do while loop causing 100% CPU usage with curl_multi_exec

This is a loop used in our script with curl. It's causing CPU usage to shoot up to 100%. A friend said "Your computer is looping so fast here it doesn't have time to process the request since it is constantly checking for a finish."... So my question is how can this loop be re-written to slow down? Thanks

$running = null;
do {
  curl_multi_exec($mh, $running);
} while($running > 0);