> Although I wish to initially  transmit approximately once every
> five seconds, eventually I would like to speed it up. Otherwise I
> could restart the measurement after the transmission has finished.
>
> I plan to transmit the signal at a random interval, averaged at
> once every 5 seconds. It would be nice to have the "Current"
> value ready for when the transmission is required.
>
> The reason for the random transmission is that there is more than one
> unit transmitting. A poor mans ethernet with no collision detection
> by the sender. If the receiver get a screwed message it ignores it
> and picks up the value on the next transmission.

What would happen if you were to, after each input pulse, randomly decide
to output a packet [possibly with some random delay beforehand]?  If you
have observed the input frequency as 60bpm, you could wait up to about 800ms
after an input transition before sending your packet and still be done
in time for the next input; if your frequency is faster (e.g. 240bpm) you
can't wait as long but you have a better selection of pulses to trigger
on.