TWO-DIMENSIONAL ECHOCARDIOGRAPHIC ANATOMY OF THE SNAKE HEART (PYTHON MOLURUS BIVITTATUS)

PATTI S. SNYDER, DVM, MS, NEIL G. SHAW, DVM, DARRYL J. HEARD, BSc, BVMS, PHD

Two-dimensional echocardiography was performed on Burmese pythons (Python molurus bivittutus) to determine an optimal echocardiographic imaging technique for snakes and to describe the echocardio- graphic anatomy of the snake heart. Five snakes immobilized with tiletamine/zolazepam and main- tained on isoflurane in oxygen were imaged in dorsal recumbency. The portion of the snake's body containing the heart was submerged in warm water to reduce the artifact created by air trapped between and under the scales. Imaging in sagittal planes demonstrated the caudal vena cava, sinus venous valve, right atrium, various portions of the ventricle, horizontal septum, the left aortic arch, and pulmonary artery. Transverse imaging depicted the spatial relationship of the left and right aortic arches and pulmonary artery and the horizontal septum. Basic knowledge of cardiac blood flow in the reptile was necessary to understand the echocardiographic anatomy. Veterinary Radiology & Ultra- sound, Vol. 40, No. 1, 1999, p p 66-72.

Key words: echocardiography, snake, heart, reptile.

Introduction

E evaluation of humans, dogs, and cats. Ultrasonography is a useful, noninvasive diagnostic technique in the nondo- mestic animal as well, and there are few reports of its use in reptiles.',2 However, little emphasis has been placed on ul- trasonographic examination of the reptilian heart.

In reptiles, published reports on ultrasound imaging have described the position and general structures of the heart of the California desert tortoise (Xerobates agassazi), Bosc monitor (Varanus exanthematicus), and Boa constrictor (Boa con~trictor). '-~ However, no paper has focused spe- cifically on either cardiac anatomy or the difference in car- diac anatomy of reptiles, as compared to mammals. Despite the plethora of information written about the mammalian heart, scientists still struggle to understand the anatomy, embryology, and physiology of amphibian and reptilian heart^.^-^ Although echocardiography could provide impor- tant information about cardiac function and disease in rep- tiles, it will remain underutilized until normal echocardio- graphic anatomy is defined.

Cardiovascular disease in reptiles is reported infre- quently, and most studies have relied on postmortem ex-

CHOCARDIOGRAPHY IS AN integral part Of the cardiac

From the Department of Small Animal Clinical Sciences, University of Florida, Gainesville, FL 32610-0126. Dr. Shaw's current address is Florida Veterinary Services, 5016 Gunn Hwy., Tampa, FL 33624.

Address correspondence and reprint requests to Patti Snyder, DVM, MS, Box 100126, Gainesville, FL 32610.

This study was funded by a grant from the University of Florida College of Veterinary Medicine.

Received September 23, 1997; accepted for publication April 29, 1998.

amination for a definitive d i agn~s i s .~ -~ One potential reason for the paucity of information on cardiac disorders is a lack of diagnostic imaging techniques applied to the reptile. In only one study was echocardiography used as a diagnostic aid.7 In that report, two-dimensional echocardiography was used to visualize a large space occupying mass in the right atrium of a female Burmese python (Python molurus bivit- tatus).

The aims of this study were to determine an optimal echocardiographic scanning technique for the snake heart, using the Burmese python (Python molurus bivittatus) as a model and to use anatomic specimens to identify important cardiac structures found by echocardiography.

Reptilian Circulation'

Blood flow and cardiac anatomy in reptiles differ from that of mammals. An understanding of cardiac blood flow patterns is essential to identify anatomic landmarks during echocardiography of the reptilian heart. The information presented below is an overview of the circulatory pattern of snakes. Variations in blood flow and intracardiac shunting exist between families of snakes, and blood flow may change during submersion and tree climbing. lo,' '

Deoxygenated blood from the systemic venous circula- tion empties into the sinus venosus. Blood enters the right atrium via a bicuspid sinus venosus valve. Blood is directed from the right atrium through the right atrioventricular (AV) valve into the cavum venosum (Fig. 1) . During diastole, the cavum venosum acts primarily as a channel directing blood into the cavum pulmonale (also called the cavum ventrale).

