Enhanced stroke volume and divergence of the human left ventricle

Technical Summary: Enhanced Stroke Volume and Divergence of the Human Left Ventricle

Problem Statement
Humans possess exceptionally high metabolic rates compared to other great apes, necessitating a cardiovascular system capable of generating and sustaining a large cardiac output. While it is established that humans exhibit a greater cardiac output than chimpanzees (Pan troglodytes) primarily due to a larger left ventricular stroke volume (SV) rather than heart rate, the specific structural and functional determinants of this enhanced SV remain unknown. Previous studies have identified differences in left ventricular (LV) traits between humans and chimpanzees but have not explicitly tested whether these differences coalesce into a distinct ventricular phenotype that directly supports higher stroke volume. Furthermore, without broader comparative data from other primates, it is unclear which human cardiac features represent derived adaptations versus ancestral traits shared across the hominid lineage.

Methodology
The study utilized a large-scale comparative echocardiographic dataset involving three primate species: humans (Homo sapiens, n=121), chimpanzees (Pan troglodytes, n=143), and rhesus macaques (Macaca mulatta, n=110) as an evolutionary outgroup. Exploratory data from gorillas (n=8) and bonobos (n=4) were also included.

• Data Collection: Transthoracic echocardiography was performed on healthy, adult subjects. Humans were awake; non-human primates were anesthetized using various protocols (e.g., medetomidine/ketamine, tiletamine-zolazepam). The authors acknowledged potential anesthetic effects on functional metrics but performed regression analyses and PCA clustering checks to confirm that anesthetic protocols did not significantly alter the relationship between ventricular parameters and stroke volume or the overall phenotypic clustering.
• Parameters Measured: The study assessed a comprehensive suite of LV structural and functional variables, including:
  • Structure: Ventricular length, sphericity index, wall thickness, internal diameter, and myocardial trabeculation (trabecular-to-compact myocardium ratio).
  • Function: Tissue Doppler velocities (S', E', A'), inflow velocities, isovolumic times, and rotational mechanics (twist, untwisting).
  • Mechanics: Longitudinal and circumferential strain and strain rates.
• Statistical Analysis: To account for body size, cardiac parameters were scaled to body mass using a quarter-power scaling model. The core analytical approach was Principal Component Analysis (PCA) applied to 29 structural and functional variables to identify distinct phenotypic clusters without prior bias regarding species differences. One-way ANOVAs and Kruskal-Wallis tests were used for univariate comparisons.

Key Contributions
This study provides one of the largest comparative echocardiographic datasets of adult chimpanzees to date and integrates them with human and macaque data to define the evolutionary trajectory of the primate left ventricle. By employing multivariate PCA, the authors move beyond isolated trait comparisons to characterize the LV as an integrated functional unit.

Results

• Stroke Volume: Humans exhibited significantly greater relative stroke volume (0.99 ± 0.19 ml/mass^1.0) compared to both chimpanzees (0.80 ± 0.15) and rhesus macaques (0.91 ± 0.17).
• Phenotypic Divergence: PCA revealed clear phenotypic separation among the three species.
  • Rhesus Macaques: Characterized by smaller left ventricles relative to body mass and rapid rates of myocardial deformation.
  • Chimpanzees and Humans: Both showed a shift toward larger ventricles with relatively thicker walls and wider chambers, but with slower myocardial deformation rates compared to macaques.
  • Humans: Humans displayed a unique, derived LV phenotype distinct from both chimpanzees and macaques. This phenotype is defined by a specific combination of traits: an elongated chamber, proportionately greater compact myocardium, and enhanced apical rotation and myocardial deformation.
• Correlation with Function: This unique human phenotype was directly associated with the greater relative stroke volume observed across the primates.

Significance
The paper concludes that the human left ventricle has diverged from that of other primates, evolving a distinct structural and functional phenotype. This derived phenotype, characterized by an elongated chamber and enhanced rotational mechanics, appears to be the specific mechanism underlying the human species' capacity for higher stroke volume. These findings help explain how the human heart evolved to meet the augmented metabolic and thermoregulatory demands required by our high metabolic rates, distinguishing human cardiac physiology not just from distant relatives like macaques, but also from our closest living relatives, chimpanzees.