Sinoatrial node
Pacemaker · 60–100/min RCA ~60% LCx ~40%
Crescent of specialised pacemaker myocytes at the junction of the superior vena cava and right atrium, lying subepicardially in the upper sulcus terminalis. It is the dominant pacemaker because its phase 4 slope is the steepest, so it reaches threshold first and overdrive-suppresses every subordinate site.
Detail
- Intrinsic rate
- 60–100/min: phase 4 'funny' current (I_f, HCN4) plus T- and L-type Ca2+ current; slow Ca2+ upstroke, no fast Na+ channels, no plateau
- Supplies
- The sinus impulse to both atria via the preferential internodal pathways (to the AV node) and Bachmann's bundle (to the left atrium)
- Arterial supply
- SA nodal artery: from the proximal RCA in ~60% (55–70%), from the LCx in the remainder
- Autonomic
- Vagus (right vagus preferentially; M2, K+ efflux, reduced I_f) slows; sympathetic (beta-1, cAMP raises I_f and I_Ca) accelerates
- Conduction velocity
- ~0.05 m/s inside the node, the slowest tissue in the heart together with the AV node
When it goes wrong
Fibrosis or ischaemia of the node and its surroundings (sick sinus syndrome, usually elderly)
Sinus bradycardia, sinus pauses or arrest, sinoatrial exit block; tachy-brady syndrome with runs of AF alternating with long pauses; presyncope and syncope; permanent pacemaker if symptomatic
Inferior MI (RCA occlusion) with the Bezold–Jarisch vagal reflex
Sinus bradycardia and hypotension in the first hours; usually transient and atropine-responsive
High vagal tone (athletes, sleep), beta-blockers, calcium-channel blockers, digoxin, hypothyroidism, hypothermia, raised intracranial pressure
Sinus bradycardia below 60/min with a normal P–QRS relationship
Sympathetic drive: pain, fever, hypovolaemia, anaemia, thyrotoxicosis, pulmonary embolism
Sinus tachycardia above 100/min with gradual onset and offset and a normal upright P in II; treat the cause, not the rate
Practise this structure
1 question in the bank tagged Pacemaker Automaticity.