The crocodilian heart; more controlled than we thought?

Transesophageal echocardiography of cardiac function in Nile crocodiles - A novel tool for assessing complex hemodynamic patterns

BACKGROUND

The crocodilian heart is unique among reptiles with its four-chambered structure and complete intracardiac separation of pulmonary and systemic blood flows and pressures. Crocodiles have retained two aortic arches; one from each ventricle, that communicate via Foramen of Panizza, immediately distally from the aortic valves. Moreover, crocodiles can regulate vascular resistance in the pulmonary portion of the right ventricular outflow tract (RVOT). These unique features allow for a complex regulation of shunting between the pulmonary and systemic circulations. Studies on crocodile shunting have predominantly been based on invasive measurements, but here we report on the use of echocardiography.

METHODS

Experiments were performed on seven pentobarbital anaesthetized juvenile Nile crocodiles (length and mass of 192 ± 13 cm and 26 ± 5 kg, respectively). Echocardiographic imaging was performed using a transesophageal (TEE) approach. All images were EKG-gated.

RESULTS

We obtain excellent views of cardiac structures and central vasculature through the esophagus. Standard imaging planes were defined for both long- and short axis views of the left ventricle and truncus arteriosus. For the RV, only a short axis view could be obtained. Color Doppler was used to visualize flow. Pulsed waved Doppler for measuring flow profiles across the atrioventricular valves, in the two RVOTs and the left ventricular outflow tract. Shunting across the Foramen of Panizza could be visualized and gated to the EKG.

CONCLUSION

TEE can be used to image the unique features of the crocodile heart and allow for in-vivo imaging of the complex shunting hemodynamics, including timing of cardiac shunts.

Exposure to hypoxia during embryonic development affects blood flow patterns and heart rate in juvenile American alligators during digestion

The developmental environment can alter an organism's phenotype through epigenetic mechanisms. We incubated eggs from American alligators in 10% O₂ (hypoxia) to investigate the functional plasticity of blood flow patterns in response to feeding later in life. Digestion is associated with marked elevations of metabolism, and we therefore used the feeding-induced stimulation of tissue O₂ demand to determine whether there are lasting effects of developmental hypoxia on the cardiovascular response to digestion later in life. In all animals studied, digestion elicited tachycardia and an elevation of blood flow in the right aorta, left aorta, and the pulmonary artery, whereas flows in the carotid and subclavian artery did not change. We found that heart rate and systemic blood flow remained elevated for a longer time period in juvenile alligators that had been incubated in hypoxia; we also found that the pulmonary blood flow was elevated at 24, 36, and 48 h. Collectively, our findings demonstrate that exposure to hypoxia during incubation has lasting effects on the hemodynamics of juvenile alligators 4 years after hatching.

Comparative and Functional Anatomy of the Ectothermic Sauropsid Heart

The heart development, form, and functional specializations of chelonians, squamates, crocodilians, and birds characterize how diverse structure and specializations arise from similar foundations. This review aims to summarize the morphologic diversity of sauropsid hearts and present it in an integrative functional and phylogenetic context. Besides the detailed morphologic descriptions, the integrative view of function, evolution, and development will aid understanding of the surprising diversity of sauropsid hearts. This integrated perspective is a foundation that strengthens appreciation that the sauropsid hearts are the outcome of biological evolution; disease often is linked to arising mismatch between adaptations and modern environments.

Notes on the gross anatomy of the heart of the broad-snouted caiman, Caiman latirostris (Daudin, 1802)

The broad-snouted caiman, Caiman latirostris (Daudin, 1802), is one of the six crocodilian species from Brazil. The topography, morphology and morphometry of the broad-snouted caiman heart were studied. Data were obtained from the necropsy of four adult animals, three females and one male. The hearts were removed from the coelomic cavity and fixed in 10% formalin for 48 hr for morphological and morphometric description. The heart is in the cranial mediastinum. It is caudally involved by the liver cranial margins, and ventrally by the ribs, intercostal muscles, and sternum and dorsally by the lungs. The four-chambered morphology is typical with two (right and left) atria and ventricles. Right and left aortic, pulmonary and subclavian arteries branch from the truncus arteriosus. Gubernaculum cordis is present as ligamentous folds uniting the heart apex to the pericardium. Main morphometric means are the apex-to-base length (49.86 mm), circumference (105.25 mm) and heart weight (45.03 g). The right atrium is craniocaudally longer with thicker walls, whereas the left ventricle is narrower. The topography, morphology and morphometry of the heart of C. latirostris are consistent with the anatomy of other crocodilian species.

