Question Summary:
Increased heart beat in Salaah?

Question Detail:

What's the cause of increased heart beat during Salah? Good or bad? It makes me uneasy sometimes as to why its goes so fast but makes me concentrate better the other times. Shaytan Puts evil thoughts, that one does not want to think, yet they just come. 

Answer :

In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
Your first query is health and medical related. You may consult your physician for diagnosis. 
You also enquire regarding Waswasa (Evil whispers and thoughts from Shaytan). From the outset we should understand that Allah Ta’ala has given Shaytan the power of whispering into the heart of a person However, there is no need to become worried and perturbed about them; the whisperings should merely be ignored. Furthermore, one should adopt the following ways to overcome Waswasa:
1. Whenever any doubt/Waswasa arises, tell yourself that your Salah, Wudhu, Ghusl and everything else is correct and valid no matter what evil whisperings enter the mind. As a remedy, tell yourself that every action of mines is valid irrespective of the evil whisperings. This may difficult for the first few times, it will become much easier after effort and hard work.
2. Remind yourself constantly that these thoughts are from Shaytan, how can I listen to them and give them credence? How can they be correct and authentic?
3. Implore Allah Ta’ala in Du’aa to alleviate you from your predicament. Du’aa is the most powerful weapon of the believer.
4. All that is needed is resolute courage and firm determination to extricate oneself from such a problem. Never give up hope. Always tell yourself that there is light at the end of the tunnel.
5. Recite  أعوذ بالله من الشيطان الرجيمexcessively.
6. Recite لا حول و لا قوة الاالله excessively.
7. Recite Surah Falaq, Surah Naas and Surah Ikhlaas thrice as much as possible.
8. Allah Ta’ala says in the Qur’an:                                              
الا بذكر الله تطمئن القلوب
“Behold! The hearts attain tranquillity through the Zikr of Allah” (13:28) 
The above verse expounds on the importance of Zikr (Remembrance of Allah Ta’ala). Make excessive Zikr and Insha’Allah, your heart will find tranquility and peace from such thoughts. Shaytan flees from the person who makes Zikr. 
9. Read the book “Combating the whisperings of Shaytan” by Hadhrat Moulana Yunus Patel Sahib (Rahmatullahi Alayhi). Kindly refer to the following link to download the book:
http://islamicexorcism.files.wordpress.com/2010/10/combating-the-whisperings-of-shaytaan.pdf
10. We also advise you to listen to the talk of Mufti Ebrahim Desai Sahib (Hafidhahullah) on the same subject. Kindly refer to the following link to download the talk: 
http://www.daruliftaa.net/index.php/resources/downloads/finish/58-2013/571-07consider-the-shaytwaan-as-your-enemy-aug-1-2013-11-59-50-am
 
And Allah Ta’āla Knows Best
Abdullah ibn Mohammed Aijaz
Student Darul Iftaa
Baltimore, USA
Checked and Approved by,
Mufti Ebrahim Desai.