What is the effect of persistent hypothermia on cardiac performance?
Winter swimming and infarcts

The publication Long-term risks of cardiac and cerebral vascular diseases increased following winter swimming in the cold seawater which came out just some weeks ago shows a link between long-term winter swimming and CVD. 
I think this is the best evidence so far about the possible link. 

WYSIWYG's comment has some good general idea:


  Many processes are affected by temperature including metabolism and
  growth rates. Reaction rates increase with temperature but not in case
  of catalysed reactions where the catalyst can operate at only an
  optimum temperature.


Some related topics


temperature effect on the rate of protein degradation


So further studies


protein degradation in the heart in persistent hypothermia
...


Effect of persistent hypothermia on processes of cAF

[1]

Let's have cAF (chronic atrial fibrillation) here as an target. 
Electrophysiological study shows that cAF-related remodeling is typically divided into three categories: 1) electrical, 2) contractile and 3) structural. 
They cause respectively


1) shortening of both the action potential (AP) duration and effective refractory period (ERP)
2) possibly a result of impaired Ca2+ intracellular handling
3) changes of cellular level (hypertrophy, glycogen accumulation and modified mitochondrial morphology) and tissue fibrosis


cAF's processes in persistent hypothermia

I cannot say now more anything about persistent hypothermia and cAF by confidence. 
Those mechanisms of Koivumäki et al are cellular processes, while the representation of the heart requires integrative analysis in the cellular, tissue and organ levels. 

In cellular level 


Only some catalyzed reactions can work in persistent hypothermia. TODO consider which enzymes of the heart and body work in low temperature in short-term and long-term cases. Depletion cases too. 
Effect of persistent hypothermia on the outflux of Calcium (probably, slows down). 


In tissue level


I can say without rigor that electrical coordination is uncoordinated in tissue fibrosis. 


In organ level 


I say say without rigor that the mean electrical axis should be determined and fixed in the study. 
Effect of Arterial hypertension must be fixed. Sokolew-Lyon index can a starting point for the indicator but better indicator is required with higher specificity. 


Persistent hypothermia from the extreme side (Sauna)


[2] Regular Sauna is with better hemodynamic function; however, the association of sauna bathing with cardiovascular and all-cause mortality is not known.


so possible effect of persistent hypothermia can be 


lower hemodynamic function if no compensation mechanism
better hemodynamic function if some compensation mechanism (genetics; unknown)


Some research topics


Effect of Persistent Hypothermia on the Arterial Hypertension. This may be associated with coagulopathy, especially in the lower limb. 
Effect of Persistent Hypothermia on Atherosclerosis. It would be very nice to see long-term study about the effect of regular 4-7 winter swimming on the cardiovascular risk score, SCORE and others.


Sources


In Silico Screening of the Key Cellular Remodeling Targets in Chronic Atrial Fibrillation. Koivumäki et al. Computational Biology. 
Association Between Sauna Bathing and Fatal Cardiovascular and All-Cause Mortality Events, here


Please, see the reference of the publication for more better view.