66

VOL. 40, No. 1 ECHOCARDIOGRAPHIC ANATOMY OF THE SNAKE HEART 67

FIG. 1. Adapted and printed with permission from Comparative Cardiac Anatomy of the Reptilia: The Chambers and Septa of the Varanid Ven- tricle, Webb G, Heatwole H, De Bevay 5, J Morph 1971;134:335-350. Schematic longitudinal section of the partitions of ventricles, atria, and great vessels of the snake heart during diastole. SV = sinus venosus, RA = right atrium, LA = left atrium, ivc = interventricular canal, vs = vertical septum, CA = cavum arteriosum, CAVUM VENOSUM = ca- vum venosum, CP = cavum plumonale, hs = horizontal septum, raa = right aortic arch, laa = left aortic arch, pa = pulmonary artery. Note: The great vessels (raa, laa, and pa) are located dorsal to the cardiac chambers. In this diagram, they are shown in the same plane in order to demonstrate the direction of blood flow through the cardiac chambers.

During systole, blood moves around an incomplete muscu- lar septum termed the horizontal septum into the cavum pulmonale and into the pulmonary artery (Fig. 2). A smaller, less developed, and incomplete vertical septum subdivides the cavum dorsale into the cavum arteriosum and cavum venosum (Fig. 1). Oxygenated blood from the pulmonary veins enters the left atrium and is directed into the cavum arteriosum (Fig. 1). This is the only entrance to the cavum arteriosurn. During systole, blood is directed from the ca- vum arteriosum over the vertical septum through the inter- ventricular canal into the cavum venosum (Fig. 2). Oxygen- ated blood then exits into the left and right aortic arches from the cavum venosum.

Unlike the atrioventricular valves of mammals, the rep- tilian AV valves are composed of a fibrous medial cusp.5 When present, the lateral cusps are usually rudimentary. The pulmonic and the two aortic valves are bicuspid, which differs from the tricuspid structure of these valves in mam- mals. Although the snake is described as having only one ventricle, it is actually subdivided into three by the incom- plete septae. When the AV valves open during diastole, they cover the interventricular canal.' This helps to direct blood

'CP FIG. 2. Adapted and printed with permission from Comparative Cardiac

Anatomy of the Reptilia: The Chambers and Septa of the Varanid Ven- tricle, Webb C, Heatwole H, De Bevay J, J Morph 1971;134:335-350. Schematic long axis diagram of the partitions of ventricles, atria, and great vessels of the snake heart during systole. Abbreviations, see Fig. 1; svv = sinus venosus valve, pv = pulmonary vein opening. Note: The great ves- sels (raa, laa, and pa) are located dorsal to the cardiac chambers. In this diagram, they are shown in the same plane in order to demonstrate the direction of blood flow through the cardiac chambers.

from the atria into the specific ventricular cavities so that the degree of mixing of oxygenated and deoxygenated blood is considered to be quite ~ma11.~-~ Variations in blood flow and intracardiac shunting that have been observed are usually related to alterations in either systemic or pulmo- nary arterial vascular resistance.lZJ3

Materials and Methods Four males and one female Burmese snake (Python molu-

rus bivittatus) weighing 4 to 63 kg and measuring 1.5 to 5.8 m culled from a breeding colony for poor reproductive per- formance (female) or chronic respiratory disease were stud- ied. Snakes were immobilized with tiletamine HCL/ zolazepam (20 mg/kg IM),* intubated and maintained on 2% isoflurane administered in oxygen to prevent movement during the echocardiographic studies. During anesthesia, the heart rates ranged from 25 to 45 beats per minute.

Scanning Technique Once immobilized, the snakes were placed in dorsal re-

cumbency, and the heart was located by visualization of

*Telazol, Fort Dodge Laboratories, Fort Dodge, IA.

