Why a racing heart can end up pumping less blood, not more
In adults, a resting pulse above 100 beats per minute counts as tachycardia. A fast heart during exercise is normal, but when the rate climbs too high the heart pumps less efficiently, delivers less blood to the body and to itself, and demands extra oxygen, which can starve its own muscle.
Doctors sort tachycardias on an electrocardiogram by the width of the QRS complex, the spike marking each ventricular beat. Narrow complexes, lasting 0.1 seconds or less, usually start in the atria and are called supraventricular; wide ones usually arise in the ventricles. Each kind can also be regular or irregular. The most common form is sinus tachycardia, driven from the heart's natural pacemaker, the sino-atrial node near the superior vena cava.
Often the fast rate is a response rather than a fault. When blood pressure drops, from dehydration, bleeding or standing up too quickly, the heart speeds up to compensate, a pattern called reflex tachycardia. Fever, diarrhoea and severe infection raise metabolic demand. Stress triggers sympathetic nerves and adrenaline in the fight-or-flight response, and stimulants such as ephedrine, amphetamines or cocaine have the same effect, as can hyperthyroidism or an adrenaline-secreting tumour called a pheochromocytoma. A rough upper limit for sinus rate is 220 beats per minute minus a person's age.
Some metabolic muscle diseases produce a telling pattern. In McArdle's disease, muscles cannot readily draw on stored glycogen, so the heart races abnormally early in exercise; after about 6 to 10 minutes of gentle activity, a second wind arrives and the rate falls.
Ventricular tachycardia is the serious end of the spectrum: usually a regular, wide rhythm at 120 to 250 beats per minute, it can tip into ventricular fibrillation if it persists. A variant called torsades de pointes, French for twisting of the points, tends to follow a long QT interval. Very fast flow also adds turbulence in vessels, one of the three conditions in Virchow's triad that favour blood clots.
Source: Tachycardia
