The 1/1 refers to the ventricular response, atrioventricular conduction.

http://medinfo.ufl.edu/~ekg/Rate and Rhythm.html

Atrial Flutter (A Flutter)

Atrial flutter is characterized by a special pattern of regular atrial activity that in adults almost always occurs at a rate of 300/minute. Atrial flutter typically manifests a sawtooth appearance that is usually best seen in the inferior leads. At times, flutter waves may be very subtle (arrows in figure).

The most common ventricular response to atrial flutter (by far!) is with 2:1 AV conduction. This means that the ventricular rate with untreated atrial flutter is usually close to 150/min (i.e., 300 ÷ 2).

Less commonly with flutter there is 4:1 AV conduction (vent. rate 75/minute)or a variable (irregular) ventricular response. Odd conduction ratios (i.e., 1:1, 3:1, 5:1) are rare. Note how much easier it is to identify flutter with 4:1 conduction (figure left) compared with 2:1 (figure above).



--------------------------------------------------
Note added at 9 mins (2005-06-19 21:38:03 GMT)
--------------------------------------------------

http://www.emedicine.com/med/topic185.htm

Variable AV conduction can also be seen (commonly present with 2:1 or 3:1 AV conduction). With 1:1 AV conduction, hemodynamic collapse may occur.


--------------------------------------------------
Note added at 11 mins (2005-06-19 21:40:08 GMT)
--------------------------------------------------

Sorry, I shouldn\'t have written AF as it can be confused with atrial fibrillation.