68 SNYDER ET AL 1999

movement of the ventral scutes over the heart and by pal- pation. A 25 to 35 cm section of the body containing the heart was then immersed in a shallow water bath (24-30°C) sufficient to cover the snake’s body by 0.5 to 1 cm of water. The tip of the transducer (7.5 MHz focused phased array sector transducer?) was applied directly to the ventral sur- face of the snake without the aid of coupling gel. Depending upon the size of the snake, the image zone was set between 4 to 9 cm. The frame rate was 35 to 40 frameds.

Two-dimensional echocardiographic images were ob- tained by placing the transducer head directly on the ventral midline over the heart. Long axis (sagittal) images were obtained by aligning the ultrasound beam with the long axis of the snake on the midline and then angling the head to the right (Fig. 3) and then left (Figs. 5 and 8). While maintain- ing the midline position, the transducer head was rotated 90” from the midsagittal plane and angled cranially from the apex to the base (Figs. 9 and 11) of the heart to obtain the short axis (transverse) images. Real-time images were stored on VHS videotape for review at a later time. In addition, still images of’ the female snake were captured digitally, stored, and printed using commercially available software$ and slide maker programs.§

An intravenous catheter11 was placed in the palatine vein in two snakes and 5 to 10 mls of agitated 0.9% NaCl solu- tion was injected into the catheter to produce echodense bubbles (“bubble study”) that could be visualized by ul- trasonography. Four to five such studies were performed in each animal to identify structures in both the sagittal and transverse imaging planes. Echocardiography with continu- ous videorecording was performed during these contrast studies.

At the conclusion of the study, the snakes were eutha- nized by an intracoelomic injection of pentobarbital# so- dium (100 mgkg). The hearts were flushed clear of clotted blood using 0.9% NaCl and then fixed in 10% formalin. Using external landmarks (position of atria, great arteries, and vena cava), dissections were made of the fixed hearts in sagittal or transverse planes to mimic those obtained by two-dimensional echocardiography. Photographs were taken of the sectioned snake’s heart in planes corresponding to the echocardiographic images.

Results

In the current study, a water bath was employed in place of coupling gel to minimize artifacts created by air trapped under and around the snakes’ scutes. Consistently good quality echocardiograms were obtained using water as the

?Apogee CX, ATL Interspec, Ambler, PA. $Powerpoint, Microsoft Corp., Cambridge, MA. $Superprint, Zenographics Corp., Irvine, CA. ((Angiocath 18 g 2 inch, Becton-Dickinson Corp., Sandy, LIT. #Beuthanasia-D Special, Schering-Plough Co., Kenilworth, NJ

FIG. 3. Right, lateral, sagittal echocardiographic image of the female snake’s heart. CVC = caudal vena cava. Other abbreviations, see previous figures. The pulmonary artery can be seen to the right of the tight atrium. The relationship of the pulmonary artery and the right atrium is also seen in Figs. 6 and 8.

coupling medium. The same echocardiographic technique has since been attempted in sedated snakes with limited success because of movement.

Echocardiographic Anatomy

The identification of the cardiac structures was made in the following manner. First, necropsy specimens of other Burmese pythons were available for examination, dissec- tion, and comparison prior to and during the study. Second, the characteristic shape and spatial relationship of cardiac structures have been well described, and these references were referred to during the s t ~ d y . ~ - ~ Third, agitated saline injected intravenously assisted in verifying the following structures: right atrium, cavum pulmonale, and pulmonary artery. Fourth, after euthanasia, the extracardiac structures (pulmonary artery, left and right aortic arches, and cranial and caudal vena cavae) were identified and tagged, and using these landmarks, dissections of fixed hearts were made in transverse and sagittal planes. These specimens were available for comparison during the remainder of the study.

