sick sinus syndrome
The sinus node is the heart's own master clock, setting a steady pace and speeding up or slowing down to match the body's needs. In sick sinus syndrome that clock becomes unreliable: it may run too slowly, pause for long stretches, fail to speed up during exercise, or swing erratically between slow and fast. The result is a metronome that keeps losing and gaining time.
Symptoms flow directly from these failures — fatigue, light-headedness, breathlessness on exertion, or fainting during a long pause. A particularly characteristic pattern is the “tachy-brady syndrome,” where bursts of a fast atrial rhythm such as atrial fibrillation alternate with dangerously slow rates, often with a long pause when the fast rhythm stops.
It is largely a disease of the ageing conduction system, more common in older people. Because the underlying tissue cannot be repaired, treatment of symptomatic disease is a permanent pacemaker to guarantee a minimum rate; rate-controlling or rhythm drugs may still be needed for the fast episodes, which the pacemaker then makes safe to use.
An older woman has spells of fatigue and one faint; a multi-day monitor reveals long sinus pauses alternating with bursts of atrial fibrillation, and a pacemaker plus a rate-control drug resolve her symptoms.
Alternating fast and slow rhythms with pauses is the hallmark of tachy-brady syndrome.
The tachy-brady pattern is a trap: drugs used to slow the fast episodes can worsen the slow ones, which is why a pacemaker often has to come first.