Heart rate variability at ICU admission in Covid-19 patients in sitting position: a prospective study

Purposeto describe HRV metrics in Covid-19 patients at admission in the ICU and its relationship with mortality and invasive mechanical ventilation (IMV). Heart rate variability (HRV) in sitting position in critically ill patients has not been explored. Methodswe conducted a prospective single-centre observational study. Adult patients admitted in the ICU with respiratory failure due to RT-PCR-confirmed SARS-CoV-2 but not under IMV were included. Electrocardiogram was recorded at least for 15 minutes at 500 Hz during a stable sitting condition. Power spectrum was obtained using wavelets. Very low frequency (VLF), low frequency (LF) and high frequency (HF) powerbands were calculated and then normalized to total power (VLFn%, LFn% and HFn%). We also analyzed non-linear HRV dynamics. Results27 patients were included. LFn% was lower in non-survivors (4.5 vs 8 %, p=0.015) and related to 28-day mortality (OR 0.61; 95% CI: 0.32 to 0.9, P=0.05). In a robust generalized Gamma linear model, we found that detrended fluctuation analysis alpha 2 (DFA2) (RR 0.092; bootstrapped 95% CI: 0.031-0.361, P=0.003), admission APACHE II score (RR 1.084; bootstrapped 95% CI: 1.046 to 1.123, P=0.002) and SAF index (RR 0.985; bootstrapped 95% CI: 0.982 - 0.990, P<0.001) were associated with longer ICU LOS. Conclusionsdiminished normalized LF power was associated with 28-day mortality in univariate analysis among critically ill COVID-19 patients on spontaneous ventilation, potentially reflecting an altered autonomic response in early severe COVID-19. Normalized and absolute VLF power should be considered when analyzing HRV in ICU patients. DFA2 was the HRV variable with the strongest association with ICU LOS. These exploratory results could be helpful to design newer tools for early prognostication in COVID-19 patients.