Sinus Rhythms | Too fast, too slow and just right

Sinus rhythms

When we talk about sinus rhythms, we’re not just talking about “normal sinus rhythm.” We’re actually talking about the standard rhythms that originate in the sinus node…could be too fast, could be too slow and could be juuuuuuust right.

As you recall from your physiology class, the sinus node is the main pacemaker for the heart. This is the one that we WANT to be working 24-7, 365 days a year. It’s intrinsic rate is 60-100 BPM and it’s what you expect to see in a healthy, uncompromised patient. So far so good?

The Components of a Sinus Rhythm

In order for a rhythm to be deemed a “sinus rhythm” it has to meet two criteria all the time, every time:

  1. All the P waves have to look exactly the same (we call this the wave’s “morphology” when we are referring to its shape); AND…
  2. All the PR intervals have to be the same (the length between the P wave and the QRS). If your rhythm doesn’t meet these two simple criteria, you can kick it out of the “sinus rhythm” club.

Sinus Tachycardia

When the heart is beating at a rate greater than 100 BPM, and the rhythm originates in the SA node, and meets those two criteria listed above, we say that the patient is in “sinus tachycardia.”  A great example of sinus tachycardia occurs when you run up a flight of stairs to get to class. Your heart rate goes up, but it’s a normal physiological response to your increased energy expenditure. Your P waves are all the same, your PR intervals are consistent…your heart is simply beating faster than 100 BPM.

  • Rate: Above 100 (if the rate is really fast…above 180-ish, then you typically won’t be able to see the P waves. In this case, we refer to it as one of the “supraventricular tachycardias”…more on these in another post.)
  • Regularity: Yep…it’s regular! To determine this, check out your R to R interval and march them out. If they vary by more than a box or two…something else is likely going on…(a-fib maybe?)
  • P waves: Yep…every QRS has a P wave
  • QRS: Normal width (0.08-0.11) or possibly slightly wider due to rate-related aberrancy or electrical alternans (but those are advanced topics so we’re not going to go into them here).
  • It basically looks like normal sinus rhythm (NSR) only faster. Easy!

What causes sinus tachycardia?

  • Physical exertion
  • Fever
  • Hypoxia
  • Stress, pain
  • Dehydration or low-volume states (blood loss)
  • Infection
  • Hyperthyroidism
  • Acute MI
  • Drugs, stimulants, caffeine

What are we going to do about sinus tachycardia?

With most sinus tachycardias, you can usually treat the underlying cause as your main line of attack. For example, if the patient has a fever, give ’em some Tylenol. If they’re hypoxic, toss on some oxygen. If they’re dehydrated, give ’em some volume. So, the question you’re probably asking is when do we get nervous?

In general, you can consider the “max heart rate” for most people to be 220 minus their age in years. So, if your patient is 50, then their “max” is 170…anything higher than that is going to cause some major concern. If your patient is 85, then you’re max is going to be more like 135…and how often do you see folks plugging along at 145? Exactly…you gotta get in there and do something about it! Super Nurse to the rescue!

The other thing you want to look at is the patient’s blood pressure. When the heart is beating quickly, filling times are reduced which can lead to a drop in cardiac output. Compare your patient’s blood pressure to what it was before they became tachycardic (if the tachycardia is new). If the patient came in tachycardic or “he’s been tachy all night” (my BIGGEST PET PEEVE) then simply look at what their blood pressure is now. If the MAP is at least 65 then you’re probably OK (for now, at least). If the BP is low, then the patient is not handling the tachycardia well and you HAVE  to do something about it ASAP!

Step 1: Get some oxygen on the patient and get them in bed if they’re out of bed. You don’t want them to exert themselves any more than necessary.

Step 2: Determine the underlying cause if you can. Are they post surgical? Maybe they’re having pain OR maybe they’re bleeding OR maybe they’re sprouting an infection. Your patient might need some fluid, blood products or a sepsis workup. Put your detective hat on and have at it, Watson!

Step 3: Consider meds…patient may need a beta blocker like metoprolol or a calcium channel blocker like nicardipine or diltiazem. Typically you’ll see orders for metoprolol 5mg q 10 min x 3 and then to let the MD know if it doesn’t work. The patient may do OK with a diltiazem push or may need to go on a nicardipine gtt. Note that all of these meds are going to also cause a decrease in blood pressure, so make sure the MD knows the full clinical picture before you go giving any of these medications (of course you would…I know I didn’t have to say that).

Most of the time, sinus tachycardia is pretty easy to treat by simply tackling the underlying cause, or giving a little bit of medication to control the rate. And it’s not nearly as scary as it’s cousin…sinus bradycardia.

