Background: The control mechanisms during general anesthesia include circulation parameters and vegetative reactions. A possible way to quantify vegetative reactions is to measure the impedance of the skin. An activation of the eccrine sweat glands via sympathetic sudomotor fibers induces a secretion of sweat, which generates a drop in skin impedance. The aim of the present study was to investigate the influence which different stressors and measurement electrodes have upon skin impedance.
Material and method: The changes in skin impedance and were measured after application of various stimuli (T1 value at rest, T2 acoustic stimulus, T3 visual stimulus, T4 tactile stimulus, T5 pain stimulus, T6 Valsalva manoeuvre, T7 forced inspiration/expiration). About 62 awake subjects underwent four standardized test sequences, during which several types of electrodes and recording sites (palmarly, plantarly) were explored.
Results: All physiological (T6-T7) and external stimuli (T2-T5) led to significant changes in skin impedance (14.9 +/- 18.2 kOmega) and heart rate. These changes happened independently of BMI, gender and measurement electrode types. The time it took to react to the stimuli was significantly shorter for palmar applications than that obtained from plantar sites. The reaction times were as follows: palmarly 1.2 +/- 0.5 seconds for solidgel electrodes and 1.15 +/- 0.5 seconds for hydrogel electrodes, plantarly 2.3 +/- 1.0 seconds for solidgel electrodes and 2.21 +/- 1.2 seconds for hydrogel electrodes. The forced inspiration and expiration manoeuvres generated greater variations in skin impedance than did pain stimulus and acoustic stimulus. Measurements that were performed with solidgel electrodes revealed significantly greater average decreases in skin impedance following exposure to a stimulus.
Conclusion(s): External, but primarily also physiological stressors, generate direct and reproducible variations in skin impedance. Solidgel ECG electrodes should be used for all measurements.