A 65-year-old gentleman arrives in your emergency department and is rapidly diagnosed with sigmoid diverticulitis and a free perforation. His BP is 80/60 with a heart rate of 120 and a temperature of 39°C. You initiate goal-directed therapy for septic shock with two boluses of 500 mL of Ringer’s lactate and place a central venous line. As your catheter enters the right atrium, his heart rate drops to 30 and, to your surprise, this is a “sinus” bradycardia. You infuse 0.6 mg atropine for its vagolytic effect and his rate returns to 110.
Following successful resuscitation and within 45 minutes you are in the operating room with the patient. You are not surprised when, with intubation, he becomes bradycardic again. While you call for some additional medication, you double check to make sure that the pacing pads are placed appropriately.
1. What heart rate is “too slow?”
2. What is the most likely reason the patient became bradycardic a second time?
Your patient’s heart is just a ball of muscle with some electrical wiring in it that tells it when to beat. If the heart rate is either too fast (traditional rule of thumb is 220 minus your patient’s age) or too slow (by definition, less than 60), you should be able to improve cardiac output with better rhythm control. Problematic fast heart rates are much more common than problematic slow heart rates. This chapter will discuss what to do for patients whose heart rate is too slow. Other chapters will deal with the topic of heart rates that are too fast.
“Too slow” is not the same thing as bradycardia. Bradycardia is a definition; “too slow” is a clinical assessment. Bradycardia is defined as a heart rate less than 60, while “too slow” is any heart rate that does not adequately preserve cardiac output. Remember:
Cardiac output = heart rate × stroke volume
An Olympic triathlete probably has a massive stroke volume and might be able to perfuse his or her end organs with a heart rate of 30. But a 95-year-old diabetic man with coronary artery disease, hypertension, and a dilated cardiomyopathy doesn’t have a lot of inotropic reserve. Instead, he relies on his heart rate to increase his cardiac output—and even though 60 is not defined as bradycardia it might still be “too slow” for him. In the presented patient with freely perforated diverticulitis, a heart rate of 30 is both bradycardic and “too slow.”
You can use your understanding of “too slow” when evaluating postoperative patients as well. For example, when a nurse calls you about a patient who has a heart rate of 55, the most important thing is not—surprise, surprise—the number. Instead, you must make a clinical assessment of the patient to determine if the heart rate ...