Cardiovascular shunting in vertebrates: a practical integration of competing hypotheses

This review explores the long-standing question: 'Why do cardiovascular shunts occur?' An historical perspective is provided on previous research into cardiac shunts in vertebrates that continues to shape current views. Cardiac shunts and when they occur is then described for vertebrates. Nearly 20 different functional reasons have been proposed as specific causes of shunts, ranging from energy conservation to improved gas exchange, and including a plethora of functions related to thermoregulation, digestion and haemodynamics. It has even been suggested that shunts are merely an evolutionary or developmental relic. Having considered the various hypotheses involving cardiovascular shunting in vertebrates, this review then takes a non-traditional approach. Rather than attempting to identify the single 'correct' reason for the occurrence of shunts, we advance a more holistic, integrative approach that embraces multiple, non-exclusive suites of proposed causes for shunts, and indicates how these varied functions might at least co-exist, if not actually support each other as shunts serve multiple, concurrent physiological functions. It is argued that deposing the 'monolithic' view of shunting leads to a more nuanced view of vertebrate cardiovascular systems. This review concludes by suggesting new paradigms for testing the function(s) of shunts, including experimentally placing organ systems into conflict in terms of their perfusion needs, reducing sources of variation in physiological experiments, measuring possible compensatory responses to shunt ablation, moving experiments from the laboratory to the field, and using cladistics-related approaches in the choice of experimental animals.

Examples of Weak, If Not Absent, Form-Function Relations in the Vertebrate Heart

That form and function are related is a maxim of anatomy and physiology. Yet, form-function relations can be difficult to prove. Human subjects with excessive trabeculated myocardium in the left ventricle, for example, are diagnosed with non-compaction cardiomyopathy, but the extent of trabeculations may be without relation to ejection fraction. Rather than rejecting a relation between form and function, we may ask whether the salient function is assessed. Is there a relation to electrical propagation, mean arterial blood pressure, or propensity to form blood clots? In addition, how should the extent of trabeculated muscle be assessed? While reviewing literature on trabeculated muscle, we applied Tinbergen's four types of causation-how does it work, why does it work, how is it made, and why did it evolve-to better parse what is meant by form and function. The paper is structured around cases that highlight advantages and pitfalls of applying Tinbergen's questions. It further uses the evolution of lunglessness in amphibians to argue that lung reduction impacts on chamber septation and it considers the evolution of an arterial outflow in fishes to argue that reductions in energy consumption may drive structural changes with little consequences to function. Concerning trabeculations, we argue they relate to pumping function in the embryo in the few weeks before the onset of coronary circulation. In human fetal and postnatal stages, a spectrum of trabeculated-to-compact myocardium makes no difference to cardiac function and in this period, form and function may appear unrelated.

Sequential segmental analysis of the crocodilian heart

Differences between hearts of crocodilians and those of mammals and birds are only partly understood because there is no standardised approach and terminology for describing cardiac structure. Whereas most reptiles have an undivided ventricle, crocodilians have a fully septated ventricle. Their hearts, therefore, are more readily comparable with the hearts of mammals and birds. Here, we describe the heart of a crocodile (Crocodylus noliticus). We use the versatile sequential segmental approach to analysis, juxtaposing several key views of the crocodilian heart to the comparable views of human hearts. In crocodiles, the atrial and ventricular septums are complete but, unlike in placental mammals, the atrial septum is without an oval fossa. The myocardial component of the crocodilian ventricular septum dominates, but the membranous septum likely makes up a greater proportion than in any mammal. In the crocodile, the aortic trunk takes its origin from the left ventricle and is not wedged between the atrioventricular junctions. Consequently, there is a common atrioventricular junction, albeit with separate right and left atrioventricular valvar orifices. As in mammals, nonetheless, the crocodilian left atrioventricular valvar orifice is cranial to the right atrioventricular valvar orifice. By applying a method of analysis and terminology usually restricted to the human heart, we build from the considerable existing literature to show neglected and overlooked shared features, such as the offset between the left and right atrioventricular valvar orifices. Such commonalities are surprising given the substantial evolutionary divergence of the archosaur and synapsid lineages, and likely reflect evolutionarily shared morphogenetic programmes.

