A Norwegian study retrospectively compared outcomes from non-traumatic cardiac arrest between ambulances staffed by physicians (PMA) and non-physician ambulances (non-PMA). There were no differences in any of the clinical outcome measures used in this study of 977 patients, in which 13% (PMA) and 11% (non-PMA) survived to hospital discharge.

Out-of hospital advanced life support with or without a physician: Effects on quality of CPR and outcome
Resuscitation. 2009 Nov;80(11):1248-52