As mentioned before, the heart rate is determined by a constant tug-of-war between the sympathetic and the parasympathetic divisions of the autonomic nervous system. If the parasympathetic dominates, the rhythm is slowed. If the sympathetic dominates, the rhythm is sped up. Sinus tachycardia is, therefore, just a physiologic response to the autonomic stress and should not really be considered a pathologic rhythm. Let’s clarify that statement.
Cardiac output is maintained by heart rate times stroke volume (amount of blood ejected by the heart during each mechanical contraction). Stroke volume depends on the mechanical filling of the heart, which is both a passive process and an active process. The heart passively fills when the AV valves open and the blood from the atria floods into the ventricular chambers. The atria actively “overfill” the ventricles when the atria mechanically contract. This pushes the remaining atrial contents into the already filled ventricles, the so-called atrial kick.
Tachycardias cause a decrease in cardiac output when their rates become so high as to affect the stroke volume. Tachycardias decrease the amount of passive filling time that is available because of a shorter diastolic phase, and they decrease the amount of blood that is expelled during the atrial kick. In other words, as the amount of time needed to fill the ventricles decreases with increasing heart rates, the less the ventricles are filled. This decreased ejection fraction lowers the stroke volume, which in turn drops the cardiac output and blood pressure.
In general, tachycardias are an abnormality of the rhythm that are caused by some inherent problem or set of circumstances in the heart itself. Sinus tachycardia, as we have seen, is not due to some inherent cardiac problem (in the vast majority of cases) but to some general stressor that stimulates sympathetic activity. Therefore, we should treat tachycardia by treating the underlying cause and not the rhythm itself. Put another way, treat what is causing the sinus tachycardia and not the tachycardia itself.
Let’s look at a couple of examples. Fevers cause sinus tachycardia. Should you give beta-blocking agents like propranolol to everyone with a fever? Of course not! You can treat it simply by giving acetaminophen and bringing the fever down a bit. Hypoxemia causes sinus tachycardia. How do you treat it? By giving supplemental oxygen. If a person is having a myocardial infarction (MI), should you give beta-blocking agents to slow the heart down? Yes, as long as there are no contraindications! Why? Because tachycardias cause an increase in myocardial oxygen demand that can make the heart deteriorate further in this circumstance. But notice, we give the beta blockers not as a treatment of the tachycardia in sinus tachycardia but to protect the myocardium. How do you treat the tachycardia? Stop the infarction.
This is an important concept to understand before we go on to discuss the other tachycardic rhythms. Generally, you should focus your clinical attention or treatment directly at the tachycardia. Sinus tachycardia is the exception to the rule. Don’t treat the arrhythmia, treat the cause.