Studies on Pulsed-wave Doppler Echocardiographic Parameters of Blood Flow through Mitral, Tricuspid and Aortic Valves in Healthy Indian Spitz Dogs

Doppler echocardiography is a non-invasive means of demonstrating velocity of blood flow across cardiac valves. Intracardiac blood flow velocities and variables obtained from Doppler waveform provide information on systolic as well as diastolic ventricular function. 
       
There is wide variation in Doppler echocardiographic values derived from mixed population of dogs (Brown et al., 1991; Schober et al., 1998; Pereira et al., 2009; Jeyaraja et al., 2016) due to differences in their breed, gender and body weight. Above reference values when used for a particular breed of dog may result in erroneous interpretation of cardiac data. Thus, there is a need to establish reference limits defining normalcy so as to differentiate between the normal and abnormal Doppler findings as well as accurately interpret Doppler indices of cardiac function.
       
Pulsed-wave Doppler measures blood flow at a specific point and provides information on velocity, direction and uniformity of blood flow throughout the cardiac cycle (Boon, 2011). Breed specific pulsed-wave Doppler echocardiographic reference values are available for German shepherd and Beagle (Kirberger et al., 1992a), Boxer (Schober et al., 2002; Cavalcanti et al., 2007), Retriever (Kobal and Petric, 2007), Doberman pinscher (O’Sullivan et al., 2007) and Indian mongrel dogs (Bodh et al., 2020). The study on cardiac parameters in Indian Spitz dogs using pulsed-wave Doppler echocardiography has not been reported so far.
       
Due to lack of reference values of Doppler echocardiography in Indian Spitz dogs the present study was designed to determine the reference intervals for pulsed-wave Doppler derived parameters of blood flow through mitral, tricuspid and aortic valves and, to further study the effect of gender and body weight on Doppler echocardiographic variables in Indian Spitz dogs.

The present study is a part of first authors PhD thesis submitted in year 2015 to Division of Surgery, ICAR-Indian Veterinary Research Institute (IVRI), Izatnagar, Bareilly.
 
Animals
 
Client owned healthy Indian Spitz (12 males and 12 females), aged 3-5 yr (mean age= 4.25±0.54 yr) and weighing 10-13 kg (mean body weight= 11.88±0.68 kg) presented to Referral Veterinary Polyclinics, Indian Veterinary Research Institute for routine clinical check up were studied. All dogs had normal cardiovascular and physical examination findings, no obvious signs of cardiac disease and showing normal findings of routine six-lead ECG, M-mode, two dimensional and Doppler echocardiography. Owners consent was obtained before each examination.
 
Echocardiographic examination
 
Echocardiographic examination was performed using a Digital Colour Doppler Ultrasound System (Chison iVis 60 Expert Vet^®), Chison Medical Imaging Co., Ltd. equipped with a 3.5-5.5 MHz multi frequency phased array transducer. All examinations were performed in non-sedated awake dogs. Standard procedures for Doppler examination, imaging plane and sample volume positions were used (Kirberger et al., 1992a).
       
Trans-mitral flow pattern with E and A wave velocities were obtained from left caudal (apical) four chamber view of heart, with sample volume positioned in the left ventricle just distal to mitral annulus at the point of maximal opening of mitral valves. Doppler curves were traced to determine peak early and late diastolic filling velocities (E peak and A peak, respectively), ratio of peak early to peak late diastolic flow (E/A ratio), velocity time integral of early and late diastolic inflow (VTI_E and VTI_A, respectively), deceleration time of early diastolic flow (DT_E) and duration of early and late diastolic inflow waves (E duration and A duration, respectively). Isovolumic relaxation time (IVRT) was determined from left caudal (apical) parasternal long axis five chamber view with sample volume positioned between the septal mitral leaflet and left ventricular outflow tract. Imaging planes providing best flow profile with highest velocities, least spectral broadening and good definition to E and A peaks were selected.
       
Tricuspid flow pattern with E and A wave velocities were obtained from left caudal (apical) four chamber view of heart with sample volume positioned in right ventricle, just distal to tricuspid annulus at the point of maximal opening of tricuspid valve. Peak E and A wave velocities and E/A ratio was measured from pulsed-wave Doppler tracings of tricuspid inflow.
       
