Assessment of cardiovascular regulation after burns by nonlinear analysis of the electrocardiogram

Critical illness and hypovolemia are associated with loss of complexity of the R-to-R interval (RRI) of the electrocardiogram, whereas recovery is characterized by restoration thereof. Our goal was to investigate the dynamics of RRI complexity in burn patients. We hypothesized that the postburn period is associated with a state of low RRI complexity, and that successful resuscitation restores it. Electrocardiogram was acquired from 13 patients (age 55 ± 5 years, total body surface area burned 36 ± 6%, 11 ± 5% full thickness) at 8, 12, 24, and 36 hours during postburn resuscitation. RRI complexity was quantified by approximate entropy (ApEn) and sample entropy (SampEn) that measure RRI signal irregularity, as well as by symbol distribution entropy and bit-per-word entropy that assess symbol sequences within the RRI signal. Data (in arbitrary units) are means ± SEM. All patients survived resuscitation. Changes in heart rate and blood pressure were not significant. ApEn at 8 hours was abnormally low at 0.89 ± 0.06. ApEn progressively increased after burn to 1.22 ± 0.04 at 36 hours. SampEn showed similar significant changes. Symbol distribution entropy and bit-per-word entropy increased with resuscitation from 3.63 ± 0.22 and 0.61 ± 0.04 respectively at 8 hours postburn to 4.25 ± 0.11 and 0.71 ± 0.02 at 24 hours postburn. RRI complexity was abnormally low during the early postburn period, possibly reflecting physiologic deterioration. Resuscitation was associated with a progressive improvement in complexity as measured by ApEn and SampEn and complementary changes in other measures. Assessment of complexity may provide new insight into the cardiovascular response to burns.