When assessing acute post-operative pain, the validity, reproducibility and reliability of Visual Analogue Scale (VAS) and verbal Numerical Rating Scale (NRS) have been investigated extensively. The use of electronic version of VAS has been published in the recent 10 years yet publication related to its use in pain medicine were few. Jamison et al have shown good correlation between electronic VAS vs paper VAS on healthy volunteers using weight as the stimulus. In normal subjects as well as fibromyalgia patients, electronic form of VAS could be used to describe different levels of experimental heat stimulation.

Long term study had been done where electronic pain diary were compared against paper diary in recording pain in 36 patients with chronic low back pain. The 2 scales showed good correlation. This author use a software that is Palm-top computer based. Following these few studies, validation study in chronic pain patient were published. A Palm-top computer based VAS was compared with paper NRS in 200 chronic pain patients. The pain level recorded by the two methods were considered equivalent by the author. However, Palm-top computer has almost disappeared from the market nowadays.

Recent handheld electronic devices (iPad®) were incorporated with touch screen input interface which allow users to use their finger as the input device. To our knowledge, there is no published data concerning iPad-based VAS, especially in post-operative pain. We would like to compare the data obtained from electronic VAS on iPad with verbal numerical rating scale when acute post-operative pain is assessed.

Hypothesis The null hypothesis is that the two pain measurement method do not correlate. The Spearman correlation coefficient between electronic VAS and verbal NRS will be reported.

Objective:

To assess the agreement between the pain level measured by electronic VAS and verbal NRS in acute pain situation. Secondary objective is to determine the test-re-test reliability of electronic VAS.

Method:

All Chinese or English speaking patients 18 years or above, who are taken care by the acute pain service in Queen Mary Hospital are eligible for inclusion. Patients suffer from dementia, visual or hearing impairment will be excluded. Written consent will be obtained.

Pain specialist responsible for the acute pain service will assess patients' condition during daily ward round while patients are using patient controlled analgesia (PCA) device, receiving epidural infusion or peripheral nerve local anaesthetic infusion. For the pain assessment, patient will be asked to rate their current pain level using verbal numerical rating scale (NRS) as a standard or care. (0 being no pain, 10 being worst pain imaginable) Within one minute, the patient will be asked by the investigator, who is blinded to the verbal NRS, to rate the current pain level again on an electronic version of visual analogue scale on iPad® (vasQ Clinical©, Maki NAKATA 2011) The electronic VAS displayed on iPad® is a horizontal scale with "No Pain" on the left, "Worst Pain Imaginable" on the right. Patient can use the finger to mark the level of pain on the iPad® screen by moving a diamond shape marker alone the horizontal line. The length of the scale is divided electronically into 100 equal parts. There will be 101 possible score (0 - 10.0, which indicate the distance of the diamond marker from the left end of the scale. The score will be stored together with the patient's identifier.

Patients' clinical pain management will not be altered by the data obtained from the electronic VAS.

Patients will be asked to rate the current pain level after 1 minute using the electronic VAS again to evaluate the test-retest reliability of the tool.

Sample size Previous studies have shown that when VAS assessment was repeated in a short period of time, the standard deviation of the difference between the two measurements is around 8 mm (on a 100 mm scale). A sample size of 84 patients will be able to limit the 95% confidence interval of the mean difference between the pain assessment to +/- 3 mm. 92 patients will be recruited assuming a drop out rate of 10%.

Analysis The agreement between electronic VAS and verbal NRS will be evaluated using Spearman rank correlation6 and Bland-Altman method. The Spearman coefficient will be presented together with 95% confident interval (CI). The Bland-Altman method involves plotting the mean of the electronic VAS and NRS against the difference between the 2 scores for each pair of data. Mean difference +/- standard deviation of the differences between electronic VAS and verbal NRS will be presented.

The analysis will be repeated for the 2 sets of electronic VAS obtained 1 minute apart to evaluate the test-retest reliability of the electronic VAS.

Inclusion Criteria:

• All Chinese or English speaking patients 18 years or above, who are taken care by the acute pain service in Queen Mary Hospital, Hong Kong are eligible for inclusion.

Exclusion Criteria:

• Patients suffer from dementia, visual or hearing impairment will be excluded.

This study will be conducted at the population of Hong Kong.