ventricle.

TAPSE measurements were obtained using an M-mode recording of the tricuspid valve annulus’s lateral aspect from a left parasternal apical four chamber view optimized for the right heart (25). The M-mode cursor was aligned parallel to the right ventricular free wall and recorded at end-diastole and end-systole using the leading-edge method, as shown in Fig. 2. Normalized TAPSE (TAPSEn) was calculated as the ratio of TAPSE to BW using allometric scaling (TAPSEn = TAPSE/BW0.297), which was derived from a previous study (35).

Figure 2. Representative measurement of M-mode derived tricuspid annular plane systolic excursion (TAPSE). M-cursor was aligned with the lateral segment of the tricuspid annulus and obtained from left apical 4-chamber view optimized for the right heart.

RV S’ was measured to determine the peak systolic annular velocity, as depicted in Fig. 3.

Figure 3. Representative measurement of color tissue Doppler imaging (TDI)-derived peak systolic longitudinal myocardial motion velocity of the lateral tricuspid annulus (RV S’). M-cursor was aligned with the lateral segment of the tricuspid annulus and obtained from left apical 4-chamber view optimized for the right heart.

The apical four chamber view was selected to minimize the incidence angle between the Doppler beam and the longitudinal wall motion, thereby enabling a quantitative assessment of regional wall motion. To avoid underestimating velocity, the cursor was aligned as parallel as possible to the longitudinal plane of the RV free wall. Measurements included the myocardial systolic wave (S’) and two diastolic waves: early diastole (E’) and atrial contraction (A’), but only S’ was included in this study.

All dogs were assessed for the probability of PH according to the ACVIM guidelines. Echocardiographic evaluations were conducted to assess anatomical changes and measure the tricuspid regurgitation velocity (TR) for PH probability estimation (Table 1).

Table 1 . PH probability of each group.

PH probability	Group 1 (HW-free)	Group 2 (HW resided in PA)	Group 3 (intracardiac HW)
Low	12	6	-
Intermediate	-	4	-
High	-	1	9
Unclear	-	4	-
Total	N = 12	N = 15	N = 9

PH, pulmonary hypertension; HW, heartworm; PA, pulmonary artery..

Statistical Analysis

Statistical analyses were conducted using commercial statistical software (IBM SPSS 16.0, SPSS Inc, USA).

The distribution of data relating to patient signalment and echocardiographic values was assessed for normality using the Kolmogorov-Smirnov test and visual inspection of dot plots. RV systolic function indices among the three groups were compared using one-way ANOVA. Post hoc comparisons were performed using Tukey’s honest significant difference test or the Dunnett T3 test when significant differences were detected by ANOVA. A p-value <0.05 was considered statistically significant.

Data were normally distributed. Data are expressed as mean ± standard deviation. ANOVA derived p-values. p < 0.05 indicates statistical significance. Within each row, superscripts with different letters indicate a statistical difference in post hoc tests (p < 0.05) between groups.

The probability of PH is summarised in Table 1 by group. In group 1, there were no dogs with PH probability. In group 2, out of 15 dogs, six dogs had low, four dogs had intermediate, and one dog had high probability. The other four dogs had missing data and could not be assessed clearly. In group 3, all dogs had high probability (Table 1).

Age, sex, body weight were summarized in Table 2 with mean ± SD. Twelve healthy dogs and 24 dogs with heartworm infection were retrospectively enrolled in this study, including males and females. The sex distribution was 17 male and 19 female. The mean of age for each group was 6.8 in group 1, 3.9 in group 2, and 7.4 in group 3. The mean of BW for each group was 9.0 in group 1, 10.2 in group 2, and 12.6 in group 3 (Table 2). The breed composition included 14 mixed breed dogs, six Beagles, three Maltese, two Pomeranians, and two each of Poodle, Spitz, and Jindo. Other represented breeds (Schnauzer, Sapsal, Husky, Border Collie, and Great Pyrenees) each had one representative.

Table 2 . Clinical characteristics of each group.

	Group 1 (HW-free)	Group 2 (HW resided in PA)	Group 3 (intracardiac HW)
Total	12	15	9
Sex (M:F)	3:9	9:6	5:4
Age (year)	6.8 ± 3.3 (1.0-12.0)	3.9 ± 1.8 (1.0-9.0)	7.4 ± 3.3 (3.0-11.0)
BW (kg)	9.0 ± 5.1 (2.3-17.3)	10.2 ± 7.0 (3.5-28.0)	12.6 ± 8.6 (2.2-24.3)
R-CHF	0	0	8

Data are expressed as mean ± standard deviation (lower bound-upper bound)..

M, male; F, female; BW, body weight; R-CHF, right congestive heart failure; HW, heartworm; PA, pulmonary artery..

