srcset: define a file to retina desktop

This is how my image tag look like:

<img 
    src="img/thumb-teste-270x190.jpg" alt="usro" 
    sizes="(max-width: 20em) 100vw, 
           (min-width: 50em) 10vw"
    srcset="img/thumb-teste-270x190.jpg 270w,
            img/thumb-teste-690x486.jpg 690w"
>

For desktop device, I use a small thumb (270x190). For mobile devices, I use the big thumb (690x486). But how can I create a rule for retina desktop use a medium thumb (540x380)?