Uncoupling of Feedforward Control on Blood Pressure by Heart Rate in Patients with Recent Myocardial Infarction

Abstract

BackgroundMechanically induced respiratory synchronous thoracic pressure fluctuations produce High Frequency(HF) BP oscillations. In addition, HF oscillations in BP are also modulated through feedforward coupling with Heart Rate(HR). Reduced baroreflex sensitivity, heart rate variability (HRV) and blood pressure variability (BPV) is known to be associated with increased mortality after Myocardial Infarction(MI).ObjectiveThe objective of the study was to assess the LF and HF oscillations in HR and BP and their associations in patients of recent MI with and without scar.MethodsNon‐invasive beat‐to‐beat BP measurement using Portapres® with simultaneous recording of Lead II ECG was used to assess short term (5min) HRV and BPV. Thirty‐two patients of recent MI (≤6 months) were divided into those with scar (S) (n = 21, age 54.3±10.7years) and those without scar (NS) (n = 11, age 54.7±9.22years) on the basis of myocardial perfusion imaging (MPI) using single photon emission computed tomography. Transfer function analysis was done using BP as the input variable and HR as the output variable. The signals were linearly interpolated and resampled at 1Hz. The series was subdivided into 256‐point segments with 50% overlap for spectral estimation. The coherence, gain and phase was calculated for the LF (0.04–0.15) and HF (0.15–0.4) band. LF and HF gain and phase was evaluated only if coherence between the two signals was >0.3.ResultsLF and HF HRV were similar between the two groups (LF normalized units (nu) S vs NS 53.6±22.4 vs 54.7±18.1, HF nu S vs NS 35.9±17.9 vs 36.0±12.0). Similarly, LF nu Systolic BPV was comparable between the two groups (S vs NS 66.7±17.2 vs 57.9±15.9). HF nu systolic BPV was significantly lower in the S group compared to the NS group [S vs NS 28.3(13.4–39.3) vs 39.1(19.8–46.5) p = 0.04]. LF and HF gain was also similar between the two groups [LF gain S vs NS 5.24±2.25 vs 5.86±3.09, HF gain S vs NS 7.19(3.26–10.4) vs 8.40(5.94–13.5) ms/mmHg]. There was no significant difference in the LF phase between the two groups (S vs NS 1.55±0.39 vs 1.13±0.61 radians). However, HF phase coupling of BP with HR of the S group was significantly more positive than the NS group (S vs NS 0.39±0.92 vs −0.61±1.0 radians p = 0.02).ConclusionHRV was comparable between the scar and no scar group however HF oscillations in BPV are attenuated in patients with irreversible damage to cardiac tissue when compared to those in whom it is preserved. The positive phase in HF in the scar group could possibly be indicative of a feedforward uncoupling between HR and BP in the scar group while the same is preserved in the no scar group.