INTRAMUSCULAR EPINEPHRINE RESULTS IN REDUCED ANESTHETIC RECOVERY TIME IN AMERICAN ALLIGATORS (ALLIGATOR MISSISSIPPIENSIS) UNDERGOING ISOFLURANE ANESTHESIA

Inhalants are commonly used to anesthetize reptiles, but volatile anesthetics have been associated with prolonged recovery times. The objective of this study was to determine the effects of intramuscular (IM) epinephrine on anesthetic recovery times following isoflurane anesthesia in a population of subadult American alligators ( Alligator mississippiensis ). In this prospective randomized crossover study, five clinically healthy alligators were anesthetized for 90 min with the use of isoflurane. Alligators were randomly assigned into one of two treatment groups: Group E received IM epinephrine (0.1 mg/kg), and Group S received an equal volume of 0.9% saline administered after isoflurane was discontinued. Time from the end of inhalant administration to return of spontaneous ventilation, return of the palpebral reflex, movement in response to a standardized toe pinch, and spontaneous movement was recorded. The time of extubation was noted and occurred following the return of spontaneous ventilation and movement. Pulse rate, surface body temperature, and airway gases including expiratory and inspiratory isoflurane concentrations and end-tidal carbon dioxide were measured every 5 min throughout the study. The time from the end of anesthesia to extubation was significantly faster in Group E (51.2 ± 16.7 min) compared to Group S (107.4 ± 43.7 min). Pulse rate was significantly higher within the first 15 min following epinephrine injection compared to the saline group at these time points. Therefore, IM epinephrine administered at the end of general anesthesia can significantly hasten anesthetic recovery from isoflurane in alligators.

Surgical removal of right-to-left cardiac shunt in the American alligator (Alligator mississippiensis) causes ventricular enlargement but does not alter apnoea or metabolism during diving

Crocodilians have complete anatomical separation between the ventricles, similar to birds and mammals, but retain the dual aortic arch system found in all non-avian reptiles. This cardiac anatomy allows surgical modification that prevents right-to-left (R-L) cardiac shunt. A R-L shunt is a bypass of the pulmonary circulation and recirculation of oxygen-poor blood back to the systemic circulation and has often been observed during the frequent apnoeic periods of non-avian reptiles, particularly during diving in aquatic species. We eliminated R-L shunt in American alligators (Alligator mississippiensis) by surgically occluding the left aorta (LAo; arising from right ventricle) upstream and downstream of the foramen of Panizza (FoP), and we tested the hypotheses that this removal of R-L shunt would cause afterload-induced cardiac remodelling and adversely affect diving performance. Occlusion of the LAo both upstream and downstream of the FoP for approximately 21 months caused a doubling of RV pressure and significant ventricular enlargement (average approximately 65%) compared with age-matched, sham-operated animals. In a separate group of recovered, surgically altered alligators allowed to dive freely in a dive chamber at 23 degrees C, occlusion of the LAo did not alter oxygen consumption or voluntary apnoeic periods relative to sham animals. While surgical removal of R-L shunt causes considerable changes in cardiac morphology similar to aortic banding in mammals, its removal does not affect the respiratory pattern or metabolism of alligators. It appears probable that the low metabolic rate of reptiles, rather than pulmonary circulatory bypass, allows for normal aerobic dives.

Evidence for endothermic ancestors of crocodiles at the stem of archosaur evolution

Physiological, anatomical, and developmental features of the crocodilian heart support the paleontological evidence that the ancestors of living crocodilians were active and endothermic, but the lineage reverted to ectothermy when it invaded the aquatic, ambush predator niche. In endotherms, there is a functional nexus between high metabolic rates, high blood flow rates, and complete separation of high systemic blood pressure from low pulmonary blood pressure in a four-chambered heart. Ectotherms generally lack all of these characteristics, but crocodilians retain a four-chambered heart. However, crocodilians have a neurally controlled, pulmonary bypass shunt that is functional in diving. Shunting occurs outside of the heart and involves the left aortic arch that originates from the right ventricle, the foramen of Panizza between the left and right aortic arches, and the cog-tooth valve at the base of the pulmonary artery. Developmental studies show that all of these uniquely crocodilian features are secondarily derived, indicating a shift from the complete separation of blood flow of endotherms to the controlled shunting of ectotherms. We present other evidence for endothermy in stem archosaurs and suggest that some dinosaurs may have inherited the trait.