Examination of the autonomic nervous system in the ICU: a pilot study

Introduction

The most widely used test for autonomic dysfunction in the ICU is the heart rate variability (HRV) test [1]. HRV is thought to be a very sensitive but less specific test [1]. Several other tests are available. For this pilot study we have investigated the ability of two tests, the skin wrinkle test (SWT), a test for postganglionic sympathetic function, and the cold face test (CFT), a reflex slowing heart rate after cold application to the forehead, to detect autonomic dysfunction in critically ill patients alongside the HRV.

Methods

ICU patients mechanically ventilated for at least 3 days were included. Exclusion criteria: polynomic or autonomic neuropathy, admission after stroke or cardiac arrest. HRV was investigated using power spectral analysis of continuous 5-minute ECG recordings [1]. The simulated SWT was used and wrinkling was assessed on a five-point scale [2]. Under continuous ECG recording a cold pack was applied to measure the CFT [3]. Changes in SWT and CFT results over time were compared to the changes in the SOFA score. Studies procedures were also performed in 17 healthy controls.

Results

Twelve patients were included (mean age: 54 (SD: 15)). HRV analysis showed decreased heart rate variability in all patients (median total power: 32 ms² (IQR: 11 to 320)). The SWT could be performed in 10 patients. SWT results were abnormal (score ≤2) in 60% of cases (6% in healthy controls; P < 0.01). The CFT was done in nine patients. Critically ill patients showed a blunted response on the CFT (2.5% increase in RR length (95% CI: -0.2% to 5.2%) vs. 7.1% in healthy controls (95% CI: 3.7% to 10.5%; P = 0.03)). Figure 1 displays the CFT results over time.

Figure 1

Changes in cold face test (CFT) results over time.

Conclusion

CFT detected autonomic dysfunction in critically ill patients better than the SWT and was easier to perform. Diagnostic accuracy and prognostic value need to be investigated.