Entropy measures of heart rate variation in conscious dogs

Our goal was to determine the contributions of sympathetic and parasympathetic activity to entropy measures of heart rate variability (HRV). We compared our results with two commonly used methods to analyze HRV: standard deviation (SDNN) and power spectral analysis (HF norm). Beat-by-beat analysis of R-R intervals was performed in conscious dogs. The R-R intervals were analyzed with approximate entropy (ApEn) and entropy of symbolic dynamics (SymDyn) to assess the effects of reducing system complexity. This was achieved by pharmacologically inhibiting sympathetic, parasympathetic, and total autonomic nervous system regulation of heart rate. Three conditions were examined: rest, standing, and systemic hypotension. At rest or standing, sympathetic inhibition (propranolol) had no effect on ApEn or SymDyn, whereas parasympathetic (atropine) and combined (propranolol + atropine) inhibition reduced both entropy measures to near zero. Systemic hypotension reduced both entropy measures in intact dogs. When hypotension was induced after sympathetic inhibition, ApEn was increased compared with hypotension alone, whereas parasympathetic inhibition with hypotension resulted in near-zero ApEn. Changes in the entropy measures of HRV were directionally similar to changes in SDNN and HF norm. These results indicate that the entropy of R-R intervals reflects parasympathetic modulation of heart rate.

Heart rate, heart rate variability, and blood pressure during perioperative stressor events in abdominal surgery

STUDY OBJECTIVE

To define the behavior of power spectral heart rate variability (PSHR) during potentially stressful events in the perioperative period, and relate it to changes in blood pressure (BP) and heart rate (HR).

DESIGN

Longitudinal clinical study.

SETTING

Operating room and recovery suites of a large tertiary care referral center.

PATIENTS

26 ASA physical status I, II, and III patients undergoing elective abdominal surgery.

INTERVENTIONS

Anesthesia was induced with thiopental sodium and fentanyl, and maintained with isoflurane/nitrous oxide (N2O)/relaxant or enflurane/N2O/relaxant. The trachea was intubated and intraabdominal surgery was performed.

MEASUREMENTS AND MAIN RESULTS

Observations consisted of HR, noninvasive blood pressure, and PSHR. They were made before and after induction of anesthesia, tracheal intubation, and surgical incision, and during maximal surgical stimulation and skin closure. HR and mean arterial pressure (MAP) maxima were also recorded for one hour before and after emergence from anesthesia. PSHR was obtained using a special algorithm and data acquisition system for real time spectral analysis of the instantaneous HRversus time function. The HR power spectrum parameters analyzed were low-frequency (LFA; powerband = 0.04 to 0.10 Hz), respiratory-induced frequency (RFA; powerband = respiratory frequency +/- 0.06 Hz), and the ratio of LFA to RFA. With induction of anesthesia, only RFA power decreased significantly. LFA power reduction became significant only after intubation and continued so until after incision. Immediately after induction, the decline in RFA power (vs. preinduction) was more pronounced when compared with the decline in LFA power (76% vs. 34%; p = 0.01). Hence, the ratio LFA/RFA increased significantly after induction of anesthesia. It was significantly higher than at postintubation, preincision, or skin closure. A significant elevation in LFA, LFA/RFA ratio, and BP occurred with maximal abdominal surgical stimulation. Only preinduction LFA, RFA, and LFA/ RFA ratio were predictive of MAP changes with induction of anesthesia (p = 0.006). In 8 of the 15 patients who had MAP changes of at least 10 mmHg with induction, PSHR indices correctly predicted a change of this magnitude. Late intraoperative HR maxima were positively correlated with the change in HR and incision (r2 = 0.58; p < 0.01). The change in BP with incision was positively correlated with early postoperative HR maxima (r2 = 0.60; p < 0.01).

CONCLUSIONS

On anesthetic induction, preoperative, but not intraoperative, spectral indices were predictive of BP changes. Power spectral analysis of HR may provide information about the autonomic state that is not evident from BP or HR. The HR power spectrum, in particular, indicated a striking autonomic imbalance immediately after the induction of anesthesia despite stable HR and BP. LFA and LFA/RFA ratio appeared to track sympathetic autonomic activation during abdominal surgical stimulation, but not during other perioperative stressor events.