Cardiovascular reserve as an integrative physiological construct: Current evidence, emerging technologies and future directions

Cardiovascular reserve represents the integrated physiological capacity of the cardiovascular system to maintain tissue perfusion during increasing physiological stress. Although reserve has long been recognized conceptually, its direct assessment remains challenging. No universally accepted definition or gold-standard measure of cardiovascular reserve currently exists. Nevertheless, if cardiovascular reserve can be estimated reliably, reserve-based monitoring could permit earlier recognition of impending circulatory deterioration before conventional physiological indicators become abnormal. Among the reserve-monitoring technologies currently under investigation, the Compensatory Reserve Metric (CRM) and the Cardiovascular Reserve Index (CVRI) represent two distinct approaches to estimating remaining cardiovascular reserve. This review examines the physiological basis of reserve-based monitoring, critically evaluates the available experimental and clinical evidence supporting CRM and CVRI, assesses their strengths and limitations and outlines priorities for future research and potential clinical applications. CRM has demonstrated strong performance in experimental haemorrhage models and remains the most extensively studied reserve-monitoring technology in low output conditions. Although evidence supporting CVRI is more limited, findings from cardiopulmonary exercise testing, haemorrhage and heat stress suggest that it may reflect progressive depletion of cardiovascular reserve across diverse forms of circulatory and physiological stress. Although seemingly promising, current evidence remains limited by small clinical cohorts, heterogeneous study designs and limited independent validation.