Many preoperative stressors, including pain, excitation and preoperative anxiety, may influence the response to anaesthesia. The correlation between preoperative anxiety and anaesthetic requirements has been supported in a previous controlled study . Several biochemical changes, such as activation of the sympathetic system resulting in elevated plasma levels of epinephrine, norepinephrine and cortisol occur related to stress . Additionally, reports link anxiety to increased activity of plasma cholinesterase [3,4] and this may affect the dose requirement for drugs such as mivacurium which are dependant upon plasma cholinesterase for their metabolism [5,6]. Several tools for the evaluation of preoperative anxiety have been described, including the hospital anxiety and depression scale which is divided into two subscales measuring anxiety and depression [7,8], the self-rating anxiety scale  and a visual analogue scale (VAS) [10,11]. In addition to these psychological measures, one can objectively describe physiological alterations related to preoperative anxiety such as changes in sympathetic-vagal balance induced by preoperative anxiety by determinations of the heart rate (HR) variability [12,13] as well as catalytic activity of plasma cholinesterase.
The aim of our study was to evaluate in an objective way the influence of preoperative anxiety on plasma cholinesterase activity and HR variability. Quantitative determination of those effects allows future studies on the influence of different premedication and anaesthetic regimens on pre- and perioperative anxiety and stress. In addition, knowledge of the level of anxiety may allow for better adjustment of the anaesthetic technique.
Materials and methods
After approval of our local Ethics Committee and written informed consent 25 male patients (American Society of Anesthesiologists (ASA) Grade I-II, minor elective ear, nose and throat (ENT) surgery) were recruited on the preoperative day. Twenty five healthy male volunteers served as a control group.
Patients aged under 18 or above 65, with problems of consent due to mentally handicap or language, ASA > II, body weight beyond the normal range (±30%) (Broca-Index), with liver or kidney failure or any other disease known to interact with plasma cholinesterase activity were excluded from the study. We decided to investigate only male subjects as medication with oral contraceptives is known to interact with plasma cholinesterase.
Patients were visited after hospital admission and after general information regarding anaesthesia between 14:00 and 18:00 h on the day before surgery. Control subjects were examined at the same time of day. All examinations were carried out by the same person. Before starting the tests a short description and time table was given to each subject. The order in every case was the same. Each was asked to fill in the two multiple choice questionnaires (hospital anxiety and depression scale, self-rating anxiety scale) and to self-rate their level of anxiety on a 0-100 mm VAS (0 = no anxiety at all, 100 = maximum anxiety). A venous blood sample was next taken for measurement of plasma cholinesterase activity. This was followed by a 10 min rest period, a further 4 min in the supine position then determination of HR variability for 5 min.
Blood samples were centrifuged and analysed within 30 min. The enzyme activity was measured photometrically at 405 nm (Synchron CX, Beckman Coulter, Fullerton, California, USA; substrate by Roche Diagnostics, Mannheim, Germany) by the following reaction:
For determination of HR variability two electrodes were fixed with a belt on the subjects' chests and they were asked to relax in the supine position. After 4 min rest, HR variability was recorded for a period of 256 beat-to-beat intervals with an acquisition rate of 1000 Hz (VariaCardio TF4, Sima Media, Olomouc, Czech Republic). The data from the entire period was used for further analysis - artefacts were automatically recognized and removed manually. Data was analysed by power spectrum analysis into very low frequency (VLF: 0.02-0.04 Hz), low frequency (LF: 0.04-0.15 Hz) and high frequency (HF: 0.15-0.4 Hz) bands. LF/HF ratio and total power (0.02-0.4 Hz) were also calculated.
A statistically significant difference of plasma cholinesterase activity between patients and controls was set as the primary end-point of the study. Power analysis (power of 80%) was performed according to Modai and colleagues  (α = 0.05; β = 0.2).
Data were analysed using the t-test, χ2-test or U-test as appropriate. For analysis of regression the correlation coefficient of Pearson (r) was used. Numerical data are presented as mean ± standard deviation (SD) except for HR variability which is expressed as median because it was found not to be normally distributed.
Data from 50 subjects aged 18-49 yr were analysed. There were no significant differences regarding height, body weight, body mass index (BMI) and ASA status. The volunteers in the control group were 6.6 yr younger than the patient group (P < 0.05; Table 1).
The patients had higher scores for depression on the hospital anxiety and depression scale than did the controls (2.84 ± 2.90 vs. 1.16 ± 2.53; P < 0.01) but there was no significant difference in anxiety. There was no significant difference between the groups with respect to the self-rating anxiety scale.
The patients had significantly higher scores in VAS than did the controls (21.04 ± 23.25 vs. 4.40 ± 5.16 mm; P < 0.01). The VAS correlated positively with the anxiety component of the hospital anxiety and depression scale (r = 0.280; P < 0.05) and negatively with total power (r = −0.293; P < 0.05). There was no correlation with the other parameters of HR variability or with the self-rating anxiety scale.
Plasma cholinesterase activity was significantly higher in patients than controls (6646 ± 924 vs. 5324 ± 1082 units L−1; P < 0.001). Plasma cholinesterase activity showed no correlation with age. There was no significant correlation with hospital anxiety and depression scale, self-rating anxiety scale and VAS.
