The digital lines on the 4451 DSA device do not directly support any syncrhonization or hardware timing. However, you could start an analog output operation based on a state change in one of the digital lines. This just requires digital triggering for your analog output. You would need to physically route one of the DIO pins to the WFMTRIG pin (PFI6) on your digital connector block.
In LabVIEW, you should make a call to AO Trigger and Gate Config.VI. Use the following parameters:
Gate or Trigger Source: PFI Pin
Trigger or Gate Condition: Trigger on Rising edge (or falling if you want to trigger when a digital line is turned off).
Trigger or Gate Source Specification: 6
If you are programming in the NI-DAQ text API, use the following call:
i Status = Select_Signal(iDevice, ND_OUT_START_TRIGGER, ND_PFI_6, ND_HIGH_TO_LOW)