معدل ضربات القلب منخفض اليوم.

In the present tense, Arabic usually does not use a verb for to be.

So:

معدل ضربات القلب منخفض اليوم
literally looks like: the heart rate low today

But in natural English, we translate it as: The heart rate is low today.

This is a very common Arabic pattern called a nominal sentence.

The sentence is a nominal sentence made of:

• معدل ضربات القلب = the heart rate
• منخفض = low
• اليوم = today

So the structure is:

• subject/topic (مبتدأ): معدل ضربات القلب
• predicate/comment (خبر): منخفض
• time expression: اليوم

So the whole sentence means: The heart rate is low today.

It is an iḍāfa (possessive/genitive chain), and literally it breaks down like this:

• معدل = rate
• ضربات = beats
• القلب = the heart

So:

• ضربات القلب = the heart’s beats / heartbeats
• معدل ضربات القلب = the rate of the heartbeats = heart rate

This is a very common way Arabic builds noun phrases.

Because in an iḍāfa construction, the first noun usually does not take الـ.

So Arabic says:

• معدل ضربات القلب

not:

• المعدل ضربات القلب ❌

The whole phrase becomes definite because the last noun is definite:

• القلب = the heart

That makes the whole chain understood as a definite phrase: the heart rate.

Because they are inside an iḍāfa chain, where the nouns after the first one are in the genitive relationship.

With full case endings, the sentence can be written as:

مُعَدَّلُ ضَرَباتِ القَلْبِ مُنْخَفِضٌ اليَوْمَ

Here:

• معدلُ is nominative because it is the subject
• ضرباتِ is genitive because it is linked to معدل
• القلبِ is genitive because it is linked to ضربات

Even if you do not pronounce the case endings in everyday speech, the grammatical relationship is still there.

Because منخفض agrees with the head noun معدل, not with ضربات.

The main noun here is:

• معدل = rate

And معدل is:

• masculine
• singular

So the predicate is also masculine singular:

• منخفض = low

Even though ضربات is plural, it is not the main noun of the whole subject phrase.

منخفض means low, reduced, or decreased, depending on context.

In this sentence, it means:

• low
• or lower than normal

So معدل ضربات القلب منخفض means the heart rate is low.

It is an adjective, but in this sentence it functions as the predicate.

اليوم means today, and it gives the time of the statement.

So:

• منخفض اليوم = low today

Arabic often uses words like اليوم directly, without a preposition.
So you do not need في here.

This is very natural Arabic.

Yes, it can move, because Arabic word order is somewhat flexible.

These are all possible:

• معدل ضربات القلب منخفض اليوم
• اليوم معدل ضربات القلب منخفض
• معدل ضربات القلب اليوم منخفض

The version you were given is very natural and straightforward. Putting اليوم at the end often sounds neutral and clear.

A careful MSA pronunciation with case endings is approximately:

muʿaddalu ḍarabāti l-qalbi munḫafiḍun al-yawma

Without full case endings, which is closer to how many people would actually say it, you might hear something like:

muʿaddal ḍarabāt il-qalb munḫafiḍ il-yawm

A few sound notes:

• ع in معدل is a deep throat sound with no exact English equivalent
• خ in منخفض is like the ch in German Bach or Scottish loch
• ق in القلب is a deep q sound, not an English k

Yes. A very common alternative is:

معدل نبضات القلب منخفض اليوم

Here:

• نبضات = pulses / beats

This is often a little more specifically associated with pulse/heartbeat language, while ضربات literally means beats or strikes and is also commonly used.

Both are understandable and natural in medical or general contexts.

A fully vowelled MSA version is:

مُعَدَّلُ ضَرَباتِ القَلْبِ مُنْخَفِضٌ اليَوْمَ

Breakdown:

• مُعَدَّلُ = nominative
• ضَرَباتِ = genitive
• القَلْبِ = genitive
• مُنْخَفِضٌ = nominative predicate
• اليَوْمَ = accusative adverb of time

In normal unvowelled writing, however, you will usually just see:

معدل ضربات القلب منخفض اليوم

Yes, it is natural and correct.

It sounds like standard written Arabic, especially in a medical, health, or formal context.

Other natural MSA variations are possible, such as:

• معدل نبضات القلب منخفض اليوم
• نبض القلب منخفض اليوم

But the given sentence is perfectly good Modern Standard Arabic.