Aortic flow pattern was obtained from left caudal (apical) long axis view of left ventricular outflow region and occasionally from an apical five chamber view. Sample volume was positioned distal to the aortic valve. Aortic peak velocity, velocity time integral and ejection time (ET) i.e. time interval between start to end of aortic blood flow signal were recorded.
 
Statistical analysis
 
Statistical analysis was performed using SPSS version 17 (SPSS Inc., Chicago, IL). Mean value of male and female dogs were compared using unpaired student’s t-test. A value of P<0.05 was considered as significant. Linear regression analysis and Pearson correlation coefficient (r) was calculated to determine influence of body weight on Doppler echocardiographic variables. Correlation was positive and significant when correlation coefficient was ≥0.40 and significance was ≤0.05.

Data are presented as mean values±standard deviation (SD). Pulsed-wave Doppler echocardiographic parameters of blood flow through mitral, tricuspid and aortic valves are summarized in Table 1. Subjective comparison of reference values of Indian Spitz dogs with values obtained from mixed dog population and breed specific reference values are presented in Table 2 and 3, respectively. Doppler echocardiographic parameters differed non-significantly between male and female dogs. Except isovolumic relaxation time, none of the Doppler echocardiographic parameters correlated significantly with body weight (Table 4).
 

 

 

 

       
The pattern of blood flow through mitral valve during diastole was positive and laminar with E and A waves, both having a spiked triangular appearance. Peak E velocity seen during rapid filling phase of early diastole was similar to values reported by Darke et al., (1993) and Jayaraja et al., (2016) but lower than values reported by Kirberger et al., (1992a) and Schober et al., (1998). Peak A velocity seen in late diastole due to atrial contraction was similar to value reported by Darke et al., (1993), but lower than values reported by Kirberger et al., (1992a), Schober et al., (1998) and Jayaraja et al., (2016). Mitral E/A ratio was within the reference range published for healthy dogs (Kirberger et al., 1992a), but slightly greater than values reported by Darke et al., (1993), Yamamoto et al., (1993), Schober et al., (1998) and Jayayraja et al., (2016). In normal animals, the E:A ratio is always greater than 1, because the rapid ventricular filling peak (E-wave) is higher than atrial contraction peak (A-wave). In impaired ventricular relaxation, ventricles fail to relax completely until late in diastole and atrial contraction contributes more to ventricular filling resulting in higher peak A velocities and E:A ratio less than 1. In addition, lower peak E velocity might also be due to smaller pressure gradients between left atrium and ventricle. In restrictive pattern, high filling pressures are predominant as most of ventricular filling occurs early in diastole and less occurs during atrial systole resulting in very high pressure within left ventricle leading to a high E:A ratio.
       
Velocity time integral (VTI) represented area covered under two velocity peaks measured by manually tracing the modal velocity envelop of Doppler signal. Velocity time integral of mitral E-wave (VTI_E) was similar to value reported by Schober et al., (1998), but smaller than value reported by Yamamoto et al., (1993) in normal dogs. Velocity time integral of mitral A-wave (VTI_A) was greater than value reported by Schober et al., (1998). An increase in VTI represents increased volume or restriction to flow whereas decreased VTI indicates poor blood flow (Boon, 2011). Peak E and A-wave velocities of mitral flow and their VTI’s correlated non-significantly with gender and body weight, in line with findings of Pereira et al., (2009) and Bodh et al., (2020), but contrary to findings of O’Sullivan et al., (2007) in Doberman Pinschers where majority of Doppler echocardiographic variables correlated significantly with body weight.
       
