Similar pressure receptors are found in the atria. When the atria distend, as in heart failure or overtransfusion, there is a reflex increase in the heart rate to pump the extra blood returning to the heart. When there is a sudden reduction in the pressure in the atria the heart slows. This is called the Bainbridge Reflex and is the cause for the marked bradycardia sometimes seen during spinal anaesthesia. It is best treated by raising the legs to increase the venous return.
Circulatory substances can also affect the heart rate.
Catecholamines, like adrenaline, are released during stress, and will cause an increase in heart rate. Drugs are another common cause of change in the heart rate and most anaesthetic drugs can do this. Halothane affects the SA node and will also depress the force of contraction of the heart. Isoflurane, by contrast has little direct affect on the heart, but causes peripheral vasodilation of the blood vessels. This will then decrease the blood pressure, and hence produce a reflex tachycardia as explained above.