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

Abstract

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.

Fig. 1.

Pressures and flows recorded simultaneously before and after acute occlusion of the left aorta (LAo) in a 2.33 kg American alligator. Pressures were recorded in the right ventricle (RV; blue) and pulmonary artery (PA; purple) and blood flows were recorded in the LAo (orange) and PA (purple) immediately before (A,C) and after (B,D) occlusion of the LAo with a suture upstream of the foramen of Panizza (FoP).

Fig. 2.

(A–C) Representative traces of voltage differentials, recorded using the H₂-electrode technique, used to validate chronic left aorta (LAo) occlusion for S-LAo (successful occlusion; A), unsuccessful occlusion (US-LAo; B), and sham-operated animals (C). (D) Ventral-view schematic of chronic LAo occlusion and the H₂-electrode technique. Chronic occlusion of the LAo at two locations (grey bars), both upstream and downstream to the foramen of Panizza, created a crocodilian heart with fully-divided pulmonary (solid arrows) and systemic (dashed arrows) circulation. Arrows in traces (A–C) denote sequential 0.2 ml injections into the left and right ventricles (LV and RV) of saline saturated with hydrogen gas (indicated by syringes in D). For all sham (B) and US-LAo animals (C), venous blood passing from the RV into right aorta (RAo) via the foramen of Panizza was demonstrated (a R–L cardiac shunt) as a voltage differential detected via a platinum-tipped wire located in the RAo (D). No R–L cardiac shunt was demonstrated in S-LAo animals (A). Schematic adapted from Axelsson et al. (Axelsson et al.,1996). CCA, common carotid artery; RS, right subclavian artery; LPA; left pulmonary artery; RPA, right pulmonary artery.

Fig. 3.

Mean and peak pressure (kPa) in the left and right ventricle of successful occlusion of the left aorta (S-LAo) animals and sham-operated animals (error bars indicate s.e.m.). Peak and mean pressure in both ventricles was significantly elevated in experimental animals (^*t-test P<0.012).

Fig. 4.

Representative traces of simultaneous pressures in left ventricle (LV; red) and right ventricle (RV; blue) for a successful occlusion of the left aorta (S-LAo) animal and sham-operated animal (Sham) approximately 20 months after the operation.

Fig. 5.

Mean mass-specific wet ventricular mass (g kg^–1; combined left and right ventricle) for sham-operated, unsuccessful occlusion (US-LAo) and successful occlusion of the left aorta (S-LAo) animals (error bars indicate s.e.m.). S-LAo animals showed a 65% enlargement of ventricular mass compared with sham animals. All groups were statistically different from each other, denoted by uppercase letters (ANOVA on mass-specific values P<0.0001; SNK α=0.05^A,B,C). Mean values (±s.e.m.) were 1.92±0.14 g kg^–1 for sham animals, 2.34±0.10 g kg^–1 for unsuccessful experimental animals and 3.17±0.19 g kg^-–1 for successful experimental animals. US-LAo animals showed 22.1% increase in ventricular mass relative to sham animals, and S-LAo animals showed a 35.5% increase in ventricular mass relative to US-LAo animals.

Fig. 6.

Mean DNA content (ng mg^–1) for the left (LV) and right ventricle (RV) of sham, unsuccessful occlusion (US-LAo) and successful occlusion of the left aorta (S-LAo) animals. Uppercase letters above error bars indicate significant differences for each ventricle, derived from SNK post-hoc test (α=0.05^A,B,C) following ANOVA. Error bars indicate s.e.m.

Fig. 7.

A histogram of mean number of apnoeas versus voluntary apnoea time (1–27 min bins) for sham-operated and experimental alligators with the left aorta (LAo) occluded taken during 6 h of a diving trial (28–34 h after the animal was put into the dive chamber). An apnoeic period was defined as absence of pressure transducer signal concordant with an absence of gas exchange for at least 30 s (i.e. successive breaths that occurred within 30 s were considered the same `breathing event'). The mean number of apnoeas is based on the number of apnoea events observed for each 1-min bin divided by the number of animals (N=7 experimental or N=6 sham). Error bars indicate s.e.m.

Fig. 8.

Ventral-view schematic of central blood flow pattern and hypothesised ventricular wall thicknesses in sham, unsuccessful occlusion (US-LAo) and successful occlusion of the left aorta (S-LAo) alligators. Sham and US-LAo schematics are drawn during R–L cardiac shunt. Schematic adapted from Axelsson et al. (Axelsson et al., 1996).