When heat becomes a toxin: A narrative review on heatwaves, cardiac vulnerability, and population risk profiles in low- and middle-income countries

Climate change has intensified global heat exposure, particularly through more frequent and severe heatwaves. These conditions pose substantial risks to cardiovascular health. Low- and middle-income countries (LMICs) are disproportionately affected because of elevated baseline cardiovascular disease (CVD) burdens and limited adaptive capacity. The aim of this study was to synthesize epidemiological evidence linking extreme heat and heatwaves to CVD morbidity and mortality in LMICs, explore biological mechanisms underlying heat-induced cardiac stress, and examines social and environmental factors contributing to population vulnerability. Evidence from studies consistently demonstrates that extreme heat increases CVD events, with effect often emerging within 0–3 days following exposure. Proposed mechanisms include thermoregulatory strain, dehydration-related hemoconcentration, endothelial dysfunction, heightened sympathetic activation, and exacerbation of pre-existing CVD. Older adults, individuals with comorbidities, outdoor workers, and households with poor housing conditions are disproportionately affected. Despite established risks, most LMICs lack a structured heat-health protection system, clinical protocols for managing vulnerable cardiovascular patients during heatwaves, and labor protections for heat-exposed workers. Strengthening climate-resilient health systems and developing targeted preventive strategies are essential to mitigate rising cardiovascular risks associated with extreme heat in LMICs.