An implantable medical device (IMD) such as a pacemaker may need to communicate with an external device, such as a programmer or monitor.  An IMD typically includes wireless communication circuitry for non-invasively communicating with the external device.  Wireless communication between the IMD and the external device may be initiated by either device.  Communication may be periodic or should be initiated in response to one or more events. 
The IMD, the external device or both may apply techniques in which communications capabilities are enabled at some times and disabled at others.  The IMD, for example, may be battery-powered.  To preserve battery life, the IMD may assume a low-power mode during which communication capabilities are disabled, and may assume a high-power mode to engage in communication with the external device. 
When the IMD seeks to communicate with the external device, the IMD may poll the external device to establish a wireless communication session.  Conventional polling techniques generally take time and put a drain on the power supply of the IMD. 
Many IMDs and external devices are equipped with internal clocks.  For various reasons, it may be advantageous for the internal clock of the IMD to be in synchrony with the internal clock of the external device.  Due to a variety of factors, however, the internal clocks in the IMD and the external device may run at different rates, causing the internal clocks to “drift” with respect to one another.  Over time, the internal clocks lose synchronicity.  To compensate time-based drift, a conventional IMD may periodically poll the external device, establish a communication session, and use the communication session to re-synchronize the internal clock of the IMD to the internal clock of the external device.  Unfortunately, this periodic polling and re-synchronization consumes battery power.