There is no need to add ANDs into your search strategy. PubMed will automatically add these when it interprets your search.
If PubMed has failed to map your search terms to MeSH, try using the search operator OR to add synonymous search terms to see if this improves your results. You should use brackets to nest your synonymous search terms to ensure PubMed carries our your search correctly
e.g. (aspirin OR acetylsalicylic acid) (heart attack OR stemi)
The words AND and OR are used in searching to combine search terms together.
OR retrieves records which contain any of your concepts. For example, you might search for cognitive behavior therapy OR CBT to retrieve all articles on the topic.
PubMed uses a process called Automatic Term Mapping to determine what you are looking for and matches this to subjects (using MeSH). This helps to expand and improve the quality of your search. PubMed also automatically searches for plural forms and British/American translations.
When you search PubMed it automatically looks for papers covering all the search concepts you enter. However, PubMed does not simply search for the exact words you use. Instead it 'translates' your terms in a sophisticated way, and searches for Medical Subject Headings (MeSH), as well as textword terms (words found anywhere in the record, such as the title or abstract).
You can see exactly how PubMed carries out your search by clicking on the Advanced link just below the simple search box. Scroll down and you will see that PubMed stores your History and Search Details.
To view how PubMed interprets your search, click on the arrow under Details:
PubMed has done quite a lot of complex work behind the scenes. You can see that each word has been searched as a phrase as well as individual words. Synonymous search terms and relevant MeSH terms have also been identified and searched for. For this example you can see that PubMed has translated heart attack into the MeSH term myocardial infarction.
It is always worth checking the Search Details after you carry out a search to make sure PubMed has interpreted your search terms correctly and has identified relevant MeSH terms.
(Created April 21, 2020): https://youtu.be/MQ-ttxlkN2U