Rasmus Sandin made it 5-1 at 3:51, one-timing a drop pass from Sgarbossa into an open net.
McMichael's power-play goal at 10:20 made it 6-1. Washington entered the game ranked last in the NHL on the power play (4-for-46) but was 2-for-2 in the game.
Sgarbossa made it 7-1 at 15:27, and Brandon Duhaime scored at 16:45 for the 8-1 final.
"To be honest, I don't really have an answer for that," Blues defenseman Justin Faulk said of allowing five third-period goals. "It's completely unacceptable. It's just not right. I don't think that should ever happen, a situation like that. We need to have respect for each other, the game. You can't just go out there and play summer hockey for a period and think that's alright at any point. We're grown men in this league and need to put in an effort that's acceptable. It's not going to go your way every night but you can't let ... that's just unacceptable."
NOTES: Ovechkin has 38 points (21 goals, 17 assists) in 27 games against St. Louis. He has 13 points (eight goals, five assists) in the past seven games. ... Strome has 13 points (one goal, 12 assists) in the past seven games. He also has had an assist on all 10 of Ovechkin's goals this season. ... Chychrun was plus-3 in 18:44 after missing the past five games with an upper-body injury. ... Thompson, who is 5-0-1 all-time against the Blues, set a Capitals record for most consecutive wins to start a season with seven (Tomas Vokoun, six in 2011).