Sinus Bradycardia

Sinus bradycardia occurs when your sinus rhythm is below 60 bpm.

  • Rate: Below 60
  • Regularity: Yes…your R-to-R intervals all match up
  • P waves: You betcha…every QRS has a P wave
  • QRS: Normal width (0.08-0.11)
  • It basically looks like normal sinus rhythm (NSR) only slower.

What causes sinus bradycardia?

  • Athleticism…many endurance athletes will have a resting heart rate in the 40s-50s. If your patient looks really fit and has a low resting heart rate, be sure to ask them what they usually run. If their heart rate is typically low, they’ll usually know.
  • Hypothyroidism
  • Sick sinus syndrome (a disease of the SA node)
  • Certain medications (beta-blockers, digoxin, calcium-channel blockers)
  • Increased vagal tone, which stimulates the parasympathetic nervous system.
  • Hypothermia
  • Increased intracranial pressure
  • Acute myocardial infarction

What are we going to do about sinus bradycardia?

The very first thing I do when I notice my patient “bradying down” is ascertain if they are symptomatic. Get in the room and LOOK at the patient. How’s their skin color, their LOC? Run a quick blood pressure while you pop on some O2. If they are out of bed and you can get them back into bed without exerting them do it. Get help if you need to.

Step 1: Determine if symptomatic. If they are, then pop on some oxygen, get them back in bed if you can do so without causing further deterioration.

Step 2: Determine the underlying cause. Lots of time, patients will “vagal” which causes the sudden onset of bradycardia. When this occurs, the bradycardia is usually self-limiting, but sometimes not. If they’re sleeping, go in and wake them up if it makes you feel better…watch to see if their heart rate comes up (and if you get cursed at in the process). If they’re complaining of chest pain…get some help in there STAT, he may be having an MI.

Step 3: Consider medication…the ACLS procedure for symptomatic bradycardia is atropine 0.5 mg. If they’re crashing, you’re going to go straight to electricity and pace that patient (to pace, make sure you put the patches on front/back instead of front/side).

My rhythm looks sinus, but it’s a little irregular…what is it?

If you’ve got what looks like normal impulses that originate in the SA node, but your rhythm is still irregular then a couple of things could be going on:

Sinus rhythm with PACs: You might simply have a sinus rhythm with PACs (premature atrial contractions). Those extra QRS complexes are going to make your overall rhythm look a bit irregular. How do you know the irregular beat is a PAC? Take a look at the P wave…it’s going to look different than your other P waves (in other words, it will have different morphology). With PACs you essentially just monitor your patient and make note of how many PACs they are having. If they start having a large number of PACs, you probably want to check for an underlying cause: stimulants, heart disease, hypertension, digitalis toxicity and abnormal K or Mag levels. But, for the most part, PACs don’t cause problems and your patient will rarely notice they are having them.

Sinus arrhythmia: This is a “normal” irregular rhythm that is often seen in kids and young adults. If you look closely at the rhythm, you’ll see that it speeds up and slows down slightly in variation with the respiratory cycle. If you’ve measured out your R-to-R intervals, and there’s more than a 0.16 variation, take a look at it in relation to the respirations…if it follows a pattern, then you most likely  have sinus arrhythmia. It is completely benign and should be no cause for concern.

Sinus Block, Pause or Sinus Arrest: Occasionally, the ol’ ticker just needs a little rest. Ok, just kidding (making sure you’re still reading!). However, a sinus pause or sinus arrest is basically a period where there is no conduction occurring and there’s a pretty decent gap in between sinus beats.

  • In a Sinus Block, there’s a total breakdown in the conduction. The sinus node does not trigger the atria to depolarize. It just doesn’t happen. It can be for one beat, or even two, three or more…but the biggest tip is that your P to P interval will remain consistent. So, if your P to P interval was 2 large boxes and you had a block of one beat, that interval would be consistent (4 large boxes). When the sinus node finally decides to come back to work, it will show up right on schedule.
  • In Sinus Pause, the impulse in the sinus node is delayed, so what you’ll see is a longer-than usual P-P interval…when the sinus node decides to show up for work again, it might work at a faster rate, the same rate as it was previously or a slower rate.
  • With a Sinus Arrest, the period of non-conductivity is typically longer. If you’re trying to differentiate a Sinus Arrest from a Sinus Block, note that in Sinus Arrest, the P-P interval will not be a multiplier in Arrest (like it is in Block). And, if you’re trying to differentiate a Sinus Pause from a Sinus Arrest, the general rule of thumb is that a Sinus Pause becomes a Sinus Arrest when the pause is longer than 3x the P-P interval.

In our next post we’ll go over even more atrial rhythms, including atrial fibrillation and atrial flutter…so check back for more!

 

 

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