Which is the scariest tachyarrhythmia?
Is it ventricular tachycardia (VT)? Maybe atrioventricular re-entrant tachycardia (AVRT)? How about pre-excited atrial fibrillation (AF)?
I personally find the following tachyarrhythmia to be the scariest:1
It’s scary because it’s not straightforward
You may perceive something as scary because you have been repeatedly told so or because you can see fear in others' eyes when they talk about it (like VT).
You may find something scary simply because it is uncommon, and you are not prepared to deal with it (like pre-excited AF).
However, in all of these circumstances, there are established algorithms you can follow (see Advanced Life Support - ALS) that offer straightforward treatments. These treatments can buy you time until a specialist arrives to take over. Worst case scenario, a tachyarrhythmia that leads to shock needs to be shocked!
In sinus tachycardia (which is the rhythm in the ECG shown above), you don’t have straightforward algorithms. Because sinus tachycardia is not really something you treat but something you investigate.
Don’t get me wrong; other tachycardias need to be investigated as well. But specialists won’t take over to investigate sinus tachycardia.
It’s the cause that matters!
Let me tell you my approach to thinking about the most important causes of sinus tachycardia.
In my post about the 4 ECG findings you MUST look for in bradycardias, I mentioned the following equation that described tissue oxygen delivery2:
Where:
DO2: Tissue oxygen delivery
CO: Cardiac output
CaO2: Oxygen concentration (arterial)
Imagine you have trucks carrying boxes from point A to point B3. You want to calculate how many boxes arrive at point B per unit of time (oxygen delivery). What do you need to know?
How many trucks arrive at point B per minute (cardiac output)
How many boxes are in each truck (oxygen concentration)
Higher cardiac output means more trucks arriving at point B per minute:
Higher oxygen concentration in the arteries means more boxes per truck:
To get more insight into the causes of sinus tachycardia, we need to dive deeper into this equation.
Cardiac output (CO) and arterial oxygen concentration (CaO2) are described by the following formulas:
Cardiac output (CO):
Where:
HR: heart rate
SV: stroke volume (i.e. the volume of blood ejected from the heart in each heartbeat)
Oxygen concentration (arterial) (CaO2):
Where:
1.31: Hüfner's constant (the maximum oxygen-carrying capacity per gram of haemoglobin)
Hb: haemoglobin (Hb) level (g/dl)
SaO2: oxygen saturation (arterial) (expressed in decimal form)
0.0225: solubility coefficient of oxygen at body temperature (the number of millilitres of oxygen dissolved per 100 ml of plasma per kPa)
PaO2: partial pressure of oxygen (arterial) (kPa)
As you can see from the coefficient in the dissolved oxygen part of the equation (0.0225), bound oxygen plays a tremendously more important role in oxygen delivery. However, although you may think that SaO2 is more important than PaO2, for oxygen delivery, based on this equation, remember that these two variables are directly related.
Taking all of the above into account, we arrive at the following equation for oxygen delivery:
Therefore, if you think about the pathologies that cause a reduction in any of the variables in this equation (except for the HR, of course), you can figure out why your patient has sinus tachycardia.
So, many of the most common and important causes of sinus tachycardia have to do with the fact that they decrease oxygen delivery, and sinus tachycardia is a compensatory response (as can be seen in the equation above).
Would you use this approach when considering the causes of sinus tachycardia?
Other causes not explained by the equation above:
Endocrine causes (e.g. hyperthyroidism and pheochromocytoma)
Stimulants (e.g. nicotine and caffeine)
Abrupt withdrawal of medications such as beta blockers
Fever
Pain
Anxiety
Sleep deprivation
Another way to think of the broader spectrum of causes (less specific) is to think about sinus tachycardia as the result of an autonomic response (for example, an autonomic response to pain).
Although this is beyond the scope of this post, there are rare conditions where actually controlling sinus tachycardia with beta-blockers might be beneficial, like in the case of inappropriate sinus tachycardia (requires specialist input).
Summary
Overall, investigating and treating the underlying causes is key for managing sinus tachycardia.
Since it is a common condition, I find the oxygen delivery equation helpful in my thought process for investigating sinus tachycardia. This way, I can remember to consider all the important causes.
And this is also a reminder that sinus tachycardia is NOT treated with bisoprolol (most times)!
Ewingdo, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
J-OC Dunn, MB ChB BAO FRCA, MG Mythen, MBBS MD FRCA FFICM FCAI (Hon), MP Grocott, BSc MBBS MD FRCA FRCP FFICM, Physiology of oxygen transport, BJA Education, Volume 16, Issue 10, October 2016, Pages 341–348, https://doi.org/10.1093/bjaed/mkw012
This analogy is attributed to my professor in medical school, Prof. Dimitrios Georgopoulos. He always used analogies like this to make his teachings more memorable!