Time interval from peak to end of mitral E-wave represented mitral deceleration time (DT_E). Value in Indian Spitz were lower than German Shepherd (Muzzi et al., 2006), Doberman Pinscher (O’ Sullivan et al., 2007) and Indian mongrel dogs (Bodh et al., 2020). Mitral DT_E correlated non-significantly with gender and body weight similar to findings in Indian mongrel dogs (Bodh et al., 2020). To its, contrary, Schober and Fuentes (2001) and O’ Sullivan et al., (2007) reported a significant effect of body weight on mitral DT_E. Insignificant effect of body weight on DT_E in present study might be due to narrow range of body weights used. In impaired relaxation as in hypertrophic cardiomyopathy, left ventricle takes longer time to relax and allow filling leading to prolonged deceleration time. While in restriction to ventricular filling as in restrictive, dilated, hypertrophic or ischemic cardiomyopathy, deceleration time is reduced due to rapid equalization of atrial and ventricular pressures (Schober and Fuentes, 2001).
       
The time interval between end of aortic ejection and the beginning of ventricular filling obtained by simultaneously recording aortic and mitral flows represent isovolumic relaxation time (IVRT). The values recorded in Indian Spitz dogs were lower than healthy dogs (Schober et al., 1998), Boxers (Schober et al., 2002), Doberman Pinschers (O’Sullivan et al., 2007) and Indian mongrel dogs (Bodh et al., 2020). Gender had no influence on IVRT. Body weight correlated significantly (r=0.42) with IVRT, in line with findings of O’ Sullivan et al., (2007) but contrary to findings of Bodh et al., (2020). Fast heart rate in small sized dogs cause early left ventricular filling resulting in premature opening of mitral leaflets and thus shortening of IVRT (Pereira et al., 2009).
       
The duration of E-wave was greater than normal dogs (Schober et al., 1998) and the duration of A-wave was also greater than values reported previously (Schober et al., 1998; O’Sullivan et al., 2007 and Bodh et al., 2020).
       
Peak tricuspid E-wave velocity was similar to values reported by Darke et al., (1993) and Jayaraja et al., (2016) but smaller than values reported by Kirberger et al., (1992a) and Yamamoto et al., (1993).Tricuspid peak A-wave velocity was similar to value reported by Darke et al., (1993) but lower than values reported by Kirberger et al., (1992a), Yamamoto et al., (1993) and Jayaraja et al., (2016). Tricuspid E/A ratio was within the reference range published for healthy dogs by Kirberger et al., (1992a) but lower than value reported by Darke et al., (1993). An increase in the right atrial pressure or volume secondary to tricuspid insufficiency resulted in increased tricuspid peak E velocity. The E and A peaks of tricuspid flow and their VTI’s showed no correlation with gender and body weight, similar to finding in normal dogs (Kirberger et al., 1992a).
       
Peak E velocity of mitral flow was significantly (P<0.05) greater than peak E velocity of tricuspid flow (Table 1). Comparison of peak A velocities of mitral and tricuspid flows was not statistically significant at 5% level. During early diastole, higher left atrial pressures cause higher pressure difference between left and right atrium resulting in higher mitral peak E velocity. Early filling rate of right ventricle is lower compared to left ventricle (Kirberger et al., 1992b), resulting in lower tricuspid peak E velocity. Further, breed influence or difference in intercept angle between ultrasound beam and blood flow for tricuspid and mitral flows might have contributed to higher peak tricuspid flow velocity (Vajhi et al., 2013).
       
The velocity curve for aortic flow was negative indicating blood flow away from transducer. Peak aortic flow velocity was within reference range published by Brown et al., (1991), but lower than values reported by Darke et al., (1993) and Jayaraja et al., (2016). Aortic VTI was lower than value reported by Brown et al., (1991). Aortic ejection time was greater than value reported by Kirberger et al., (1992a). Similar to findings of Kirberger et al., (1992a), aortic peak velocity did not correlated significantly with gender and body weight. Reduction in peak aortic flow velocity correlates to decreased left ventricular systolic function as seen in severe dilated cardiomyopathy. This could be due to good correlation between aortic peak flow velocity and left ventricular fractional shortening.

Reference values of blood flow velocities through mitral, tricuspid and aortic valves in healthy Indian Spitz dogs was determined using pulsed-wave Doppler echocardiography. Establishing normal values of important pulsed-wave Doppler derived blood flow velocities will help clinicians to interpret these indices of cardiac function in clinical cases and diagnose a variety of pathological cardiac conditions.

The authors would like to thank the Director, ICAR-Indian Veterinary Research Institute for providing necessary facility for the conduct of research work.