episodes throughout the season, portraying patient Denny Duquette, who begins a relationship with Izzie Stevens, but ultimately dies following an unsuccessful heart transplant surgery. Loretta Devine acted as Adele Webber, Richard's wife, who is revealed to have been aware of her husband's affair since its inception. Other guest-stars include Sarah Utterback in the role of nurse Olivia Harper, love-interest of both George O'Malley and Alex Karev, Kali Rocha portraying fifth-year resident Sydney Heron, who replaces Miranda Bailey temporarily when she takes a maternity leave, Jeff Perry portraying Meredith Grey's father, Thatcher Grey, Mare Winningham in the role of Susan Grey, Tsai Chin in the role of Helen Yang Rubenstein, Cristina's mother, Mandy Siegfried portraying Molly Grey Thompson, Meredith's half-sister and Tessa Thompson portraying Camille Travis, Richard Webber's niece, Christina Ricci portraying paramedic Hannah Davies, and Kyle Chandler in the role of Dylan Young, head of the bomb squad. Eric Dane, who would be promoted to a series-regular in the third season, appeared in the eighteenth episode, portraying attending physician, otolaryngologist and plastic surgeon Mark Sloan, Addison Montgomery's former lover, whose affair with her is presented as the reason behind the estrangement of her husband, Derek Shepherd. Reception Ratings The first 5 episodes of the season were initially planned to be within the first season with the episode "Bring the Pain" as the season 1 finale. According to Rhimes, after airing the ninth episode of the show, the ratings, the time-slots and the really great audience meant that they have to end the season with that episode, and the cliffhanger with the coming of Derek's estranged wife fit perfectly. The season received high critical acclaim, and it performed better than the first season. Due to the success of the first season, Grey's Anatomy took over the Sunday night timeslot along with Desperate Housewives, replacing Boston Legal. The second season averaged 21.07 million viewers, making it the highest-rated season of the series to date. It was ranked #5 in the 2005-2006 television season. The season also includes the series' highest-rated episode, "It's the End of the World" which was watched by 37.88 million viewers. Critical response The season opened up to high critical acclaim with many critics calling it "one of the best shows on TV" and was included in the top 10 for numerous "best of television" lists of 2006. Emily VanDerWerff of The A.V. Club called the show a "Pokémon"