Introduction: Why Respiratory Rate Matters in Physiological Monitoring   Respiratory rate (RR) is a vital indicator of physiological stability—often the first parameter to shift in response to metabolic stress, infection, or hypoxia. Yet, continuous and unobtrusive respiratory monitoring remains challenging outside of controlled environments.  With the rise of wearable and optical sensing technologies, electrocardiography (ECG) and photoplethysmography (PPG) have emerged as