Caring for the smallest hearts: cardiovascular phenotypes and assessment in tiny babies.

With improving survival, periviable neonates (≤25 weeks' gestation) represent a dynamic, but under-studied population in neonatal care, with persistently high cardiopulmonary and vascular vulnerability. Immature cardiovascular structure and function as a consequence of immature myocardial architecture, altered calcium handling, relative adrenal insufficiency, and persistent fetal shunts contribute to complex and dynamic cardiovascular physiology in this population. This may be present clinically in the form of hypotension and low end-organ perfusion. Traditional paradigms of blood pressure-based definitions of hypotension are poorly validated in this population and do not accurately reflect systemic blood flow or end-organ perfusion. Emerging evidence supports a phenotype-based multi-parametric approach to cardiovascular assessment, to distinguish ongoing physiological changes such as ductal physiology, pulmonary hypertension, low systemic vascular resistance and primary myocardial dysfunction phenotypes. However, significant knowledge gaps remain, including lack of normative hemodynamic values, and limited evidence guiding pharmacologic therapies. This narrative review focuses on the cardiovascular challenges in the management of periviable neonates as they transition to extrauterine life, delineating cardiac phenotypes, describing modalities of cardiovascular assessment and identifying existing knowledge gaps. We propose a physiology-based approach to cardiovascular management strategies based on existing, albeit limited, evidence. IMPACT: Periviable neonates present unique hemodynamic challenges due to structural and functional cardiovascular system immaturity, which can be categorized into different hemodynamic phenotypes dictated by baseline cardiac function, lung compliance and directionality of intracardiac shunts, especially the patent ductus arteriosus. In the absence of established normative reference values for common modalities of cardiac assessment, optimal care should consist of early identification of cardiac phenotypes, continuous surveillance, physiology-based management strategies, and frequent reassessment to guide individualized treatment.