Single ventricle: amphibians and human beings

aolo Angelini P· Bruno Marino · Antonio F. Corno

Survival to adult life in patients born with “functionally”single ventricle, without any surgical treatment, as natural history, is extremely rarely reported [ 1— 7]. In general, these group of patients with “functionally” single ventricle are symptomatic from the f irst weeks of life, and require two to three staged operations to obtain that the single ventricle pumps the oxygenated blood to the systemic circulation,while the less oxygenated blood is deviated by gravity from the superior and inferior vena cava directly to the lungs [ 1,8, 9]. The result of this artif icial “Fontan circulation” is high systemic venous pressure, and the chronic elevation of the systemic venous pressure may results in liver failure, renal failure, protein-losing enteropathy, and plastic bronchitis[ 10— 17]. As a consequence, the long-term surgical outcomes of these patients are complicated by the combination of heart failure and cyanosis, substantially reducing their life expectancy and severely compromising their quality of life [ 10— 19].Mathematical and computational f luid dynamic models were used to study the blood f low distribution in single ventricle[ 20, 21], and a better design of circulation, altering the traditional surgical options [ 22]. Research could greatly benef it from studies performed on animals born with single ventricle,even if the only animal models available in nature are amphibians, like axolotl salamander (Ambystoma mexicanum) [ 7] and frogs (Xenopus laevis) [ 23, 24], and reptiles.

These experimental studies on amphibians, performed with echocardiography and cardiac magnetic resonance imaging, revealed that salamanders and frogs have two elements in common in their double inlet form of single ventricle: intact interatrial septum and excessively trabeculated ventricular chamber.

Intact atrial septum

This characteristic has been verif ied in frogs through microscopic dissections, echocardiography and cardiac magnetic resonance: in all frogs investigated, the interatrial septum was completely intact, without any pinpoint communication. This evidently is responsible for preventing any mixing of oxygenated pulmonary venous drainage and desaturated systemic venous drainage at the atrial level, before entering in the double inlet left ventricle [ 23].

Excessively trabeculated ventricular chamber

Once the separated pulmonary and venous drainages enter in the double inlet type of single ventricle, they encounter, in both frogs and salamanders, in an excessively trabeculated ventricular cavity, non-conducive for homogeneous mixing,or at least making complete blood mixing extremely unlikely[ 7, 23].

What seem extremely interesting are the observations of the similarities between the anatomical and physiological f indings in frogs and salamanders and the few reported cases with single ventricle who survived until adult age without surgery [ 7, 19, 23, 24]. The presence of intact interatrial septum, by preventing any mixing at atrial level, may well justify that, in the absence of complete mixing in the single ventricular chamber, and with certain morphological characteristics, the amphibians can live unrestrictive long life.The same is true because of the excessive trabeculated single ventricular chamber, certainly non-conducive for homogeneous mixing.

With regard to the characteristics of the single ventricular chamber, a large spectrum of ventricular architectural organization exists in nature, acceptable to the lives of diff erent animals, diff erent in their cardiac working requirements and performance. Some animals adaptations include to run quite fast while having large bodies (vertebrates, mammals), while others have small bodies and are mainly sedentary (like salamanders) or only intermittently jump (like frogs). This observation should imply that oxygen requirements and capacity at baseline and with exertion are diff erent for diff erent animals living circumstances and habits, but these have not yet been studied well during exertion.

In general, it is currently quite clear that the evolution of species in the animal kingdom occurred by genetic variants (or sequential spontaneous genetic errors), that become tested by environmental responses, resulting in progressive adaptation in animals habits. The critical recent acquisitions in this f ield, from experimental gene deletion in animal models (for the human heart, the current preference is in mice),have greatly helped in studying the result of many diseases,like left ventricular non-compaction cardiomyopathy, where 25 viable models were reported up to year 2018 [ 25, 26].Some of the experiments have followed indications from genetic studies in family clustering of left ventricular noncompaction cardiomyopathy.

Review of the evolution in the animal species can be probably best summarized in claiming that more evolved species (larger animals, vertebrates, ectothermic) have taken advantage of split systemic and pulmonary circulatory systems in parallel, which seems to be the most eff ective and common model [ 27]. Life under water, level of activities,diet and access to favorite food, have all selected specif ic arrangements, especially in the organization of the heart and respiratory systems (lungs or gills or skin) [ 23]. For example, frogs are usually prone to jumping in place better than running, whereas salamanders have sedentary habits:the resting heart rate of 22 beats/minute is a good indication of that lifestyle [ 7]. In addition, as a consequence of the relative bradycardia in salamanders, the cardiac ejection fraction is higher than expected, due to increased duration of the cardiac f illing phase [ 7].

Split circulatory systems imply that the two, systemic and pulmonary, are completely separated: the desaturated systemic venous return only should go to the respiratory system, whereas the oxygenated pulmonary venous return will maintain the higher oxygen concentration exclusively for the systemic arterial circulation [ 19].

In the circulatory system of frogs and salamanders, the two circulations are fused at the level of single ventricle, but then they are split at the pulmonary and systemic branches,where the amount of blood f low distributed between the outlet arteries is determined by the ration of the peripheral resistances in the two territories [ 24]. The relative oxygen concentration in the pulmonary and systemic arteries has not yet been measured in relationship to intra-cardiac anatomy,but we must assume that it is possible and hopefully it will be soon available [ 7].

