Ventritex, based in Sunnyvale, Calif., became a public company on Jan. 22 when it sold 4.7 million shares at $18 each. A competitor, Medtronic Inc., plans to submit a similar heart-pacing device to the F.D.A. later in the quarter. Ventritex's shares gained $2 today, closing at $30.25 in over-the counter trading; Medtronics' rose $1, to $88.125, on the New York Stock Exchange