Shortcode creation – need loop or if statement?

Hi guys, i have created a shortcode for the Twitter Bootstrap carousel, with the following as part of it:

function j72_carousel_img($atts, $content = null) {
            return '<div class="item">' . do_shortcode( $content ) . '</div>';
  }

What i need is for the above to be looped through so that the first instance of this line has the class “active” added to it like this:

<div class="item active">

This indicates the first image in the carousel – can anyone let me know what needs to be done to achieve this?

Thanks