Overview

There are three tricks we can play on the body to try and fire parasympathetic reflexes to slow the heart in a patient with tachycardia. These are generally attempted in patients with supraventricular tachycardia, before pharmacological or electrocardioversion options are explored.


The Vasalva monoeuver

Although other historical figures are contenders for the “true inventor” of this manoeuver, involving forced expiratory effort against a closed glottis, the 18th century physician Antonio Maria Vasalva is generally credited (rightly or wrongly). Vasalva recommended it for expulsion of foreign objects in the middle ear. These days, most people are familiar with the manoeuver as an effective way to equalise pressures in the ear after high altitude travel. The changes in intrathoracic pressure that the procedure produces have a complex effect on the cardiovascular system. In tachycardia, it’s used a bit like a hammer against a misfiring engine.

Physicians sometimes get patients to perform the Vasalva manoeuver by asking them to blow hard into a large (say, 20 ml) syringe in order to push the plunger back. This isn’t actually possible (you can’t blow a large syringe plunger back with forced expiration), but provides a competitive incentive for patients to make a concerted effort. The other way to explain it to patients is that it is just like straining on the toilet when constipated.

The increased intrathoracic pressure during the manoeuver compresses the larger veins, reducing venous return to the heart. In the face of reduced venous return, the heart cannot produce sufficient output and arterial blood pressure falls. In response, the sympathetic reflex increase of blood pressure and heart rate is activated. At the end of the manoeuver, when intrathoracic pressure abruptly falls, the high blood pressure and heart rate fire the parasympathetic reflex which slows the heart until blood pressure stabilises. In tachycardia, the Vasalva manoeuver is employed to exploit this parasympathetic phase, in the hope that profound parasympathetic inhibition of the heart will “reboot” the normal rhythm and conduction.


Carotid sinus massage

Baroreceptors in the carotid sinus and aortic arch are important sensors for rapid (<2 sec) reflexes that modulate blood pressure. An acute increase in arterial pressure (to about 150/90) triggers withdrawal of all CVS sympathetic activity (leading to vasodilatation and reduced cardiac output) and increased parasympathetic activity to the heart (bradycardia). Massaging the carotid sinus above the baroreceptors in the carotid body, increases the blood pressure behind the resultant obstruction and fires this reflex. Again, the idea is to “reboot” the heart.


Immersing the face in cold water

Placing your face in cold water activates an evolutionarily ancient parasympathetic reflex to slow the heart (reducing oxygen demand) during diving. Expert diving species such as seals slow their heart down to increase the length of a dive (blood flow is also diverted to keep the muscles and brain functioning). Although we are not generally held by biologists to be a professional diving species, the reflex is present in humans and it can be exploited as another way to attempt to abort tachycardia.



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