in placebo group, patients were cannulated with 20 gauge intra-arterial catheter. In experiment groups, patients were cannulated with 22 gauge or 24 gauge intra-arterial catheter.

The accuracy of arterial blood pressure waveform is depedent on damping coefficient (DC) and natural frequency (NF). The extent to which a monitoring system is damped is expressed by the DC, while the frequency of oscillation that a monitoring system resonates at is known as the NF. The NF and DC are required to be within certain rages for accurate blood pressure monitoring, and they can be affected by different arterial catheter size. Adequately damped arterial waveform can be defined as NF and DC being within certain ranges. For example, waveforms with DC of 0.5 at a NF of 15Hz can be regarded as adequately damped arterial waveform. However, if waveforms have DC of 0.1 at a NF of 15Hz, these can be regarded as not having optimal dynamic response. (There is a reference figure explaining these factors in detail in a previous article. Gardner RM: Direct blood pressure measurement--dynamic response requirements. Anesthesiology 1981; 54:227-3)

At 1 minute after cannulating radial artery, we will obtain information about the mean blood pressure measured in both invasive blood pressure and non-invasive blood pressure. Invasive blood pressure will be measured from radial artery cannulation, and non-invasive blood pressure will be measured by blood pressure cuff at Rt. upper arm. Thereafter, the difference of mean blood pressure between two values will be used for data analysis.