Total power, LF and HF power were significantly lower in patients than controls. VLF power showed no significant difference (Fig. 1). The LF/HF power ratio was not different although there was a trend towards higher values in patients than controls (median 1.74 vs. 0.98 ms2). Hospital anxiety and depression scale and LF/HF power ratio exhibited a positive correlation (anxiety component r = 0.324, P < 0.05; depression component r = 0.415, P < 0.01). Total power negatively correlated with age (r = −0.37; P < 0.005).
Preoperative stress such as anxiety or pain leads to a number of biochemical changes including activation of the sympathetic nervous system with increased plasma levels of catecholamines and cortisol. The results of the present study indicate that preoperative anxiety influences plasma cholinesterase activity and HR variability.
A common way to determine preoperative anxiety is the use of psychological assessment scores such as the hospital anxiety and depression scale [7,8] and self-rating anxiety scale  or, less time-consuming but less differentiated, a VAS [10,11]. All scoring systems used in our study have been established in the literature [7-11] for determination of preoperative anxiety and depression. In our study the hospital anxiety and depression scale and the VAS showed a significant correlation with HR variability and correlated with each other. These results match with the findings of Millar and colleagues  who demonstrated a good agreement between hospital anxiety and depression scale and VAS.
We did not find a difference between groups using the self-rating anxiety scale. Papanikolaou and colleagues  investigated the preoperative attitude of surgical patients to regional anaesthesia and found it to be significantly correlated with preoperative fear but could not demonstrate a correlation with the self-rating anxiety scale. For that reason the self-rating anxiety scale seems to be less suitable for determination of preoperative anxiety.
Though we were able to demonstrate significant differences between patients and controls in the hospital anxiety and depression scale and VAS, the differences regarding the hospital anxiety and depression scale were not as distinct as expected.
One reason for these findings may have been a tendency for dissimulation (faking good) in our groups of healthy young men who were examined by a young, female member of our staff. It demonstrates a disadvantage of all questionnaires which only allow a more or less subjective evaluation of preoperative anxiety. Furthermore all assessment scores presume the capability to understand and answer the questions - this may exclude certain individuals and some elderly or mentally handicapped patients. For that reason more objective instruments which are independent of the patients' compliance are favourable.
Plasma cholinesterase is known to be influenced by state and trait anxiety [3,4] or chronic pain . Modai and colleagues  demonstrated higher levels of enzyme activity in medical students before compared to after an examination. Mathew and colleagues  found, in comparison with a control group, a higher plasma cholinesterase activity in trait or state anxiety patients, but no changes between the enzyme activity before and after relaxation training. Cameron and colleagues  showed an increased plasma cholinesterase activity in patients with chronic spinal pain in which the enzyme activity correlated positively with a visual assessment score for pain.
In agreement with these studies we demonstrated a significantly higher plasma cholinesterase activity in preoperative patients, however, no correlation with the assessment scores used in our study was found. The primary end-point of this study was the determination of changes of plasma cholinesterase activity in patients with preoperative anxiety. Thus the number of patients may have been too small to achieve a significant correlation of plasma cholinesterase activity with the psychological tests used.
HR variability measures sympathetic-vagal balance and its changes with mental or physical stress. Therefore, HR variability may be of clinical use to get more precise information about the patients stress level than with self-assessment scores alone.
In our investigation there was a significant difference in age between patients and controls and we could demonstrate a significant correlation of age and total power.
It has been previously shown that HR variability indices decline with increasing age . Therefore one could argue that the differences in HR variability were influenced by the higher age of the patients. Umetani and colleagues  investigated 260 patients aged 10-99 yr, but found no significant differences between the decades 20-29 yr and 30-39 yr. For that reason the small difference in age of 6.6 yr seems unimportant. Watkins and colleagues  could demonstrate a correlation between anxiety and baroreflex control of HR and, in a previous study, between trait anxiety and a reduction of vagal tone . We found the LF and HF band (0.04-0.4 Hz) to be significantly smaller in the patient group. In addition we demonstrated a trend towards a higher LF/HF ratio in the patients. These findings indicate a global decrease of HR variability in a state of preoperative anxiety. Supposing that the HF band correlates with vagal tone , preoperative anxiety in our study leads to a decrease of vagal control of the HR as mentioned by Watkins and colleagues [13,16]. These results match with the physiological stress model with an increase of sympathetic tone caused by an increased catecholamine level . In contrast Sleigh and Henderson  found no correlation between preoperative anxiety and the spectral power in the mid-frequency (0.05-0.15 Hz), but an increase of the relative power in the HF band (0.15-0.5 Hz) with increasing anxiety. They concluded that preoperative anxiety would often be associated with a relative vagal predominance. The authors suggested that these contrasting results could be due to an anxiety-dependent hyperventilation (HR variability was recorded directly before operation started) which may have influenced the power of the HF band.
Plasma cholinesterase activity increases while HR variability and vagal control of the HR decrease with preoperative anxiety. The changes of HR variability correlate with the results of the hospital anxiety and depression scale and a VAS. Measurement of HR variability and plasma cholinesterase activity may thus represent tools for a more objective way to study the preoperative condition of a patient. This may not only be useful for plasma cholinesterase-dependent drugs such as mivacurium or succinylcholine but also for hypnotics and analgesics which are known to have higher dose requirements in anxious patients with an increased sympathetic tone.
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