Lag and Causality in Baroreceptor Response during Passive Head‐up Tilt

Abstract

Multiple methods measure baroreflex sensitivity as effect of blood pressure (BP) on heart rate or interbeat interval (IBI). However, most consider only magnitude, not phase or lag. Also, in the closed‐loop IBI‐BP system, many methods do not differentiate between cause and effect. We postulated that long lag or loss of causality between changes in BP and IBI would predispose to unstable BP. Beat‐to‐beat systolic BP (Finometer) and IBI signals were considered as input and output respectively. Cross covariance (XCV) functions calculated using 40 to 80 heartbeats from cardiovascular steady state were examined for lag for one quarter of their length. Lag, the beat number at maximum XCV with changes in BP leading, was converted to time using average IBI. Significant (p<0.05) correlation at that lag indicated causality. Of 21 subjects with 15 minutes of 70° head‐up tilt, correlation in early tilt (ET) was significant in 19, with lag (mean ± SEM) 0.9±0.2 s. In late tilt, correlation was significant in 16, with lag 2.2±0.4 s. After 5 days of 6‐hour water immersion in 7 subjects, 2 became presyncopal during tilt and lost correlation. ET causal lag was 2.2±0.4 vs. 0.4±0.2 s pre‐immersion (p<0.01).Support: ONR and NAVSEA DSBDP