Scanning in the sagittal plane allowed the best visualiza- tion of major vessels, chambers, septae, and valves. In ad- dition, these views were more anatomically familiar to the sonographer than the transverse views. From a right lateral sagittal view, the caudal vena cava could be seen entering the right atrium through the sinus venosus valve (Fig. 3). The fixed specimen shows the caudal vena cava laying dor- sal along the ventricular muscle of the cavum dorsale (Fig. 4). The pulmonary artery overlies the right atrium in Fig. 3 and Fig. 8. This is also seen in the fixed specimen (Fig. 6).

Moving medially in the sagittal plane, the cavum pulmo-

VOL. 40. No. 1 ECHOCARDIOGRAPHIC ANATOMY OF THE SNAKE HEART 69

FIG. 4. Fixed specimen of the female snake’s heart sectioned in a plane similar to Fig. 3 . Ve = ventricular muscle. Other abbreviations, see pre- vious figures.

FIG. 6. Fixed specimen of the female snake’s heart sectioned in a plane similar to Fig. 5. Other abbreviations, see previous figures.

nale is observed, with the pulmonary artery exiting from it (Figs. 5 , 6, 8c). The left aortic arch and cavum venosum are separated from the pulmonary artery and cavum pulmonale by the horizontal septum (Figs. 5 and 6). The left aortic arch exits the cavum venosum. The leaflets of the left aortic arch were present in Fig. 5. Although the cavum pulmonale ex- tends further apically than the cavum venosum (Figs. 6, 7) , this was not observed echocardiographically because of the extensive trabecular nature of the ventricle of the python.

The spatial relationship of the great vessels in the sagittal planes is displayed in a series of images in Fig. 8. The images were obtained by scanning right (Fig. 8a) to left (Fig. 8d). At the edge of Figs. 8a and b, a portion of the right aortic arch is visualized. However, because of its position (adjacent and dorsal to the left aortic arch, Fig. 7), the right aortic arch is not imaged in its long axis when the pulmo-

nary artery and left aortic arch are observed adjacent to each other (Figs. 5, 6, 8). Likewise when the valve leaflets of the pulmonary artery and left aortic arch are seen in the sagittal view, the right aortic arch is not observed (Fig. 5). In Figs. 8c and d and Fig. 6, the pulmonary artery is ob- served as it traverses craniodorsally.

Transverse imaging best portrayed the extensive trabec- ular nature of the ventricle apically and the structure and position of the horizontal septum. By scanning from the apex toward the base of the heart in the transverse plane, the horizontal septum is found. It is a comma-shaped incom- plete septum that separates the cavum venosum and cavum arteriosum from the cavumpulmonale (Figs. 9, lo). Moving further cranially to the base of the heart, the relative ana- tomic position of the pulmonary artery, left aortic arch, and right aortic arch at the base of the heart is seen in Figs. 11 and 12.

The cranial vena cava, left atrium, vertical septum, and

FIG. 7. Fixed specimen of the female snake’s heart sectioned to the left and lateral of specimen shown in Fig. 6 . LAA and RAA are seen, but PA is not visualized. Other abbreviations, see previous figures.

FIG. 5 . Midline sagittal echocardiographic image of the female snake’s heart. Other abbreviations, see previous figures.

70 SNYDER ET AL 1999

FIG. 8. Series of echocardiographic images obtained by scanning right (Fig. 8a) to left (Fig. 8d) in the sagittal plane. PV = pulmonic valve. Other abbreviations, see previous figures.

FIG. 9. Midventricular, transverse echocardiographic image of the fe- male snake’s heart. Other abbreviations, see previous figures.

FIG. 10. Fixed specimen sectioned of the female snake’s heart in a plane similar to Fig. 9. Other abbreviations, see previous figures.

VOL. 40, No. 1 ECHOCARDIOGRAPHIC ANATOMY OF THE SNAKE HEART 71

FIG. 11. Basilar, transverse echocardiographic image of the female snake’s heart. Other abbreviations, see previous figures.

small pulmonary veins were not consistently observed by echocardiography. Echocardiographic landmarks that may be useful during scanning include the caudal vena cava entering the right atrium via the sinus venosus valve in the sagittal plane and the position of the pulmonary artery and aortic arches in the transverse plane.