The values and boxplots of RV systolic function indices (FACn, TAPSEn, RV S’) for each group are presented in Table 3 and Fig. 4, respectively. FACn and TAPSEn were significantly different between group 3 and other groups (FACn, p < 0.001; TAPSEn, p = 0.002). The FACn of group 3 was significantly smaller than that of the other groups (p < 0.001). More negative values of FACn were observed in group 3 than in group 2 (p < 0.001), but the difference between groups 2 and 1 was not significant (p = 0.95). The TAPSEn value of group 3 was significantly smaller than that of group 1 (p = 0.001). More negative values of TAPSEn were identified in group 3 than in group 2 (p = 0.02), but the difference between groups 2 and 1 was not significant (p = 0.41). The values of RV S’ showed no significant differences between groups (p = 0.7).

Table 3 . Echocardiographic measures of right ventricular systolic function in each group.

RV function index	Group 1 (HW-free, n = 12)	Group 2 (HW resided in PA, n= 15)	Group 3 (intracardiac HW, n = 9)	Post-hoc		p
FACn	56.3 ± 10.1	54.5 ± 21.1	24.1 ± 7.5	G1-G2	0.95	<0.001*
				G1-G3	<0.001
				G2-G3	<0.001
TAPSEn	7.1 ± 0.7	6.5 ± 1.2	5.2 ± 1.4	G1-G2	0.41	0.002*
				G1-G3	0.001
				G2-G3	0.02
RV S’ (cm/s)	11.9 ± 4.8	11.2 ± 3.3	10.5 ± 3.3	G1-G2	0.88	0.7
				G1-G3	0.68
				G2-G3	0.9

Data were normally distributed. Data are expressed as mean ± standard deviation. ANOVA derived p-values. *p < 0.05 indicates statistical significance. Within each row, superscripts with different letters indicate a statistical difference in post hoc tests (p < 0.05) between groups..

FACn, fractional area change normalized for body weight; TAPSEn, tricuspid annular plane systolic excursion normalized for body weight; RV S’, systolic myocardial velocity of the lateral tricuspid annulus..

Figure 4. Boxplots between groups by index of right ventricular systolic function. Group 1 (HW-free, n = 12), Group 2 (HW resided in PA, n = 15), and Group 3 (Intracardiac HW, n = 9). Solid bars represent a 95% CI for the mean and the horizontal lines represent median. *FACn was significantly lower in intracardiac group compared to other groups, p < 0.001; **TAPSEn was significantly lower in intracardiac group compared to other groups, p = 0.002.

A major finding of this study is that the presence of intracardiac heartworm, caused by the retrograde migration of heartworms from the PA, leads to reduced RV systolic function. There was a general tendency for all indices of RV systolic function to decrease with the progression of HWD. Among the RV systolic function indices, FACn and TAPSEn were significantly decreased in the intracardiac heartworm group, while no significant differences were observed in other groups. Consequently, the reduction in FACn and TAPSEn values suggests impaired RV systolic function in dogs with intracardiac heartworms.

In HWD, echocardiography provides definitive evidence of heartworm infection with images characterized by short parallel sides, resembling “equal signs,” and also allows assessment of cardiac anatomical and functional consequences of the disease (24). Heartworm resides in the PA and causes various pathological issues. As the disease progresses, it affects right heart function, potentially leading to caval syndrome and R-CHF at the end-stage. Therefore, even in HWD, accurate evaluation of the right heart is necessary to identify and address right heart dysfunction at an early stage.

In this study, FACn, TAPSEn, and RV S’ were utilized to monitor changes in RV systolic function. Cardiac indices such as TAPSE, FAC, and RV S’ are routinely used to assess RV systolic function in various pathological conditions. They are also the best predictors of RV systolic function in humans. And this is likely to be similar in veterinary medicine. In addition, given the results of this study, we speculate that measuring FACn and TAPSEn in dogs with heartworm may allow us to visualize the deterioration of the systolic function of the right ventricle and assess the likelihood of developing critical diseases such as R-CHF.

However, the measurement of RV systolic function via echocardiography has several limitations. FAC is a parameter that can be quickly and easily obtained, and it is recommended in veterinary medicine. However, as a 2D measurement, it cannot fully represent the actual global RV function like RVEF and is prone to underestimation due to foreshortening and interference from trabeculations. TAPSE is easily obtainable and represents a measure of RV longitudinal function. However, TAPSE is relatively load- and angle-dependent, and may either under- or overestimate RV function when cardiac translation occurs. RV S’, like other indicators, is one of the most reliable and reproducible methods for assessing RV systolic function. However, RV S’ is measured relative to the transducer and may be influenced by overall heart motion. It also has angular dependencies and is not fully representative of global RV function (19). Each parameter has significant limitations and pitfalls. Therefore, all measurable parameters should be evaluated and compared to accurately determine RV systolic function.