The simplest f inal product of the cardiac dysfunction in single ventricle is expressed by cyanosis and dyspnea [ 1— 6,28]. It is likely that the pulmonary vascular resistance are variable in diff erent functional states, especially in frogs and salamanders, with the resulting QP:QS, ration between the pulmonary blood f low (QP) and systemic blood f low (QS),depending upon variable circumstances and environments[ 7, 24], as in human beings with single ventricle. In general,the cardiac hemodynamics is primarily controlled by pressures and resistance, and not by metabolic markers, such as oxygen concentration.

The important phenomenon of favorable versus unfavorable blood streaming in diff erent carriers with double inlet single ventricle is interesting and probably real, but incompletely studied as yet. The initial and simple evidence of possible importance at this regard is the systemic oxygenation, typically measured by percutaneous oximeter. Two recent articles on the intra-ventricular anatomy in double inlet single ventricle, unfortunately lacked essential clinical correlations to the very diligent anatomic description of their specimens [ 28, 29].

Incidentally, the complex and transitional model of the cardiorespiratory system in amphibians was reported in detail in a recent study on theChinese giant salamander[ 30]. This study showed that the larvae are aquatic (gillsbased) and the terrestrial adults have lungs, developing under the action of multiple successive miRNAs genes and proteins. A further, detailed study of the conal valve (or bulbus cordis) was recently performed, using high-resolution micro-tomography, of wholeBufospecimens [ 31].

The discussion about separation of blood streams seems still open and requires in vivo high-def inition video, besides the measurements of oxygen saturations at diff erent sites,diffi cult to obtain in small animals. The exact structure and function of the conal valve, as we reported [ 23], with independent timing of “functional opening and closing”, add to the complexity of the hemodynamic model in frogs and salamanders [ 7, 23]. Incidentally, the conal myocardium is thick and compact, much more than the epicardial compact layer of the main ventricle, and apparently it is perfused only by the coronary arteries, small for they are, of such heart.

Considering the presence of exceptionally trabeculated ventricular chamber observed in frogs and salamanders,dealing with the other objects of comparison in human pathology, the left ventricular non-compaction should be better understood in detail. The results of recent cardiac magnetic resonance screening of a large adolescent population [ 32, 33] are being studied in detail; current indication exists that some degree of left ventricular non-compaction exists in a general asymptomatic population is welldescribed and conf irmed, and only at the non-compaction to compacted myocardial thickness ratio ＞ 2:1 it is accompanied by compact layer hypoplasia, with thinning of the subjacent compact layer of more than 25% of the interventricular compact layer. Whether this could be associated with prognostic increased risk of developing dilated cardiomyopathy is being under current investigation, but it seems unlikely, when based on the multi-ethnic study of atherosclerosis (MESA trial) [ 34]. Such case of benign, isolated left ventricular non-compaction is not genetically identif iable,and only the rare cases of severe, congenital dilated cardiomyopathy (usually, a congenital, familiar, genetic anomaly)are frequently associated with hyper-trabecular heart.

A recent study reported experimental gene deletion of factors known to be involved with the development of the normal left ventricular compaction in mice embryos, that have succeeded in reproducing non-compaction dilated cardiomyopathy by using several alternative genes [ 25]. Both these human variants, solitary left ventricular non-compaction or pathological entities left ventricular non-compaction dilated cardiomyopathy, have very little to do with the understanding of the structure and function of double inlet single ventricle in human patients. To the best of our knowledge, such associated anomaly has not been documented in humans with double inlet single ventricle, and it is only matter of speculation if it could inf luence the blood streaming in the single ventricular chamber. In particular, in double inlet single ventricle dilated cardiomyopathy has not been observed, because the mean ventricular ejection fraction is usually in the range of 50%, or almost normal. In fact,Fontan failure is generally caused by progressive increase of atrio-ventricular valve(s) regurgitation, increased pulmonary vascular resistance, arrhythmias, but very unusually by impairment of ventricular ejection fraction and/or cardiomyopathy [ 10— 19]. The presence of non-compaction of the single ventricular cavity is usually not even mentioned in the context of congenital heart defects with single ventricle, and we do not expect that any surgical treatment could be reasonably off ered for modulation of ventricular non compaction.

The main questions at this regard remain if we need to further investigate the frogs and salamanders heart in order to advance the understanding, and possibly the treatment of human complex congenital heart defects like the single ventricle?

At present, the anatomic and physiologic context of the amphibian heart does not seem to be even remotely applicable, either to explain the human cardiac pathophysiology, or to modify the currently available surgical options in patients born with double inlet ventricle. We have to accept that,either God or nature, have created, through the evolution,the “ultimate machine” allowing the amphibians to live an unrestricted life style for many years.

However, the evolutionary origin of normal and abnormal morphogenesis of the human heart previously suggested[ 35], has been more recently demonstrated for some aspects[ 36— 38]. As genetics, anatomy and function are strictly linked, the morphologic and functional studies of animal hearts, and comparisons with normal and abnormal human f indings, become increasingly important.

Author contributionsAll authors contributed to conceptualization,writing - original draft, writing - review and editing and to the f inal approval of the version to be published.

Funding None.

Declarations

Ethical approval Not required.

Conflict of interestNo f inancial or non-f inancial benef its have been received or will be received by all authors from any party related directly or indirectly to the subject of this article.