The successful dissemination of emergency messages in vehicular ad hoc networks can make a difference between life and death. To achieve the life-saving goals, emergency message dissemination needs timely and lossless medium access in vehicular ad hoc networks. Although there are existing medium access control (MAC) protocols that support priority medium access in the literature, these protocols focus on providing statistical priority for unicast flows instead of strict priority for individual packets. This paper proposes a new MAC scheme to address this issue. With its novel pulse-based control mechanism, the proposed MAC scheme realizes strict packet-level priority scheduling for emergency packets in a fully distributed way. With the same mechanism, the proposed scheme supports multiple levels of strict priority for emergency packets. The comprehensive simulation results in this paper show the effectiveness of the proposed MAC scheme in serving emergency messages in vehicular ad hoc networks.