A patient is found to have unexpected tachycardia.  What is the cause?  We use associated symptoms and signs to guide the diagnostic approach.  When that isn’t clear, a physiologic approach might enhance bedside diagnostic reasoning:


Cardiac output (CO) = stroke volume (SV) x heart rate (HR)


Although the simple equation in a living organism belies much deeper complexity, we can reason that an increase in heart rate means one of three things:


1. Stroke volume is decreased

a. Decreased preload (hypovolemia, pulmonary embolism)

b. Decreased inotropy (congestive heart failure)

c. Increased afterload (hypertensive congestive heart failure)

2. Cardiac output is increased

3. Primary disturbance in heart rate


Using this approach generates its own differential diagnosis, and may help in subtle presentations.


Take home points:

-CO = SV x HR

-An increased heart rate suggests increased cardiac output or decreased stroke volume


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