Cardiology In Critical Care
Impulses originate in an atrial pacemaker at a rate 240-340/min but some of them are blocked regularly at the AV junction. Atrial activity is represented by sawtooth-like deflections (F waves) not separated by an isoelectric segment and best seen in L2, L3 and AVF. Usually only every second F is conducted to the ventricles (2:1 AV block) with the resulting ventricular rate half the atrial rate; rarely the AV conduction is 3:1, 4:1, 1:1 or variable. Vagal stimulation may have no effect or slow ventricular rate by increasing AV block or terminate flutter. Exercise increases HR by decreasing the block.
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