<prompt> The lead of ECG marked as X is called?Lewis leadV4RaVRVg  <response>: And. B. V4R15 - Lead ECGV4R - 5th intercostal space midclavicular line same as V4 but on the other sideV8 - On the back, 5th intercostal space midscapular line.V9 - On the Back, 5th intercostal space between V8 and the spine 