  We analyze the effects of time dependent magnetic and RF fields on the spin
dynamics of the Carr-Purcell-Meiboom-Gill (CPMG) sequence. The analysis is
based on the decomposition of the magnetization into the eigenmodes of the
propagator of a single refocusing cycle. For sufficiently slow changes in the
external fields, the magnetization follows the changing eigenmodes
adiabatically. This results in echo amplitudes that show regular modulations
with time. Faster field changes can induce transitions between the eigenmodes.
Such non-adiabatic behavior occurs preferentially at particular offsets of the
Larmor frequency from the RF frequency where the eigenmodes become nearly
degenerate. We introduce the instantaneous adiabaticity parameter ${\cal A}(t)$
that accurately predicts the crossover from the adiabatic to the non-adiabatic
regime and allows the classification of field fluctuations. ${\cal A}(t)$ is
determined solely by the properties of a single refocusing cycle under static
conditions and the instantaneous value of the field offset and its temporal
derivative. The analytical results are compared with numerical simulations.
