Sinus tachycardia is a deceptively difficult rhythm to interpret in some cases. I like to think of it like the chameleon of arrhythmias because it can appear at times like many rhythms. You should approach impersonators with respect and trepidation because you never know what you are going to get.
The definition is simple. Sinus tachycardia is a rapid sinus rhythm that is above 100 beats per minute (BPM), typically seen at or below 160 BPM, but can reach rates as high as 220 BPM. The more rapid the rhythm, the more this chameleon changes colors. The main cause of the morphologic changes is due to the fact that the higher the rate, the shorter the PR, QRS, and QT intervals and the more compact things become. At very rapid tachycardia, the P waves can actually be buried in the previous T wave. This can give the appearance of atrioventricular (AV) nodal reentrant tachycardia.
Another trick that sinus tachycardia has up its sleeve is that at rapid rates, the right bundle branch could be caught in a refractory state, causing an aberrantly conducted morphology that could be confused with ventricular tachycardia or the other wide-complex tachycardias.
The maximum heart rate is calculated by the following formula: Max heart rate = 220 − Age. In general, young, healthy patients can tolerate pretty high rates. However, the max heart rate for a 60-year-old man when calculated by the formula is 160 BPM. This can be, once again, well tolerated by a healthy 60-year-old with no serious medical complications. But, what happens if the man has a 95% occlusion of the left anterior descending coronary artery or is severely anemic? The cardiac reserves for that patient may be very low, and a heart rate of 160 BPM, even at rest, could kill him. What if his ventricular wall is thickened because of long-standing hypertension? The decreased filling times and pressures of a heart rate of 160 BPM could produce a massive decrease in cardiac output. The decreased cardiac output could lead to many complications, including death.
This brings me to one of my father’s favorite clinical sayings: Always look at the company it keeps. To put it simply, look at the other things occurring in your patient at the same time as the rhythm disturbance or medical complication. Those are the greatest bedside predictors of morbidity, mortality, and the seriousness of the clinical scenario. If you are told that a 60-year-old patient has a heart rate of 160 BPM, take a quick look at your patient. Is he in distress? Is he using accessory muscles of respiration? Does he look pale or cyanotic? Is he mentating? Do the patient’s fingers show normal blanching when you apply pressure? Is he making urine? Does he have chest pain? Symptomatic shortness of breath? Palpitations? There are more questions, but we’ll leave it there. If your answer to any of these questions is troubling, treat the underlying cause immediately. Do not just say, “Well, he’s in sinus tach. He’ll be alright.”
I’m going to close this section with the two most important clinical pearls I’ve ever heard: “Always follow your clinical gut” and “always look at the company it keeps.”
—Daniel J. Garcia