Intra-atrial and pulmonary arterial swirling of blood (termed “spontaneous contrast’ ’ or “smoke” by sonogra- phers) was observed during the echocardiographic studies in all snakes. This has also been described in humans with heart disease and may be the predecessor to throm- bus formation in humans.14 However, it has also been re- ported in normal reptiles3 and in normal horses.15 Because the snake’s lungs are located caudal to the heart, and the trachea lays dorsal to the heart, there was no discern- ible lung interference associated with the studies in the snakes.2

Discussion

The large size of the snakes in this study aided in pro- viding good quality images and anatomic specimens. These animals were chosen for two reasons. First, it was important to image animals of sufficient size so that the anatomic structures could be easily identified and photographed for the illustrations and second, they were being culled from a breeding facility. Imaging snakes of smaller size is possible using the same technique. The main limitation is the foot- print size (surface area) of the transducer and shallow depth of field in small snakes.

Imaging in both sagittal and transverse planes was im- portant in maximizing the number of identifiable structures by echocardiography. Because of differences in reptilian and mammalian cardiac anatomy, direct comparison to for- malin-fixed specimens sectioned in similar echocardio-

FIG. 12. Fixed specimen sectioned of the female snake’s heart in a plane similar to Fig. 11. Other abbreviations, see previous figures.

graphic planes was beneficial. Only midline sagittal and transverse images were obtained in the study. Additional transducer orientations may have been useful to provide better visualization of the left atrium, the cranial vena cava, and the septae.

No attempt was made to perform m-mode echocardio- grams to determine ventricular wall motion, wall muscle thickness, or atrial or ventricular chamber size. The snake’s ventricular muscle is highly trabeculated, making such mea- surements difficult.

Previous studies attempting to validate ultrasonographic images in other species have used various techniques to verify correct positioning of the transducer. Intramedullary pins have been inserted into the heart percutaneously along the imaging planes (sagittal and transverse), and later the hearts were sectioned with the pins in place.16 Unless a guide for the pins is prepositioned on the transducer head in the imaging plane, correct positioning of the pins cannot be verified during or after placement. No guide was available for the transducer in our study, so this technique was not employed. Another validation technique involves position- ing catheters into the various cardiac chambers and great vessels to verify the echocardiographic images by contrast echocardiography or pressure monitoring. In the present study, these would have been useful in documenting the cavum ventrale, cavum dorsale, and the left and right aortic arches. In another imaging study of the coelomic cavity of the Boa constrictor,2 the imaging plane was marked by cu- taneous sutures, and postmortem transverse sections were made along the sutures. We regret that we did not utilize this relatively simple technique in our study to document the transverse images, because it would have strengthened the validity of our results.

72 SNYDER ET AL 1999

Placement of a rubber sleeve filled with coupling gel over the transducer is a common practice when performing in- traoperative ultrasonography, sonography per rectum, or when biopsies are obtained. l 7 Positioning the transducer inside a rubber sleeve before submerging the tip of the transducer in water will help protect it from the potential hazards of submersion. Subsequent to the study, a rubber sleeve was used to image several patients without reduction of image quality. * *

Summary Echocardiography is a useful, noninvasive diagnostic tool

to evaluate the anatomy of the reptilian heart. Understand-

**Personal observation, PS Snyder.

ing of normal anatomy is imperative and should improve the ultrasonographer's ability to identify such cardiac abnor- malities as congenital heart defects, endocarditis, cardiac neoplasia, thrombi, and cardiomyopathy. Once clinicians become familiar with using echocardiography in reptiles, use of this diagnostic modality will increase in clinical prac- tice and improve the opportunity to obtain diagnoses other than by necropsy . Lastly, Doppler echocardiography could be used to assist researchers studying blood flow patterns of snakes. ' O,

ACKNOWLEDGMENTS

The authors acknowledge the assistance of E. Jacobson and Mark Hof- fenberg on the project.

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