A previous study comparing heartworm-infected dogs with and without pulmonary hypertension (PH) reported significant reductions in TAPSEn and RV S’ in dogs with PH (21). Another study found that FACn showed borderline significance (p-value = 0.06 > 0.05), though not statistically significant, in precapillary PH cases (11). In contrast, our analysis indicated significant reductions in FACn and TAPSEn across groups, while RV S’ did not reach significance. This difference may stem from our grouping based on heartworm presence and location rather than PH probability. Another possible factor is tissue Doppler imaging (TDI) characteristics; TDI measures myocardial velocity at a single point, but values can vary with cursor alignment, introducing subjectivity that may contribute to differing results from previous studies.

It was confirmed that FACn and TAPSEn showed significantly lower values in the intracardiac heartworm group (p < 0.001, p = 0.002, respectively). This indicates that the presence of intracardiac HW may deteriorate RV systolic function. Even slight variations in pulmonary pressure can impair RV function and overall cardiac performance in echocardiography, even with normal preload (12). In HWD, PH and mechanical obstruction due to HW lead to increased afterload, which is thought to impair RV systolic function. In a pulmonary hypertensive state, the RV enlarges, leading to dilatation of the tricuspid annulus and consequently, tricuspid regurgitation. This condition may be exacerbated by disruption of the tricuspid annulus, mechanical damage to muscle fibers, and physical resistance caused by intracardiac HW. The pressure or volume overload created by these processes can reduce cardiac output. Changes in myocardial intrinsic properties may occur, and RV end-diastolic and RA pressures may rise, resulting in clinical signs of R-CHF. In this study, R-CHF was identified in eight dogs, all part of group 3 (intracardiac heartworm group). This may be a result of cardiac dysfunction caused by the pathological effects of intracardiac HW, but further research is necessary to evaluate this conclusion.

Another possible reason for the decrease in FACn and TAPSEn within the intracardiac heartworm group is the reduced RV function due to HW-induced secondary PH. The Doppler-estimated peak systolic tricuspid regurgitation pressure gradient serves as a representative noninvasive parameter for assessing PA systolic pressure (6). In Group 2, tricuspid regurgitation was unconfirmed in over half of the subjects, preventing statistical comparison, yet relatively higher tricuspid regurgitation pressure gradient measurements were observed in the intracardiac HW group. As previously stated, decreased FACn and TAPSEn values were noted in the intracardiac HW group. In essence, the group displaying higher pulmonary artery pressure exhibited lower FACn and TAPSEn values. Veterinary literature presents conflicting data on RV systolic function in dogs with PH. Consistent with this study, significant reductions in TAPSE and FAC were reported in two studies involving dogs with severe PH (25,34). Conversely, other studies on PH in dogs showed no significant differences in TAPSEn and FACn between dogs with and without PH, regardless of severity (33). Differences in sample population size and the impact of severe TR on Doppler estimation of pulmonary arterial pressure, which tends to increase TAPSE regardless of RV function, are possible reasons for these discrepancies. Another potential cause for varied results is the differing etiologies of PH. Thus, a larger prospective study is necessary to confirm the effects on FAC and TAPSE in dogs with HW-induced secondary PH. Further research is also essential to explore the relationship between RV function reduction and prognosis in HWD.

This study has several limitations. Firstly, this study did not evaluate inter-observer and intra-observer reproducibility in measuring each parameter of RV function, which is necessary to minimize measurement errors. Lastly, the limited data quantity necessitates further data accumulation to enhance statistical accuracy. Furthermore, as this study did not exclusively involve dogs completely free of left-sided heart disease, additional research is warranted to clarify these findings.

In this study, the RV systolic function indices, FAC, TAPSE, and RV S’, were measured in dogs with intracardiac heartworms, dogs with heartworm residing in the PA, and dogs without heartworm infection. The analysis confirmed that FACn and TAPSEn are significantly reduced in dogs with intracardiac HW, supporting the hypothesis that intracardiac HW impairs RV systolic function. These indices are recommended in veterinary medicine and prove to be useful for evaluating RV systolic function in dogs with intracardiac HW. Despite several limitations and a relatively small data set, further investigation into RV systolic function indices, particularly FAC and TAPSE, is warranted to assess their prognostic value in dogs with intracardiac HW.

This work was supported by the Korea Institute of Planning and Evaluation for Technology in Food, Agriculture, Forestry (IPET) through the Companion Animal Life Cycle Industry Technology Development Program funded by the Ministry of Agriculture, Food and Rural Affairs (MAFRA) (322094).

The authors have no conflicting interests.