Different Loop Gain Phenotypes in Patients With Chronic Systolic Heart Failure and Periodic Breathing

Central sleep apnoea (CSA) is common in patients with chronic systolic heart failure (HFrEF). Various trials have shown a prevalence of 21 - 37% in this group of people. Up to 66% of patients with CSA and HFrEF present with periodic breathing (PB), which is considered being a marker of HF severity and poor prognosis. Brack et al. summarized data from cohorts, longitudinal studies and retrospective analyses showing an independently increased risk of death in HF patients with PB (HR 2.1-5.7 in five of seven studies). Furthermore, PB in HF patients is known to reduce quality of life and exercise performance and to increase sympathetic nerve activity as well as the probability of malignant cardiac arrhythmias. The pathogenesis of PB is characterized by an instability of ventilatory drive. The level of carbon dioxide (CO2) in blood and cerebrospinal fluid correlates linearly with minute ventilation. A high level of CO2 increases ventilation while hypocapnia dampens it. This control theory is based on the loop gain (LG), which represents the sensitivity and reactivity of the ventilatory system and comprises three components: The plant gain defines the capacity of the system to change PaCO2 in response to a change in ventilation (metabolic response). It is influenced by the lung volume as well as the anatomy of the thorax and the upper airways. The feedback gain is defined by the chemoreceptor responsiveness in reaction to blood gas changes. The controller gain is represented by the respiratory control center in the brain stem and defines the capacity of the system to change ventilation in response to a change in PaCO2 (ventilatory response). Sands et al. proposed and validated a mathematical model based on the ventilatory cycle pattern that quantifies the feedback loop. The ratio of ventilatory and cycle duration within the PB pattern is defined as the duty ratio (DR), which is the basis to calculate the LG. Any temporary breathing disturbance causing a PB pattern with