Heart Rate

Why does heart rate increase in most types of shock?

Cardiac output (CO) is heart rate (HR) x stroke volume (SV). A reduction in CO causes reduced activity of arterial baroreceptors causing increased sympathetic and reduced parasympathetic activity [11]. Heart rate increases and cardiac output is restored towards normal.

In what types of shock does the heart rate not increase?

An increase in heart rate is an early sign of compensation but its absence does not exclude significant compromise. While the majority of patients with acute blood loss demonstrate the typical tachycardic response, it is important to be aware that as many as 30-35% may present with an initial relative bradycardia, i.e. heart rate not in excess of 100 beats/min [12].

This is in addition to patients on beta-blockers who are pharmacologically prevented from mounting a tachycardia, and those in whom a tachycardia may go unrecognised as their normal resting heart rate is lower than average, for example athletes. Bradycardia may, of course, be the cause of the shock state, for example complete heart block, beta-blocker or calcium antagonist overdose. Neurogenic shock is usually typified by an absence of tachycardia, too.

Learning bite

Absence of tachycardia does not rule out haemorrhagic shock.