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What is pulsus alternans ?

Pulsus alternans is characterised by high and low volume of pulse which is due to alternate high and low stroke
In pulses alternans the alternate pulse waves are weak that is of low volume. In contrast to the ectopics, the rhythm remains regular. In a patient with severe left ventricular failure, pulsus alternans is said to be present.This is better demonstrated in radial arteries in LVF. This is seen due to the some ventricular muscle fibres are healthy and others are degenerated and so, produces normal and weak beat respectively. 
You should always search for gallop rhythm and basal crepitations when pulsus alternans is felt.
What is the mechanism of pulses alternanas ?
This is a reflex mediated process, initiated by an extra systole in LVF, that is sensed by the sensitive baroreceptors of the Carotid sinus. Left ventricular contraction following an extrasystole is strong as there is more diastolic filling during compensatory pause,  which increase the stroke volume hence the pulse volume is increased. This is sensed by the baroreceptors of the Carotid sinus and in turn send inhibitory impulse to atria via Carotid vagoatrial reflex resulting in weak contraction of atria, ventricular filling is reduced and stroke volume and pulse volume is reduced , which is again sensed by the baroreceptor which send the facilitator impulses via Carotid sympathetic atrial reflex resulting in increased atrial contraction and ventricular filling is increased  and also stroke volume is increased hence pulse volume is increased. This reflex activity occuring alternatively producing high and low pulse volume.
How will you demonstrate the Pulses alternans ?
It can be can be felt by palpating fingers but definite demonstration is by sphygmomanometer. Occlude the pulse by raising the pressure, then you should slowly reduce the pressure. Initially the Korotkoff sounds due to the passage of the high volume pulse is heard, further reduction will allow the passage of the weak beat also.
This will produce sudden doubling of the Korotkoff sounds called  as  Gallavardin sign.