oukyuuteate ni saisite, kyuukyuutai wo yobu.

A natural reading is:

• 応急手当 = おうきゅうてあて
• に際して = にさいして
• 救急隊 = きゅうきゅうたい
• を呼ぶ = をよぶ

So the full sentence is read:

おうきゅうてあて に さいして、きゅうきゅうたい を よぶ。

に際して is a formal grammar pattern that means something like:

• when ...
• on the occasion of ...
• at the time of ...
• in dealing with ...

Here, 応急手当に際して means when giving first aid or in a first-aid situation.

It is more formal and official-sounding than a simple とき.

Yes. This is a set pattern:

• Noun + に際して
• sometimes also Verb dictionary form + に際して

So:

• 応急手当 is a noun
• に際して attaches to that noun

That is why it is 応急手当に際して.

You do not need の here. This is just how the grammar works.

に際して sounds more formal, official, and written.

Compare:

• 応急手当をするとき、救急隊を呼ぶ。
  = more plain and everyday
• 応急手当に際して、救急隊を呼ぶ。
  = more formal, like a manual, safety notice, or official instruction

So if this sentence appears in training material, instructions, or a handbook, に際して makes sense.

応急手当 means first aid or emergency treatment.

It is a noun, not a full verb phrase by itself.

So:

• 応急 = emergency / temporary response
• 手当 = treatment / care

Together, 応急手当 means the act or process of giving first aid.

That is why it can be followed directly by a grammar pattern for nouns, like に際して.

Because 呼ぶ takes a direct object marked by を.

In Japanese, 呼ぶ can mean:

• to call
• to summon
• to send for

So 救急隊を呼ぶ means call/summon the emergency team.

The thing being summoned is treated as the direct object, so it gets を.

Here it means to call for / summon them, not just to say their name out loud.

呼ぶ has a wider meaning than English call sometimes does. It can mean:

• call someone over
• send for someone
• summon help

So in this sentence, 救急隊を呼ぶ means contact emergency services / summon the rescue team.

Not exactly.

• 救急隊 = the emergency team / paramedics / rescue crew
• 救急車 = the ambulance vehicle

In real-life Japanese, people often say 救急車を呼ぶ for call an ambulance.

So 救急隊を呼ぶ is understandable, but depending on context, 救急車を呼ぶ may sound more common in everyday speech.

That is normal in Japanese. The subject is often omitted when it is obvious from context.

In a sentence like this, the implied subject is usually:

• you
• the person reading the instructions
• or one should

So the sense is something like:

• When giving first aid, call the emergency team.

Japanese often leaves that unstated.

The dictionary form 呼ぶ is often used in:

• manuals
• safety instructions
• headings
• procedural steps
• concise written directions

It can sound like a neutral instruction rather than a casual statement about what someone happens to do.

So even though 呼ぶ is the plain dictionary form, in this kind of context it can function like:

• call the emergency team
• you should call the emergency team

A fuller, more direct instruction would be:

• 救急隊を呼んでください。

The comma separates the introductory phrase from the main action.

So the sentence structure is:

• 応急手当に際して、 = when giving first aid,
• 救急隊を呼ぶ。 = call the emergency team.

The comma makes it easier to read and shows where the background condition ends and the main instruction begins.

Some parts of Japanese word order are flexible, but this order is natural and clear.

This sentence follows a common pattern:

• situation/condition first
• main action second

So:

• 応急手当に際して、救急隊を呼ぶ。

is a very normal structure.

You could rearrange things in some contexts, but for an instructional sentence, this order is the most straightforward.

Yes, it is understandable and grammatically fine, especially in a formal or instructional context.

That said, depending on the exact situation, native speakers might more commonly say something like:

• 応急手当をするときは、救急車を呼ぶ。
• 応急手当に際しては、救急車を呼ぶ。
• 必要に応じて救急車を呼ぶ。

So the original sentence is fine, but it has a somewhat formal, written, manual-like feel.