Mackie was a sweet girl, a six-month-old rescue, maybe a bit Rottweiller and a little bit of Husky. She woke up rambunctious and stayed that way, keeping her big sister Rosie, the grumpy grandma dog of the household, happy and active. Last Tuesday, the dog’s owner, Jamie Simms, who lives on the Venice Canals, came home and watched her dogs do laps around the patio, as usual. But by 8:30 pm, something was drastically wrong. As Simms reached for their leashes, Mackie collapsed. She raced Mackie to an emergency room, where her “sweet girl” went into inexplicable heart failure. Within hours, the puppy had passed, Simms…..