The term “tachycardia” means fast heart rate, usually defined in an adult person as faster than 100 beats per minute. An electrocardiogram recorded from a patient with tachycardia is shown in Figure 13–1. This electrocardiogram is normal except that the heart rate, as determined from the time intervals between QRS complexes, is about 150 per minute instead of the normal 72 per minute.
The general causes of tachycardia include increased body temperature, stimulationof the heart by the sympathetic nerves, or toxic conditions of the heart.
The heart rate increases about 10 beats per minute for each degree Fahrenheit (18 beats per degree Celsius) increase in body temperature, up to a body tempera-ture of about 105°F (40.5°C); beyond this, the heart rate may decrease because of progressive debility of the heart muscle as a result of the fever. Fever causes tachy-cardia because increased temperature increases the rate of metabolism of the sinus node, which in turn directly increases its excitability and rate of rhythm.
Many factors can cause the sympathetic nervous system to excite the heart, as we discuss at multiple points in this text. For instance, when a patient loses blood and passes into a state of shock or semishock, sympathetic reflex stimulation of the heart often increases the heart rate to 150 to 180 beats per minute.
Simple weakening of the myocardium usually increases the heart rate because the weakened heart does not pump blood into the arterial tree to a normal extent, and this elicits sympathetic reflexes to increase the heart rate.