Nocturnal variabilities of tidal airflow and heart rate show mutual association in young children with asthma symptoms.

Introduction Wearable impedance pneumography (IP) -based technique was recently introduced to describe tidal airflow variability (TAFV) during night sleep at home. These indices were associated with risk of persistent asthma in a cohort of young children (Eur Respir J 2016; 47:1687-96). From the same overnight recordings heart rate variability (HRV) indices were derived and found as well to associate with risk of persistent asthma (under review). In the present study, recordings were successful in 34 young children (age 3-7 y) with recurrent or persistent lower airway symptoms. Most of the children were without regular medication and were considered to have either high (n=14) or lower (n=13) risk of persistent asthma according to modified asthma predictive index (mAPI); additionally, 7 children were on inhaled corticosteroids due to asthma. Signals for both TAFV and HRV analysis were collected with custom-made wearable recorders (design of Tampere University of Technology) through four skin electrodes at home during night sleep. The airflow parameters of minimal curve shape correlation (CSRmin) and minimal noise limit (NLmin) reflect overnight change in expired flow-volume curve shape, and smallest detected respiratory chaoticity, respectively. HRV parameters represent high (HF) and normalised low (LFn) frequency power. HRV organisation index (OI) describes regularity of the HF components of HRV spectrum. Objectives The objective of this study was to assess the association between novel IP-derived indices of TAFV and HRV indices measured during sleep. Results HRV indices LFn and OI correlated significantly with NLmin, but none of the HRV indices correlated with CSRmin (Table 1). Exclusion of the ICS group (n=7) did not change significances of the correlation analysis. Conclusions The results show that indices of TAFV and HRV are related in young children with asthmatic symptoms. Specifically, reduced NLmin indicating lowered chaoticity in tidal breathing flows relates to reduced LFn indicating parasympathetic dominance and increased organization of the HF band of the HRV spectrum, which may suggest lack of adaptability of the parasympathetic nervous system. It is possible that the same neural mechanisms contribute to both heart rate and airflow variability, or changes in HRV spectrum reflect altered respiratory modulation in asthma. Table 1. Spearman rank correlation between indices of tidal airflow variability (TAFV) and heart rate variability (HRV) * : p<0.05, § : p<0.005   HRV indices HF LFn OI (NLmin) TAFV indices CSRmin 0.02 -0.01 -0.13 (0.48 §) NLmin -0.05 0.36 * -0.50 §