Autonomic activity in trauma patients based on variability of heart rate and respiratory rate.

Objective: To evaluate the effects of sympathetic and parasympathetic nervous system activity on the heart rate and other hemodynamic variables in acute emergency patients with mild to moderately severe trauma.

Design: Clinical study.

Setting: Level 1 university-run trauma service.

Patients: Fourteen trauma patients studied immediately after admission to the emergency department.

Interventions: We measured heart rate and respiratory rate variability by spectral analysis in the early period of mildly to moderately injured patients and compared the patterns of the low- (Lfa) and high-frequency (Hfa) areas of variability.

Measurements And Main Results: The Lfa is the area under the spectral analysis curve within the frequency range of 0.04-0.10 Hz. This area reflects primarily the tone of the sympathetic nervous system as mediated by the cardiac nerve. The respiratory area or Hfa is a 0.12 Hz-wide frequency range centered around the fundamental respiratory frequency defined by the peak mode of the respiratory power spectrum. It is indicative of vagal outflow reflecting parasympathetic nervous system activity. The Lfa/Hfa, or "L/R ratio," reflects the balance between the sympathetic and parasympathetic nervous systems. The hemodynamic effects of bursts of autonomic activity in response to injury were explored by heart rate and respiratory rate variability measured with non-invasive hemodynamic monitoring consisting of bioimpedance cardiac output, heart rate, and mean arterial pressure to measure cardiac function and transcutaneous oxygen (PtcO2) to reflect tissue perfusion. During sudden surges of autonomic activity, we described increased heart rate variability reflecting increased Lfa and to a lesser degree to Hfa. Slightly later there was increased heart rate, mean arterial pressure, and cardiac index but decreased tissue perfusion indicated by the decreased PtcO2/FIO2 ratio.

Conclusions: Surges in autonomic activity in the period immediately after emergency department admission of trauma patients were associated with pronounced increases in cardiac index, mean arterial pressure, and heart rate and reduced tissue oxygenation.