4.55 million viewers watched Flatline, according to unofficial overnight figures.
Doctor Who was fourth for the day, slipping behind Pointless Celebrities which managed 4.94 million viewers at the earlier time of 5.45pm.
Top for the day was Strictly Come Dancing with 9.03 million, while on ITV The X Factor had 7.71 million viewers.
Doctor Who had a share of 19.9% of the total television audience.
Final figures should be available next Sunday.