there is no general, simple, absolute answer to this question. no
activity is inherently "cardio"/aerobic or anaerobic. if you are
grossly out of shape and go for a jog (a presumptive "cardio"
activity), your heartrate might be way out of its aerobic threshold
and you will be effectivly doing an anaerobic activity.
so, it really depends on the particular format of your class, the
poses you perform, how you transition through them, your personal
fitness level, etc.
the best you can do is judge for yourself whether or not your
particular Bikram Yoga class meets the American College of Sports
Medicine (ACSM) criteria for "cardio" (i.e. aerobic) activity: this
is basically any activity that uses large muscle groups, can be
maintained continuously, and is rhythmic in nature.
another useful rule of thumb is to assess whether or not your heart
rate is maintained at 60-80% of its maximum for an excess of 20
so, my suggestion would be to take your heartrate (or consult your
heart rate monitor) periodically during your yoga class. if it is
continuously maintained within that 60-80% of maximum range for more
than 20 minutes, then, YES, you are getting some "cardio" benefit from
if your heartrate fluctuates up and down quite a bit (ie between
"resting" and "working" states) or you practice yoga for only 10
minutes then i would say that NO it does not provide a cardio benefit.